THE GENERAL EFFECT OF THE MORE IMPORTANT ELEMENTS IN TOOL STEELS.
We know that all metals of engineering nature are crystalline in
character, that is, the crystals form when the metal solidifies. If
these crystals were free it would be easy to determine definitely just
what properties the metal would have. However, the crystals are not
free, but exist in the steel in combination with many other types of
crystals. This results in many complicated and complex possibilities in
the finished product, and will bring us presently to the subject of
“Alloy Steels”.
CARBON STEELS.
Carbon Steels are those which do =not= contain enough of any element
=other= than carbon to materially affect the physical properties which
the steel will have when hard. Carbon is one element used above all
others by manufacturers in getting required physical properties. An
increase of one hundredth of one per cent (.01%) gives a tensile
strength of about one thousand pounds per square inch, but even this
amount of carbon also regularly decreases the ductility of the finished
product. When steel is heated red hot and plunged into water, the
carbon in the metal unites with the iron in some peculiar way so that
it produces a compound of extreme hardness. If the steel contains
nine-tenths of one per cent (.90%) of carbon, a sharp point so quenched
will almost scratch glass. With one per cent (1.00%) of carbon it
reaches nearly its limit of hardness. Now carbon steels with this
percentage carbon can be used for some of the harder tools, which do
not require much ductility or toughness, but with higher carbon
contents than this percentage, the brittleness increases so fast that
the usefulness of the metal is decidedly limited.
Therefore, when the steel must meet requirements other than just that
of hardness, such as, strength, ductility, toughness, resistance to
repeated shock, “red hardness”, etc., then it is necessary to resort to
other means and combinations for obtaining the required needs. It is to
be remembered that such methods and combinations will materially
increase the cost of the final product.
ALLOY STEELS.
What is an alloy steel? The general definition of an alloy steel is, “a
solidified solution of two or more metallic substances”. The
International Committee upon the nomenclature of iron and steel defines
alloy steels as “those steels which owe their properties chiefly to the
presence of an element (or elements) =other= than carbon”.
This latter definition more nearly applies to our case, but it must be
born in mind that the distinction between an element added merely to
produce a slight benefit to ordinary carbon steel, and the very same
element added to produce an alloy steel itself, is sometimes a very
delicate one. For example: Manganese is added in amounts usually less
than 1.50% to all Bessemer and Open-Hearth Steels, for the purpose of
getting rid of oxygen, and neutralizing the effect of the sulphur. But
this does not produce an Alloy Steel. When we make “manganese steel”
containing 10 to 20% manganese, the material then has properties quite
different from the same steel without the manganese, and we then have a
Manganese Alloy Steel.
Thus, for our purpose, we may consider an alloy steel as being one to
which some element =other= than carbon has been added in sufficient
amount to materially affect the physical properties which the steel
will have when hard.
HIGH SPEED STEELS.
High Speed Steels are perhaps the most important of alloy steels, and
derive their name from the fact that they can be used as cutting tools
when the cut on the machined member is being made at a high speed.
This, of course, subjects the tool to severe operating conditions,
which simple carbon steels could not stand. These steels have other
notable characteristics, among which is that of “self-hardening” or
“air-hardening”, as it is sometimes called. This means, when the steel
cools naturally in the air, from a red heat or above, it is not soft
like ordinary steel, but is hard and capable of cutting other metals.
Another striking characteristic of high speed steels is their ability
to maintain a sharp cutting edge while heated to a temperature far
above that which would at once destroy the cutting ability of a simple
tool steel. Because of this property, a tool made of high speed steel
can be made to cut continuously at speeds three to five times as great
as that practicable with other tools. The result of the friction of the
chip on the tool may cause the tool to become red hot at the point on
top where the chip rubs hardest, and the chip may, itself, by its
friction on the tool, and the internal work done on it, by upsetting
it, be heated to a blue heat, or even hotter.
ELEMENTS WHICH OCCUR IN ALL STEELS.
There are certain elements which are practically always found in =any=
kind of steel. These elements are capable of producing many varied
effects on the finished product. They are Iron, Carbon, Manganese,
Silicon, Phosphorous and Sulphur.
IRON.
The base of all steels is Iron. It goes without saying that this
element should be obtained in the best and purest state possible.
Probably the best “base” iron comes largely from Sweden, which country
seems to have produced the highest quality of iron on the market today.
CARBON.
