HIGH SPEED STEELS.
After the processes of hardening and drawing our sample of simple
carbon tool steel have become thoroughly mastered, it might seem that
all which was desired had been accomplished and that we could go on
indefinitely making and using our simple carbon steel tools. However,
when the extraordinary demands of modern industry required faster and
faster cutting speeds, and deeper and deeper cuts, we commenced to
realize that our familiar carbon tool steels would not fill the bill.
This was due to the fact that as the tools became pressed with the
faster speeds and deeper cuts, they could not do their work without
becoming over-heated by the friction caused by the work of upsetting
the chip and therefore the critical temperature was rapidly approached.
Of course we know that if this temperature should be reached the steel
would quickly lose its hardness and its cutting edge would therefore be
completely ruined.
Therefore, it was necessary to develop a new kind of steel to meet a
new and severe condition and accordingly the mother of experiment and
invention gave birth to the now famous “High Speed” Steel.
The general principles applying to the hardening and drawing of High
Speed Steel are in many ways the same as described above for the simple
carbon steel, except that as we begin to add various elements other
than carbon to the melt, the resulting alloy becomes more and more
complex in its form and reactions and therefore its heat treatment
causes greater and greater study and skill in its successful
undertaking.
It is generally known among tool hardeners that it is necessary to heat
the tool to a higher degree of temperature in order to secure proper
hardness when using High Speed Steel than it is when a simple Carbon
Tool Steel is employed. We are told that the introduction of certain
elements into the melt of a simple Carbon Tool Steel has the tendency
to change the critical range. Of course, the formulas used in the
manufacture of any high grade High Speed Steel contain very appreciable
amounts of various elements other than Carbon which materially effect
the property which the steel will have when hard. The effect which
these elements appear to produce in the period of critical range can be
seen from figure 7.
[Illustration: ≈HEATING AND COOLING OF HIGH SPEED STEEL SHOWN IN FIG.
12.≈]
In this case an experiment was made with a piece of High Tungsten High
Speed Steel similar to the experiment which was described in detail
above with the test piece of simple Carbon Tool Steel. The readings of
the pyrometer were carefully recorded and when plotted on the graph
sheet produced the picture under discussion.
Here it will be noticed that the vivid reaction, which we might have
expected would occur as the temperature indicating the first critical
range was reached, was materially reduced. This might lead us to
suspect that the desired allotropic change had not completely taken
place at this point. In fact we noticed that the pyrometer needle did
not record a vivid critical point until a very much higher temperature
was reached. All of these observations serve as a possible explanation
or indication of why it is necessary to employ very much higher
temperatures in the hardening of High Speed Steel than it is in the
hardening of a piece of simple Carbon Tool Steel.
In a later chapter of this little volume we define Carbon Steels as
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. High Speed Steels which are one of a very important group of
special alloy steels, are those steels to which some element =other=
than carbon has been added in sufficient amount to materially effect
the physical properties which the steel will have when hard.
The element which stands out alone as the most vital and important one
as affecting the wonderful and highly desirable features looked for in
High Speed Steels is Tungsten. We will discuss the various effects
which the different elements give to the different alloy steels in a
later chapter, but for the present we will confine ourselves to a brief
discussion of the heat treatment of the now famous modern High Speed
Steel.
[Illustration: High Speed Steel. Carbon .58%. Structure: Very fine
pearlitic condition, with particles of free carbide. Mag. 500x]
As previously suggested the pressing demand of modern industry for
quicker work, greater efficiency and enormously increased out-put of
product, gave rise to the necessity of producing far more remarkable
tools than was possible with the old fashioned carbon tool steel.
Therefore it became necessary to produce a steel which could be
rendered sufficiently hard to cut deep furrows in the various metals
which have to be machined and, which could be made sufficiently tough
to stand the enormous cutting strains and chatter and vibration of the
machine, and at the same time maintain all these characteristics when
the work done by upsetting the chip of the machined member actually
rendered the cutting edge of the tool red hot.
After the seemingly impossible task of producing a steel to meet these
terrific conditions had been successfully accomplished, the next
question which arose was to produce a machine which was sufficiently
powerful to stand the work done by the tool, and so fast has been the
progress made by the tool steel producer, that many of our modern
manufacturing industries of today are constantly having to introduce
new and heavier machinery into their various machine shop and tool
rooms in order to keep pace with the possibilities of the tool made
from the modern High Speed Steel.
Now, if we were to run an experiment with a test piece made from High
Speed Steel similar to the one which we ran on the simple Carbon Tool
Steel, we would find that many of the same phenomena previously noticed
would again be recorded.
Probably the most important difference would be the fact that instead
of having to quench the same in water it would be desirable to use a
bath of oil. In fact, water would cause the High Speed Steel to cool
off far too quickly so that it would be likely to crack and be rendered
useless.
A peculiar action of the various elements in High Speed Steel is very
likely to materially retard the change of one allotropic form into
another. In fact, the change is so slow that after a piece of High
Speed Steel has been heated above the critical temperature, it will
actually retain its hardened or austenitic condition even if allowed to
cool in the air, and it would only be possible to get it back into its
softened condition by the lengthy process of annealing.
Annealing is the process of undoing exactly what the act of hardening
accomplished. Long tubes are filled with the tool steel bars and sealed
from the air and then placed into the annealing furnaces, wherein the
annealing temperature is maintained for a sufficient number of hours,
until the steel has had an opportunity to become thoroughly softened.
As before stated “drawing” or “tempering” means the careful re-heating
of the steel to 400 degrees Fahr. to 600 degrees Fahr., thus allowing a
slight “slipping” of enough of the higher allotropic solution to a
lower form, which it is always eager to accomplish at temperatures near
the point of recalescence. This, of course, relieves the excess
brittleness of the hardened steel.
Annealing is the complete release of the higher allotropic form of the
solution and the “trapped” carbon which allows of their return to the
normal condition of pearlite and alpha iron. Therefore, it is necessary
to heat the steel above the point of recalescence and cool more or less
slowly. Different speeds of cooling give different grain, size,
structure and physical property.
This explanation of hardening, which is known as the “allotropic
theory” is not universally accepted, although it is difficult to find a
better or more complete explanation of the remarkable phenomena
involved. However, the fact remains that the great accomplishments
which have been made by the men of science and understanding have
caused remarkable results to have taken place in the manufacturing
world of today and the fine and obscure lines which these patient and
careful laborers are continually drawing upon the map of knowledge are
doing much to make the world a better and safer and more wonderful
place in which to live.