← catalogue

The Working of Steel

by Fred H. Colvin · English · Gutenberg #20282 · 478,536 chars · public domain (US)

The Project Gutenberg eBook of The Working of Steel
    
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.

Title: The Working of Steel

Author: Fred H. Colvin
        K. A. Juthe


        
Release date: January 4, 2007 [eBook #20282]

Language: English

Other information and formats: www.gutenberg.org/ebooks/20282

Credits: Produced by Robert J. Hall


*** START OF THE PROJECT GUTENBERG EBOOK THE WORKING OF STEEL ***




Produced by Robert J. Hall




THE

WORKING OF STEEL


ANNEALING, HEAT TREATING

AND

HARDENING OF CARBON AND ALLOY STEEL


BY

FRED H. COLVIN

Member American Society of Mechanical Engineers and Franklin Institute;
Editor of the _American Machinist_, Author of "_Machine Shop
Arithmetic_," "_Machine Shop Calculations_," "_American Machinists'
Hand Book_."

AND

K. A. JUTHE, M.E.

Chief Engineer, American Metallurgical Corp. Member American Society
Mechanical Engineers, American Society Testing Materials, Heat
Treatment Association, Etc.


SECOND EDITION

THIRD IMPRESSION


McGRAW-HILL BOOK COMPANY, Inc.

NEW YORK: 370 SEVENTH AVENUE

LONDON: 6 & 8 BOUVERIE ST., E. C. 4




PREFACE TO SECOND EDITION

Advantage has been taken of a reprinting to revise, extensively,
the portions of the book relating to the modern science of
metallography. Considerable of the matter relating to the influence
of chemical composition upon the properties of alloy steels has
been rewritten. Furthermore, opportunity has been taken to include
some brief notes on methods of physical testing--whereby the
metallurgist judges of the excellence of his metal in advance of
its actual performance in service.

NEW YORK, N. Y.,

  _August, 1922._




PREFACE TO FIRST EDITION

The ever increasing uses of steel in all industries and the necessity
of securing the best results with the material used, make a knowledge
of the proper working of steel more important than ever before.
For it is not alone the quality of the steel itself or the alloys
used in its composition, but the proper working or treatment of
the steel which determines whether or not the best possible use
has been made of it.

With this in mind, the authors have drawn, not only from their
own experience but from the best sources available, information
as to the most approved methods of working the various kinds of
steel now in commercial use. These include low carbon, high carbon
and alloy steels of various kinds, and from a variety of industries.
The automotive field has done much to develop not only new alloys but
efficient methods of working them and has been drawn on liberally
so as to show the best practice. The practice in government arsenals
on steels used in fire arms is also given.

While not intended as a treatise on steel making or metallurgy in
any sense, it has seemed best to include a little information as
to the making of different steels and to give considerable general
information which it is believed will be helpful to those who desire
to become familiar with the most modern methods of working steel.

It is with the hope that this volume, which has endeavored to give
due credit to all sources of information, may prove of value to
its readers and through them to the industry at large.

_July_, 1921.

  THE AUTHORS.




CONTENTS

PREFACE

INTRODUCTION

CHAPTER
    I. STEEL MAKING
   II. COMPOSITION AND PROPERTIES OF STEELS
  III. ALLOYS AND THEIR EFFECT UPON STEEL
   IV. APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY
    V. THE FORGING OF STEEL
   VI. ANNEALING
  VII. CASE-HARDENING OR SURFACE-CARBURIZING
 VIII. HEAT TREATMENT OF STEEL
   IX. HARDENING CARBON STEEL FOR TOOLS
    X. HIGH SPEED STEEL
   XI. FURNACES
  XII. PYROMETRY AND PYROMETERS

APPENDIX

INDEX




INTRODUCTION

THE ABC OF IRON AND STEEL

In spite of all that has been written about iron and steel there
are many hazy notions in the minds of many mechanics regarding
them. It is not always clear as to just what makes the difference
between iron and steel. We know that high-carbon steel makes a
better cutting tool than low-carbon steel. And yet carbon alone
does not make all the difference because we know that cast iron
has more carbon than tool steel and yet it does not make a good
cutting tool.

Pig iron or cast iron has from 3 to 5 per cent carbon, while good
tool steel rarely has more than 1-1/4 per cent of carbon, yet one
is soft and has a coarse grain, while the other has a fine grain
and can be hardened by heating and dipping in water. Most of the
carbon in cast iron is in a form like graphite, which is almost pure
carbon, and is therefore called graphitic carbon. The resemblance
can be seen by noting how cast-iron borings blacken the hands just
as does graphite, while steel turnings do not have the same effect.
The difference is due to the fact that the carbon in steel is not
in a graphitic form as well as because it is present in smaller
quantities.

In making steel in the old way the cast iron was melted and the
carbon and other impurities burned out of it, the melted iron being
stirred or "puddled," meanwhile. The resulting puddled iron, also
known as wrought iron, is very low in carbon; it is tough, and on
being broken appears to be made up of a bundle of long fibers.
Then the iron was heated to redness for several days in material
containing carbon (charcoal) until it absorbed the desired amount,
which made it steel, just as case-hardening iron or steel adds
carbon to the outer surface of the metal. The carbon absorbed by
the iron does not take on a graphitic form, however, as in the
case of cast iron, but enters into a chemical compound with the
iron, a hard brittle substance called "cementite" by metallurgists.
In fact, the difference between the hard, brittle cementite and
the soft, greasy graphite, accounts for many of the differences
between steel and gray cast iron. Wrought iron, which has very
little carbon of any sort in it, is fairly soft and tough. The
properties of wrought iron are the properties of pure iron. As
more and more carbon is introduced into the iron, it combines with
the iron and distributes itself throughout the metal in extremely
small crystals of cementite, and this brittle, hard substance lends
more and more hardness and strength to the steel, at the expense
of the original toughness of the iron. As more and more carbon is
contained in the alloy--for steel is a true alloy--it begins to
appear as graphite, and its properties counteract the remaining
brittle cementite. Eventually, in gray cast iron, we have properties
which would be expected of wrought iron, whose tough metallic texture
was shot through with flakes of slippery, weak graphite.

But to return to the methods of making steel tools in use 100 years
ago.

The iron bars, after heating in charcoal, were broken and the carbon
content judged by the fracture. Those which had been in the hottest
part of the furnace would have the deepest "case" and highest carbon.
So when the steel was graded, and separated into different piles,
a few bars of like kind were broken into short lengths, melted
in fire-clay crucibles at an intense white heat, cast carefully
into iron molds, and the resulting ingot forged into bars under
a crude trip hammer. This melting practice is still in use for
crucible steel, and will be described further on page 4.




THE WORKING OF STEEL


ANNEALING, HEAT TREATING AND HARDENING

OF

CARBON AND ALLOY STEEL


CHAPTER I

STEEL MAKING

There are four processes now used for the manufacture of steel.
These are: The Bessemer, Open Hearth, Crucible and Electric Furnace
Methods.

BESSEMER PROCESS

The bessemer process consists of charging molten pig iron into
a huge, brick-lined pot called the bessemer converter, and then
in blowing a current of air through holes in the bottom of the
vessel into the liquid metal.

The air blast burns the white hot metal, and the temperature increases.
The action is exactly similar to what happens in a fire box under
forced draft. And in both cases some parts of the material burn
easier and more quickly than others. Thus it is that some of the
impurities in the pig iron--including the carbon--burn first, and
if the blast is shut off when they are gone but little of the iron
is destroyed. Unfortunately sulphur, one of the most dangerous
impurities, is not expelled in the process.

A bessemer converter is shown in Fig. 1, while Fig. 2 shows the
details of its construction. This shows how the air blast is forced
in from one side, through the trunnion, and up through the metal.
Where the steel is finished the converter is tilted, or swung on
its trunnions, the blast turned off, and the steel poured out of
the top.

OPEN HEARTH PROCESS

The open hearth furnace consists of a big brick room with a low
arched roof. It is charged with pig iron and scrap through doors
in the side walls.

[Illustration: FIG. 1.--A typical Bessemer converter.]

Through openings at one end of the furnace come hot air and gas,
which burn in the furnace, producing sufficient heat to melt the
charge and refine it of its impurities. Lime and other nonmetallic
substances are put in the furnace. These melt, forming a "slag"
which floats on the metal and aids materially in the refining
operations.

In the bessemer process air is forced _through_ the metal. In the
open-hearth furnace the metal is protected from the flaming gases
by a slag covering. Therefore it is reasonable to suppose that
the final product will not contain so much gas.

[Illustration: FIG. 2.--Action of Bessemer converter.]

[Illustration: FIG. 3.--Regenerative open hearth furnace.]

A diagram of a modern regenerative furnace is shown in Fig. 3.
Air and gas enter the hearth through chambers loosely packed with
hot fire brick, burn, and exit to the chimney through another pair
of chambers, giving to them some of the heat which would otherwise
waste. The direction is reversed about every twenty minutes by
changing the position of the dampers.

CRUCIBLE STEEL

Crucible steel is still made by melting material in a clay or graphite
crucible. Each crucible contains about 40 lb. of best puddled iron,
40 lb. of clean "mill scrap"--ends trimmed from tool steel bars--and
sufficient rich alloys and charcoal to make the mixture conform to
the desired chemical analysis. The crucible is covered, lowered
into a melting hole (Fig. 4) and entirely surrounded by burning
coke. In about four hours the metal is converted into a quiet white
hot liquid. Several crucibles are then pulled out of the hole, and
their contents carefully poured into a metal mold, forming an ingot.

[Illustration: FIG. 4.--Typical crucible furnace.]

If modern high-speed steel is being made, the ingots are taken
out of the molds while still red hot and placed in a furnace which
keeps them at this temperature for some hours, an operation known
as annealing. After slow cooling any surface defects are ground
out. Ingots are then reheated to forging temperature, hammered
down into "billets" of about one-quarter size, and 10 to 20 per
cent of the length cut from the top. After reheating the billets
are hammered or rolled into bars of desired size. Finished bars are
packed with a little charcoal into large pipes, the ends sealed,
and annealed for two or three days. After careful inspection and
testing the steel is ready for market.

THE ELECTRIC PROCESS

The fourth method of manufacturing steel is by the electric furnace.
These furnaces are of various sizes and designs; their size may be
sufficient for only 100 lb. of metal--on the other hand electric
furnaces for making armor-plate steel will hold 40 tons of steel.
Designs vary widely according to the electrical principles used.
A popular furnace is the 6-ton Heroult furnace illustrated in Fig. 5.

It is seen to be a squat kettle, made of heavy sheet steel, with
a dished bottom and mounted so it can be tilted forward slightly
and completely drained. This kettle is lined with special fire
brick which will withstand most intense heat and resist the cutting
action of hot metal and slag. For a roof, a low dome of fire brick
is provided. The shell and lining is pierced in front for a pouring
spout, and on either side by doors, through which the raw material
is charged.

Two or three carbon "electrodes"--18-in. cylinders of specially
prepared coke or graphite--extend through holes in the roof. Electrical
connections are made to the upper ends, and a very high current
sent through them. This causes tremendous arcs to form between
the lower ends of the electrodes and the metal below, and these
electric arcs are the only source of heat in this style of furnace.

Electric furnaces can be used to do the same work as is done in
crucible furnaces--that is to say, merely melt a charge of carefully
selected pure raw materials. On the other hand it can be used to
produce very high-grade steel from cheap and impure metal, when
it acts more like an open-hearth furnace. It can push the refining
even further than the latter furnace does, for two reasons: first
the bath is not swept continuously by a flaming mass of gases;
second, the temperature can be run up higher, enabling the operator
to make up slags which are difficult to melt but very useful to
remove small traces of impurities from the metal.

Electric furnaces are widely used, not only in the iron industry,
but in brass, copper and aluminum works. It is a useful melter of
cold metal for making castings. It can be used to convert iron
into steel or vice versa. Its most useful sphere, however, is as a
refiner of metal, wherein it takes either cold steel or molten steel
from open hearth or bessemer furnaces, and gives it the finishing
touches.

[Illustration: FIG. 5.--"Slagging off" an electric furnace.]

[Illustration: FIG. 6.--Pouring the ingots.]

As an illustration of the furnace reactions that take place the
following schedule is given, showing the various stages in the
making of a heat of electric steel. The steel to be made was a
high-carbon chrome steel used for balls for ball bearings:

6-TON HEROULT FURNACE

11:50 A.M.--Material charged:
  Boiler plate   5,980 lb.
  Stampings      5,991 lb.
               -----------
                11,971 lb.
  Limestone        700 lb.
12:29 P.M.--Completed charging (current switched on).
 3:20 P.M.--Charge melted down.
  Preliminary analysis under black slag.
    Analysis:
      Carbon      Silicon     Sulphur     Phosphorus  Manganese
       0.06        0.014       0.032        0.009       0.08
           Note the practical elimination of phosphorus.
 3:40 P.M.--The oxidizing (black) slag is now poured and skimmed off as
   clean as possible to prevent rephosphorizing and to permit of adding
   carburizing materials. For this purpose carbon is added in the form
   of powdered coke, ground electrodes or other forms of pure carbon.

The deoxidizing slag is now formed by additions of lime, coke and
fluorspar (and for some analyses ferrosilicon). The slag changes
from black to white as the metallic oxides are reduced by these
deoxidizing additions and the reduced metals return to the bath.
A good finishing slag is creamy white, porous and viscous. After
the slag becomes white, some time is necessary for the absorption
of the sulphur in the bath by the slag.

The white slag disintegrates to a powder when exposed to the atmosphere
and has a pronounced odor of acetylene when wet.

Further additions of recarburizing material are added as needed to
meet the analysis. The further reactions are shown by the following:

3:40 P.M.--Recarburizing material added:
  130 lb. ground electrodes.
   25 lb. ferromanganese.
    Analysis:
      Carbon   Silicon  Sulphur  Phosphorus  Manganese
       0.76     0.011    0.030     0.008       0.26

To form white slag there was added:

  225 lb. lime.
   75 lb. powdered coke.
   55 lb. fluorspar.
4:50 P.M.--
  Analysis:
      Carbon   Silicon  Sulphur  Phosphorus  Manganese
       0.75     0.014    0.012     0.008       0.28
           Note reduction of the sulphur content.

During the white-slag period the following alloying additions were
made:

  500 lb. pig iron.
   80 lb. ferrosilicon.
    9 lb. ferromanganese.
  146 lb. 6 per cent carbon ferrochrome.

The furnace was rotated forward to an inclined position and the
charge poured into the ladle, from which in turn it was poured
into molds.

5:40 P.M.--Heat poured.
  Analysis:
      Carbon   Silicon  Sulphur  Phosphorus  Manganese  Chromium
       0.97     0.25     0.014     0.013       0.33       0.70

Ingot weight poured  94.0 per cent
Scull                 2.7 per cent
Loss                  3.3 per cent

Total current consumption for the heat, 4,700 kW.-hr. or 710 kw.-hr.
per ton.

Electric steel, in fact, all fine steel, should be cast in big-end-up
molds with refractory hot tops to prevent any possibility of pipage
in the body of the ingot. In the further processing of the ingot,
whether in the rolling mill or forge, special precautions should
be taken in the heating, in the reduction of the metal and in the
cooling.

No attempt is made to compare the relative merits of open hearth
and electric steel; results in service, day in and day out, have,
however, thoroughly established the desirability of electric steel.
Ten years of experience indicate that electric steel is equal to
crucible steel and superior to open hearth.

The rare purity of the heat derived from the electric are, combined
with definite control of the slag in a neutral atmosphere, explains
in part the superiority of electric steel. Commenting on this recently
Dr. H. M. Howe stated that "in the open hearth process you have such
atmosphere and slag conditions as you can get, and in the electric
you have such atmosphere and slag conditions as you desire."

Another type of electric furnace is shown in Figs. 7 and 8. This
is the Ludlum furnace, the illustrations showing a 10-ton size.
Figure 7 shows it in normal, or melting position, while in Fig.
8 it is tilted for pouring. In melting, the electrodes first rest
on the charge of material in the furnace. After the current is
turned on they eat their way through, nearly to the bottom. By
this time there is a pool of molten metal beneath the electrode
and the charge is melted from the bottom up so that the roof is
not exposed to the high temperature radiating from the open arc.
The electrodes in this furnace are of graphite, 9 in. in diameter
and the current consumed is about 500 kw.-hr. per ton.

[Illustration: FIG. 7.--Ludlum electric furnace.]

[Illustration: FIG. S.--The furnace tilted for pouring.]

One of the things which sometimes confuse regarding the contents
of steel is the fact that the percentage of carbon and the other
alloys are usually designated in different ways. Carbon is usually
designated by "points" and the other alloys by percentages. The
point is one ten-thousandth while 1 per cent is one one-hundredth
of the whole. In other words, "one hundred point carbon" is steel
containing 1 per cent carbon. Twenty point carbon, such as is used
for carbonizing purposes is 0.20 per cent. Tool steel varies from
one hundred to one hundred and fifty points carbon, or from 1.00
to 1.50 per cent.

Nickel, chromium, etc., are always given in per cent, as a 3.5
per cent nickel, which means exactly what it says--3-1/2 parts in
100. Bearing this difference in mind all confusion will be avoided.

CLASSIFICATIONS OF STEEL

Among makers and sellers, carbon tool-steels are classed by "grade"
and "temper." The word grade is qualified by many adjectives of
more or less cryptic meaning, but in general they aim to denote
the process and care with which the steel is made.

_Temper_ of a steel refers to the carbon content. This should preferably
be noted by "points," as just explained; but unfortunately, a 53-point
steel (containing 0.53 per cent carbon) may locally be called something
like "No. 3 temper."

A widely used method of classifying steels was originated by the
Society of Automotive Engineers. Each specification is represented
by a number of 4 digits, the first figure indicating the class, the
second figure the approximate percentage of predominant alloying
element, and the last two the average carbon content in points.
Plain carbon steels are class 1, nickel steels are class 2,
nickel-chromium steels are class 3, chromium steels are class 5,
chromium-vanadium steels are class 6, and silico-manganese steels
are class 9. Thus by this system, steel 2340 would be a 3 per cent
nickel steel with 0.40 per cent carbon; or steel 1025 would be a
0.25 plain carbon steel.

Steel makers have no uniform classification for the various kinds
of steel or steels used for different purposes. The following list
shows the names used by some of the well-known makers:

Air-hardening steel       Chrome-vanadium steel
Alloy steel               Circular saw plates
Automobile steel          Coal auger steel
Awl steel                 Coal mining pick or cutter steel
Axe and hatchet steel     Coal wedge steel
Band knife steel          Cone steel
Band saw steel            Crucible cast steel
Butcher saw steel         Crucible machinery steel
Chisel steel              Cutlery steel
Chrome-nickel steel       Drawing die steel (Wortle)

Drill rod steel           Patent, bush or hammer steel
Facing and welding steel  Pick steel
Fork steel                Pivot steel
Gin saw steel             Plane bit steel
Granite wedge steel       Quarry steel
Gun barrel steel          Razor steel
Hack saw steel            Roll turning steel
High-speed tool steel     Saw steel
Hot-rolled sheet steel    Scythe steel
Lathe spindle steel       Shear knife steel
Lawn mower knife steel    Silico-manganese steel
Machine knife steel       Spindle steel
Magnet steel              Spring steel
Mining drill steel        Tool holder steel
Nail die shapes           Vanadium tool steel
Nickel-chrome steel       Vanadium-chrome steel
Paper knife steel         Wortle steel

Passing to the tonnage specifications, the following table from
Tiemann's excellent pocket book on "Iron and Steel," will give
an approximate idea of the ordinary designations now in use:

                  Approximate
  Grades          carbon range             Common uses

Extra soft         0.08-0.18    Pipe, chain and other welding purposes;
(dead soft)                     case-hardening purposes; rivets; pressing
                                and stamping purposes.
Structural (soft)  0.15-0.25    Structural plates, shapes and bars for
(medium)                        bridges, buildings, cars, locomotives;
                                boiler (flange) steel; drop forgings; bolts.
Medium             0.20-0.35    Structural purposes (ships); shafting;
                                automobile parts; drop forgings.
Medium hard        0.35-0.60    Locomotive and similar large forgings; car
                                axles; rails.
Hard               0.60-0.85    Wrought steel wheels for steam and electric
                                railway service; locomotive tires; rails;
                                tools, such as sledges, hammers, pick points,
                                crowbars, etc.
Spring             0.85-1.05    Automobile and other vehicle springs; tools,
                                such as hot and cold chisels, rock drills
                                and shear blades.
Spring             0.90-1.15    Railway springs; general machine shop tools.




CHAPTER II

COMPOSITION AND PROPERTIES OF STEEL

It is a remarkable fact that one can look through a dozen text
books on metallurgy and not find a definition of the word "steel."
Some of them describe the properties of many other irons and then
allow you to guess that everything else is steel. If it was difficult
a hundred years ago to give a good definition of the term when the
metal was made by only one or two processes, it is doubly difficult
now, since the introduction of so many new operations and furnaces.

