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March, 1925
cumstances. It etches rather slowly, but faster than
austenite. The typical structure of martensite consists
of small, hard needles running in three general
directions, at about 60 degrees with each other. It is
very well illustrated in the lighter portions of Fig. 57.
(The dark spots are another constituent, which will
be discussed presently.)
If the steel is of sufficiently high carbon content,
and is cooled fast enough to retain some austenite,
then a mixture of austenite and martensite may result,
as shown in Fig. 58. This photomicrograph is
taken at exceptionally high magnification (2200X)
and shows needles of martensite (dark) in a ground
mass of austenite (white). In this case the martensite
needles have etched more rapidly than the
austenitic matrix in which they are imbedded, and
FIG. 58—MARTENSITE IN AUSTENITE. Carbon 0.70
per cent. Very rapidly quenched. (2200x.) (F. W. Lucas,
Western Electric Company.)
therefore appear dark. Steel which consists partly of
austenite and partly of martensite is not so hard as
the same steel when it is entirely martensitic. Since
it requires faster cooling to retain austenite than to
form martensite, the fastest cooling does not always
produce the hardest steel.
Troostite.
If the cooling is a little slower than that which
would produce a structure consisting entirely of martensite,
dark, rounded patches will be distributed
through the ground mass of martensite as shown in
Fig. 57. These dark patches are a constituent known
as "Troostite."* Troostite etches much more rapidly
than martensite, and therefore appears dark on a
martensitic background. If etching is brief the martensite
may not be visibly attacked, and will appear
white, or structureless, as in Fig. 59, in which the
dark spots are troostite. This constituent is slightly
softer and more ductile than martensite. The slower
the rate of cooling in quenching, the more troostite
will be present. This is illustrated in (a), (b) and
(c) of Fig. 59, which were taken at different points in
the same specimen, (a) having been the most rapidly
cooled, and (c) the most slowly cooled portion. When
the cooling is still slower, the structure may consist
*After the French Chemist, Troost.
Forging- Stamping - Heat Treating 9,
entirely of troostite as shown in Fig. 60. It is often
somewhat difficult to distinguish between finely divided
martensite, shown in Fig. 56a, and an area consisting
of troostite, as in Fig. 60. It may be that each
of these consists partly of martensite (white needles)
and partly of troostite (dark background). The
rapidity with which troostite etches is one of its distinguishing
characteristics.
FIGS. 59a, 59b, 59c—TROOSTITE IN MARTENSITE.
Carbon 0.90 per cent. Showing areas unevenly quenched
in same piece. Hardness decreases with increasing troostite
(dark). (lOOx.)
Sorbite.
If the cooling through the critical range is slower
than would produce troostite, still another constituent
may be found which consists of a mass of very
finely divided, rounded or curved particles, as illustrated
in Figs. 61 and 62. This is known as Sorbite.f
It etches faster than martensite, but more slowly than
troostite, and therefore appears lighter than the latter.
It is softer and more ductile than troostite, and is, in
fact, the constituent generally desired in structural
tAfter Sorby, English Metallurgist.