Carbon has already been discussed under Carbon Steels, although, of
course, its importance in Alloy Steels must not be under-estimated. The
proportion of carbon aimed at in high speed tool steels is about 0.65%,
which in simple steel would not be enough to give the maximum hardness,
even if the steel were heated above the critical point and quenched in
water, and still less so when the steel is cooled as slowly as these
steels are in their treatment. This shows that the carbon element acts
in a different way from what it does in simple carbon steels as
previously discussed.
MANGANESE.
Manganese Steel is a typical self-hardening steel and so, obviously, is
any steel which is in the austenitic condition at atmospheric
temperatures, that is to say, whose critical temperature is below
atmospheric temperature. Thus, self-hardening steels are non-magnetic.
Because of its low-yield point, manganese steel does not give
satisfaction in many lines, for which otherwise it might be eminently
fitted.
Manganese used in =small= quantities (.30% to 1.50%) will produce
certain desired effects. Under these conditions it acts as a purifier.
And when added in the form of Ferro Manganese to a heat of steel it
unites with the oxygen and transforms it to slag as oxide of manganese.
There is also good reason for believing that manganese prevents the
coarse crystallization, which impurities such as Phosphorus and Sulphur
would otherwise produce. Five per cent to 14% manganese renders the
steel non-magnetic as well as a poor conductor of electricity.
SILICON.
The dividing line between silicon-treated steels and silicon-alloy
steels is not clearly defined, but the latter are used for several
important purposes.
Such steel has been used in springs of the leaf type for automobiles
and other vehicles, the silicon being considered to add slightly to the
toughness of the springs. However, the most important use of steels of
this type is probably in the manufacture of electrical machinery. It is
possible to produce a silicon-alloy steel which has not only a greater
magnetic permeability than the purest iron, but also, a high electrical
resistance. Its hysteresis is, of course, low, this property always
accompanying a high permeability. It therefore is a very valuable
material for use in electro-magnets, and in electric generating
machinery, is the most efficient material known.
In silicon-treated steels, the silicon is used somewhat as a scavenger,
although it also produces results somewhat similar to manganese.
PHOSPHORUS.
Phosphorus has little effect upon the hot properties, but in the cold
state makes the steel brittle and is of course highly undesirable
although some writers have claimed that it adds to the tensile strength
in about the same degree as carbon.
SULPHUR.
Sulphur has just the opposite effect of Phosphorus, and makes the steel
crack while it is being hot worked, although after the metal is cold it
seems to have no particular effect upon the physical properties.
ELEMENTS WHICH HAVE BECOME
ESPECIALLY ASSOCIATED
WITH SPECIAL
ALLOY STEELS.
Such elements are:—Chromium, Tungsten, Molybdenum, Vanadium, Cobalt,
Uranium, Titanium, Aluminum, etc.
CHROMIUM.
Chromium is an indispensable constituent in modern high speed steel,
and does not make a poor high speed steel, even when used alone. The
chief effect which chromium produces in high speed steels is
undoubtedly that of “hardening”. However, chromium, like carbon, will
produce brittleness, if added in too large quantities, although if kept
down to between 2 to 5% it seems to allow the lowering of the carbon
element, while at the same time maintaining the desired hardening
effect, without causing undue brittleness. The great hardness in the
face of an armor plate, and the great toughness in the back of the
plate, also the superb properties in the projectile which attempts to
pierce the plate, can all be induced in chromium steels to a degree
unattainable by the use of any other single element.
As a simple chromium steel the product may be used in five-ply plates
for the manufacture of safes. These plates are made of five alternate
layers, two of chrome steel and three of soft steel, and after having
been hardened, offer resistance to the drilling tools employed by
burglars. Hardened chromium rolls are manufactured for use in
cold-rolling metals. Files, ball and roller-bearings are other noted
products of this type of steel. It is the essential constituent of
those steels which neither rust nor tarnish.
TUNGSTEN.
It was soon found that the composition of “self-hardening” steels was
not the best one for high speed steels. Tungsten was discovered as an
element which gave the steel properties of hardness and toughness at a
red heat. After the peculiar heat treatment had been learned, and the
presence of manganese or chromium in addition to the tungsten was shown
to be unnecessary in appreciable amounts, it was found that more
durable qualities could be obtained by increasing the percentage of
tungsten, while at the same time the carbon element was greatly
reduced.