We are in better shape to know what steel is than our forefathers.
They went through certain operations and they got a soft malleable,
weldable metal which would not harden; this they called iron. Certain
other operations gave them something which looked very much like
iron, but which would harden after quenching from a red heat. This
was steel. Not knowing the essential difference between the two,
they must distinguish by the process of manufacture. To-day we
can make either variety by several methods, and can convert either
into the other at will, back and forth as often as we wish; so
we are able to distinguish between the two more logically.

We know that iron is a chemical element--the chemists write it
Fe for short, after the Latin word "ferrum," meaning iron--it is
one of those substances which cannot be separated into anything
else but itself. It can be made to join with other elements; for
instance, it joins with the oxygen in the air and forms scale or
rust, substances known to the chemist as iron oxide. But the same
metal iron can be recovered from that rust by abstracting the oxygen;
having recovered the iron nothing else can be extracted but iron;
_iron is elemental_.

We can get relatively pure iron from various minerals and artificial
substances, and when we get it we always have a magnetic metal,
almost infusible, ductile, fairly strong, tough, something which
can be hardened slightly by hammering but which cannot be hardened
by quenching. It has certain chemical properties, which need not be
described, which allow a skilled chemist to distinguish it without
difficulty and unerringly from the other known elements--nearly
100 of them.

Carbon is another chemical element, written C for short, which is
widely distributed through nature. Carbon also readily combines
with oxygen and other chemical elements, so that it is rarely found
pure; its most familiar form is soot, although the rarer graphite and
most rare diamond are also forms of quite pure carbon. It can also
be readily separated from its multitude of compounds (vegetation,
coal, limestone, petroleum) by the chemist.

With the rise of knowledge of scientific chemistry, it was quickly
found that the essential difference between iron and steel was that
the latter was _iron plus carbon_. Consequently it is an alloy,
and the definition which modern metallurgists accept is this:

"Steel is an iron-carbon alloy containing less than about 2 per
cent carbon."

Of course there are other elements contained in commercial steel,
and these elements are especially important in modern "alloy steels,"
but carbon is the element which changes a soft metal into one which
may be hardened, and strengthened by quenching. In fact, carbon,
of itself, without heat treatment, strengthens iron at the expense
of ductility (as noted by the percentage elongation an 8-in. bar
will stretch before breaking). This is shown by the following table:

--------------------------------------------------------------------------
                  |           |            |Elastic |Ultimate|Percentage.
  Class by use.   | Class by  |  Per cent  | limit  |strength|elongation
                  | hardness. |  carbon.   |lb. per |lb. per |in 8 inches.
                  |           |            |sq. in. |sq. in. |
------------------|-----------|------------|--------|--------|------------
Boiler rivet steel|Dead soft  |0.08 to 0.15| 25,000 | 50,000 |     30
Struc. rivet steel|Soft       |0.15 to 0.22| 30,000 | 55,000 |     30
Boiler plate steel|Soft       |0.08 to 0.10| 30,000 | 60,000 |     25
Structural steel  |Medium     |0.18 to 0.30| 35,000 | 65,000 |     25
Machinery steel   |Hard       |0.35 to 0.60| 40,000 | 75,000 |     20
Rail steel        |Hard       |0.35 to 0.55| 40,000 | 75,000 |     15
Spring steel      |High carbon|1.00 to 1.50| 60,000 |125,000 |     10
Tool steel        |High carbon|0.90 to 1.50| 80,000 |150,000 |      5
--------------------------------------------------------------------------

Just why a soft material like carbon (graphite), when added to
another soft material like iron, should make the iron harder, has
been quite a mystery, and one which has caused a tremendous amount
of study. The mutual interactions of these two elements in various
proportions and at various temperatures will be discussed at greater
length later, especially in Chap. VIII, p. 105. But we may anticipate
by saying that some of the iron unites with all the carbon to form a
new substance, very hard, a carbide which has been called "cementite."
The compound always contains iron and carbon in the proportions
of three atoms of iron to one atom of carbon; chemists note this
fact in shorthand by the symbol Fe3C (a definite chemical compound
of three atoms of iron to one of carbon). Many of the properties
of steel, as they vary with carbon content, can be linked up with
the increasing amount of this hard carbide cementite, distributed
in very fine particles through the softer iron.

SULPHUR is another element (symbol S) which is always found in
steel in small quantities. Some sulphur is contained in the ore
from which the iron is smelted; more sulphur is introduced by the
coke and fuel used. Sulphur is very difficult to get rid of in
steel making; in fact the resulting metal usually contains a little
more than the raw materials used. Only the electric furnace is
able to produce the necessary heat and slags required to eliminate
sulphur, and as a matter of fact the sulphur does not go until
several other impurities have been eliminated. Consequently, an
electric steel with extremely low sulphur (0.02 per cent) is by
that same token a well-made metal.

Sulphur is of most trouble to rolling and forging operations when
conducted at a red heat. It makes steel tender and brittle at that
temperature--a condition known to the workmen as "red-short." It
seems to have little or no effect upon the physical properties
of cold steel--at least as revealed by the ordinary testing
machines--consequently many specifications do not set any limit
on sulphur, resting on the idea that if sulphur is low enough not
to cause trouble to the manufacturer during rolling, it will not
cause the user any trouble.

Tool steel and other fine steels should be very low in sulphur,
preferably not higher than 0.03 per cent. Higher sulphur steels
(0.06 per cent, and even up to 0.10 per cent) have given very good
service for machine parts, but in general a high sulphur steel
is a suspicious steel. Screw stock is purposely made with up to
0.12 per cent sulphur and a like amount of phosphorus so it will
cut freely.

Manganese counteracts the detrimental effect of sulphur when present
in the steel to an amount at least five times the sulphur content.

PHOSPHORUS is an element (symbol P) which enters the metal from
the ore. It remains in the steel when made by the so-called acid
process, but it can be easily eliminated down to 0.06 per cent
in the basic process. In fact the discovery of the basic process
was necessary before the huge iron deposits of Belgium and the
Franco-German border could be used. These ores contain several
per cent phosphorus, and made a very brittle steel ("cold short")
until basic furnaces were used. Basic furnaces allow the formation
of a slag high in lime, which takes practically all the phosphorus
out of the metal. Not only is the resulting metal usable, but the
slag makes a very excellent fertilizer, and is in good demand.

SILICON is a very widespread element (symbol Si), being an essential
constituent of nearly all the rocks of the earth. It is similar to
carbon in many of its chemical properties; for instance it burns
very readily in oxygen, and consequently native silicon is unknown--it
is always found in combination with one or more other elements.
When it bums, each atom of silicon unites with two atoms of oxygen
to form a compound known to chemists as silica (SiO2), and to the
small boy as "sand" and "agate."

Iron ore (an oxide of iron) contains more or less sand and dirt
mixed in it when it is mined, and not only the iron oxide but also
some of the silicon oxide is robbed of its oxygen by the smelting
process. Pig iron--the product of the blast furnace--therefore
contains from 1 to 3 per cent of silicon, and some silicon remains
in the metal after it has been purified and converted into steel.

However, silicon, as noted above, burns very readily in oxygen,
and this property is of good use in steel making. At the end of
the steel-making process the metal contains more or less oxygen,
which must be removed. This is sometimes done (especially in the
so-called acid process) by adding a small amount of silicon to
the hot metal just before it leaves the furnace, and stirring it
in. It thereupon abstracts oxygen from the metal wherever it finds
it, changing to silica (SiO2) which rises and floats on the surface
of the cleaned metal. Most of the silicon remaining in the metal
is an excess over that which is required to remove the dangerous
oxygen, and the final analysis of many steels show enough silicon
(from 0.20 to 0.40) to make sure that this step in the manufacture
has been properly done.

MANGANESE is a metal much like iron. Its chemical symbol is Mn. It
is somewhat more active than iron in many chemical changes--notably
it has what is apparently a stronger attraction for oxygen and
sulphur than has iron. Therefore the metal is used (especially in
the so-called basic process) to free the molten steel of oxygen,
acting in a manner similar to silicon, as explained above. The
compound of manganese and oxygen is readily eliminated from the
metal. Sufficient excess of elemental manganese should remain so
that the purchaser may be sure that the iron has been properly
"deoxidized," and to render harmless the traces of sulphur present.
No damage is done by the presence of a little manganese in steel,
quite the reverse. Consequently it is common to find steels containing
from 0.3 to 1.5 per cent.

ALLOYING ELEMENTS.--Commercial steels of even the simplest types
are therefore primarily alloys of iron and carbon. Impurities and
their "remedies" are always present: sulphur, phosphorus, silicon
and manganese--to say nothing of oxygen, nitrogen and carbon oxide
gases, about which we know very little. It has been found that other
metals, if added to well-made steel, produce definite improvements
in certain directions, and these "alloy steels" have found much
use in the last ten years. Alloy steels, in addition to the
above-mentioned elements, may commonly contain one or more of the
following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium
(Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed
at more length in Chapters III and IV.

PROPERTIES OF STEEL

Steels are known by certain tests. Early tests were more or less
crude, and depended upon the ability of the workman to judge the
"grain" exhibited by a freshly broken piece of steel. The cold-bend
test was also very useful--a small bar was bent flat upon itself,
and the stretched fibers examined for any sign of break. Harder
stiff steels were supported at the ends and the amount of central
load they would support before fracture, or the amount of permanent
set they would acquire at a given load noted. Files were also used
to test the hardness of very hard steel.

These tests are still used to a considerable extent, especially in
works where the progress of an operation can be kept under close
watch in this way, the product being periodically examined by more
precise methods. The chief furnace-man, or "melter," in a steel
plant, judges the course of the refining process by casting small
test ingots from time to time, breaking them and examining the
fracture. Cutlery manufacturers use the bend test to judge the
temper of blades. File testing of case-hardened parts is very common.

However there is need of standardized methods which depend less
upon the individual skill of the operator, and which will yield
results comparable to others made by different men at different
places and on different steels. Hence has grown up the art of testing
materials.

TENSILE PROPERTIES

Strength of a metal is usually expressed in the number of pounds
a 1-in. bar will support just before breaking, a term called the
"ultimate strength." It has been found that the shape of the test
bar and its method of loading has some effect upon the results,
so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.
in diameter for a central length of 2-3/8 in., ending the turn
with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so
the load it bears at rupture multiplied by 5 will represent the
"ultimate strength" in pounds per square inch.

Such a test bar is stretched apart in a machine like that shown
in Fig. 9. The upper end of the bar is held in wedged jaws by the
top cross-head, and the lower end grasped by the movable head.
The latter is moved up and down by three long screws, driven at
the same speed, which pass through threads cut in the corners of
the cross-head. When the test piece is fixed in position the motor
which drives the machine is given a few turns, which by proper
gearing pulls the cross-head down with a certain pull. This pull
is transmitted to the upper cross-head by the test bar, and can
be weighed on the scale arm, acting through a system of links and
levers.

Thus the load may be increased as rapidly as desirable, always
kept balanced by the weighing mechanism, and the load at fracture
may be read directly from the scale beam.

This same test piece may give other information. If light punch
marks are made, 2 in. apart, before the test is begun, the broken
ends may be clamped together, and the distance between punch marks
measured. If it now measures 3 in. the stretch has been 1 in. in 2,
or 50 per cent. This figure is known as the elongation at fracture,
or briefly, the "elongation," and is generally taken to be a measure
of ductility.

When steel shows any elongation, it also contracts in area at the
same time. Often this contraction is sharply localized at the fracture;
the piece is said to "neck." A figure for contraction in area is
also of much interest as an indication of toughness; the diameter
at fracture is measured, a corresponding area taken out from a
table of circles, subtracted from the original area (0.200 sq.
in.) and the difference divided by 0.2 to get the percentage
contraction.

[Illustration: FIG. 9.--Olsen testing machine.]

Quite often it is desired to discover the elastic limit of the
steel, in fact this is of more use to the designer than the ultimate
strength. The elastic limit is usually very close to the load where
the metal takes on a permanent set. That is to say, if a delicate
caliper ("extensometer," so called) be fixed to the side of the
test specimen, it would show the piece to be somewhat longer under
load than when free. Furthermore, if the load had not yet reached the
yield point, and were released at any time, the piece would return
to its original length. However, if the load had been excessive, and
then relieved, the extensometer would no longer read exactly 2.0
in., but something more.

Soft steels "give" very quickly at the yield point. In fact, if
the testing machine is running slowly, it takes some time for the
lower head to catch up with the stretching steel. Consequently at
the yield point, the top head is suddenly but only temporarily
relieved of load, and the scale beam drops. In commercial practice,
the yield point is therefore determined by the "drop of the beam."
For more precise work the calipers are read at intervals of 500 or
1,000 lb. load, and a curve plotted from these results, a curve
which runs straight up to the elastic limit, but there bends off.

A tensile test therefore gives four properties of great usefulness:
The yield point, the ultimate strength, the elongation and the
contraction. Compression tests are seldom made, since the action
of metal in compression and in tension is closely allied, and the
designer is usually satisfied with the latter.

IMPACT TESTS

Impact tests are of considerable importance as an indication of
how a metal will perform under shock. Some engineers think that
the tensile test, which is one made under slow loading, should
therefore be supplemented by another showing what will happen if
the load is applied almost instantaneously. This test, however, has
not been standardized, and depends to a considerable extent upon
the type of machine, but more especially the size of the specimen
and the way it is "nicked." The machine is generally a swinging
heavy pendulum. It falls a certain height, strikes the sample at
the lowest point, and swings on past. The difference between the
downward and upward swing is a measure of the energy it took to
break the test piece.

FATIGUE TESTS

It has been known for fifty years that a beam or rod would fail
at a relatively low stress if only repeated often enough. It has
been found, however, that each material possesses a limiting stress,
or endurance limit, within which it is safe, no matter how often
the loading occurs. That limiting stress for all steels so far
investigated causes fracture below 10 million reversals. In other
words, a steel which will not break before 10,000,000 reversals
can confidently be expected to endure 100,000,000, and doubtless
into the billions.

About the only way to test one piece such a large number of times
is to fashion it into a beam, load it, and then turn the beam in
its supports. Thus the stress in the outer fibers of the bar varies
from a maximum stretch through zero to a maximum compression, and
back again. A simple machine of this sort is shown in Fig. 10,
where _B_ and _E_ are bearings, _A_ the test piece, turned slightly
down in the center, _C_ and _D_ ball bearings supporting a load
_W_. _K_ is a pulley for driving the machine and _N_ is a counter.

[Illustration: FIG. 10.--Sketch of rotating beam machine for measuring
endurance of metal.]

HARDNESS TESTING

The word "hardness" is used to express various properties of metals,
and is measured in as many different ways.

"Scratch hardness" is used by the geologist, who has constructed
"Moh's scale" as follows:

  Talc      has a hardness of  1
  Rock Salt has a hardness of  2
  Calcite   has a hardness of  3
  Fluorite  has a hardness of  4
  Apatite   has a hardness of  5
  Feldspar  has a hardness of  6
  Quartz    has a hardness of  7
  Topaz     has a hardness of  8
  Corundum  has a hardness of  9
  Diamond   has a hardness of 10

A mineral will scratch all those above it in the series, and will
be scratched by those below. A weighted diamond cone drawn slowly
over a surface will leave a path the width of which (measured by
a microscope) varies inversely as the scratch hardness.

"Cutting hardness" is measured by a standardized drilling machine,
and has a limited application in machine-shop practice.

"Rebounding hardness" is commonly measured by the Shore scleroscope,
illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter,
3/4 in. in length, and weighing about 1/12 oz. is dropped a distance
of 10 in. upon the test piece. The height of rebound in arbitrary
units represents the hardness numeral.

[Illustration: FIG. 11.--Shore scleroscope.]

Should the hammer have a hard flat surface and drop on steel so hard
that no impression were made, it would rebound about 90 per cent
of the fall. The point, however, consists of a slightly spherical,
blunt diamond nose 0.02 in. in diameter, which will indent the steel
to a certain extent. The work required to make the indentation
is taken from the energy of the falling body; the rebound will
absorb the balance, and the hammer will now rise from the same
steel a distance equal to about 75 per cent of the fall. A permanent
impression is left upon the test piece because the impact will
develop a force of several hundred thousand pounds per square inch
under the tiny diamond-pointed hammer head, stressing the test
piece at this point of contact much beyond its ultimate strength.
The rebound is thus dependent upon the indentation hardness, for
the reason that the less the indentation, the more energy will
reappear in the rebound; also, the less the indentation, the harder
the material. Consequently, the harder the material, the more the
rebound.

"Indentation hardness" is a measure of a material's resistance
to penetration and deformation. The standard testing machine is
the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter,
is forced into the test piece with a pressure of 3,000 kg. (3-1/3
tons). The resulting indentation is then measured.

[Illustration: FIG. 12.--Hydraulic testing machine. (Brinell
principle.)]

While under load, the steel ball in a Brinell machine naturally
flattens somewhat. The indentation left behind in the test piece is
a duplicate of the surface which made it, and is usually regarded
as being the segment of a sphere of somewhat larger radius than
the ball. The radius of curvature of this spherical indentation
will vary slightly with the load and the depth of indentation.
The Brinell hardness numeral is the quotient found by dividing the
test pressure in kilograms by the spherical area of the indentation.
The denominator, as before, will vary according to the size of the
sphere, the hardness of the sphere and the load. These items have
been standardized, and the following table has been constructed
so that if the diameter of the identation produced by a load of
3,000 kg. be measured the hardness numeral is found directly.

                      TABLE FOR BRINELL BALL TEST
------------------------------------------------------------------------
Diameter of Ball | Hardness Number | Diameter of Ball | Hardness Number
Impression, mm.  |  for a Load of  | Impression, mm.  |  for a Load of
                 |    3,000 kg.    |                  |    3,000 kg.
-----------------|-----------------|------------------|-----------------
      2.0        |       946       |       4.5        |       179
      2.1        |       857       |       4.6        |       170
      2.2        |       782       |       4 7        |       163
      2.3        |       713       |       4.8        |       156
      2.4        |       652       |       4.9        |       149
      2.5        |       600       |       5.0        |       143
                 |                 |                  |
      2.6        |       555       |       5.1        |       137
      2.7        |       512       |       5.2        |       131
      2.8        |       477       |       5.3        |       126
      2.9        |       444       |       5.4        |       121
      3.0        |       418       |       5.5        |       116
                 |                 |                  |
      3.1        |       387       |       5.6        |       112
      3.2        |       364       |       5.7        |       107
      3.3        |       340       |       5.8        |       103
      3.4        |       321       |       5.9        |        99
      3.5        |       302       |       6.0        |        95
                 |                 |                  |
      3.6        |       286       |       6.1        |        92
      3.7        |       269       |       6.2        |        89
      3.8        |       255       |       6.3        |        86
      3.9        |       241       |       6.4        |        83
      4.0        |       228       |       6.5        |        80
                 |                 |                  |
      4.1        |       217       |       6.6        |        77
      4.2        |       207       |       6.7        |        74
      4.3        |       196       |       6.8        |        71.5
      4.4        |       187       |       6.9        |        69
------------------------------------------------------------------------




CHAPTER III

ALLOYS AND THEIR EFFECT UPON STEEL

In view of the fact that alloy steels are coming into a great deal
of prominence, it would be well for the users of these steels to
fully appreciate the effects of the alloys upon the various grades
of steel. We have endeavored to summarize the effect of these alloys
so that the users can appreciate their effect, without having to
study a metallurgical treatise and then, perhaps, not get the crux
of the matter.

NICKEL

Nickel may be considered as the toughest among the non-rare alloys
now used in steel manufacture. Originally nickel was added to give
increased strength and toughness over that obtained with the ordinary
rolled structural steel and little attempt was made to utilize its
great possibilities so far as heat treatment was concerned.

The difficulties experienced have been a tendency towards laminated
structure during manufacture and great liability to seam, both
arising from improper melting practice. When extra care is exercised
in the manufacture, particularly in the melting and rolling, many
of these difficulties can be overcome.

The electric steel furnace, of modern construction, is a very important
step forward in the melting of nickel steel; neither the crucible
process nor basic or acid open-hearth furnaces give such good results.

Great care must be exercised in reheating the billet for rolling
so that the steel is correctly soaked. The rolling must not be
forced; too big reduction per pass should not be indulged in, as
this sets up a tendency towards seams.

Nickel steel has remarkably good mechanical qualities when suitably
heat-treated, and it is preeminently adapted for case-hardening. It
is not difficult to machine low-nickel steel, consequently it is
in great favor where easy machining properties are of importance.

Nickel influences the strength and ductility of steel by being
dissolved directly in the iron or ferrite; in this respect differing
from chromium, tungsten and vanadium. The addition of each 1 per
cent nickel up to 5 per cent will cause an approximate increase of
from 4,000 to 6,000 lb. per square inch in the tensile strength and
elastic limit over the corresponding steel and without any decrease
in ductility. The static strength of nickel steel is affected to
some degree by the percentage of carbon; for instance, steel with
0.25 per cent carbon and 3.5 per cent nickel has a tensile strength,
in its normal state, equal to a straight carbon steel of 0.5 per
cent with a proportionately greater elastic limit and retaining
all the advantages of the ductility of the lower carbon.

To bring out the full qualities of nickel it must be heat-treated,
otherwise there is no object in using nickel as an alloy with carbon
steel as the additional cost is not justified by increased strength.

