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September, 1925
the space lattice of the ferrite grains, thereby interfering
with slip. He believes that this is an important
cause of hardness.
Formation of Troostite and Sorbite on Quenching.
We have seen in Chapter III that martensite is not
always formed on quenching, but that slower cooling,
or lower carbon content will produce the softer constituents,
troostite and sorbite. Slower cooling and
less carbon does not lower the transformation point so
much as rapid cooling and high carbon. A higher temperature
and longer time favor the formation of larger
grains of Alpha iron, and the precipitation and growth
of cementite particles. There is therefore less resistance
to slip, and consequently less hardness, than in
martensite.
Migration of Carbon in Alpha Iron.
Below the Al point, iron (that is, Alpha iron or
ferrite), will dissolve very little carbon. The amount
it will retain in solid solution after slow cooling from
above the critical point, is certainly less than about .05
per cent, as is evident from the fact that steel containing
this much carbon will show islands of pearlite after
annealing. It is therefore usually said that cementite
or carbon is insoluble in Alpha iron. This, however,
cannot be strictly true. Troostite is known to consist
of Alpha iron in which are embedded innumerable
crystalline particles of cementite, too small to be seen
FIG. 120—Grain growth by strain. Tapered bar of mild steel,
broken in tension and then heated to about 600 deg. C. for 2
hours. Longitudinal section. Low magnification. (C.
Chappell.)
Forging- Sf amping - Heaf Treating
325
Grain Growth Below Critical Range.
We have seen that grain growth takes place in steel
when it is heated above the critical range, but that,
ordinarily, none occurs below the critical range. There
are exceptions to this rule, which should be noted.
Pronounced grain growth may occur in low carbon
steel (carbon about 0.04 to 0.12 per cent) at temperatures
below the critical range, if it is in a cold worked
state — that is, if strains have been set up in the
metal. For a certain temperature there is a certain
degree of strain that will produce the greatest grain
growth. For instance, if a tapered bar is annealed,
so as to remove all strain and produce a fairly fine
grain structure, then broken in tension and finally
heated to some temperature below the Al point, such
as 600 deg. C, quite large grains may be produced at
some point along its length. Since the bar was tapered,
the stress, and therefore the strain, will have increased
gradually from the thickest part to the thinnest.
The section at which the strain was most favorable
for grain growth at 600 deg. C, will have the largest
grains. Higher and lower strains will have produced
less grain growth. For a different temperature,
maximum grain growth would have occurred at some
other section. The experiment is illustrated in Fig.
120.
A "temperature gradient", that is a condition
wherein the temperature increases from one point to
another, in the piece, may also cause grain growth in
steel.
The reasons for these exceptions to the ordinary
laws of grain growth, are still uncertain. The subject
is discussed at some length in ref. 13.
In view of the pronounced effect of grain size on
the properties of metal the importance of these exceptional
cases of grain growth is plain.
Volume Changes.
When austenite changes to martensite on quench­
under the microscope. If troostite is heated to a temping, there is an increase in volume camparable to that
erature slightly below the Al point, say 700 deg. C. which takes place in soft iron on slow cooling through
for several hours, cementite particles large enough to Ar3. This expansion during quenching, takes place
be seen under high magnification, will be formed. The at a temperature below 300 deg. C. It is accompanied
structure will have changed to sorbite. Continued by reappearance of magnetism and the development
heating below the critical range, will cause the cemen­ of pronounced hardness. If austenite, which has been
tite particles to increase in size, but decrease in num­ preserved at ordinary temperatures, is caused to
ber, resulting in the structure known as spheroidized
change into martensite, by tempering, a similar ex­
cementite or granular pearlite. The particles of cemenpansion
takes place.
tite in troostite or sorbite are not connected, and are When freshly formed martensite is allowed to stand
completely surrounded by ferrite. In order for some at room temperature for a time, or is slightly heated
of them to grow, carbon or carbide from neighboring as by placing in boiling water, contraction takes place.
particles must travel to them through the ferrite. The Tempering causes further contraction. This contrac­
large particles grow by taking material from smaller tion is attributed to the formation (precipitation) of
particles, through the intervening ferrite. It is cementite. The individual carbon atoms, entrapped
considered that the small particles gradually dis­ in Alpha iron during sudden cooling, probably take up
solve in the surrounding ferrite, and that carbon atoms more room than they do when combined with iron
then migrate through the ferrite to the larger particles. atoms in the form of cementite, Fe3C.
combining there with iron atoms to form carbide The amount of contraction which takes place, in­
(Fe3C) again, and becoming part of the larger cemencreases with the carbon content. In steel containing
tite particle. This means that the larger particles have 1.0 per cent carbon, the contraction due to the forma­
a stronger tendency to grow than do the small partition of cementite is sufficient to counterbalance the
cles. (We have seen that the same principle holds true expansion which accompanies the formation of Alpha
in the crystalline grains of metal, and accounts for iron from Gamma iron. Such steel, when slowly cooled
grain growth.) The cementite particles could not grow through the critical range, undergoes no volume
in this way, unless carbide or carbon is soluble, to change except the gradual contraction due to cooling.
some extent, in ferrite. The solubility is no doubt If it is suddenly cooled, so as to form martensite, ex­
greater just below Al than at lower temperatures, and pansion occurs, and this is followed by contraction on
is probably about 0.10 per cent.
standing or on tempering.