The best grade of High Speed Steel ought to have a tungsten content of
about 18.00% and a carbon content of about 0.65%. Thus whenever a steel
is needed which must operate under especially severe conditions, this
would be the steel to use. Such conditions are usually met in the case
of rapid turning, boring, planing, slotting and shaping tools, also
with twist drills and all forms of milling cutters, gear cutters, taps,
reamers, special dies, etc.
MOLYBDENUM.
Molybdenum was once thought of as being somewhat in a class with
tungsten, but its use in high speed tool steels is being generally
discontinued. The reason for this is that it was found that in rapid
steels this element caused irregular performance, such as large
variations in the cutting speeds which they would stand. This element
is also likely to make the steels seamy and contain physical
imperfections. Molybdenum steels were also found to crack on quenching,
and possess decided variations in internal structure.
VANADIUM.
Vanadium steels are still in their infancy. Therefore, the true value
of this element in rapid steels must probably be held as not yet fully
determined. With the single exception of carbon, no element has such a
powerful effect upon steel as vanadium, for it is only necessary to use
from 0.10 to 0.15% in order to obtain very noticeable results. In
addition to acting as a very great strengthener of steel, especially
against dynamic strains, vanadium also serves as a scavenger in getting
rid of oxygen and possibly nitrogen. It is also said to decrease
segregation, which we may readily believe, as most of the elements
which quiet the steel have this effect.
“Vanadium Steels” demand a somewhat higher price than do those steels
which do not contain this element in appreciable amounts. It is, of
course, especially useful for all purposes where strength and lightness
are desired, such as springs, axles, frames and other parts of railroad
rolling stock, and automobiles.
COBALT.
The valuable effect of cobalt is claimed to be that it increases the
red hardness of high speed tool steel, enabling the steel to cut at a
higher speed. However, this element much resembles nickel, which has
been largely condemned as not being a desirable ingredient for high
speed tool steels, because it has the effect of making the edge of the
finished tool soft or “leady”.
URANIUM, TITANIUM AND ALUMINUM.
These elements are generally classed as scavengers, although recently
important claims have been advanced for their effect upon the physical
properties of steel. This is especially true for the first two. In
present practice, however, they are used almost entirely as deoxidizers
or cleansers, and are added to the metal for this purpose only.
IMPURITIES.
Phosphorus, Sulphur and Copper are the most noted impurities which
occur in steel. The first two are practically always present in greater
or smaller amounts as the case may be. The best processes of tool steel
manufacture are capable of producing steels with no copper. While
Aluminum is not generally classed as an impurity, it nevertheless
sometimes shows up in the finished product when its presence is not
desired, and therefore, might be considered an impurity.
Combinations of iron with some or all of the above elements in the form
of slags and oxides are other well known impurities.
From the forgoing pages it must be evident that producing a steel with
exactly the correct chemical content is only =one= step towards
securing a satisfactory product. However, it might be well if we were
to briefly sum up a few of the more important features of our
discussion on this interesting subject.
HEAT TREATMENT.
The heat treatment of tool steels is of the utmost importance. Tool
makers of the old school proved their ability to accomplish certain
desired results in the art of heat treatment without really fully
understanding exactly how or why they were able to do so. Today,
however, progressive manufacturers are using the results of research
and such thorough scientific investigation that the process has become
far more complicated and complex, and the results obtained are
correspondingly more remarkable.
Chemically perfect steel may be easily and completely ruined during the
process of melting, cogging, rolling, hammering, annealing, heat
treating and tempering. It is the business of the steel manufacturer to
carefully guard his product up through the process of annealing, but it
usually falls to the tool maker to undertake the delicate operations of
heat treatment and tempering.
HARDENING.
The application of heat alone to steel can very materially affect the
condition of the structure of the metal, either with or without
simultaneous mechanical treatment. Depending upon the degree of heat,
the rate of heating and cooling and the duration of such treatment,
this application may be decidedly beneficial or harmful as the case may
be.
We now know that when steel is heated above the critical point, and is
then allowed to rapidly cool, a very marked hardness in the metal is
produced. The degree of hardness so attained will, in general, vary
directly with (1) the percentage of carbon, (2) the rate of cooling,
(3) and the temperature above the critical point from which the cooling
takes place. When the steel comes from the rolling mill and from the
finishing hammers it is in this hardened condition. Therefore, in order
to render it soft and ductile enough to cut and work up into certain
desired shapes, sizes and tools, it is necessary to subject the steel
to the process of annealing. This operation is usually undertaken by
the steel producer, under which circumstances he is able to control his
product through this delicate procedure, and deliver the same to his
customers in the best possible condition for their use.