Nickel has a peculiar effect upon the critical ranges of steel,
the critical range being lowered by the percentage of nickel; in
this respect it is similar to manganese.

Nickel can be alloyed with steel in various percentages, each percentage
having a very definite effect on the microstructure. For instance, a
steel with 0.2 per cent carbon and 2 per cent nickel has a pearlitic
structure but the grain is much finer than if the straight carbon
were used. With the same carbon content and say 5 per cent nickel,
the structure would still be pearlitic, but much finer and denser,
therefore capable of withstanding shock, and having greater dynamic
strength. With about 0.2 per cent carbon and 8 per cent nickel, the
steel is nearing the stage between pearlite and martensite, and
the structure is extremely fine, the ferrite and pearlite having
a very pronounced tendency to mimic a purely martensite structure.
Steel with 0.2 per cent carbon and 15 per cent nickel is entirely
martensite. Higher percentages of nickel change the martensitic
structure to austenite, the steel then being non-magnetic. The
higher percentages, that is 30 to 35 per cent nickel, are used
for valve seats, valve heads, and valve stems, as the alloy is a
poor conductor of heat and is particularly free from any tendency
towards corrosion or pitting from the action of waste gases of
the internal-combustion engine.

Nickel steels having 3-1/2 per cent nickel and 0.15 to 0.20 per
cent carbon are excellent for case-hardening purposes, giving hard
surfaces and tough interiors.

To obtain the full effect of nickel as an alloy, it is essential
that the correct percentage of carbon be used. High nickel and
low carbon will not be more efficient than lower nickel and higher
carbon, but the cost will be much greater. Generally speaking,
heat-treated nickel alloy steels are about two to three times stronger
than the same steel annealed. This point is very important as many
instances have been found where nickel steel is incorrectly used,
being employed when in the annealed or normal state.

CHROMIUM

Chromium when alloyed with steel, has the characteristic function
of opposing the disintegration and reconstruction of cementite.
This is demonstrated by the changes in the critical ranges of this
alloy steel taking place slowly; in other words, it has a tendency
to raise the _Ac_ range (decalescent points) and lower the _Ar_
range (recalescent points). Chromium steels are therefore capable
of great hardness, due to the rapid cooling being able to retard
the decomposition of the austenite.

The great hardness of chromium steels is also due to the formation
of double carbides of chromium and iron. This condition is not
removed when the steel is slightly tempered or drawn. This additional
hardness is also obtained without causing undue brittleness such as
would be obtained by any increase of carbon. The degree of hardness
of the lower-chrome steels is dependent upon the carbon content,
as chromium alone will not harden iron.

The toughness so noticeable in this steel is the result of the
fineness of structure; in this instance, the action is similar
to that of nickel, and the tensile strength and elastic limit is
therefore increased without any loss of ductility. We then have
the desirable condition of tough hardness, making chrome steels
extremely valuable for all purposes requiring great resistance
to wear, and in higher-chrome contents resistance to corrosion.
All chromium-alloy steels offer great resistance to corrosion and
erosion. In view of this, it is surprising that chromium steels
are not more largely used for structural steel work and for all
purposes where the steel has to withstand the corroding action
of air and liquids. Bridges, ships, steel building, etc., would
offer greater resistance to deterioration through rust if the
chromium-alloy steels were employed.

Prolonged heating and high temperatures have a very bad effect upon
chromium steels. In this respect they differ from nickel steels,
which are not so affected by prolonged heating, but chromium steels
will stand higher temperatures than nickel steels when the period
is short.

Chromium steels, due to their admirable property of increased hardness,
without the loss of ductility, make very excellent chisels and
impact tools of all types, although for die blocks they do not give
such good results as can be obtained from other alloy combinations.

For ball bearing steels, where intense hardness with great toughness
and ready recovery from temporary deflection is required, chromium
as an alloy offers the best solution.

Two per cent chromium steels; due to their very hard tough surface,
are largely used for armor-piercing projectiles, cold rolls, crushers,
drawing dies, etc.

The normal structure of chromium steels, with a very low carbon
content is roughly pearlitic up to 7 per cent, and martensitic
from 8 to 20 per cent; therefore, the greatest application is in
the pearlitic zone or the lower percentages.

NICKEL-CHROMIUM

A combination of the characteristics of nickel and the characteristics
of chromium, as described, should obviously give a very excellent
steel as the nickel particularly affects the ferrite of the steel
and the chromium the carbon. From this combination, we are able to
get a very strong ferrite matrix and a very hard tough cementite.
The strength of a strictly pearlitic steel over a pure iron is due
to the pearlitic being a layer arrangement of cementite running
parallel to that of a pure iron layer in each individual grain. The
ferrite _i.e._, the iron is increased in strength by the resistance
offered by the cementite which is the simple iron-carbon combination
known to metallurgists as Fe3C. The cementite, although adding
to the tensile strength, is very brittle and the strength of the
pearlite is the combination of the ferrite and cementite. In the
event of the cementite being strengthened, as in the case of strictly
chromium steels, an increased tensile strength is readily obtained
without loss of ductility and if the ferrite is strengthened then
the tensile strength and ductility of the metal is still further
improved.

Nickel-chromium alloy represents one of the best combinations available
at the present time. The nickel intensifies the physical characteristics
of the chromium and the chromium has a similar effect on the nickel.

For case-hardening, nickel-chromium steels seem to give very excellent
results. The carbon is very rapidly taken up in this combination,
and for that reason is rather preferable to the straight nickel steel.

With the mutually intensifying action of chromium and nickel there
is a most suitable ratio for these two alloys, and it has been found
that roughly 2-1/2 parts of nickel to about 1 part of chromium
gives the best results. Therefore, we have the standard types of
3.5 per cent nickel with 1.5 per cent chromium to 1.5 per cent
nickel with 0.6 per cent chromium and the various intermediate
types. This ratio, however, does not give the whole story of
nickel-chromium combinations, and many surprising results have
been obtained with these alloys when other percentage combinations
have been employed.

VANADIUM

Vanadium has a very marked effect upon alloy steels rich in chromium,
carbon, or manganese. Vanadium itself, when combined with steel very
low in carbon, is not so noticeably beneficial as in the same carbon
steel higher in manganese, but if a small quantity of chromium
is added, then the vanadium has a very marked effect in increasing
the impact strength of the alloy. It would seem that vanadium has
the effect of intensifying the action of chromium and manganese, or
that vanadium is intensified by the action of chromium or manganese.

Vanadium has the peculiar property of readily entering into solution
with ferrite. If vanadium contained is considerable it also combines
with the carbon, forming carbides. The ductility of carbon-vanadium
steels is therefore increased, likewise the ductility of chrome-vanadium
steels.

The full effect of vanadium is not felt unless the temperatures to
which the steel is heated for hardening are raised considerably.
It is therefore necessary that a certain amount of "soaking" takes
place, so as to get the necessary equalization. This is true of all
alloys which contain complex carbides, i.e., compounds of carbon,
iron and one or more elements.

Chrome-vanadium steels also are highly favored for case hardening.
When used under alternating stresses it appears to have superior
endurance. It would appear that the intensification of the properties
due to chromium and manganese in the alloy steel accounts for this
peculiar phenomenon.

Vanadium is also a very excellent scavenger for either removing
the harmful gases, or causing them to enter into solution with the
metal in such a way as to largely obviate their harmful effects.
Chrome-vanadium steels have been claimed, by many steel manufacturers
and users, to be preferable to nickel-chrome steels. While not
wishing to pass judgment on this, it should be borne in mind that
the chrome-vanadium steel, which is tested, is generally compared
with a very low nickel-chromium alloy steel (the price factor entering
into the situation), but equally good results can be obtained by
nickel-chromium steels of suitable analysis.

Where price is the leading factor, there are many cases where a
stronger steel can be obtained from the chrome and vanadium than
the nickel-chrome. It will be safe to say that each of these two
systems of alloys have their own particular fields and chrome-vanadium
steel should not be regarded as the sole solution for all problems,
neither should nickel-chromium.

MANGANESE

Manganese adds considerably to the tensile strength of steel, but
this is dependent on the carbon content. High carbon materially adds
to the brittleness, whereas low-carbon, pearlitic-manganese steels
are very tough and ductile and are not at all brittle, providing the
heat-treating is correct. Manganese steel is very susceptible to
high temperatures and prolonged heating.

In low-carbon pearlitic steels, manganese is more effective in
increasing ultimate strength than is nickel; that is to say, a
0.45 carbon steel with 1.25 per cent manganese is as strong as a
0.45 carbon steel with 1.5 per cent nickel. The former steel is
much used for rifle barrels, and in the heat-treated condition will
give 80,000 to 90,000 lb. per square inch elastic limit, 115,000 to
125,000 lb. per square inch tensile strength, 23 per cent elongation,
and 55 per cent reduction in area.

Manganese when added to steel has the effect of lowering the critical
range; 1 per cent manganese will lower the upper critical point
60°F. The action of manganese is very similar to that of nickel
in this respect, only twice as powerful. As an instance, 1 per
cent nickel would have the effect of lowering the upper critical
range from 25 to 30°F.

Low-carbon pearlitic-manganese steel, heat-treated, will give dynamic
strength which cannot be equaled by low-priced and necessarily
low-content nickel steels. In many instances, it is preferable to use
high-grade manganese steel, rather than low-content nickel steel.

High-manganese steels or austenite manganese steels are used for a
variety of purposes where great resistance to abrasion is required,
the percentage of manganese being from 11 to 14 per cent, and carbon
1 to 1.5 per cent. This steel is practically valueless unless
heat-treated; that is, heated to about yellow red and quenched
in ice water. The structure is then austenite and the air-cooled
structure of this steel is martensite. Therefore this steel has to
be heated and very rapidly cooled to obtain the ductile austenite
structure.

Manganese between 2 and 7 per cent is a very brittle material when
the carbon is about 1 per cent or higher and is, therefore, quite
valueless. Below 2 per cent manganese steel low in carbon is very
ductile and tough steel.

The high-content manganese steels are known as the "Hadfield manganese
steels," having been developed by Sir Robert Hadfield. Small additions
of chrome up to 1 per cent increase the elastic limit of low-carbon
pearlitic-manganese steels without affecting the steel in its resistance
to shock, but materially decrease the percentage of elongation.

Vanadium added to low-carbon pearlitic manganese steel has a very
marked effect, increasing greatly the dynamic strength and changing
slightly the susceptibility of this steel to heat treatments, giving
a greater margin for the hardening temperature. Manganese steel
with added vanadium is most efficient when heat-treated.

TUNGSTEN

Tungsten, as an alloy in steel, has been known and used for a long
time. The celebrated and ancient damascus steel being a form of
tungsten-alloy steel. Tungsten and its effects, however, did not
become generally realized until Robert Mushet experimented and
developed his famous mushet steel and the many improvement made
since that date go to prove how little Mushet himself understood
the peculiar effects of tungsten as an alloy.

Tungsten acts on steel in a similar manner to carbon, that is,
it increases its hardness, but is much less effective than carbon
in this respect. If the percentage of tungsten and manganese is
high, the steel will be hard after cooling in the air. This is
impossible in a carbon steel. It was this combination that Mushet
used in his well-known "air-hardening" steel.

The principal use of tungsten is in high-speed tool steel, but
here a high percentage of manganese is distinctly detrimental,
making the steel liable to fire crack, very brittle and weak in
the body, less easily forged and annealed. Manganese should be
kept low and a high percentage of chromium used instead.

Tools of tungsten-chromium steels, when hardened, retain their
hardness, even when heated to a dark cherry red by the friction of
the cutting or the heat arising from the chips. This characteristic
led to the term "red-hardness," and it is this property that has
made possible the use of very high cutting speeds in tools made
of the tungsten-chromium alloy, that is, "high-speed" steel.

Tungsten steels containing up to 6 per cent do not have the property
of red hardness any more than does carbon tool steel, providing
the manganese or chromium is low.

When chromium is alloyed with tungsten, a very definite red-hardness
is noticed with a great increase of cutting efficiency. The maximum
red-hardness seems to be had with steels containing 18 per cent
tungsten, 5.5 per cent chromium and 0.70 per cent carbon.

Very little is known of the actual function of tungsten, although
a vast amount of experimental work has been done. It is possible
that when the effect of tungsten with iron-carbon alloys is better
known, a greater improvement can be expected from these steels.
Tungsten has been tried and is still used by some steel manufacturers
for making punches, chisels, and other impact tools. It has also
been used for springs, and has given very good results, although
other less expensive alloys give equally good results, and are
in some instances, better.

Tungsten is largely used in permanent magnets. In this, its action
is not well understood. In fact, the reason why steel becomes a
permanent magnet is not at all understood. Theories have been evolved,
but all are open to serious questioning. The principal effect of
tungsten, as conceded by leading authorities, is that it distinctly
retards separation of the iron-carbon solution, removing the lowest
recalescent point down to atmospheric temperature.

A peculiar property of tungsten steels is that if a heating temperature
of 1,750°F. is not exceeded, the cooling curves indicate but one
critical point at about 1,350°F. But when the heating temperature
is raised above 1,850°F., this critical point is nearly if not
quite suppressed, while a lower critical point appears and grows
enormously in intensity at a temperature between 660 and 750°F.

The change in the critical ranges, which is produced by heating
tungsten steels to over 1,850°F., is the real cause of the red-hard
properties of these alloys. Its real nature is not understood,
and there is no direct evidence to show what actually happens at
these high temperatures.

It may readily be understood that an alloy containing four essential
elements, namely: iron, carbon, tungsten and chromium, is one whose
study presents problems of extreme complexity. It is possible that
complex carbides may be formed, as in chromium steels, and that
compounds between iron and tungsten exist. Behavior of these
combinations on heating and cooling must be better known before
we are able to explain many peculiarities of tungsten steels.

MOLYBDENUM

Molybdenum steels have been made commercially for twenty-five years,
but they have not been widely exploited until since the war. Very
large resources of molybdenum have been developed in America, and
the mining companies who are equipped to produce the metal are
very active in advertising the advantages of molybdenum steels.

It was early found that 1 part molybdenum was the equivalent of from
2 to 2-1/2 parts of tungsten in tool steels, and magnet steels. It
fell into disrepute as an alloy for high-speed tool steel, however,
because it was found that the molybdenum was driven out of the
surface of the tool during forging and heat treating.

Within the last few years it has been found that the presence of
less than 1 per cent of molybdenum greatly enhances certain properties
of heat-treated carbon and alloy steels used for automobiles and
high-grade machinery.

In general, molybdenum when added to an alloy steel, increases the
figure for reduction of area, which is considered a good measure
of "toughness." Molybdenum steels are also relatively insensible
to variations in heat treatment; that is to say, a
chromium-nickel-molybdenum steel after quenching in oil from 1,450°F.
may be drawn at any temperature between 900 and 1,100°F. with
substantially the same result (static tensile properties and hardness).

SILICON

Silicon prevents, to a large extent, defects such as gas bubbles
or blow holes forming while steel is solidifying. In fact, steel
after it has been melted and before it has been refined, is "wild"
and "gassy." That is to say, if it would be cast into molds it
would froth up, and boil all over the floor. A judicious amount
of silicon added to the metal just before pouring, prevents this
action--in the words of the steel maker, silicon "kills" the steel.
If about 1.75 per cent metallic silicon remains in a 0.65 carbon
steel, it makes excellent springs.

PHOSPHORUS

Phosphorus is one of the impurities in steel, and it has been the
object of steel makers for years to eliminate it. On cheap grades
of steel, not subject to any abnormal strain or stress, 0.1 per
cent phosphorus is not objectionable. High phosphorus makes steel
"cold short," i.e., brittle when cold or moderately warm.

SULPHUR

Sulphur is another impurity and high sulphur is even a greater
detriment to steel than phosphorus. High sulphur up to 0.09 per
cent helps machining properties, but has a tendency to make the
steel "hot short," i.e., subject to opening up cracks and seams
at forging or rolling heats. Sulphur should never exceed 0.06 per
cent nor phosphorus 0.08 per cent.

Steel used for tool purposes should have as low phosphorus and sulphur
contents as possible, not over 0.02 per cent.

We can sum up the various factors something as follows for ready
reference.

The ingredient        Its effect

  Iron            The basis of steel
  Carbon          The determinative
  Sulphur         A strength sapper
  Phosphorus      The weak link
  Oxygen          A strength destroyer
  Manganese       For strength
  Nickel          For strength and toughness
  Tungsten        Hardener and heat resister
  Chromium        For resisting shocks
  Vanadium        Purifier and fatigue resister
  Silicon         Impurity and hardener
  Titanium        Removes nitrogen and oxygen
  Molybdenum      Hardener and heat resister
  Aluminum        Kills or deoxidizes steel

PROPERTIES OF ALLOY STEELS

The following table shows the percentages of carbon, manganese,
nickel, chromium and vanadium in typical steel alloys for engineering
purposes. It also gives the elastic limit, tensile strength, elongation
and reduction of area of the various alloys, all being given the same
heat treatment with a drawing temperature of 1,100°F. (600°C.). The
specimens were one inch rounds machined after heat treatment.

Tungsten is not shown in the table because it is seldom used in
engineering construction steels and then usually in combination
with chromium. Tungsten is used principally for the magnets of
magnetos, to some extent in the manufacture of hacksaws, and for
special tool steels.

TABLE I.--PROPERTIES OF ALLOY STEELS
------------------------------------------------------------------------------
     \Manganese,/    \Chromium,/       |Elastic|Tensile  |Elongation|Reduction
Carbon,\ per  /Nickel,\ per  /Vanadium,|limit, |Strength,|in 2 in., | of area,
  per  | cent | per   | cent |per cent |lb. per|lb. per  |per cent  | per cent
  cent |      | cent  |      |         |sq. in.|sq. in.  |          |
-------|------|-------|------|---------|-------|---------|----------|---------
  0.27 | 0.55 |       |      |         | 49,000|  80,000 |    30    |    65
  0.27 | 0.47 |       |      |  0.26   | 66,000|  98,000 |    25    |    52
  0.36 | 0.42 |       |      |         | 58,000|  90,000 |    27    |    60
  0.34 | 0.87 |       |      |  0.13   | 82,500| 103,000 |    22    |    57
  0.45 | 0.50 |       |      |         | 65,000|  96,000 |    22    |    52
  0.43 | 0.60 |       |      |  0.32   | 96,000| 122,000 |    21    |    52
  0.47 | 0.90 |       |      |  0.15   |102,000| 127,500 |    23    |    58
  0.30 | 0.60 | 3.40  |      |         | 75,000| 105,000 |    25    |    67
  0.33 | 0.63 | 3.60  |      |  0.25   |118,000| 142,000 |    17    |    57
  0.30 | 0.49 | 3.60  | 1.70 |         |119,000| 149,500 |    21    |    60
  0.25 | 0.47 | 3.47  | 1.60 |  0.15   |139,000| 170,000 |    18    |    53
  0.25 | 0.50 | 2.00  | 1.00 |         |102,000| 124,000 |    25    |    70
  0.38 | 0.30 | 2.08  | 1.16 |         |120,000| 134,000 |    20    |    57
  0.42 | 0.22 | 2.14  | 1.27 |  0.26   |145,000| 161,500 |    16    |    53
  0.36 | 0.61 | 1.46  | 0.64 |         |117,600| 132,500 |    16    |    58
  0.36 | 0.50 | 1.30  | 0.75 |  0.16   |140,000| 157,500 |    17    |    54
  0.30 | 0.50 |       | 0.80 |         | 90,000| 105,000 |    20    |    50
  0.23 | 0.58 |       | 0.82 |  0.17   |106,000| 124,000 |    21    |    66
  0.26 | 0.48 |       | 0.92 |  0.20   |112,000| 137,000 |    20    |    61
  0.35 | 0.64 |       | 1.03 |  0.22   |132,500| 149,500 |    16    |    54
  0.50 | 0.92 |       | 1.02 |  0.20   |170,000| 186,000 |    15    |    45
------------------------------------------------------------------------------

NON-SHRINKING, OIL-HARDENING STEELS

Certain steels have a very low rate of expansion and contraction
in hardening and are very desirable for test plugs, gages, punches
and dies, for milling cutters, taps, reamers, hard steel bushings
and similar work.

It is recommended that for forging these steels it be heated slowly
and uniformly to a bright red, but not in a direct flame or blast.
Harden at a dull red heat, about 1,300°F. A clean coal or coke
fire, or a good muffle-gas furnace will give best results. Fish
oil is good for quenching although in some cases warm water will
give excellent results. The steel should be kept moving in the bath
until perfectly cold. Heated and cooled in this way the steel is
very tough, takes a good cutting edge and has very little expansion
or contraction which makes it desirable for long taps where the
accuracy of lead is important.

The composition of these steels is as follows:

               Per cent
  Manganese  1.40 to 1.60
  Carbon     0.80 to 0.90
  Vanadium   0.20 to 0.25

[Illustration: FIG. 13.--Effect of copper in steel.]

