BBBBflt] «BlJIUrIrlr - Clpdigital.org

324 Forging - Sfamping - Heaf Treating
takes place, and very fine grains are temporarily produced.
But grain growth proceeds rapidly at this high
temperature. When the Gamma iron is cooled through
Arl, a good deal of heat is evolved. This tends to retard
cooling and allows time for the grains of Alpha
iron to grow. It is therefore difficult or impossible to
produce a very fine grain size in pure iron, even by
severe quenching.
Anything which will lower the Ar3 transformation
of iron, hinders grain growth. This produces a finer
grained Alpha iron, which is consequently harder and
stronger. Xickel has this effect, and so does carbon.
When carbon (or cementite) goes unto solution in
austenite, it probably does so as atoms of carbon, distributed
among the atoms of iron. The tendency for
the carbon to distribute itself uniformly through the
austenite will cause the carbon atoms to take positions
as far from each other as possible.
When the change from Gamma iron to Alpha iron
takes place, there is a strong tendency for the carbon
to be rejected from solution. In so doing it will combine
with atoms of iron, forming the carbide Fe3C.
"The change from Gamma iron to Alpha iron involves
only minor movements of the iron atoms (face
centered to body centered pattern). Formation of
cementite from iron atoms and carbon atoms, the latter
being as far apart as the}- can get in the space
lattice, involves the diffusion of carbon. This requires
much more time than that required for the
transformation of iron from one space lattice to another."
For this reason the carbon may not be precipitated
when Gamma iron is changed to Alpha iron
by rapid cooling, but may be trapped in the form of
carbon atoms, distributed through the crystalline
grains of Alpha iron.
When austenite, high in carbon, is rapidly cooled,
as by quenching in water, the transformation from
Gamma to Alpha iron is lowered to about 300 deg. C.
Little or no grain growth would occur at this temperature.
The Alpha iron grains are therefore probably
extremely small.
Martensite.
Jeffries and Archer consider martensite, the hardest
state of steel, to be the product of such a quenching
operation and to consist of extremely fine grains of
ferrite in which the carbon is temporarily held as individual
atoms (atomic dispersion).
"Martensite has a variable carbon content, being
the same as that of the austenite from which it was
formed at the moment of the allotropic transformation
(change from Gamma to Alpha iron). Its properties
vary with the carbon content, and with physical differences,
such as those due to different quenching temperature,
or maximum temperature of treatment, before
quenching. Carbon steels containing less than
about 0.15 per cent carbon, do not form hard martensite,
when quenched from above the upper critical temperature.
Carbon steels containing about 0.20 per
cent carbon, form a medium hard but ductile martensite.
The hardness increases with the carbon content
up to about 0.70 per cent carbon, above which there is
a less marked change, up to about 1.50 per cent carbon.
The lower the carbon content, the more drastic
must be the quench, in order to produce martensite.
For example the mildest quench which will produce
100 per cent martensite in a 0.90 per cent carbon steel,
will produce free ferrite, martensite and troostite (or
September, 1925
sorbite) in a 0.20 per cent carbon steel, and martensite
and troostite in a 0.50 per cent carbon steel."
Microstructure of Martensite.
The ferrite grains in martensite probably form
along the cleavage planes of austenite when the latter
is breaking down, and consist of very small thin plates.
This would account for the needle-like appearance of
martensite, and for the peculiar triangular arrangement
of the needles*. (See Figs. 57 and 58, Chapter
III.)
Martensite inherits its appearance, to a still further
extent, from the austenite grains from which it was
formed. If these were large, due to overheating, the
martensite may appear to have distinct grain boundaries,
which, in reality, are merely traces of the austenite
grain boundaries. The larger the parent grains of
austenite, the more pronounced is the needle-like appearance
of martensite. Martensite formed from fine
grained austenite (i.e., which has not been overheated),
has a much less pronounced needle-like structure.
Tempering Martensite.
Since carbon is very much less soluble in Alpha
iron than in Gamma iron, there is a strong tendency
for the entrapped carbon in martensite to precipitate
out, with the formation of carbide. This actually occurs,
when martensite is heated to moderate temperatures,
or even when it is allowed to stand for a time
at room temperature. The cementite so formed gathers
into very minute, submicroscopic particles. As
the temperature is raised, these particles grow in -size,
by the migration of carbon atoms from neighboring
points, and the structure changes to that known as
troostite. A higher temperature results in the forma*
tion of still larger carbide particles, producing the
structure called sorbite. If the tempering temperature
is carried still higher (but below Al), and is held for
a long enough time, relatively large particles are
formed, and we have the structure known as spheroidized
carbide or globular cementite, or sometimes
granular pearlite.
During the first stage of tempering martensite,
perhaps on standing at room temperature, the carbide
particles will grow to the critical size at which they
exert the maximum interference with slip in the ferrite
grains. There is an increase in the hardness of
the martensite up to this point, and it here attains its
maximum hardness. Further increase in the size of
the cabride particles is accompanied by a decrease in
their number, and by a decrease in their effectiveness
as key particles to resist slip. Progressive tempering,
with the formation of troostite, sorbite and globular
cementite therefore reduces the hardness of the steel.
Causes of Hardness of Martensite.
Jeffries and Archer conclude that the hardness of
martensite is due chiefly to the fineness of the ferrite
grains and consider that the presence of great numbers
of minute crystalline particles of cementite, which act
as keys, is an additional source of hardness. Rosenhain
suggests that the presence of insoluble carbon
atoms or the particles of rejected cementite, distorts
•Some microscopic work recently done by Lucas at very hi
magnifications, throws new light on the structure of martensite,
and appears to support the view that it consists of extremely
small plates following the cleavage planes of the parent austenite
grain. See "The Microstructure of Austenite and Martensite "—
F. F. Lucas, Trans. A. S. S. T., Dec, 1924, vol. VI, No. 6.