ANNEALING.
Annealing has for its object: (1) Completely undoing the effect of
hardening, leaving the steel soft and ductile (2) refining the grain,
in which case the crystals are allowed to re-arrange and re-adjust
themselves, usually growing to a rather large size (3) and removing
strains and stresses caused by too rapid cooling. Such cooling strains
are particularly likely to exist where the rate of cooling is different
in different parts of the bar, but the process of annealing ought to
remedy any such condition, leaving the steel soft, ductile and of
refined and uniform crystalline structure throughout.
The process of annealing is easier to explain than it is to actually
put into practice. The steel is first packed in lime, charcoal, fine
dry ashes or sand, and then sealed in long air-tight tubes or boxes.
The whole receptacle is next slowly brought up to a dull red heat, of
about 1500 degrees Fahrenheit.
It is very important to heat the material uniformly all the way
through, and then hold it in this condition from three to eight hours.
Thus, allowing the slipping of one allotropic condition into another.
The receptacle must be cooled equally slowly, either allowing the
packed steel to cool slowly down with the furnace, or by placing the
same in a soaking or cooling pit, which also accomplishes the desired
result.
After the receptacle has become entirely cooled it is opened and the
steel unpacked and removed. The steel is then ready for its final
inspection before shipping to the tool maker.
TEMPERING.
The process of tempering usually has to be undertaken by the tool maker
or user after the annealed steel, which he received from the steel
mill, has been cut up and shaped into the desired form and size.
The main object of tempering steel is to re-harden the material to such
an extent that it will cut other metals, retaining its desired shape
size and cutting edge, while at the same time it must not possess too
much brittleness. The treatment varies materially with different brands
of steels.
For the average grade of the best High Speed Steel containing from 16%
to 18% tungsten, the tool should be brought very slowly up to a dull
cherry red. It is usually considered good practice to first place the
tool near or on top of the pre-heating furnace before actually placing
it in the pre-heater, in order that the heating might be effected just
as slowly as possible. The pre-heating operation should bring the tool
up to about 1600 to 1800 degrees Fahrenheit, after which the tool
should be placed in the high heating furnace and brought up to 2300 to
2400 degrees Fahrenheit, or a white sweating heat. Care should be taken
not to allow the tool to remain in this condition for more than an
instant, as it is then in a very critical condition and could be easily
burned or ruined.
Therefore, the tool should be immediately pulled from the furnace and
plunged into a good clean oil bath, keeping it constantly in motion.
As High Speed Steels are air-hardening steels, it is also the practice
to harden these steels by simply placing the cutting edge in an air
blast, which produces maximum hardness in the desired point and allows
the body of the tool to cool at a little slower rate, thus slightly
relieving the cooling strains and producing a little less brittleness
therein. Such cooling strains can be relieved throughout the whole tool
by drawing the same back to about 400 to 500 degrees Fahrenheit, and
sometimes as high as 1050 degrees Fahrenheit, depending upon the
particular tool and its use.
The treatment of Carbon Steels varies with each particular brand. Great
care must always be taken to heat the steel uniformly, as a material
which is heated unevenly will expand and contract unevenly and, in
consequence, will crack when quenched.
The steel should always be hardened on the rising heat, in general
bringing the same slowly up to a dull cherry red, or to about 1450
degrees Fahrenheit, and then quenching in clear cold water, keeping the
same in motion until the steel is cold. The temper should then be drawn
according to the purpose of the tool, which could only be discussed for
each particular case. The following range of temperatures are
interesting, as being approximately indicated by the thin film of oxide
tints which occur on the tool undergoing a tempering operation:
Pale Yellow 428 Degrees Fahrenheit
Golden Yellow 469 Degrees Fahrenheit
Purple 531 Degrees Fahrenheit
Bright Blue 550 Degrees Fahrenheit
Dark Blue 601 Degrees Fahrenheit
CONCLUSION.
The effects of annealing, rolling, hammering, treating and tempering
are best understood by those manufacturers who make a specialty of
supplying a high grade tool steel, and in general it would be well if
customers would consult freely with the producers of these steels,
before attempting the delicate undertaking of Heat Treatment.