EFFECT OF A SMALL AMOUNT OF COPPER IN MEDIUM-CARBON STEEL

This shows the result of tests by C. R. Hayward and A. B. Johnston
on two types of steel: one containing 0.30 per cent carbon, 0.012
per cent phosphorus, and 0.860 per cent copper, and the other 0.365
per cent carbon, 0.053 per cent phosphorus, and 0.030 per cent
copper. The accompanying chart in Fig. 13 shows that high-copper
steel has decided superiority in tensile strength, yield point and
ultimate strength, while the ductility is practically the same.
Hardness tests by both methods show high-copper steel to be harder
than low-copper, and the Charpy shock tests show high-copper steel
also superior to low-copper. The tests confirm those made by Stead,
showing that the behavior of copper steel resembles that of nickel
steel. The high-copper steels show finer grain than the low-copper.
The quenched and drawn specimens of high-copper steel were found
to be slightly more martensitic.

HIGH-CHROMIUM OR RUST-PROOF STEEL

High-chromium, or what is called stainless steel containing from
11 to 14 per cent chromium, was originally developed for cutlery
purposes, but has in the past few years been used to a considerable
extent for exhaust valves in airplane engines because of its resistance
to scaling at high temperatures.

                                Percentage
  Carbon                       0.20 to 0.40
  Manganese, not to exceed         0.50
  Phosphorus, not to exceed        0.035
  Sulphur, not to exceed           0.035
  Chromium                    11.50 to 14.00
  Silicon, not to exceed           0.30

The steel should be heated slowly and forged at a temperature above
1,750°F. preferably between 1,800 and 2,200°F. If forged at temperatures
between 1,650 and 1,750°F. there is considerable danger of rupturing
the steel because of its hardness at red heat. Owing to the
air-hardening property of the steel, the drop-forgings should be
trimmed while hot. Thin forgings should be reheated to redness
before trimming, as otherwise they are liable to crack.

The forgings will be hard if they are allowed to cool in air. This
hardness varies over a range of from 250 to 500 Brinell, depending
on the original forging temperature.

ANNEALING can be done by heating to temperatures ranging from 1,290
to 1,380°F. and cooling in air or quenching in water or oil. After
this treatment the forgings will have a hardness of about 200 Brinell
and a tensile strength of 100,000 to 112,000 lb. per square inch.
If softer forgings are desired they can be heated to a temperature
of from 1,560 to 1,650°F. and cooled very slowly. Although softer
the forgings will not machine as smoothly as when annealed at the
lower temperature.

HARDENING.--The forgings can be hardened by cooling in still air
or quenching in oil or water from a temperature between 1,650 and
1,750°F.

The physical properties do not vary greatly when the carbon is
within the range of composition given, or when the steel is hardened
and tempered in air, oil, or water.

When used for valves the following specification of physical properties
have been used:

  Yield point, pounds per square inch       70,000
  Tensile strength, pounds per square inch  90,000
  Elongation in 2 in., per cent                 18
  Reduction of area, per cent                   50

The usual heat treatment is to quench in oil from 1,650°F. and
temper or draw at 1,100 to 1,200°F. One valve manufacturer stated
that valves of this steel are hardened by heating the previously
annealed valves to 1,650°F. and cooling in still air. This treatment
gives a scleroscope hardness of about 50.

In addition to use in valves this steel should prove very satisfactory
for shafting for water-pumps and other automobile parts subject to
objectionable corrosion.

      TABLE 2.--COMPARISON OF PHYSICAL PROPERTIES FOR HIGH-CHROMIUM
                STEELS OF DIFFERENT CARBON CONTENT --------------------------------------------------------------------------
                                         | C   0.20 | C   0.27 | C   0.50
                                         | Mn  0.45 | Mn  0.50 |
                                         | Cr 12.56 | Cr 12.24 | Cr 14.84
-----------------------------------------|----------|----------|----------
Quenched in oil from degrees Fahrenheit  |   1,600  |   1,600  |   1,650
Tempered at degrees Fahrenheit           |   1,160  |   1,080  |   1,100
Yield point, pounds per square inch      |  78,300  |  75,000  |  91,616
Tensile strength, pounds per square inch | 104,600  | 104,250  | 123,648
Elongation in 2 in., per cent            |   25.0   |   23.5   |   14.5
Reduction of area, per cent              |   52.5   |   51.4   |   33.5
--------------------------------------------------------------------------

TABLE 3.--COMPARISON OF PHYSICAL PROPERTIES BETWEEN AIR, OIL AND
          WATER-HARDENED STEEL HAVING CHEMICAL ANALYSIS IN
          PERCENTAGE OF
-------------------------------------------------------------------------
  Carbon       0.24
  Manganese    0.30
  Phosphorus   0.035
  Sulphur      0.035
  Chromium    12.85
  Silicon      0.20

-------------------------------------------------------------------------
         | Hardened |           | Elastic | Tensile |          |
Hardening|  from,   | Tempered  | limit,  |strength,|Elongation|Reduction
medium   | degrees  |at, degrees| per lb. |lb.  Per | in 2 in. |of area,
         |Fahrenheit|Fahrenheit | sq. in. | sq. in. | per cent |per cent
---------|----------|-----------|---------|---------|----------|---------
         |          |     930   | 158,815 | 192,415 |   13.0   |  40.5
         |          |   1,100   |  99,680 | 120,065 |   21.0   |  59.2
  Air    |  1,650   |   1,300   |  70,785 | 101,250 |   26.0   |  64.6
         |          |   1,380   |  66,080 |  98,335 |   28.0   |  63.6
         |          |   1,470   |  70,785 |  96,990 |   27.0   |  64.7
---------|----------|-----------|---------|---------|----------|---------
         |          |     930   | 163,070 | 202,720 |    8.0   |  18.2
  Oil    |  1,650   |   1,100   |  88,255 | 116,480 |   20.0   |  56.9
         |          |   1,300   |  77,950 | 105,505 |   25.5   |  63.8
         |          |   1,380   |  88,255 |  98,785 |   27.0   |  66.3
---------|----------|-----------|---------|---------|----------|---------
         |          |     930   | 158,815 | 202,050 |   12.0   |  34.2
  Water  |  1,650   |   1,100   |  90,270 | 120,735 |   22.0   |  59.8
         |          |   1,300   |  66,080 | 102,590 |   25.8   |  64.8
         |          |   1,380   |  67,200 |  97,890 |   27.0   |  65.2
-------------------------------------------------------------------------

This steel can be drawn into wire, rolled into sheets and strips
and drawn into seamless tubes.

CORROSION.--This steel like any other steel when distorted by cold
working is more sensitive to corrosion and will rust. Rough cut
surfaces will rust. Surfaces finished with a fine cut are less
liable to rust. Ground and polished surfaces are practically immune
to rust.

When chromium content is increased to 16 to 18 per cent and silicon
is added, from 2 to 4 per cent, this steel becomes rust proof in
its raw state, as soon as the outside surface is removed. It does
not need to be heat-treated in any way. These compositions are
both patented.

S. A. E. STANDARD STEELS

The following steel specifications are considered standard by the
Society of Automotive Engineers and represents automobile practice in
this country. These tables give the S. A. E. number, the composition
of the steel and the heat treatment. These are referred to by
letter--the heat treatments being given in detail on pages 134
to 137 in Chap. 8. It should be noted that the percentage of the
different ingredients desired is the mean, or halfway between the
minimum and maximum.

TABLE 4.--CARBON STEELS
------------------------------------------------------------------------------
   S. A. E.  |   Carbon    |  Manganese  |          |         |
Specification|(minimum and |(minimum and |Phosphorus| Sulphur |     Heat
     no.     |   maximum)  |   maximum)  |(maximum) |(maximum)|  treatment
-------------|-------------|-------------|----------|---------|---------------
   1,010     | 0.05 to 0.15| 0.30 to 0.60|  0.045   |  0.05   |Quench at 1,500
   1,020     | 0.15 to 0.25| 0.30 to 0.60|  0.045   |  0.05   |    A or B
   1,025     | 0.20 to 0.30| 0.50 to 0.80|  0.045   |  0.05   |      H
             |             |             |          |         |
   1,035     | 0.30 to 0.40| 0.50 to 0.80|  0.045   |  0.05   |  H, D or E
   1,045     | 0.40 to 0.50| 0.50 to 0.80|  0.045   |  0.05   |  H, D or E
   1,095     | 0.90 to 1.05| 0.25 to 0.50|  0.040   |  0.05   |      F
------------------------------------------------------------------------------

TABLE 5.--SCREW STOCK
---------------------------------------------------------------------------
    S. A. E.     |    Carbon    |  Manganese   | Phosphorus |   Sulphur
Specification no.|              |              | (maximum)  |
-----------------|--------------|--------------|------------|--------------
     1,114       | 0.08 to 0.20 | 0.30 to 0.80 |   0.12     | 0.06 to 0.12
---------------------------------------------------------------------------

TABLE 6.--NICKEL STEELS
-----------------------------------------------------------------------------
   S. A. E.   |       |          | Phosphorus|       |            |
Specification |       |          | (maximum) |       |            |
    no.   ----        |           \         /        |            |
         |   Carbon   |  Manganese |       | Sulphur |   Nickel   |  Heat
         |(minimum and|(minimum and|       |(maximum)|(minimum and|treatment
         |   maximum) |   maximum) |       |         |  maximum)  |
---------|------------|------------|-------|---------|------------|----------
  2,315  |0.10 to 0.20|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75|G, H or K
  2,320  |0.15 to 0.25|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75|G, H or K
  2,330  |0.25 to 0.35|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75| H or K
         |            |            |       |         |            |
  2,335  |0.30 to 0.40|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75| H or K
  2,340  |0.35 to 0.45|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75| H or K
  2,345  |0.40 to 0.50|0.50 to 0.80|  0.04 |  0.045  |3.25 to 3.75| H or K
-----------------------------------------------------------------------------

TABLE 7.--NICKEL-CHROMIUM STEELS
-------------------------------------------------------------------------------
   S. A. E.  |     |     | Phosphorus| Sulphur  |       |            |
Specification|     |     |  (maximum)|(maximum) |       |            |  Heat
  no.  ------      |      ------     |      ----        |            |treatment
      |   Carbon   |  Manganese |    |     |   Nickel   |  Chromium  \
      |(minimum and|(minimum and|    |     |(minimum and|(minimum and |
      |  maximum)  |   maximum) |    |     |  maximum)  |  maximum)   |
------|------------|------------|----|-----|------------|-------------|--------
 3,120|0.15 to 0.25|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|G,H or D
 3,125|0.20 to 0.30|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
 3,130|0.25 to 0.35|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
      |            |            |    |     |            |             |
 3,135|0.30 to 0.40|0.50 to 0 80|0.04|0.045|1.00 to 1.50|0.45 to 0 75*|H,D or E
 3,140|0.35 to 0.45|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
 3,220|0.15 to 0.25|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 |G,H or D
      |            |            |    |     |            |             |
 3,230|0.25 to 0.35|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | H or D
 3,240|0.35 to 0.45|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | H or D
 3,250|0.45 to 0.55|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | M or Q
      |            |            |    |     |            |             |
X3,315|0.10 to 0.20|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 |   G
X3,335|0.30 to 0.40|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 | P or R
X3,350|0.45 to 0.55|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 | P or R
      |            |            |    |     |            |             |
 3,320|0.15 to 0.25|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 |   L
 3,330|0.25 to 0.35|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 | P or R
 3,340|0.35 to 0.45|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 | P or R
-------------------------------------------------------------------------------
* Another grade of this type of steel is available with chromium content
of 0.15 per cent to 45 per cent. It has somewhat lower physical properties.

TABLE 8.--CHROMIUM STEELS
-------------------------------------------------------------------------------
   S. A. E.  |        |            |          |         |            |
Specification|        |            |          |         |            |
    no.   ---  Carbon |  Manganese |          |         |  Chromium  |
         |(minimum and|(minimum and|Phosphorus|Sulphur  |(minimum and|  Heat
         |  maximum)  |  maximum)  |(maximum) |(maximum)|  maximum)  |treatment
---------|------------|------------|----------|---------|------------|---------
  5,120  |0.15 to 0.25|      *     |   0.04   |  0.045  |0.65 to 0.85|    B
  5,140  |0.35 to 0.45|      *     |   0.04   |  0.045  |0.65 to 0.85|  H or D
  5,165  |0.60 to 0.70|      *     |   0.04   |  0.045  |0.65 to 0.85|  H or D
         |            |            |          |         |            |
  5,195  |0.90 to 1.05|0.20 to 0.45|   0.03   |  0.03   |0.90 to 1.10|M, P or R
 51,120  |1.10 to 1.30|0.20 to 0.45|   0.03   |  0.03   |0.90 to 1.10|M, P or R
  5,295  |0.90 to 1.05|0.20 to 0.45|   0.03   |  0.03   |1.10 to 1.30|M, P or R
 52,120  |1.10 to 1.30|0.20 to 0.45|   0.03   |  0.03   |1.10 to 1.30|M, P or R
-------------------------------------------------------------------------------
--Two types of steel are available in this class, one with manganese 0.25
to 0.50 per cent (0.35 per cent desired), and silicon not over 0.20 per
cent; the other with manganese 0.60 to 0.80 per cent (0.70 per cent
desired), and silicon 0.15 to 0.50 per cent.

TABLE 9.--CHROMIUM-VANADIUM STEELS
-------------------------------------------------------------------------------
   S. A. E.  |     |         |Phosphorus| Sulphur |        |Vanadium |
Specification|     |         | (maximum)|(maximum)|        |(minimum)|
  no.  ------      |          --        |        /          -        |
      |   Carbon   |  Manganese |       |       |  Chromium  |       |   Heat
      |(minimum and|(minimum and|       |       |(minimum and|       |treatment
      |   maximum) |   maximum) |       |       |  maximum)  |       |
------|------------|------------|-------|-------|------------|-------|---------
6,120 |0.15 to 0.25|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |     S
6,125 |0.20 to 0.30|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |  S or T
6,130 |0.25 to 0.35|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |  T or U
6,135 |0.30 to 0.40|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |  T or U
6,140 |0.35 to 0.45|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |  T or U
6,145 |0.40 to 0.50|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |     U
6,150 |0.45 to 0.55|0.50 to 0.80|  0.04 |  0.04 |0.80 to 1.10|  0.15 |     U
6,195 |0.90 to 1.05|0.20 to 0.45|  0.03 |  0.03 |0.80 to 1.10|  0.15       U
-------------------------------------------------------------------------------

TABLE 10.--SILICO-MANGANESE STEELS
-----------------------------------------------------------------------------
  S. A. E.   |      |            |          |         |            |
Specification|      |            |          |         |            |
  no.   ----- Carbon|  Manganese |          |         |   Silicon  |
       |(minimum and|(minimum and|Phosphorus|Sulphur  |(minimum and|  Heat
       |  maximum)  |  maximum)  |(maximum) |(maximum)|  maximum)  |treatment
-------|------------|------------|----------|---------|------------|---------
 9,250 |0.45 to 0.55|0.60 to 0.80|  0.045*  |  0.045  |1.80 to 2.10|    V
 9,260 |0.55 to 0.65|0.50 to 0.70|  0.045*  |  0.045  |1.50 to 1.80|    V
-----------------------------------------------------------------------------
* Steel made by the acid process may contain maximum 0.05 phosphorus.

LIBERTY MOTOR CONNECTING RODS

The requirements for materials for the Liberty motor connecting rods
are so severe that the methods of securing the desired qualities
will be of value in other lines. The original specifications called
for chrome-nickel but the losses due to the difficulty of handling
caused the Lincoln Motor Company to suggest the substitution of
chrome-vanadium steel, and this was accepted by the Signal Corps. The
rods were accordingly made from chromium-vanadium steel, containing
carbon, 0.30 to 0.40 per cent; manganese, 0.50 to 0.80 per cent;
phosphorus, not over 0.04 per cent; sulphur, not over 0.04 per
cent; chromium, 0.80 to 1.10 per cent; vanadium, not less than 0.15
per cent. This steel is ordinarily known in the trade as 0.35 carbon
steel, S. A. E., specification 6,135, which provides a first-rate
quality steel for structural parts that are to be heat-treated.
The fatigue resisting or endurance qualities of this material are
excellent. It has a tensile strength of 150,000 lb. minimum per
square inch; elastic limit, 115,000 lb. minimum per square inch;
elongation, 5 per cent minimum in 2 in.; and minimum reduction
in area, 25 per cent.

The original production system as outlined for the manufacturers
had called for a heat treatment in the rough-forged state for the
connecting rods, and then semi-machining the rod forgings before
giving them the final treatment. The Lincoln Motor Company insisted
from the first that the proper method would be a complete heat
treatment of the forging in the rough state, and machining the
rod after the heat treatment. After a number of trial lots, the
Signal Corps acceded to the request and production was immediately
increased and quality benefited by the change. This method was
later included in a revised specification issued to all producers.

The original system was one that required a great deal of labor
per unit output. The Lincoln organization developed a method of
handling connecting rods whereby five workmen accomplished the
same result that would have required about 30 or 32 by the original
method. Even after revising the specification so as to allow complete
heat treatments in the rough-forged state, the ordinary methods
employed in heat-treating would have required 12 to 15 men. With
the fixtures employed, five men could handle 1,300 connecting rods,
half of which are plain and half, forked, in a working period of
little over 7 hr.

[Illustration: Fig. 14.--Rack for holding rods.]

[Illustration: Fig. 15.--Sliding rods into tank.]

The increase in production was gained by devising fixtures which
enabled fewer men to handle a greater quantity of parts with less
effort and in less time.

In heat-treating the forgings were laid on a rack or loop _A_,
Fig. 14, made of 1-1/4-in. double extra-heavy pipe, bent up with
parallel sides about 9 in. apart, one end being bent straight across
and the other end being bent upward so as to afford an easy grasp
for the hook. Fifteen rods were laid on each loop, there being
four loops of rods charged into a furnace with a hearth area of 36
by 66 in. The rods were charged at a temperature of approximately
900°F. They were heated for refining over a period of 3 hr. to
1,625°F., soaked 15 min, at this degree of heat and quenched in
soluble quenching oil.

In pulling the heat to quench the rods, the furnace door was raised
and the operator pulls one of the loops _A_, Fig. 15 forward to
the shelf of the furnace, supporting the straight end of the loop
by means of the porter bar _B_. They swung the loop of rods around
from the furnace shelf and set the straight end of the loop on
the edge of the quenching tank, then raise the curved end _C_,
by means of their hook _D_ so that all the rods on the loop slide
into the oil bath.

Before the rods cooled entirely, the baskets in the quenching tank
were raised and the oil allowed to partly drain off the forgings,
and they were stacked on curved-end loops or racks and charged into
the furnace for the second or hardening heat. The temperature of
the furnace was raised in 1-1/2 hr. to 1,550°F., the rods soaked
for 15 min. at this degree of heat and quenched in the same manner
as above.

They were again drained while yet warm, placed on loops and charged
into the furnace for the third or tempering heat. The temperature of
the furnace was brought to 1,100°F. in 1 hr., and the rods soaked at
this degree of heat for 1 hr. They were then removed from the furnace
the same as for quenching, but were dumped onto steel platforms
instead of into the quenching oil, and allowed to cool on these
steel platforms down to the room temperature.

PICKLING THE FORGINGS

The forgings were then pickled in a hot solution of either niter
cake or sulphuric acid and water at a temperature of 170°F., and
using a solution of about 25 per cent. The solution was maintained
at a constant point by taking hydrometer readings two or three
times a day, maintaining a reading of about 1.175. Sixty forked or
one hundred single rods were placed in wooden racks and immersed
in a lead-lined vat 30 by 30 by 5 ft. long. The rack was lowered
or lifted by means of an air hoist and the rods were allowed to
stay in solution from 1/2 to 1 hr., depending on the amount of
scale. The rods were then swung and lowered in the rack into running
hot water until all trace of the acid was removed.

The rod was finally subjected to Brinell test. This shows whether
or not the rod has been heat-treated to the proper hardness. If
the rods did not read between 241 and 277, they were re-treated
until the proper hardness is obtained.




CHAPTER IV

APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY[1]

[Footnote 1: Paper presented at the summer meeting of the S. A.
E. at Ottawa Beach in June, 1919.]

The success of the Liberty engine program was an engineering achievement
in which the science of metallurgy played an important part. The
reasons for the use of certain materials and certain treatments
for each part are given with recommendations for their application
to the problems of automotive industry.

The most important items to be taken into consideration in the
selection of material for parts of this type are uniformity and
machineability. It has been demonstrated many times that the ordinary
grades of bessemer screw stock are unsatisfactory for aviation
purposes, due to the presence of excessive amounts of unevenly
distributed phosphorus and sulphide segregations. For this reason,
material finished by the basic open hearth process was selected,
in accordance with the following specifications: Carbon, 0.150 to
0.250 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,
0.045 maximum per cent; sulphur, 0.060 to 0.090 per cent.

This material in the cold-drawn condition will show: Elastic limit,
50,000 lb. per square inch, elongation in 2 in., 10 per cent, reduction
of area, 35 per cent.

This material gave as uniform physical properties as S. A. E. No.
1020 steel and at the same time was sufficiently free cutting to
produce a smooth thread and enable the screw-machine manufacturers
to produce, to the same thread limits, approximately 75 per cent
as many parts as from bessemer screw stock.

