THE HIGH-SPEED STEELS
During early centuries the art of metal cutting made little progress
except in-so-far as the application of greater driving power and the use
of better machines were concerned. Lathes had been known since the sixth
century B. C., at least, but, of course, little was or could be
accomplished, comparatively speaking, before the invention of the steam
engine by Watt, which was the first contrivance to give sufficient power
for machining purposes. During the century which followed Huntsman’s
time, steel makers and smiths became very expert in the manufacture and
tempering of carbon steels. Lathe tools were made of these carbon
steels, the only available material, but even with the better machine
shop practice of the 19th century they were capable only of what we now
consider to be inefficient results. The trouble was that since carbon
tool steel gets its hardness from quenching from a red heat and then
“drawing” the temper by reheating and slowly cooling from 400° or 500°
F., tools made from it could not retain their hardness if their cutting
points became much heated, as occurred if the lathe was run too fast.
The usual cutting speed, therefore, was 20 or 30 linear feet per minute.
Speeds in excess of this took the temper out of the tools and soon made
them useless.
About 1868 Robert F. Mushet, a metallurgist of Sheffield, England, made
a momentous discovery. He found that a piece of tool steel which had
cooled in the air was as hard as some of those which he had quenched.
Being an investigator he set about discovering the reason for this
experience which was without precedent. Analysis showed that beside the
usual constituents of tool steel this particular bar contained tungsten,
a comparatively new metal. He experimented with some hundreds of
mixtures and evolved an alloy, which, in tools, would stand up under
machine speeds double those which could be used with carbon steels.
These new alloys became known as “air-hardening” or self-hardening tool
steels because they required no quenching.
The principal application of these new steels was in the cutting of
harder metal than it had before been possible to cut, and little
attention, apparently, was paid to the getting of greater outputs by
increase of machine speeds.
At this point Frederick W. Taylor of “efficiency” fame appears upon the
scene. While manager of the Bethlehem Steel Works in the nineties of the
last century, he was working upon the efficiency investigations for
which he later became so famous. During his investigations, with Maunsel
White he experimented with many air-hardening steels to determine the
best grades to use for their shop work. Getting some inconsistent
results they determined upon and made what was a most extended and
systematic investigation, one so thorough and complete that Taylor and
White have become part of the history of high-speed steels.
They produced new compositions the quenching of which could be from
temperatures greatly in excess of those which tool makers for centuries
had held to be ruinous and which really are ruinous to carbon tool
steels. The best results were obtained when the new steels were quenched
by plunging in oil from close to their melting points—a dripping or
“sweating” heat as it is called. This was something entirely new but it
developed that after proper drawing, steels quenched from these extreme
temperatures, would stand up under lathe speeds as great as 200 or 300
feet per minute.
Compare these with the miserable speeds of 20 or 30 feet per minute,
which were the average performances of the best carbon steels.
The secret of Taylor and White’s treatment was not long in coming out
and soon high-speed steel makers in Europe and America were vying with
each other in production of finer and finer high-speed steels.
The progress which has been made during the last twenty years and
particularly during the past ten has been astounding. Improvement has
followed improvement in composition, manufacture and heat treatment, so
that to-day, instead of the cut at 30 feet per minute, which was a high
figure with carbon steel tools, the modern lathe or shaper tool often
works at 300 or 400 or more feet per minute, and, with sufficient power
behind it, at somewhat lower speeds plows out ½ inch deep and ¾ inch
wide chips so fast that their removal to keep the machine clear is no
mean problem. Often 2,000 pounds of the material per hour can be thus
cut away with one tool.
As suggested, the new steels do not suffer such loss of temper from the
heat generated by the friction of the tool in the metal as occurs with
carbon steel tools. In fact, tools of high-speed steel work best after
“warming up” and they can run for a considerable period of time with the
point of the tool red-hot, though such is not advisable.
As is readily seen the essential property of the high-speed steels is
the so-called “red hardness” which is the ability to retain hardness at
red heat. This is several hundred degrees in excess of the temperatures
at which the carbon tool steels quickly lose their “temper.”
As forecast by Mushet, the essential constituent of the new steels is
the metal, tungsten. But tungsten alone cannot give the desired
property. Mushet, it will be remembered, was the metallurgist whose
patents for the use of manganese in steel Bessemer was obliged to
recognize to make his process a success, though the metal had earlier
been used in crucible steel. The air-hardening property of Mushet’s
steel was contributed by a happenstance combination of tungsten and this
same metal, manganese. It later developed that tungsten and chromium
were the best hardening elements and these have maintained their place,
though refinements of the past few years have made use of vanadium, and,
more recently, cobalt in addition. Usual amounts may be said to be
tungsten 14 to 25 per cent, chromium 2 to 7 per cent, with vanadium ½ to
1½ per cent, and cobalt up to 4 per cent, perhaps. The carbon content is
usually .6 to .8 per cent. Sometimes another comparatively rare metal,
molybdenum, is used in high-speed steels in place of part of the
tungsten, but its use does not seem to be on the increase.
Manufacturers differ considerably in formulas.
It will be noticed that at best there is left room for only 70 or 80 per
cent of iron in the alloy. From certain standpoints, the high-speed
steels might not at first thought be called “steels” at all since carbon
seems to be of so little importance. They might be considered to be low
carbon alloys somewhat similar to the newer “stellite” (an alloy from
which tools are made), which contains little or no carbon and no iron
but is made up mainly of cobalt and chromium. They fit in, however, with
the general and very comprehensive scheme of classification of the
iron-carbon alloys which has been developing over a period of twenty
years and there is no doubt among metallurgists and metallographists
that, as is the case with the alloy steels described above, they are
iron-carbon alloys—in other words, steels—the properties of which have
been greatly modified through the presence of the other elements.
Carbon, therefore, is an essential, though it is much less in amount
than in the carbon tool steels. The hardening and softening properties,
also, very definitely classify these alloys with the “tool steels.”
Stellite cannot be softened.
As with the carbon tool steels, most of the high-speed steels are made
by the crucible method, though a small but increasing amount is of late
being produced in the electric furnace. After careful pouring into small
ingots and cooling, the ingots are removed from the iron molds and
“topped” to remove any “pipe” or unsound portion. Then, if without
defect and satisfactory as to analysis, they are slowly and carefully
heated to forging temperature and are hammered out into bars. By this
method they are taken nearly down to the final size desired. The bars
are finished by rolling to size. After careful annealing they are ready
for shipment to the tool maker.
When taking heavy cuts a tool of to-day may exert as much as ten tons’
pressure against the metal it is cutting and the advent of this
wonderful material for tools necessitated the building of immensely
heavier and stronger lathes and other machines, which, alone, were
capable of giving them power to do their work. The high-speed steels,
therefore, have revolutionized metal-cutting practice and shop methods
and have very largely aided efficiency.