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ADVANCED MATERIALS & PROCESSES • JUNE 2014 33
Scientific understanding progresses
Fast forward 40 years past Taylor andWhite’s dis-
covery of high-speed steel heat treatment, and 15
years since Bain and Jeffries published their theory
of secondary hardening. While practical develop-
ment of Mo high-speed steels steadily progressed
during this time, little was done to develop a more
fundamental understanding of the materials. A
young metallurgy professor at Massachusetts Insti-
tute of Technology, Morris Cohen, set out to develop
an understanding of the nature of high-speed steel
tempering. Beginning in 1939, several studies were
published by Cohen and a series of graduate students
who wrote their doctoral theses on high-speed steel.
The first paper, by M. Cohen and P.K. Koh in 1939,
set the tone for many that followed. They reviewed the
literature on secondary hardening, which showed that
most researchers believed the high-speed phenome-
non was caused by transformation of residual austen-
ite to martensite during tempering. A considerable
amount of residual austenite was always seen in the
microstructure of as-cooled high-speed steel, but it
was replaced by martensite after tempering. Cohen
and Koh studied changes in properties after heat treat-
ing at various temperatures and times. The variable of
time at tempering temperature had never been exam-
ined adequately. In addition to x-ray diffraction of solid
samples, they studied changes in electrical and mag-
netic properties, length, volume, and hardness. They
concluded there were four stages to the reactions in
high-speed steel during tempering:
1. Formation of iron carbide (Fe3C)
2. Precipitation of carbide in retained austenite
3. Transformation of retained austenite to
martensite
4. Precipitation of alloy carbides in martensite
Cohen and Koh’s paper showed for the first time
in American technical literature that retained austen-
ite did not transform to martensite at the tempering
temperature, but rather on cooling. Time spent at the
tempering temperature conditioned residual austen-
ite for its subsequent transformation. The basic con-
clusion was that stages two and three contributed the
secondary hardening. This conclusion continued the
conventional wisdom that somehow the transforma-
tion of retained austenite was the cause of secondary
hardening, or red hardness, and that the eventual for-
mation of alloy carbides upon long exposure to tem-
peratures of 1100°F or higher was insignificant.
During the next decade, Cohen and his students
continued their research studies on steel. Some of
the work pertained directly to high-speed steel,
while other research applied more broadly to steels
in general. The work on tempering of high-speed
steel showed that little was understood about the
tempering of all hardenable steels. Such a study was
undertaken by Antia, Fletcher, and Cohen and re-
ported in 1944. One of the last papers in the high-
speed steel series, by Cohen and Blickwede,
explored the effects of vanadium and carbon on 6%
tungsten, 5% molybdenum high-speed steel. This
new steel was becoming very popular in industry
under the M-2 designation.
Walter Crafts and John Lamont of Union
Carbide and Carbon Research Laboratories
published another paper in 1948 of great impor-
tance to high-speed steel. They studied the ef-
fects of alloying elements on ordinary
engineering steels, so they increased the amount
of alloy until it approached the level found in
tool steels. Secondary hardening peaks were
found after tempering, even though these steels
did not contain residual austenite from quench-
ing. This work finally killed the retained austen-
ite theory, stubbornly held since the days of H.C.
Carpenter more than 40 years earlier. Tiny car-
bide particles were found forming at tempering
temperatures corresponding to the high-speed
steel hardening peak. These small carbides were
found by x-ray refraction to be alloy carbides of
Mo2C, W 2C, or VC in various Mo, W, and V
steels. This research supported the now 25-year-
old theory of Bain and Jeffries, which stated that
alloy carbides were the direct cause of secondary
hardening in high-speed steels.
Hats off to the tool makers
It is now nearly 150 years since the first alloy tool
steel was invented by Robert Mushet and more than
a century since Taylor and White’s discovery of spe-
cialized heat treatment. In this brief historical time,
our modern technological society came into being in
no small part due to the availability of advanced cut-
ting tools made of high-speed steel. The early pio-
neers who contributed to these special steels will
forever be known as the “tool makers.”
For more
information:
Charles R. Simcoe
can be reached at
.
For more metallurgical
history,
visit
.
Morris Cohen was
educated and served his
career at MIT. He was
awarded the ASM Gold
Medal in 1968, the
National Medal of
Science presented by
President Carter in1976,
and elected to the
National Academy of
Science, Engineering
Division. Courtesy of MIT.
During World War II,
the War Production
Board rationed
commodities such
as gasoline, metals,
rubber, paper, and
plastics, enabling
the massive
technological shift
to molybdenum by
denying the tool
steel industry the
tungsten it required
to maintain
production.