There are but seven carbon-steel carbonized parts on the Liberty
engine. The most important are the camshaft, the camshaft rocker
lever roller and the tappet. The material used for parts of this
type was S. A. E. No. 1,020 steel, which is of the following chemical
analysis: Carbon 0.150 to 0.250 per cent; manganese, 0.300 to 0.600
per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum
per cent.

The heat treatment consisted in carbonizing at a temperature of
from 1,650 to 1,700°F. for a sufficient length of time to secure
the proper depth of case, cool slowly or quench; then reheat to a
temperature of 1,380 to 1,430°F. to refine the grain of the case,
and quench in water. The only thing that should limit the rate of
cooling from the carbonizing heat is distortion. Camshaft rocker
lever rollers and tappets, as well as gear pins, were quenched
directly from the carbonizing heat in water and then case-refined
and rehardened by quenching in water from a temperature of from
1,380 to 1,430°F.

The advantage of direct quenching from the carbonizing heat is
doubtless one of economy, and in many cases will save the cost
of a reheating. Specifications for case hardening, issued by the
Society of Automotive Engineers, have lately been revised; whereas
they formerly called for a slow cooling, they now permit a quenching
from the pot. Doubtless this is a step in advance. Warpage caused
by quenching can be reduced to a minimum by thoroughly annealing
the stock before any machine work is done on it.

Another advantage obtained from rapid cooling from the carbonizing
heat is the retaining of the majority of the excess cementite in
solution which produces a less brittle case and by so doing reduces
the liability of grinding checks and chipping of the case in actual
service.

In the case of the camshaft, it is not possible to quench directly
from the carbonizing heat because of distortion and therefore excessive
breakage during straightening operations. All Liberty camshafts
were cooled slowly from carbonizing heat and hardened by a single
reheating to a temperature of from 1,380 to 1,430°F. and quenching
in water.

Considerable trouble has always been experienced in obtaining uniform
hardness on finished camshafts. This is caused by insufficient
water circulation in the quenching tank, which allows the formation
of steam pockets to take place, or by decarbonization of the case
during heating by the use of an overoxidizing flame. Another cause,
which is very often overlooked, is due to the case being ground off
one side of cam more than the other and is caused by the roughing
master cam being slightly different from the finishing master cam.
Great care should be taken to see that this condition does not occur,
especially when the depth of case is between 1/32 and 3/64 in.

CARBON-STEEL FORGINGS

Low-stressed, carbon-steel forgings include such parts as carbureter
control levers, etc. The important criterion for parts of this type
is ease of fabrication and freedom from over-heated and burned
forgings. The material used for such parts was S. A. E. No. 1,030
steel, which is of the following chemical composition: Carbon, 0.250
to 0.350 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,
0.045 maximum per cent; sulphur, 0.050 maximum per cent.

To obtain good machineability, all forgings produced from this
steel were heated to a temperature of from 1,575 to 1,625°F. to
refine the grain of the steel thoroughly and quenched in water
and then tempered to obtain proper machineability by heating to a
temperature of from 1,000 to 1,100°F. and cooled slowly or quenched.

Forgings subjected to this heat treatment are free from hard spots
and will show a Brinell hardness of 177 to 217, which is proper for
all ordinary machining operations. Great care should be taken not
to use steel for parts of this type containing less than 0.25 per
cent carbon, because the lower the carbon the greater the liability
of hard spots, and the more difficult it becomes to eliminate them.
The only satisfactory method so far in commercial use for the
elimination of hard spots is to give forgings a very severe quench
from a high temperature followed by a proper tempering heat to
secure good machine ability as outlined above.

The important carbon-steel forgings consisted of the cylinders,
the propeller-hubs, the propeller-hub flange, etc. The material
used for parts of this type was S. A. E. No. 1,045 steel, which
is of the following chemical composition: Carbon, 0.400 to 0.500
per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045
maximum per cent; sulphur, 0.050 maximum per cent.

All forgings made from this material must show, after heat treatment,
the following minimum physical properties: Elastic limit, 70,000;
lb. per square inch, elongation in 2 in., 18 per cent, reduction
of area, 45; per cent, Brinell hardness, 217 to 255.

To obtain these physical properties, the forgings were quenched in
water from a temperature of 1,500 to 1,550°F., followed by tempering
to meet proper Brinell requirements by heating to a temperature
of 1,150 to 1,200°F. and cooled slowly or quenched. No trouble
of any kind was ever experienced with parts of this type.

The principal carbon-steel pressed parts used on the Liberty engine
were the water jackets and the exhaust manifolds. The material
used for parts of this type was S. A. E. No. 1,010 steel, which
is of the following chemical composition: Carbon, 0.05 to 0.15 per
cent; manganese, 0.30 to 0.60 per cent; phosphorus, 0.045 maximum
per cent; sulphur, 0.045 maximum per cent.

No trouble was experienced in the production of any parts from
this material with the exception of the water jacket. Due to the
particular design of the Liberty cylinder assembly, many failures
occurred in the early days, due to the top of the jacket cracking
with a brittle fracture. It was found that these failures were
caused primarily from the use of jackets which showed small scratches
or die marks at this joint and secondarily by improper annealing of
the jackets themselves between the different forming operations.
By a careful inspection for die marks and by giving the jackets
1,400°F. annealing before the last forming operation, it was possible
to completely eliminate the trouble encountered.

HIGHLY STRESSED PARTS

The highly stressed parts on the Liberty engine consisted of the
connecting-rod bolt, the main-bearing bolt, the propeller-hub key,
etc. The material used for parts of this type was selected at the
option of the manufacturer from standard S. A. E. steels, the
composition of which are given in Table 11.

TABLE 11.--COMPOSITION OF S. A. E. STEELS Nos. 2,330, 3,135 AND 6,130

  Steel No              2,330   3,135   6,130
  Carbon, minimum       0.250   0.300   0.250
  Carbon, maximum       0.350   0.400   0.450
  Manganese, minimum    0.500   0.500   0.500
  Manganese, maximum    0.800   0.800   0.800
  Phosphorus, maximum   0.045   0.040   0.040
  Sulphur, maximum      0.045   0.045   0.045
  Nickel, minimum       3.250   1.000
  Nickel, maximum       3.750   1.500
  Chromium, minimum             0.450   0.800
  Chromium, maximum             0.750   1.100
  Vanadium, minimum                     0.150

All highly stressed parts on the Liberty engine must show, after
heat treatment, the following minimum physical properties: Elastic
limit, 100,000 lb. per square inch; elongation in 2 in., 16 per
cent; reduction of area, 45 per cent; scleroscope hardness, 40
to 50.

The heat treatment employed to obtain these physical properties
consisted in quenching from a temperature of 1,525 to 1,575°F., in
oil, followed by tempering at a temperature of from 925 to 975°F.

Due to the extremely fine limits used on all threaded parts for
the Liberty engine, a large percentage of rejection was due to
warpage and scaling of parts. To eliminate this objection, many
of the Liberty engine builders adopted the use of heat-treated
and cold-drawn alloy steel for their highly stressed parts. On
all sizes up to and including 3/8 in. in diameter, the physical
properties were secured by merely normalizing the hot-rolled bars
by heating to a temperature of from 1,525 to 1,575°F., and cooling
in air, followed by the usual cold-drawing reductions. For parts
requiring stock over 3/8 in. in diameter, the physical properties
desired were obtained by quenching and tempering the hot-rolled bars
before cold-drawing. It is the opinion that the use of heat-treated
and cold-drawn bars is very good practice, provided proper inspection
is made to guarantee the uniformity of heat treatment and, therefore,
the uniformity of the physical properties of the finished parts.

The question has been asked many times by different manufacturers, as
to which alloy steel offers the best machineability when heat-treated
to a given Brinell hardness. The general consensus of opinion among
the screw-machine manufacturers is that S. A. E. No. 6,130 steel
gives the best machineability and that S. A. E. No. 2,330 steel
would receive second choice of the three specified.

In the finishing of highly stressed parts for aviation engines,
extreme care must be taken to see that all tool marks are eliminated,
unless they are parallel to the axis of strain, and that proper
radii are maintained at all changes of section. This is of the
utmost importance to give proper fatigue resistance to the part
in question.

GEARS

The material used for all gears on the Liberty engine was selected
at the option of the manufacturer from the following standard S.
A. E. steels, the composition of which are given in Table 12,

TABLE 12.--COMPOSITION OF STEELS NOS. X-3,340 AND 6,140

  Steel No              X-3,340   6,140
  Carbon, minimum        0.350    0.350
  Carbon, maximum        0.450    0.450
  Manganese, minimum     0.450    0.500
  Manganese, maximum     0.750    0.800
  Phosphorus, maximum    0.040    0.040
  Sulphur, maximum       0.045    0.045
  Nickel, minimum        2.750
  Nickel, maximum        3.250
  Chromium, minimum      0.700    0.800
  Chromium, maximum      0.950    1.100
  Vanadium, minimum               0.150

All gears were heat-treated to a scleroscope hardness of from 55
to 55. The heat treatment used to secure this hardness consisted
in quenching the forgings from a temperature of 1,550 to 1,600°F.
in oil and annealing for good machineability at a temperature of
from 1,300 to 1,350°F. Forgings treated in this manner showed a
Brinell hardness of from 177 to 217.

RATE OF COOLING

At the option of the manufacturer, the above treatment of gear
forgings could be substituted by normalizing the forgings at a
temperature of from 1,550 to 1,600°F. The most important criterion
for proper normalizing, consisted in allowing the forgings to cool
through the critical temperature of the steel, at a rate not to exceed
50°F. per hour. For the two standard steels used, this consisted in
cooling from the normalizing temperature down to a temperature
of 1,100°F., at the rate indicated. Forgings normalized in this
manner will show a Brinell hardness of from 177 to 217. The question
has been repeatedly asked as to which treatment will produce the
higher quality finished part. In answer to this I will state that
on simple forgings of comparatively small section, the normalizing
treatment will produce a finished part which is of equal quality to
that of the quenched and annealed forgings. However, in the case of
complex forgings, or those of large section, more uniform physical
properties of the finished part will be obtained by quenching and
annealing the forgings in the place of normalizing.

The heat treatment of the finished gears consisted of quenching
in oil from a temperature of from 1,420 to 1,440°F. for the No.
X-3,340 steel, or from a temperature of from 1,500 to 1,540°F.
for No. 6,140 steel, followed by tempering in saltpeter or in an
electric furnace at a temperature of from 650 to 700°F.

The question has been asked by many engineers, why is the comparatively
low scleroscope hardness specified for gears? The reason for this is
that at best the life of an aviation engine is short, as compared with
that of an automobile, truck or tractor, and that shock resistance
is of vital importance. A sclerescope hardness of from 55 to 65
will give sufficient resistance to wear to prevent replacements
during the life of an aviation engine, while at the same time this
hardness produces approximately 50 per cent greater shock-resisting
properties to the gear. In the case of the automobile, truck or
tractor, resistance to wear is the main criterion and for that
reason the higher hardness is specified.

Great care should be taken in the design of an aviation engine
gear to eliminate sharp corners at the bottom of teeth as well
as in keyways. Any change of section in any stressed part of an
aviation engine must have a radius of at least 1/32 in. to give
proper shock and fatigue resistance. This fact has been demonstrated
many times during the Liberty engine program.

CONNECTING RODS

The material used for all connecting rods on the Liberty engine
was selected at the option of the manufacturer from one of two
standard S. A. E. steels, the composition of which are given in
Table 13.

TABLE 13.--COMPOSITION OF STEELS NOS. X-3,335 AND 6,135

  Steel No.             X-3,335  6,135
  Carbon, minimum        0.300   0.300
  Carbon, maximum        0.400   0.400
  Manganese, minimum     0.450   0.500
  Manganese, maximum     0.750   0.800
  Phosphorus, maximum    0.040   0.040
  Sulphur, maximum       0.045   0.045
  Nickel, minimum        2.750
  Nickel, maximum        3.250
  Chromium, minimum      0.700   0.800
  Chromium, maximum      0.950   1.100
  Vanadium minimum               0.150

All connecting rods were heat-treated to show the following minimum
physical properties; Elastic limit, 105,000 lb. per square inch:
elongation in 2 in., 17.5; per cent, reduction of area 50.0; per
cent., Brinell hardness, 241 to 277.

The heat treatment used to secure these physical properties consisted
in normalizing the forgings at a temperature of from 1,550 to 1,600°F.,
followed by cooling in the furnace or in air. The forgings were then
quenched in oil from a temperature of from 1,420 to 1,440°F. for the
No. X-3,335 steel, or from a temperature of from 1,500 to 1,525°F.
for No. 6,135 steel, followed by tempering at a temperature of from
1,075 to 1,150°F. At the option of the manufacturer, the normalizing
treatment could be substituted by quenching the forgings from a
temperature of from 1,550 to 1,600°F., in oil, and annealing for
the best machineability at a temperature of from 1,300 to 1,350°F.
The double quench, however, did not prove satisfactory on No. X-3,335
steel, due to the fact that it was necessary to remove forgings
from the quenching bath while still at a temperature of from 300
to 500°F. to eliminate any possibility of cracking. In view of the
fact that this practice is difficult to carry out in the average
heat-treating plant, considerable trouble was experienced.

The most important criterion in the production of aviation engine
connecting rods is the elimination of burned or severely overheated
forgings. Due to the particular design of the forked rod, considerable
trouble was experienced in this respect because of the necessity
of reheating the forgings before they are completely forged. As
a means of elimination of burned forgings, test lugs were forged
on the channel section as well as on the top end of fork. After
the finish heat treatment, these test lugs were nicked and broken
and the fracture of the steel carefully examined. This precaution
made it possible to eliminate burned forgings as the test lugs were
placed on sections which would be most likely to become burned.

There is a great difference of opinion among engineers as to what
physical properties an aviation engine connecting rod should have.
Many of the most prominent engineers contend that a connecting rod
should be as stiff as possible. To produce rods in this manner in
any quantity, it is necessary for the final heat treatment to be made
on the semi-machined rod. This practice would make it necessary for a
larger percentage of the semi-machined rods to be cold-straightened
after the finish heat treatment. The cold-straightening operation
on a part having important functions to perform as a connecting
rod is extremely dangerous.

In view of the fact that a connecting rod functions as a strut,
it is considered that this part should be only stiff enough to
prevent any whipping action during the running of the engine. The
greater the fatigue-resisting property that one can put into the
rod after this stiffness is reached, the longer the life of the
rod will be. This is the reason for the Brinell limits mentioned
being specified.

In connection with the connecting rod, emphasis must be laid on the
importance of proper radii at all changes of section. The connecting
rods for the first few Liberty engines were machined with sharp
corners at the point where the connecting-rod bolt-head fits on
assembly. On the first long endurance test of a Liberty engine
equipped with rods of this type, failure resulted from fatigue
starting at this point. It is interesting to note that every rod on
the engine which did not completely fail at this point had started
to crack. The adoption of a 1/32-in. radius at this point completely
eliminated fatigue failures on Liberty rods.

CRANKSHAFT

The crankshaft was the most highly stressed part of the entire
Liberty engine, and, therefore, every metallurgical precaution
was taken to guarantee the quality of this part. The material used
for the greater portion of the Liberty crankshafts produced was
nickel-chromium steel of the following chemical composition: Carbon,
0.350 to 0.450 per cent; manganese, 0.300 to 0.600 per cent; phosphorus,
0.040 maximum per cent; sulphur, 0.045 maximum per cent; nickel,
1.750 to 2.250 per cent; chromium, 0.700 to 0.900 per cent.

Each crankshaft was heat-treated to show the following minimum
physical properties: Elastic limit, 116,000 lb. per square inch;
elongation in 2 in., 16 per cent, reduction of area, 50 per cent,
Izod impact, 34 ft.-lb.; Brinell hardness, 266 to 321.

For every increase of 4,000 lb. per square inch in the elastic
limit above 116,000 lb. per square inch, the minimum Izod impact
required was reduced 1 ft.-lb.

The heat treatment used to produce these physical properties consisted
in normalizing the forgings at a temperature of from 1,550 to 1,600°F.,
followed by quenching in water at a temperature of from 1,475 to
1,525°F. and tempering at a temperature of from 1,000 to 1,100°F.
It is absolutely necessary that the crankshafts be removed from the
quenching tank before being allowed to cool below a temperature of
500°F., and immediately placed in the tempering furnace to eliminate
the possibility of quenching cracks.

A prolongation of not less than the diameter of the forging bearing
was forged on one end of each crankshaft. This was removed from
the shaft after the finish heat treatment, and physical tests were
made on test specimens which were cut from it at a point half way
between the center and the surface. One tensile test and one impact
test were made on each crankshaft, and the results obtained were
recorded against the serial number of the shaft in question. This
serial number was carried through all machining operations and
stamped on the cheek of the finished shaft. In addition to the
above tensile and impact tests, at least two Brinell hardness
determinations were made on each shaft.

All straightening operations on the Liberty crankshaft which were
performed below a temperature of 500°F. were followed by retempering
at a temperature of approximately 200°F. below the original tempering
temperature.

Another illustration of the importance of proper radii at all changes
of section is given in the case of the Liberty crankshaft. The presence
of tool marks or under cuts must be completely eliminated from an
aviation engine crankshaft to secure proper service. During the
duration of the Liberty program, four crankshafts failed from fatigue,
failures starting from sharp corners at bottom of propeller-hub
keyway. Two of the shafts that failed showed torsional spirals
running more than completely around the shaft. As soon as this
difficulty was removed no further trouble was experienced.

One of the most important difficulties encountered in connection
with the production of Liberty crankshafts was hair-line seams. The
question of hair-line seams has been discussed to greater length
by engineers and metallurgists during the war than any other single
question. Hair-line seams are caused by small non-metallic inclusions
in the steel. There is every reason to believe that these inclusions
are in the greater majority of cases manganese sulphide. There is
a great difference of opinion as to the exact effect of hair-line
seams on the service of an aviation engine crankshaft. It is the
opinion of many that hair-line seams do not in any way affect the
endurance of a crankshaft in service, provided they are parallel to
the grain of the steel and do not occur on a fillet. Of the 20,000
Liberty engines produced, fully 50 per cent of the crankshafts
used contain hair-line seams but not at the locations mentioned.
There has never been a failure of a Liberty crankshaft which could
in any way be traced to hair-line seams.

It was found that hair-line seams occur generally on high
nickel-chromium steels. One of the main reasons why the comparatively
mild analysis nickel-chromium steel was used was due to the very
few hair-line seams present in it. It was also determined that
the hair lines will in general be found near the surface of the
forgings. For that reason, as much finish as possible was allowed
for machining. A number of tests have been made on forging bars
to determine the depths at which hair-line seams are found, and
many cases came up in which hair-line seams were found 3/8 in.
from the surface of the bar. This means that in case a crankshaft
does not show hair-line seams on the ground surface this is no
indication that it is free from such a defect.

One important peculiarity of nickel-chromium steel was brought
out from the results obtained on impact tests. This peculiarity
is known as "blue brittleness." Just what the effect of this is
on the service of a finished part depends entirely upon the design
of the particular part in question. There have been no failures of
any nickel-chromium steel parts in the automotive industry which
could in any way be traced to this phenomena.

Whether or not nickel-chromium-steel forgings will show "blue
brittleness" depends entirely upon the temperature at which they
are tempered and their rate of cooling from this temperature. The
danger range for tempering nickel-chromium steels is between a
temperature of from 400 to 1,100°F. From the data so far gathered
on this phenomena, it is necessary that the nickel-chromium steel
to show "blue brittleness" be made by the acid process. There has
never come to my attention a single instance in which basic open
hearth steel has shown this phenomena. Just why the acid open hearth
steel should be sensitive to "blue brittleness" is not known.

All that is necessary to eliminate the presence of "blue brittleness"
is to quench all nickel-chromium-steel forgings in water from their
tempering temperature. The last 20,000 Liberty crankshafts that
were made were quenched in this manner.

PISTON PIN

The piston pin on an aviation engine must possess maximum resistance
to wear and to fatigue. For this reason, the piston pin is considered,
from a metallurgical standpoint, the most important part on the
engine to produce in quantities and still possess the above
characteristics. The material used for the Liberty engine piston
pin was S. A. E. No. 2315 steel, which is of the following chemical
composition: Carbon, 0.100 to 0.200 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; nickel, 3.250 to 3.750 per cent.

Each finished piston pin, after heat treatment, must show a minimum
scleroscope hardness of the case of 70, a scleroscope hardness of
the core of from 35 to 55 and a minimum crushing strength when
supported as a beam and the load applied at the center of 35,000
lb. The heat treatment used to obtain the above physical properties
consisted in carburizing at a temperature not to exceed 1,675°F.,
for a sufficient length of time to secure a case of from 0.02 to
0.04 in. deep. The pins are then allowed to cool slowly from the
carbonizing heat, after which the hole is finish-machined and the
pin cut to length. The finish heat treatment of the piston pin
consisted in quenching in oil from a temperature of from 1,525 to
1,575°F. to refine the grain of core properly and then quenching in
oil at a temperature of from 1,340 to 1,380°F. to refine and harden
the grain of the case properly, as well as to secure proper hardness
of core. After this quenching, all piston pins are tempered in oil
at a temperature of from 375 to 400°F. A 100 per cent inspection
for scleroscope hardness of the case and the core was made, and
no failures were ever recorded when the above material and heat
treatment was used.

