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ADVANCED MATERIALS & PROCESSES •
AUGUST 2014
28
A
fter the bustling 1890s, with its exciting and
productive discoveries around steel metal-
lography in England, France, Germany, Rus-
sia, Japan, and the United States, a period of quiet
consolidation occurred in the early 20th century.
By then, most metals pioneers had either joined or
established metallurgy departments or metallo-
graphic sections within existing mining or chemi-
cal engineering departments at universities
throughout the industrial world. Henry Marion
Howe, America’s earliest metals researcher, joined
Columbia University, New York, in 1897 to become
its first fulltime professor of metallurgy. By 1905,
he had a new laboratory and staff of five, including
William Campbell of Great Britain, who had come
to America to study under Howe and then re-
mained to serve a lifetime career teaching metal-
lography to students.
Increased use of heat-treated alloy steel during
and after World War I lead to the development of
a variety of different alloys with emphases on spe-
cial compositions that provide superior properties.
No two manufacturing companies seemed to use
the same alloy steel for the same application. It was
qualitative rather than quantitative, and a lot of ex-
pensive alloy elements were wasted as well. A
greater understanding of alloys in steel was desper-
ately needed to sort out the transformation of
austenite to martensite.
The first published research that questioned
longstanding ideas about steel transformation was
a paper by well-known French metallurgist Albert
Portevin and his co-author M. Garvin, in 1919.
They showed for the first time that transformation
to hard martensite did not occur until the steel
being quenched had cooled to temperatures well
below those where pearlite (layers of iron and iron
carbide) formed. In 1922, W.R. Chapin, an Ameri-
can production metallurgist, showed that a carbon
tool steel could be quenched to 570°F and still be
austenitic. He further demonstrated that with slow
cooling below 570°F, steel could be studied as it
gradually transforms to martensite with falling
temperatures. Chapin’s observations were the most
significant on martensite formation at the time and
should have ended confusion about how it formed.
However, although these two early studies did not
change the thinking of many established metals re-
searchers, they were of immediate interest to Zay
Jeffries, Marcus Grossman, and Edgar Bain.
Austenite, martensite, bainite
Edgar Bain was among the earliest Americans
to apply x-ray diffraction to the study of metals. He
showed that steel heated to the hardening temper-
ature—
austenite,
named after Sir William Chan-
dler Roberts-Austen—had a face centered cubic
(fcc) crystal structure, whereas ferrite and quench-
hardened steel—
martensite,
named after Adolf
Martens—were body centered cubic (bcc). The
next major scientific advancement in hardened
steel was the discovery through precision x-ray dif-
fraction by William Fink and Edward Campbell
that martensite was not a simple bcc structure like
iron, but a distorted tetragonal crystal structure. Fi-
nally, after 35 years of studying martensite, a ra-
tionale was discovered for its great hardness.
Unlike iron, which contains no carbon or pearlite,
martensite features carbon trapped within its crys-
tal structure on an atomic scale.
In the late 1920s, Edgar Bain and E.S. Daven-
port of the U.S. Steel Corp. Research Laboratory at
Kearny, N.J., published their world renowned
paper, “Transformation of Austenite at Constant
Subcritical Temperatures.” As part of their ground-
breaking research, Bain and Davenport quenched a
series of thin samples from the hardening temper-
ature to a transformation temperature and held
them at this temperature for various times before
water quenching to martensite. By using metallog-
raphy and thermal expansion measurements, they
were able to follow the formation process of the
new product from the beginning, through the reac-
tion period, and onto the end. They plotted indi-
vidual transformation curves of percent
transformed as a function of time at each transfor-
mation temperature.
At last, the sequence of events occurring at de-
creasing temperatures could be observed, accu-
rately described, and quantitatively measured as
the austenite transformed to a variety of structures
depending on the transformation temperature.
Data resulting from plotting the beginning and end
of the transformation is called a
time-temperature-
transformation (TTT) diagram,
and many of these
have been determined for numerous steels in
Metallurgy Lane,
authored by
ASM life member
Charles R. Simcoe
,
is a yearlong series
dedicated to the early
history of the U.S. metals
and materials industries
along with key
milestones and
developments.
The History of Alloy Steels: Part II
Throughout metal making history, nothing has exceeded the technical importance,
scientific complexity, and human curiosity involved in the hardening of steel.