APPLICATION TO THE AUTOMOTIVE INDUSTRY

The information given on the various parts of the Liberty engine
applies with equal force to the corresponding parts in the construction
of an automobile, truck or tractor. We recommend as first choice for
carbon-steel screw-machine parts material produced by the basic
open hearth process and having the following chemical composition;
Carbon, 0.150 to 0.250 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.045 maximum per cent; sulphur, 0.075 to 0.150
per cent.

This material is very uniform and is nearly as free cutting as
bessemer screw stock. It is sufficiently uniform to be used for
unimportant carburized parts, as well as for non-heat-treated
screw-machine parts. A number of the large automobile manufacturers
are now specifying this material in preference to the regular bessemer
grades.

As second choice for carbon-steel screw-machine parts we recommend
ordinary bessemer screw stock, purchased in accordance with S. A.
E. specification No. 1114. The advantage of using No. 1114 steel
lies in the fact that the majority of warehouses carry standard
sizes of this material in stock at all times. The disadvantage
of using this material is due to its lack of uniformity.

The important criterion for transmission gears is resistance to
wear. To secure proper resistance to wear a Brinell hardness of
from 512 to 560 must be obtained. The material selected to obtain
this hardness should be one which can be made most nearly uniform,
will undergo forging operations the easiest, will be the hardest
to overheat or burn, will machine best and will respond to a good
commercial range of heat treatment.

It is a well-known fact that the element chromium, when in the form
of chromium carbide in alloy steel, offers the greatest resistance to
wear of any combination yet developed. It is also a well-known fact that
the element nickel in steel gives excellent shock-resisting properties
as well as resistance to wear but not nearly as great a resistance
to wear as chromium. It has been standard practice for a number of
years for many manufacturers to use a high nickel-chromium steel
for transmission gears. A typical nickel-chromium gear specification
is as follows: Carbon, 0.470 to 0.520 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.700 to 0.950 per cent.

There is no question but that a gear made from material of such an
analysis will give excellent service. However, it is possible to
obtain the same quality of service and at the same time appreciably
reduce the cost of the finished part. The gear steel specified is
of the air-hardening type. It is extremely sensitive to secondary
pipe, as well as seams, and is extremely difficult to forge and
very easy to overheat. The heat-treatment range is very wide, but
the danger from quenching cracks is very great. In regard to the
machineability, this material is the hardest to machine of any
alloy steel known.

COMPOSITION OF TRANSMISSION-GEAR STEEL

If the nickel content of this steel is eliminated, and the percentage
of chromium raised slightly, an ideal transmission-gear material is
obtained. This would, therefore, be of the following composition:
Carbon, 0.470 to 0.520 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; chromium, 0.800 to 1.100 per cent.

The important criterion in connection with the use of this material
is that the steel be properly deoxidized, either through the use
of ferrovanadium or its equivalent. Approximately 2,500 sets of
transmission gears are being made daily from material of this analysis
and are giving entirely satisfactory results in service. The heat
treatment of the above material for transmission gears is as follows:
"Normalize forgings at a temperature of from 1,5.50 to 1,600°F.
Cool from this temperature to a temperature of 1,100°F. at the
rate of 50° per hour. Cool from 1,100°F., either in air or quench
in water."

Forgings so treated will show a Brinell hardness of from 177 to
217, which is the proper range for the best machineability. The
heat treatment of the finished gears consists of quenching in oil
from a temperature of 1,500 to 1,540°F., followed by tempering
in oil at a temperature of from 375 to 425°F. Gears so treated
will show a Brinell hardness of from 512 to 560, or a scleroscope
hardness of from 72 to 80. One tractor builder has placed in service
20,000 sets of gears of this type of material and has never had to
replace a gear. Taking into consideration the fact that a tractor
transmission is subjected to the worst possible service conditions,
and that it is under high stress 90 per cent of the time, it seems
inconceivable that any appreciable transmission trouble would be
experienced when material of this type is used on an automobile,
where the full load is applied not over 1 per cent of the time,
or on trucks where the full load is applied not over 50 per cent
of the time.

The gear hardness specified is necessary to reduce to a minimum
the pitting or surface fatigue of the teeth. If gears having a
Brinell hardness of over 560 are used, danger is encountered, due
to low shock-resisting properties. If the Brinell hardness is under
512, trouble is experienced due to wear and surface fatigue of
the teeth.

For ring gears and pinions material of the following chemical
composition is recommended: Carbon, 0.100 to 0.200 per cent; manganese,
0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.550 to 0.750 per cent; nickel,
0.400 to 0.600 per cent.

Care should be taken to see that this material is properly deoxidized
either by the use of ferrovanadium or its equivalent. The advantage
of using a material of the above type lies in the fact that it will
produce a satisfactory finished part with a very simple treatment.
The heat treatment of ring gears and pinions is as follows: "Carburize
at a temperature of from 1,650 to 1,700°F. for a sufficient length
of time to secure a depth of case of from 1/32 to 3/64 in., and
quench directly from carburizing heat in oil. Reheat to a temperature
of from 1,430 to 1,460°F. and quench in oil. Temper in oil at a
temperature of from 375 to 425°F. The final quenching operation
on a ring gear should be made on a fixture similar to the Gleason
press to reduce distortion to a minimum."

One of the largest producers of ring gears and pinions in the automotive
industry has been using this material and treatment for the last 2
years, and is of the opinion that he is now producing the highest
quality product ever turned out by that plant.

On some designs of automobiles a large amount of trouble is experienced
with the driving pinion. If the material and heat treatment specified
will not give satisfaction, rather than to change the design it is
possible to use the following analysis material, which will raise
the cost of the finished part but will give excellent service:
Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; nickel, 4.750 to 5.250 per cent.

The heat treatment of pinions produced from this material consists
in carburizing at a temperature of from 1,600 to 1,650°F. for a
sufficient length of time to secure a depth of case from 1/32 to
3/64 in. The pinions are then quenched in oil from a temperature
of 1,500 to 1,525°F. to refine the grain of the core and quenched
in oil from a temperature of from 1,340 to 1,360°F. To refine and
harden the case. The use of this material however, is recommended
only in an emergency, as high-nickel steel is very susceptible
to seams, secondary pipe and laminations.

The main criterion on rear-axle and pinion shafts, steering knuckles
and arms and parts of this general type is resistance to fatigue and
torsion. The material recommended for parts of this character is
either S. A. E. No. 6135 or No. 3135 steel, which have the chemical
composition given in Tables 9 and 7.

HEAT TREATMENT OF AXLES

Parts of this general type should be heat-treated to show the following
minimum physical properties: Elastic limit, 115,000 lb. per square
inch; elongation in 2 in., 16 per cent; reduction of area, 50 per
cent; Brinell hardness, 277 to 321.

The heat treatment used to secure these physical properties consists
in quenching from a temperature of from 1,520 to 1,540°F. in water
and tempering at a temperature of from 975 to 1,025°F. Where the
axle shaft is a forging, and in the case of steering knuckles and
arms, this heat treatment should be preceded by normalizing the
forgings at a temperature of from 1,550 to 1,600°F. It will be
noted that these physical properties correspond to those worked
out for an ideal aviation engine crankshaft. If parts of this type
are designed with proper sections, so that this range of physical
properties can be used, the part in question will give maximum
service.

One of the most important developments during the Liberty engine
program was the fact that it is not necessary to use a high-analysis
alloy steel to secure a finished part which will give proper service.
This fact should save the automotive industry millions of dollars
on future production.

If the proper authority be given the metallurgical engineer to
govern the handling of the steel from the time it is purchased
until it is assembled into finished product, mild-analysis steels
can be used and the quality of the finished product guaranteed.
It was only through the careful adherence to these fundamental
principles that it was possible to produce 20,000 Liberty engines,
which are considered to be the most highly stressed mechanism ever
produced, without the failure of a single engine from defective
material or heat treatment.

MAKING STEEL BALLS

Steel balls are made from rods or coils according to size, stock
less than 9/16-in. comes in coils. Stock 5/8-in. and larger comes
in rods. Ball stock is designated in thousandths so that 5/8-in.
rods are known as 0.625-in. stock.

Steel for making balls of average size is made up of:

  Carbon                         0.95 to 1.05 per cent
  Silicon                        0.20 to 0.35 per cent
  Manganese                      0.30 to 0.45 per cent
  Chromium                       0.35 to 0.45 per cent
  Sulphur and phosphorus not to exceed  0.025 per cent

For the larger sizes a typical analysis is:

  Carbon        1.02  per cent
  Silicon       0.21  per cent
  Manganese     0.40  per cent
  Chromium      0.65  per cent
  Sulphur       0.026 per cent
  Phosphorus    0.014 per cent

Balls 5/8 in. and below are formed cold on upsetting or heading
machines, the stock use is as follows:

 TABLE 14.--SIZES OF STOCK FOR FORMING BALLS ON HEADER
 -------------------------------------------------------
  Diameter of | Diameter of | Diameter of | Diameter of
  ball, inch  | stock inch  | ball, inch  | stock, inch
 -------------|-------------|-------------|-------------
      1/8     |    0.100    |     5/16    |    0.235
      5/32    |    0.120    |     3/8     |    0.275
      3/16    |    0.145    |     7/16    |    0.320
      7/32    |    0.170    |     1/2     |    0.365
      1/4     |    0.190    |     9/16    |    0.395
      9/32    |    0.220    |     5/8     |    0.440
 -------------------------------------------------------

For larger balls the blanks are hot-forged from straight bars.
They are usually forged in multiples of four under a spring hammer
and then separated by a suitable punching or shearing die in a
press adjoining the hammer. The dimensions are:

 -----------------------------------------------------------
  Diameter of ball, | Diameter of die, | Diameter of stock,
        inch        |       inch       |       inch
 -------------------|------------------|--------------------
         3/4        |      0.775       |       0.625
         7/8        |      0.905       |       0.729
          1         |      1.035       |       0.823
 -----------------------------------------------------------

Before hardening, the balls are annealed to relieve the stresses
of forging and grinding, this being done by passing them through a
revolving retort made of nichrome or other heat-resisting substance.
The annealing temperature is 1,300°F.

The hardening temperature is from 1,425 to 1,475°F. according to
size and composition of steel. Small balls, 5/16 and under, are
quenched in oil, the larger sizes in water. In some special cases
brine is used. Quenching small balls in water is too great a shock
as the small volume is cooled clear through almost instantly. The
larger balls have metal enough to cool more slowly.

Balls which are cooled in either water or brine are boiled in water
for 2 hr. to relieve internal stresses, after which the balls are
finished by dry-grinding and oil-grinding.

The ball makers have an interesting method of testing stock for
seams which do not show in the rod or wire. The Hoover Steel Ball
Company cut off pieces of rod or wire 7/16 in. long and subject
them to an end pressure of from 20,000 to 50,000 lb. A pressure
of 20,000 lb. compresses the piece to 3/16 in. and the 50,000 lb.
pressure to 3/32 in. This opens any seam which may exist but a
solid bar shows no seam.

Another method which has proved very successful is to pass the
bar or rod to be tested through a solenoid electro-magnet. With
suitable instruments it is claimed that this is an almost infallible
test as the instruments show at once when a seam or flaw is present
in the bar.




CHAPTER V

THE FORGING OF STEEL

So much depends upon the forging of steel that this operation must
be carefully supervised. This is especially true because of the
tendency to place unskilled and ignorant men as furnace-tenders
and hammer men. The main points to be supervised are the slow and
careful heating to the proper temperature; forging must be continued
at a proper rate to the correct temperature. The bar of stock from
which a forging was made may have had a fairly good structure, but
if the details of the working are not carefully watched, a seamy,
split article of no value may easily result.

HEATING.--Although it is possible to work steels cold, to an extent
depending upon their ductility, and although such operations are
commonly performed, "forging" usually means working _heated_ steel.
_Heating_ is therefore a vital part of the process.

Heating should be done slowly in a soaking heat. A soft "lazy"
flame with excess carbon is necessary to avoid burning the corners
of the bar or billet, and heavily scaling the surface. If the
temperature is not raised slowly, the outer part of the metal may
be at welding heat while the inner part is several hundred degrees
colder and comparatively hard and brittle.

The above refers to muffle furnaces. If the heating is done in
a small blacksmith's forge, the fire should be kept clean, and
remade at intervals of about two hours. Ashes and cinders should
be cleaned from the center down to the tuyere and oily waste and
wood used to start a new fire. As this kindles a layer of coke
from the old fire is put on top, and another layer of green coal
(screened and dampened blacksmiths' coal) as a cover. When the
green coal on top has been coked the fire is ready for use. As
the fuel burns out in the center, the coke forming around the edge
is pushed inward, and its place taken by more green coal. Thus the
fire is made up of three parts; the center where coke is burning
and the iron heating; a zone where coke is forming, and the outside
bank of green coal.

STEEL WORKED IN AUSTENITIC STATE.--As a general rule steel should
be worked when it is in the austenitic state. (See page 108.) It
is then soft and ductile.

As the steel is heated above the critical temperature the size of
the austenite crystals tends to grow rapidly. When forging starts,
however, these grains are broken up. The growth is continually
destroyed by the hammering, which should consequently be continued
down to the upper critical temperature when the austenite crystals
break up into ferrite and cementite. The size of the final grains
will be much smaller and hence a more uniform structure will result
if the "mother" austenite was also fine grained. A final steel
will be composed of pearlite; ferrite and pearlite; or cementite
and pearlite, according to the carbon content.

The ultimate object is to secure a fine, uniform grain throughout
the piece and this can be secured by uniform heating and by thoroughly
rolling it or working it at a temperature just down to its critical
point. If this is correctly done the fracture will be fine and
silky. Steel which has been overheated slightly and the forging
stopped at too high a temperature will show a "granular" fracture.
A badly overheated or "burned" steel will have iridescent colors
on a fresh fracture, it will be brittle both hot and cold, and
absolutely ruined.

STEEL CAN BE WORKED COLD.--As noted above, steel can be worked cold,
as in the case of cold-rolled steel. Heat treatment of cold-worked
steel is a very delicate operation. Cold working hardens and strengthens
steel. It also introduces internal stresses. Heat-treatments are
designed to eliminate the stresses without losing the hardness
and strength. This is done by tempering at a low heat. Avoid the
"blue" range (350 to 750°C.). Tempering for a considerable time just
under the critical is liable to cause great brittleness. Annealing
(reheating through the critical) destroys the effect of cold work.

FORGING

HIGH-SPEED STEEL.--Heat very slowly and carefully to from 1,800
to 2,000°F. and forge thoroughly and uniformly. If the forging
operation is prolonged do not continue forging the tool when the
steel begins to stiffen under the hammer. Do not forge below 1,700°F.
(a dark lemon or orange color). Reheat frequently rather than prolong
the hammering at the low heats.

After finishing the forging allow the tool to cool as slowly as
possible in lime or dry ashes; avoid placing the tool on the damp
ground or in a draught of air. Use a good clean fire for heating.
Do not allow the tool to soak at the forging heat. Do not heat any
more of the tool than is necessary in order to forge it to the
desired shape.

CARBON TOOL STEEL.--Heat to a bright red, about 1,500 to 1,550°F.
Do not hammer steel when it cools down to a dark cherry red, or
just below its hardening point, as this creates surface cracks.

OIL-HARDENING STEEL.--Heat slowly and uniformly to 1,450°F. and
forge thoroughly. Do not under any circumstances attempt to harden
at the forging heat. After cooling from forging reheat to about
1,450°F. and cool slowly so as to remove forging strains.

CHROME-NICKEL STEEL.--Forging heat of chrome-nickel steel depends
very largely on the percentage of each element contained in the
steel. Steel containing from 1/2 to 1 per cent chromium and from
1-1/2 to 3-1/2 per cent nickel, with a carbon content equal to
the chromium, should be heated very slowly and uniformly to
approximately 1,600° F., or salmon color. After forging, reheat
the steel to about 1,450° and cool slowly so as to remove forging
strains. Do not attempt to harden the steel before such annealing.

A great deal of steel is constantly being spoiled by carelessness
in the forging operation. The billets may be perfectly sound, but
even if the steel is heated to a good forging heat, and is hammered
too lightly, a poor forging results. A proper blow will cause the
edges and ends to bulge slightly outwards--the inner-most parts
of the steel seem to flow faster than the surface. Light blows
will work the surface out faster; the edges and ends will curve
inwards. This condition in extreme cases leaves a seam in the axis
of the forging.

Steel which is heated quickly and forging begun before uniform
heat has penetrated to its center will open up seams because the
cooler central portion is not able to flow with the hot metal
surrounding it. Uniform heating is absolutely necessary for the
best results.

Figure 16 shows a sound forging. The bars in Fig. 17 were burst
by improper forging, while the die, Fig. 18, burst from a piped
center.

Figure 19 shows a piece forged with a hammer too light for the size
of the work. This gives an appearance similar to case-hardening,
the refining effect of the blows reaching but a short distance
from the surface.

While it is impossible to accurately rate the capacity of steam
hammers with respect to the size of work they should handle, on
account of the greatly varying conditions, a few notes from the
experience of the Bement works of the Niles-Bement-Pond Company
will be of service.

[Illustration: FIG. 16.--A sound forging.]

[Illustration: FIG. 17.--Burst from improper forging.]

For making an occasional forging of a given size, a smaller hammer
may be used than if we are manufacturing this same piece in large
quantities. If we have a 6-in. piece to forge, such as a pinion or
a short shaft, a hammer of about 1,100-lb. capacity would answer
very nicely. But should the general work be as large as this, it
would be very much better to use a 1,500-lb. hammer. If, on the
other hand, we wish to forge 6-in. axles economically, it would
be necessary to use a 7,000- or 8,000-lb. hammer. The following
table will be found convenient for reference for the proper size
of hammer to be used on different classes of general blacksmith
work, although it will be understood that it is necessary to modify
these to suit conditions, as has already been indicated.

[Illustration: FIG. 18.--Burst from a piped center.]

[Illustration: FIG. 19.--Result of using too light a hammer.]

  Diameter of stock    Size of hammer
  3-1/2 in.             250 to   350 lb.
  4     in.             350 to   600 lb.
  4-1/2 in.             600 to   800 lb.
  5     in.             800 to 1,000 lb.
  6     in.           1,100 to 1,500 lb.

Steam hammers are always rated by the weight of the ram, and the
attached parts, which include the piston and rod, nothing being
added on account of the steam pressure behind the piston. This makes
it a little difficult to compare them with plain drop or tilting
hammers, which are also rated in the same way.

[Illustration: FIG. 20.--Good and bad ingots.]

Steam hammers are usually operated at pressures varying from 75
to 100 lb. of steam per square inch, and may also be operated by
compressed air at about the same pressures. It is cheaper, however,
in the case of compressed air to use pressures from 60 to 80 lb.
instead of going higher.

Forgings must, however, be made from sound billets if satisfactory
results are to be secured. Figure 20 shows three cross-sections
of which _A_ is sound, _B_ is badly piped and _C_ is worthless.

PLANT FOR FORGING RIFLE BARRELS

The forging of rifle barrels in large quantities and heat-treating
them to meet the specifications demanded by some of the foreign
governments led Wheelock, Lovejoy & Company to establish a complete
plant for this purpose in connection with their warehouse in Cambridge,
Mass. This plant, designed and constructed by their chief engineer,
K. A. Juthe, had many interesting features. Many features of this
plant can be modified for other classes of work.

[Illustration: FIG 21.--Cutting up barrels.]

[Illustration: FIG. 22.--Upsetting the ends.]

The stock, which came in bars of mill length, was cut off so as to
make a barrel with the proper allowances for trimming (Fig. 21).
They then pass to the forging or upsetting press in the adjoining
room. This press, which is shown in more detail in Fig. 22, handled
the barrels from all the heating furnaces shown. The men changed
work at frequent intervals, to avoid excessive fatigue.

[Illustration: FIG. 23.--Continuous heating furnace.]

Then the barrels were reheated in the continuous furnace, shown
in Fig. 23, and straightened before being tested.

The barrels were next tested for straightness. After the heat-treating,
the ends are ground, a spot ground on the enlarged end and each
barrel tested on a Brinell machine. The pressure used is 3,000 kg.,
or 6,614 lb., on a 10-millimeter ball, which is standard. Hardness
of 240 was desired.

The heat-treating of the rifle blanks covered four separate operations:
(1) Heating and soaking the steel above the critical temperature
and quenching in oil to harden the steel through to the center;
(2) reheating for drawing of temper for the purpose of meeting the
physical specifications; (3) reheating to meet the machine ability
test for production purposes; and (4) reheating to straighten the
blanks while hot.

A short explanation of the necessity for the many heats may be
interesting. For the first heat, the blanks were slowly brought
to the required heat, which is about 150°F. above the critical
temperature. They are then soaked at a high heat for about 1 hr.
before quenching. The purpose of this treatment is to eliminate
any rolling or heat stresses that might be in the bars from mill
operations; also to insure a thorough even heat through a cross-section
of the steel. This heat also causes blanks with seams or slight
flaws to open up in quenching, making detection of defective blanks
very easy.

The quenching oil was kept at a constant temperature of 100°F.,
to avoid subjecting the steel to shocks, thereby causing surface
cracks. The drawing of temper was the most critical operation and
was kept within a 10° fluctuation. The degree of heat necessary
depends entirely on the analysis of the steel, there being a certain
variation in the different heats of steel as received from the mill.

MACHINEABILITY

Reheating for machine ability was done at 100° less than the drawing
temperature, but the time of soaking is more than double. After
both drawing and reheating, the blanks were buried in lime where
they remain, out of contact with the air, until their temperature
had dropped to that of the workroom.

For straightening, the barrels were heated to from 900 to 1,000°F.
in an automatic furnace 25 ft. long, this operation taking about 2
hr. The purpose of hot straightening was to prevent any stresses
being put into the blanks, so that after rough-turning, drilling
or rifling operations they would not have a tendency to spring
back to shape as left by the quenching bath.

A method that produces an even better machining rifle blank, which
practically stays straight through the different machining operations,
was to rough-turn the blanks, then subject them to a heat of practically
1,0000 for 4 hr. Production throughout the different operations is
materially increased, with practically no straightening required
after drilling, reaming, finish-turning or rifling operations.

[Illustration: FIG. 24. FIG. 25.

FIGS. 24 and 25.--Roof system of cooling quenching oil.]

This method was tested out by one of the largest manufacturers and
proved to be the best way to eliminate a very expensive finished
gun-barrel straightening process.

[Illustration: FIG. 26.--Details of the cooler.]

The heat-treating required a large amount of cooling oil, and the
problem of keeping this at the proper temperature required considerable
study. The result was the cooling plant on the roof, as shown in
Figs. 24, 25 and 26. The first two illustrations show the plant as
it appeared complete. Figure 26 shows how the oil was handled in
what is sometimes called the ebulator system. The oil was pumped
up from the cooling tanks through the pipe _A_ to the tank _B_.
From here it ran down onto the breakers or separators _C_, which
break the oil up into fine particles that are caught by the fans
_D_. The spray is blown up into the cooling tower _E_, which contains
banks of cooling pipes, as can be seen, as well as baffies _F_. The
spray collects on the cool pipes and forms drops, which fall on
the curved plates _G_ and run back to the oil-storage tank below
ground.

The water for this cooling was pumped from 10 artesian wells at the
rate of 60 gal. per minute and cooled 90 gal. of oil per minute,
lowering the temperature from 130 or 140 to 100°F. The water as
it came from the wells averaged around 52°F. The motor was of a
7-1/2-hp. variable-speed type with a range of from 700 to 1,200
r.p.m., which could be varied to suit the amount of oil to be cooled.
The plant handled 300 gal. of oil per minute.




CHAPTER VI

ANNEALING

There is no mystery or secret about the proper annealing of different
steels, but in order to secure the best results it is absolutely
necessary for the operator to know the kind of steel which is to
be annealed. The annealing of steel is primarily done for one of
three specific purposes: To soften for machining purposes; to change
the physical properties, largely to increase ductility; or to release
strains caused by rolling or forging.

Proper annealing means the heating of the steel slowly and uniformly
to the right temperature, the holding of the temperature for a given
period and the gradual cooling to normal temperature. The proper
temperature depends on the kind of steel, and the suggestions of the
maker of the special steel being used should be carefully followed.
For carbon steel the temperatures recommended for annealing vary
from 1,450 to 1,600°F. This temperature need not be long continued.
The steel should be cooled in hot sand, lime or ashes. If heated in
the open forge the steel should be buried in the cooling material
as quickly as possible, not allowing it to remain in the open air
any longer than absolutely necessary. Best results, however, are
secured when the fire does not come in direct contact with the
steel.

Good results are obtained by packing the steel in iron boxes or
tubes, much as for case-hardening or carbonizing, using the same
materials. Pieces do not require to be entirely surrounded by carbon
for annealing, however. Do not remove from boxes until cold.

Steel to be annealed may be classified into four different groups,
each of which must be treated according to the elements contained in
its particular analysis. Different methods are therefore necessary
to bring about the desired result. The classifications are as follows:
High-speed steel, alloy steel, tool or crucible steel, and high-carbon
machinery steel.

ANNEALING OF HIGH-SPEED STEEL

For annealing high-speed steel, some makers recommend using ground
mica, charcoal, lime, fine dry ashes or lake sand as a packing
in the annealing boxes. Mixtures of one part charcoal, one part
lime and three parts of sand are also suggested, or two parts of
ashes may be substituted for the one part of lime.

To bring about the softest structure or machine ability of high-speed
steel, it should be packed in charcoal in boxes or pipes, carefully
sealed at all points, so that no gases will escape or air be admitted.
It should be heated slowly to not less than 1,450°F. and the steel
must not be removed from its packing until it is cool. Slow heating
means that the high heat must have penetrated to the very core of
the steel.

When the steel is heated clear through it has been in the furnace
long enough. If the steel can remain in the furnace and cool down
with it, there will be no danger of air blasts or sudden or uneven
cooling. If not, remove the box and cover quickly with dry ashes,
sand or lime until it becomes cold.

Too high a heat or maintaining the heat for too long a period,
produces a harsh, coarse grain and greatly increases the liability
to crack in hardening. It also reduces the strength and toughness
of the steel.

Steel which is to be used for making tools with teeth, such as
taps, reamers and milling cutters, should not be annealed too much.
When the steel is too soft it is more apt to tear in cutting and
makes it more difficult to cut a smooth thread or other surface.
Moderate annealing is found best for tools of this kind.

TOOL OR CRUCIBLE STEEL

Crucible steel can be annealed either in muffled furnace or by
being packed. Packing is by far the most satisfactory method as it
prevents scaling, local hard spots, uneven annealing, or violent
changes in shape. It should be brought up slowly to just above
its calescent or hardening temperature. The operator must know
before setting his heats the temperature at which the different
carbon content steels are hardened. The higher the carbon contents
the lower is the hardening heat, but this should in no case be
less than 1,450°F.

ANNEALING ALLOY STEEL

The term alloy steel, from the steel maker's point of view, refers
largely to nickel and chromium steel or a combination of both. These
steels are manufactured very largely by the open-hearth process,
although chromium steels are also a crucible product. It is next
to impossible to give proper directions for the proper annealing
of alloy steel unless the composition is known to the operator.

Nickel steels may be annealed at lower temperatures than carbon
steels, depending upon their alloy content. For instance, if a
pearlitic carbon steel may be annealed at 1,450°C., the same analysis
containing 2-1/2 per cent nickel may be annealed at 1,360°C. and
a 5 per cent nickel steel at 1,270°.

In order that high chromium steels may be readily machined, they
must be heated at or slightly above the critical for a very long
time, and cooled through the critical at an extremely slow rate.
For a steel containing 0.9 to 1.1 per cent carbon, under 0.50 per
cent manganese, and about 1.0 per cent chromium, Bullens recommends
the following anneal:

  1. Heat to 1,700 or 1,750°F.
  2. Air cool to about 800°F.
  3. Soak at 1,425 to 1,450°F.
  4. Cool slowly in furnace.

HIGH-CARBON MACHINERY STEEL

The carbon content of this steel is above 30 points and is hardly
ever above 60 points or 0.60 per cent. Annealing such steel is
generally in quantity production and does not require the care that
the other steels need because it is very largely a much cheaper
product and a great deal of material is generally removed from
the outside surface.

The purpose for which this steel is annealed is a deciding factor
as to what heat to give it. If it is for machineability only, the
steel requires to be brought up slowly to just below the critical and
then slowly cooled in the furnace or ash pit. It must be thoroughly
covered so that there will be no access of cool air. If the annealing
is to increase ductility to the maximum extent it should be slowly
heated to slightly over the upper critical temperature and kept at
this heat for a length of time necessary for a thorough penetration
to the core, after which it can be cooled to about 1,200°F., then
reheated to about 1,360°F., when it can be removed and put in an
ash pit or covered with lime. If the annealing is just to relieve
strains, slow heating is not necessary, but the steel must be brought
up to a temperature not much less than a forging or rolling heat
and gradually cooled. Covering in this case is only necessary in
steel of a carbon content of more than 40 points.

ANNEALING IN BONE

Steel and cast iron may both be annealed in granulated bone. Pack the
work the same as for case-hardening except that it is not necessary
to keep the pieces away from each other. Pack with bone that has
been used until it is nearly white. Heat as hot as necessary for
the steel and let the furnace cool down. If the boxes are removed
from furnace while still warm, cover boxes and all in warm ashes
or sand, air slaked lime or old, burned bone to retain heat as
long as possible. Do not remove work from boxes until cold.

ANNEALING OF RIFLE COMPONENTS AT SPRINGFIELD ARMORY

In general, all forgings of the components of the arms manufactured
at the Armory and all forgings for other ordnance establishments
are packed in charcoal, lime or suitable material and annealed
before being transferred from the forge shop.

Except in special cases, all annealing will be done in annealing
pots of appropriate size. One fire end of a thermo-couple is inserted
in the center of the annealing pot nearest the middle of the furnace
and another in the furnace outside of but near the annealing pots.

The temperatures used in annealing carbon steel components of the
various classes used at the Armory vary from 800°C. To 880°C. or
1,475 to 1,615°F.

The fuel is shut off from the annealing furnace gradually as the
temperature of the pot approaches the prescribed annealing temperature
so as to prevent heating beyond that temperature.

The forgings of the rifle barrel and the pistol barrel are exceptions
to the above general rule. These forgings will be packed in lime
and allowed to cool slowly from the residual heat after forging.




CHAPTER VII

CASE-HARDENING OR SURFACE-CARBURIZING

Carburizing, commonly called case-hardening, is the art of producing
a high-carbon surface, or case, upon a low carbon steel article.
Wrenches, locomotive link motions, gun mechanisms, balls and ball
races, automobile gears and many other devices are thereby given
a high-carbon _case_ capable of assuming extreme hardness, while
the interior body of metal, the _core_, remains soft and tough.

The simplest method is to heat the piece to be hardened to a bright
red, dip it in cyanide of potassium (or cover it by sprinkling
the cyanide over it), keep it hot until the melted cyanide covers
it thoroughly, and quench in water. Carbon and nitrogen enter the
outer skin of the steel and harden this skin but leave the center
soft. The hard surface or "case" varies in thickness according to
the size of the piece, the materials used and the length of time
which the piece remains at the carburizing temperature. Cyanide
case-hardening is used only where a light or thin skin is sufficient.
It gives a thickness of about 0.002 in.

In some cases of cyanide carburizing, the piece is heated in cyanide
to the desired temperature and then quenched. For a thicker case
the steel is packed in carbon materials of various kinds such as
burnt leather scraps, charcoal, granulated bone or some of the
many carbonizing compounds.

Machined or forged steel parts are packed with case-hardening material
in metal boxes and subjected to a red heat. Under such conditions,
carbon is absorbed by the steel surfaces, and a carburized case is
produced capable of responding to ordinary hardening and tempering
operations, the core meanwhile retaining its original softness and
toughness.

Such case-hardened parts are stronger, cheaper, and more serviceable
than similar parts made of tool steel. The tough core resists breakage
by shock. The hardened case resists wear from friction. The low cost
of material, the ease of manufacture, and the lessened breakage
in quenching all serve to promote cheap production.

For successful carburizing, the following points should be carefully
observed:

The utmost care should be used in the selection of pots for carburizing;
they should be as free as possible from both scaling and warping.
These two requirements eliminate the cast iron pot, although many
are used, thus leaving us to select from malleable castings, wrought
iron, cast steel, and special alloys, such as nichrome or silchrome.
If first cost is not important, it will prove cheaper in the end
to use pots of some special alloy.

[Illustrations: FIGS. 27 to 30.--Case-hardening or carburizing boxes.]

[Illustration: FIG. 31.--A lid that is easily luted.]

The pots should be standardized to suit the product. Pots should be
made as small as possible in width, and space gained by increasing
the height; for it takes about 1-1/2 hr. to heat the average small
pot of 4 in. in width, between 3 and 4 hr. to heat to the center
of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in.
box; and the longer the time required to heat to the center, the
more uneven the carburizing.

The work is packed in the box surrounded by materials which will
give up carbon when heated. It must be packed so that each piece
is separate from the others and does not touch the box, with a
sufficient amount of carburizing material surrounding each. Figures
27 to 31 show the kind of boxes used and the way the work should be
packed. Figure 31 shows a later type of box in which the edges can
be easily luted. Figure 30 shows test wires broken periodically to
determine the depth of case. Figure 28 shows the minimum clearance
which should be used in packing and Fig. 29 the way in which the
outer pieces receive the heat first and likewise take up the carbon
before those in the center. This is why a slow, soaking heat is
necessary in handling large quantities of work, so as to allow
the heat and carbon to soak in equally.

While it has been claimed that iron below its critical temperature
will absorb some carbon, Giolitti has shown that this absorption
is very slow. In order to produce quick and intense carburization
the iron should preferably be above its upper critical temperature
or 1,600°F.,--therefore the carbon absorbed immediately goes into
austenite, or solid solution. It is also certain that the higher
the temperature the quicker will carbon be absorbed, and the deeper
it will penetrate into the steel, that is, the deeper the "case."
At Sheffield, England, where wrought iron is packed in charcoal and
heated for days to convert it into "blister steel," the temperatures
are from 1,750 to 1,830°F. Charcoal by itself carburizes slowly,
consequently commercial compounds also contain certain "energizers"
which give rapid penetration at lower temperatures.

The most important thing in carburizing is the human element. Most
careful vigilance should be kept when packing and unpacking, and the
operator should be instructed in the necessity for clean compound
free from scale, moisture, fire clay, sand, floor sweepings, etc.
From just such causes, many a good carburizer has been unjustly
condemned. It is essential with most carburizers to use about 25 to
50 per cent of used material, in order to prevent undue shrinking
during heating; therefore the necessity of properly screening used
material and carefully inspecting it for foreign substances before
it is used again. It is right here that the greatest carelessness
is generally encountered.

Don't pack the work to be carburized too closely; leave at least
1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the
top of pots, and for a 6-hr. run, have the pieces at least 1/2
in. apart. This gives the heat a chance to thoroughly permeate
the pot, and the carburizing material a chance to shrink without
allowing carburized pieces to touch and cause soft spots.

Good case-hardening pots and annealing tubes can be made from the
desired size of wrought iron pipe. The ends are capped or welded,
and a slot is cut in the side of the pot, equal to one quarter of
its circumference, and about 7/8 of its length. Another piece of
the same diameter pipe cut lengthwise into thirds forms a cover
for this pot. We then have a cheap, substantial pot, non-warping,
with a minimum tendency to scale, but the pot is difficult to seal
tightly. This idea is especially adaptable when long, narrow pots
are desired.

When pots are packed and the carburizer thoroughly tamped down,
the covers of the pot are put on and sealed with fire clay which
has a little salt mixed into it. The more perfect the seal the
more we can get out of the carburizer. The rates of penetration
depend on temperature and the presence of proper gas in the required
volume. Any pressure we can cause will, of course, have a tendency
to increase the rate of penetration.

If you have a wide furnace, do not load it full at one time. Put
one-half your load in first, in the center of the furnace, and
heat until pots show a low red, about 1,325 to 1,350°F. Then fill
the furnace by putting the cold pots on the outside or, the section
nearest the source of heat. This will give the work in the slowest
portion of the furnace a chance to come to heat at the same time
as the pots that are nearest the sources of heat.

To obtain an even heating of the pots and lessen their tendency
to warp and scale, and to cause the contents of the furnace to
heat up evenly, we should use a reducing fire and fill the heating
chamber with flame. This can be accomplished by partially closing
the waste gas vents and reducing slightly the amount of air used
by the burners. A short flame will then be noticed issuing from
the partially closed vents. Thus, while maintaining the temperature
of the heating chamber, we will have a lower temperature in the
combustion chamber, which will naturally increase its longevity.

Sometimes it is advisable to cool the work in the pots. This saves
compound, and causes a more gradual diffusion of the carbon between
the case and the core, and is very desirable condition, inasmuch
as abrupt cases are inclined to chip out.

The most satisfactory steel to carburize contains between 0.10
and 0.20 per cent carbon, less than 0.35 per cent manganese, less
than 0.04 per cent phosphorus and sulphur, and low silicon. But
steel of this composition does not seem to satisfy our progressive
engineers, and many alloy steels are now on the market, these,
although more or less difficult to machine, give when carburized
the various qualities demanded, such as a very hard case, very tough
core, or very hard case and tough core. However, the additional
elements also have a great effect both on the rate of penetration
during the carburizing operation, and on the final treatment,
consequently such alloy steels require very careful supervision
during the entire heat treating operations.

RATE OF ABSORPTION

According to Guillet, the absorption of carbon is favored by those
special elements which exist as double carbides in steel. For example,
manganese exists as manganese carbide in combination with the iron
carbide. The elements that favor the absorption of carbon are:
manganese, tungsten, chromium and molybdenum those opposing it,
nickel, silicon, and aluminum. Guillet has worked out the effect
of the different elements on the rate of penetration in comparison
with steel that absorbed carbon at a given temperature, at an average
rate of 0.035 in. per hour.

His tables show that the following elements require an increased
time of exposure to the carburizing material in order to obtain
the same depth of penetration as with simple steel:

   When steel contains    Increased time of exposure
  2.0 per cent nickel            28 per cent
  7.0 per cent nickel            30 per cent
  1.0 per cent titanium          12 per cent
  2.0 per cent titanium          28 per cent
  0.5 per cent silicon           50 per cent
  1.0 per cent silicon           80 per cent
  2.0 per cent silicon          122 per cent
  5.0 per cent silicon         No penetration
  1.0 per cent aluminum         122 per cent
  2.0 per cent aluminum         350 per cent

The following elements seem to assist the rate of penetration of
carbon, and the carburizing time may therefore be reduced as follows:

   When steel contains      Decreased time of exposure
  0.5 per cent manganese           18 per cent
  1.0 per cent manganese           25 per cent
  1.0 per cent chromium            10 per cent
  2.0 per cent chromium            18 per cent
  0.5 per cent tungsten             0
  1.0 per cent tungsten             0
  2.0 per cent tungsten            25 per cent
  1.0 per cent molybdenum           0
  2.0 per cent molybdenum          18 per cent

The temperature at which carburization is accomplished is a very
important factor. Hence the necessity for a reliable pyrometer,
located so as to give the temperature just below the tops of the
pots. It must be remembered, however, that the pyrometer gives
the temperature of only one spot, and is therefore only an aid to
the operator, who must use his eyes for successful results.

The carbon content of the case generally is governed by the temperature
of the carburization. It generally proves advisable to have the
case contain between 0.90 per cent and 1.10 carbon; more carbon
than this gives rise to excess free cementite or carbide of iron,
which is detrimental, causing the case to be brittle and apt to chip.

T. G. Selleck gives a very useful table of temperatures and the
relative carbon contents of the case of steels carburized between
4 and 6 hrs. using a good charcoal carburizer. This data is as
follows:

   TABLE 15.--CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES

  At 1,500°F., the surface carbon content will be 0.90 per cent
  At 1,600°F., the surface carbon content will be 1.00 per cent
  At 1,650°F., the surface carbon content will be 1.10 per cent
  At 1,700°F., the surface carbon content will be 1.25 per cent
  At 1,750°F., the surface carbon content will be 1.40 per cent
  At 1,800°F., the surface carbon content will be 1.75 per cent

To this very valuable table, it seems best to add the following
data, which we have used for a number of years. We do not know
the name of its author, but it has proved very valuable, and seems
to complete the above information. The table is self-explanatory,
giving depth of penetration of the carbon of the case at different
temperatures for different lengths of time:

 ---------------------------------------------------------
                            |         Temperature
        Penetration         |-----------------------------
                            |  1,550  |  1,650  |  1,800
 ---------------------------|---------|---------|---------
  Penetration after 1/2 hr. |  0.008  |  0.012  |  0.030
  Penetration after 1 hr.   |  0.018  |  0.026  |  0.045
  Penetration after 2 hr.   |  0.035  |  0.048  |  0.060
  Penetration after 3 hr.   |  0.045  |  0.055  |  0.075
  Penetration after 4 hr.   |  0.052  |  0.061  |  0.092
  Penetration after 6 hr.   |  0.056  |  0.075  |  0.110
  Penetration after 8 hr.   |  0.062  |  0.083  |  0.130
 ---------------------------------------------------------

From the tables given, we may calculate with a fair degree of certainty
the amount of carbon in the case, and its penetration. These figures
vary widely with different carburizers, and as pointed out immediately
above, with different alloy steels.

CARBURIZING MATERIAL

The simplest carburizing substance is charcoal. It is also the
slowest, but is often used mixed with something that will evolve
large volumes of carbon monoxide or hydrocarbon gas on being heated.
A great variety of materials is used, a few of them being charcoal
(both wood and bone), charred leather, crushed bone, horn, mixtures
of charcoal and barium carbonate, coke and heavy oils, coke treated
with alkaline carbonates, peat, charcoal mixed with common salt,
saltpeter, resin, flour, potassium bichromate, vegetable fibre,
limestone, various seed husks, etc. In general, it is well to avoid
complex mixtures.

H. L. Heathcote, on analyzing seventeen different carburizers, found
that they contained the following ingredients:

                          Per cent
  Moisture             2.68 to 26.17
  Oil                  0.17 to 20.76
  Carbon (organic)     6.70 to 54.19
  Calcium phosphate    0.32 to 74.75
  Calcium carbonate    1.20 to 11.57
  Barium carbonate      nil to 42.00
  Zinc oxide            nil to 14.50
  Silica                nil to  8.14
  Sulphates (SO3)     trace to  3.45
  Sodium chloride       nil to  7.88
  Sodium carbonate      nil to 40.00
  Sulphides (S)         nil to  2.80

Carburizing mixtures, though bought by weight, are used by volume,
and the weight per cubic foot is a big factor in making a selection.
A good mixture should be porous, so that the evolved gases, which
should be generated at the proper temperature, may move freely
around the steel objects being carburized; should be a good conductor
of heat; should possess minimum shrinkage when used; and should
be capable of being tamped down.

Many "secret mixtures" are sold, falsely claimed to be able to
convert inferior metal into crucible tool steel grade. They are
generally nothing more than mixtures of carbonaceous and cyanogen
compounds possessing the well-known carburizing properties of those
substances.

QUENCHING

It is considered good practice to quench alloy steels from the pot,
especially if the case is of any appreciable depth. The texture
of carbon steel will be weakened by the prolonged high heat of
carburizing, so that if we need a tough core, we must reheat it
above its critical range, which is about 1,600°F. for soft steel,
but lower for manganese and nickel steels. Quenching is done in
either water, oil, or air, depending upon the results desired.
The steel is then very carefully reheated to refine the case, the
temperature varying from 1,350 to 1,450°F., depending on whether
the material is an alloy or a simple steel, and quenched in either
water or oil.

[Illustration: FIG. 32.--Case-hardening depths.]

There are many possibilities yet to be developed with the carburizing
of alloy steels, which can produce a very tough, tenacious austenitic
case which becomes hard on cooling in air, and still retains a
soft, pearlitic core. An austenitic case is not necessarily file
hard, but has a very great resistance to abrasive wear.

The more carbon a steel has to begin with the more slowly will it
absorb carbon and the lower the temperature required. Low-carbon
steel of from 15 to 20 points is generally used and the carbon
brought up to 80 or 85 points. Tool steels may be carbonized as
high as 250 points.

In addition to the carburizing materials given, a mixture of 40
per cent of barium carbonate and 60 per cent charcoal gives much
faster penetration than charcoal, bone or leather. The penetration
of this mixture on ordinary low-carbon steel is shown in Fig. 32,
over a range of from 2 to 12 hr.

EFFECT OF DIFFERENT CARBURIZING MATERIAL

[Illustrations: FIGS. 33 to 37.]

Each of these different packing materials has a different effect
upon the work in which it is heated. Charcoal by itself will give
a rather light case. Mixed with raw bone it will carburize more
rapidly, and still more so if mixed with burnt bone. Raw bone and
burnt bone, as may be inferred, are both quicker carbonizers than
charcoal, but raw bone must never be used where the breakage of
hardened edges is to be avoided, as it contains phosphorus and
tends to make the piece brittle. Charred leather mixed with charcoal
is a still faster material, and horns and hoofs exceed even this
in speed; but these two compounds are restricted by their cost
to use with high-grade articles, usually of tool or high-carbon
steel, that are to be hardened locally--that is, "pack-hardened."
Cyanide of potassium or prussiate of potash are also included in
the list of carbonizing materials; but outside of carburizing by
dipping into melted baths of this material, their use is largely
confined to local hardening of small surfaces, such as holes in
dies and the like.

Dr. Federico Giolitti has proven that when carbonizing with charcoal,
or charcoal plus barium carbonate, the active agent which introduces
carbon into the steel is a gas, carbon monoxide (CO), derived by
combustion of the charcoal in the air trapped in the box, or by
decomposition of the carbonate. This gas diffuses in and out of
the hot steel, transporting carbon from the charcoal to the outer
portions of the metal:

If energizers like tar, peat, and vegetable fiber are used, they
produce hydrocarbon gases on being heated--gases principally composed
of hydrogen and carbon. These gases are unstable in the presence of
hot iron: it seems to decompose them and sooty carbon is deposited
on the surface of the metal. This diffuses into the metal a little,
but it acts principally by being a ready source of carbon, highly
active and waiting to be carried into the metal by the carbon
monoxide--which as before, is the principal transfer agent.

Animal refuse when used to speed up the action of clean charcoal
acts somewhat in the same manner, but in addition the gases given
off by the hot substance contain nitrogen compounds. Nitrogen and
cyanides (compounds of carbon and nitrogen) have long been known
to give a very hard thin case very rapidly. It has been discovered
only recently that this is due to the steel absorbing nitrogen
as well as carbon, and that nitrogen hardens steel and makes it
brittle just like carbon does. In fact it is very difficult to
distinguish between these two hardening agents when examining a
carburized steel under the microscope.

One of the advantages of hardening by carburizing is the fact that
you can arrange to leave part of the work soft and thus retain
the toughness and strength of the original material. Figures 33
to 37 show ways of doing this. The inside of the cup in Fig. 34
is locally hardened, as illustrated in Fig. 34, "spent" or used
bone being packed around the surfaces that are to be left soft,
while cyanide of potassium is put around those which are desired
hard. The threads of the nut in Fig. 35 are kept soft by carburizing
the nut while upon a stud. The profile gage, Fig. 36, is made of
high-carbon steel and is hardened on the inside by packing with
charred leather, but kept soft on the outside by surrounding it
with fireclay. The rivet stud shown in Fig. 37 is carburized while
of its full diameter and then turned down to the size of the rivet
end, thus cutting away the carburized surface.

After packing the work carefully in the boxes the lids are sealed
or luted with fireclay to keep out any gases from the fire. The
size of box should be proportioned to the work. The box should
not be too large especially for light work that is run on a short
heat. If it can be just large enough to allow the proper amount
of material around it, the work is apt to be more satisfactory
in every way.

Pieces of this kind are of course not quenched and hardened in
the carburizing heat, but are left in the box to cool, just as in
box annealing, being reheated and quenched as a second operation.
In fact, this is a good scheme to use for the majority of carburizing
work of small and moderate size. Material is on the market with which
one side of the steel can be treated; or copper-plating one side
of it will answer the same purpose and prevent that side becoming
carburized.

QUENCHING THE WORK

In some operations case-hardened work is quenched from the box by
dumping the whole contents into the quenching tank. It is common
practice to leave a sieve or wire basket to catch the work, allowing
the carburizing material to fall to the bottom of the tank where it
can be recovered later and used again as a part of a new mixture.
For best results, however, the steel is allowed to cool down slowly
in the box after which it is removed and hardened by heating and
quenching the same as carbon steel of the same grade. It has absorbed
sufficient carbon so that, in the outer portions at least, it is
a high-carbon steel.

THE QUENCHING TANK

The quenching tank is an important feature of apparatus in
case-hardening--possibly more so than in ordinary tempering. One
reason for this is because of the large quantities of pieces usually
dumped into the tank at a time. One cannot take time to separate
the articles themselves from the case-hardening mixture, and the
whole content of the box is droped into the bath in short order,
as exposure to air of the heated work is fatal to results. Unless
it is split up, it is likely to go to the bottom as a solid mass,
in which case very few of the pieces are properly hardened.

[Illustration: FIG. 38.--Combination cooling tank for case-hardening.]

A combination cooling tank is shown in Fig. 38. Water inlet and
outlet pipes are shown and also a drain plug that enables the tank
to be emptied when it is desired to clean out the spent carburizing
material from the bottom. A wire-bottomed tray, framed with angle
iron, is arranged to slide into this tank from the top and rests
upon angle irons screwed to the tank sides. Its function is to
catch the pieces and prevent them from settling to the tank bottom,
and it also makes it easy to remove a batch of work. A bottomless
box of sheet steel is shown at _C_. This fits into the wire-bottomed
tray and has a number of rods or wires running across it, their
purpose being to break up the mass of material as it comes from
the carbonizing box.

Below the wire-bottomed tray is a perforated cross-pipe that is
connected with a compressed-air line. This is used when case-hardening
for colors. The shop that has no air compressor may rig up a
satisfactory equivalent in the shape of a low-pressure hand-operated
air pump and a receiver tank, for it is not necessary to use
high-pressure air for this purpose. When colors are desired on
case-hardened work, the treatment in quenching is exactly the same
as that previously described except that air is pumped through
this pipe and keeps the water agitated. The addition of a slight
amount of powdered cyanide of potassium to the packing material
used for carburizing will produce stronger colors, and where this is
the sole object, it is best to maintain the box at a dull-red heat.

[Illustration: FIG. 39.--Why heat treatment of case-hardened work
is necessary.]

The old way of case-hardening was to dump the contents of the box
at the end of the carburizing heat. Later study in the structure
of steel thus treated has caused a change in this procedure, the
use of automobiles and alloy steels probably hastening this result.
The diagrams reproduced in Fig. 39 show why the heat treatment of
case-hardened work is necessary. Starting at _A_ with a close-grained
and tough stock, such as ordinary machinery steel containing from 15
to 20 points of carbon, if such work is quenched on a carbonizing
heat the result will be as shown at _B_. This gives a core that is
coarse-grained and brittle and an outer case that is fine-grained
and hard, but is likely to flake off, owing to the great difference
in structure between it and the core. Reheating this work beyond
the critical temperature of the core refines this core, closes
the grain and makes it tough, but leaves the case very brittle;
in fact, more so than it was before.

REFINING THE GRAIN

This is remedied by reheating the piece to a temperature slightly
above the critical temperature of the case, this temperature
corresponding ordinarily to that of steel having a carbon content
of 85 points, When this is again quenched, the temperature, which
has not been high enough to disturb the refined core, will have
closed the grain of the case and toughened it. So, instead of but
one heat and one quenching for this class of work, we have three
of each, although it is quite possible and often profitable to
omit the quenching after carburizing and allow the piece or pieces
and the case-carburizing box to cool together, as in annealing.
Sometimes another heat treatment is added to the foregoing, for
the purpose of letting down the hardness of the case and giving
it additional toughness by heating to a temperature between 300°
and 500°. Usually this is done in an oil bath. After this the piece
is allowed to cool.

It is possible to harden the surface of tool steel extremely hard
and yet leave its inner core soft and tough for strength, by a
process similar to case-hardening and known as "pack-hardening."
It consists in using tool steel of carbon contents ranging from
60 to 80 points, packing this in a box with charred leather mixed
with wood charcoal and heating at a low-red heat for 2 or 3 hr.,
thus raising the carbon content of the exterior of the piece. The
article when quenched in an oil bath will have an extremely hard
exterior and tough core. It is a good scheme for tools that must
be hard and yet strong enough to stand abuse. Raw bone is never
used as a packing for this class of work, as it makes the cutting
edges brittle.

CASE-HARDENING TREATMENTS FOR VARIOUS STEELS

Plain water, salt water and linseed oil are the three most common
quenching materials for case-hardening. Water is used for ordinary
work, salt water for work which must be extremely hard on the surface,
and oil for work in which toughness is the main consideration. The
higher the carbon of the case, the less sudden need the quenching
action take hold of the piece; in fact, experience in case-hardening
work gives a great many combinations of quenching baths of these
three materials, depending on their temperatures. Thin work, highly
carbonized, which would fly to pieces under the slightest blow if
quenched in water or brine, is made strong and tough by properly
quenching in slightly heated oil. It is impossible to give any
rules for the temperature of this work, so much depending on the
size and design of the piece; but it is not a difficult matter to
try three or four pieces by different methods and determine what
is needed for best results.

The alloy steels are all susceptible of case-hardening treatment;
in fact, this is one of the most important heat treatments for such
steels in the automobile industry. Nickel steel carburizes more
slowly than common steel, the nickel seeming to have the effect
of slowing down the rate of penetration. There is no cloud without
its silver lining, however, and to offset this retardation, a single
treatment is often sufficient for nickel steel; for the core is not
coarsened as much as low-carbon machinery steel and thus ordinary
work may be quenched on the carburizing heat. Steel containing
from 3 to 3.5 per cent of nickel is carburized between 1,650 and
1,750°F. Nickel steel containing less than 25 points of carbon,
with this same percentage of nickel, may be slightly hardened by
cooling in air instead of quenching.

Chrome-nickel steel may be case-hardened similarly to the method just
described for nickel steel, but double treatment gives better results
and is used for high-grade work. The carburizing temperature is the
same, between 1,650 and 1,750°F., the second treatment consisting
of reheating to 1,400° and then quenching in boiling salt water,
which gives a hard surface and at the same time prevents distortion
of the piece. The core of chrome-nickel case-hardened steel, like
that of nickel steel, is not coarsened excessively by the first
heat treatment, and therefore a single heating and quenching will
suffice.

CARBURIZING BY GAS

The process of carburizing by gas, briefly mentioned on page 88,
consists of having a slowly revolving, properly heated, cylindrical
retort into which illuminating gas (a mixture of various hydrocarbons)
is continuously injected under pressure. The spent gases are vented
to insure the greatest speed in carbonizing. The work is constantly
and uniformly exposed to a clean carbonizing atmosphere instead
of partially spent carbonaceous solids which may give off very
complex compounds of phosphorus, sulphur, carbon and nitrogen.

Originally this process was thought to require a gas generator but
it has been discovered that city gas works all right. The gas consists
of vapors derived from petroleum or bituminous coal. Sometimes the
gas supply is diluted by air, to reduce the speed of carburization
and increase the depth.

PREVENTING CARBURIZING BY COPPER-PLATING

Copper-plating has been found effective and must have a thickness
of 0.0005 in. Less than this does not give a continuous coating.
The plating bath used has a temperature of 170°F. A voltage of
4.1 is to be maintained across the terminals. Regions which are
to be hardened can be kept free from copper by coating them with
paraffin before they enter the plating tank. The operation is as
follows:

Operation
   No.             Contents of bath                   Purpose
    1      Gasoline                           To remove grease
    2      Sawdust                            To dry
    3      Warm potassium hydroxide solution  To remove grease and dirt
    4      Warm water                         To wash
    5      Warm sulphuric acid solution       To acid clean
    6      Warm water                         To wash
    7      Cold water                         Additional wash
    8      Cold potassium cyanide solution    Cleanser
    9      Cold water                         To wash
   10      Electric cleaner, warm sodium      Cleanser to give good
             hydroxide case-iron anode          plating surface
   11      Copper plating bath of copper      Plating bath
             sulphate and potassium cyanide
             solution warm

There are also other methods of preventing case-hardening, one
being to paint the surface with a special compound prepared for
this purpose. In some cases a coating of plastic asbestos is used
while in others thin sheet asbestos is wired around the part to
be kept soft.

PREPARING PARTS FOR LOCAL CASE-HARDENING

At the works of the Dayton Engineering Laboratories Company, Dayton,
Ohio, they have a large quantity of small shafts, Fig. 40, that
are to be case-hardened at _A_ while the ends _B_ and _C_ are to
be left soft. Formerly, the part _A_ was brush-coated with melted
paraffin but, as there were many shafts, this was tedious and great
care was necessary to avoid getting paraffin where it was not wanted.

[Illustration: FIG. 40.--Shaft to be coated with paraffin.]

To insure uniform coating the device shown in Fig. 41 was made.
Melted paraffin is poured in the well _A_ and kept liquid by setting
the device on a hot plate, the paraffin being kept high enough
to touch the bottoms of the rollers. The shaft to be coated is
laid between the rollers with one end against the gage _B_, when
a turn or two of the crank _C_ will cause it to be evenly coated.

[Illustration: FIG. 41.--Device for coating the shaft.]

THE PENETRATION OF CARBON

Carburized mild steel is used to a great extent in the manufacture
of automobile and other parts which are likely to be subjected to
rough usage. The strength and ability to withstand hard knocks
depend to a very considerable degree on the thoroughness with which
the carburizing process is conducted.

Many automobile manufacturers have at one time or another passed
through a period of unfortunate breakages, or have found that for
a certain period the parts turned out of their hardening shops
were not sufficiently hard to enable the rubbing surfaces to stand
up against the pressure to which they were subjected.

So many factors govern the success of hardening that often this
succession of bad work has been actually overcome without those
interested realizing what was the weak point in their system of
treatment. As the question is one that can create a bad reputation
for the product of any firm it is well to study the influential
factors minutely.

INTRODUCTION OF CARBON

The matter to which these notes are primarily directed is the
introduction of carbon into the case of the article to be hardened.
In the first place the chances of success are increased by selecting
as few brands of steel as practicable to cover the requirements of
each component of the mechanism. The hardener is then able to become
accustomed to the characteristics of that particular material, and
after determining the most suitable treatment for it no further
experimenting beyond the usual check-test pieces is necessary.

Although a certain make of material may vary in composition from
time to time the products of a manufacturer of good steel can be
generally relied upon, and it is better to deal directly with him
than with others.

In most cases the case-hardening steels can be chosen from the
following: (1) Case-hardening mild steel of 0.20 per cent carbon;
(2) case-hardening 3-1/2 per cent nickel steel; (3) case-hardening
nickel-chromium steel; (4) case-hardening chromium vanadium. After
having chosen a suitable steel it is best to have the sample analyzed
by reliable chemists and also to have test pieces machined and pulled.

To prepare samples for analysis place a sheet of paper on the table
of a drilling machine, and with a 3/8-in. diameter drill, machine
a few holes about 3/8 in. deep in various parts of the sample bar,
collecting about 3 oz. of fine drillings free from dust. This can be
placed in a bottle and dispatched to the laboratory with instructions
to search for carbon, silicon, manganese, sulphur, phosphorus and
alloys. The results of the different tests should be carefully
tabulated, and as there would most probably be some variation an
average should be made as a fair basis of each element present,
and the following tables may be used with confidence when deciding
if the material is reliable enough to be used.

TABLE 16.--CASE-HARDENING MILD STEEL OF 0.20 PER CENT CARBON

  Carbon          0.15 to 0.25 per cent
  Silicon        Not over 0.20 per cent
  Manganese       0.30 to 0.60 per cent
  Sulphur        Not over 0.04 per cent
  Phosphorus     Not over 0.04 per cent

A tension test should register at least 60,000 lb. per square inch.

TABLE 17.--CASE-HARDENING 3-1/2 PER CENT NICKEL STEEL

  Carbon          0.12 to 0.20 per cent
  Manganese               0.65 per cent
  Sulphur       Not over 0.045 per cent
  Phosphorus     Not over 0.04 per cent
  Nickel          3.25 to 3.75 per cent

TABLE 18.--CASE-HARDENING NICKEL CHROMIUM STEEL

  Carbon          0.15 to 0.25 per cent
  Manganese       0.50 to 0.80 per cent
  Sulphur       Not over 0.045 per cent
  Phosphorus     Not over 0.04 per cent
  Nickel              1 to 1.5 per cent
  Chromium        0.45 to 0.75 per cent

TABLE 19.--CASE-HARDENING CHROMIUM VANADIUM STEEL

  Carbon         Not over 0.25 per cent
  Manganese       0.50 to 0.85 per cent
  Sulphur        Not over 0.04 per cent
  Phosphorus     Not over 0.04 per cent
  Chromium        0.80 to 1.10 per cent
  Vanadium  Not less than 0.15 per cent

Having determined what is required we now proceed to inquire into
the question of carburizing, which is of vital importance.

USING ILLUMINATING GAS

The choice of a carburizing furnace depends greatly on the facilities
available in the locality where the shop is situated and the nature
and quantity of the work to be done. The furnaces can be heated with
producer gas in most cases, but when space is of value illuminating
gas from a separate source of supply has some compensations. When
the latter is used it is well to install a governor if the pressure
is likely to fluctuate, particularly where the shop is at a high
altitude or at a long distance from the gas supply.

Many furnaces are coal-fired, and although greater care is required
in maintaining a uniform temperature good results have been obtained.
The use of electricity as a means of reaching the requisite temperature
is receiving some attention, and no doubt it would make the control
of temperature comparatively simple. However, the cost when applied
to large quantities of work will, for the present at least, prevent
this method from becoming popular. It is believed that the results
obtainable \with the electric furnace would surpass any others; but
the apparatus is expensive, and unless handled with intelligence
would not last long.

The most elementary medium of carburization is pure carbon, but
the rate of carburization induced by this material is very low,
and other components are necessary to accelerate the process. Many
mixtures have been marketed, each possessing its individual merits,
and as the prices vary considerably it is difficult to decide which
is the most advantageous.

Absorption from actual contact with solid carbon is decidedly slow,
and it is necessary to employ a compound from which gases are liberated,
and the steel will absorb the carbon from the gases much more readily.

Both bone and leather charcoal give off more carburizing gases
than wood charcoal, and although the high sulphur content of the
leather

Showing first 200,000 characters. Full text: gutenberg.org/ebooks/20282