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32. Forging- Sf amping - Heaf Treating
H E A T T R E A T M E N T and M E T A L L O G R A P H Y of STEEL
September, 1925
A P r a c t i c a l C o u r s e in t h e E l e m e n t s o f
Compounds such as iron carbide, Fe3C, have usually
been considered to go into solution in their combined
form. This would imply for example that distinct
molecules of Fe3C are dispersed in the Gamma
iron. Jeffries and Archer have reached the conclusion
that this is not the case. They believe that the carbon
in austenite is present as individual atoms of carbon.
These atoms would be held strongly to the neighboring
iron atoms, but without a permanent union. They
would be free to migrate (move about, slowly). It
is well known that carbon will diffuse (travel) through
Gamma iron (and even Alpha iron), so that if time is
allowed and the temperature is not too low, any carbon
present will be distributed uniformly through the
mass. This means that, on going into solution, carbon
atoms must move from points where there are many,
to points where there are fewer. When carbon comes
out of solution, as in the precipitation of cementite,
during the cooling of austenite, carbon atoms
must migrate to the grain boundaries or gather together
into small islands. It is considered that molecules
of Fe3C could not, on account of their size, travel
through solid iron. According to this view, cementite
has no existance except as a crystalline substance,
which is not merely precipitated but actually formed
on the coolinsr of austenite.
The author is Consulting Metallurgist, Philadelphia, Pa.
Copyright, 1925, H. C. Knerr.
P h y s i c a l M e t a l l u r g y
CHAPTER VI — Continued
The fact that solid solutions are harder and stronger
than the solvent metal, is believed to be due chiefly
PART 2 — SLIP INTERFERENCE
to two causes :
(1) Increased interatomic forces, the attraction
INI general, metals are hardened and strengthened between by unlike atoms being in general, greater than
the addition of elements which dissolve in them to between like atoms.
form solid solutions. The structure of solid solutions
(2) Roughening of the slip planes, due to the
is, at present, not completely understood, but there distorting effect which the stranger atoms have on
is good reason to believe that the atoms of the solute the crystalline pattern. (Rosenhain.)
(dissolved substance) are distributed among the atoms
of the solvent, usually taking the place of some of the Compounds.
latter in the atomic pattern.
The great hardness of compounds such as Fe3C,
which are formed in alloys, is regarded as being due:
(1) To the large interatomic forces between the
dissimilar elements in the compound.
(2) To the fact that, in compounds, the atoms
have greater reluctance to change partners during
slip.
The difficulty in forming new atomic bonds during
slip, renders these compounds brittle. But if they are
uniformly and evenly loaded, so that the stress is
tvenly distributed, they are probably very strong. It
is likely that cementite, for example, has a strength of
several hundred thousand pounds per square inch.
Small quantities of these strong, hard compounds
have a very important effect in increasing the hardness
and strength of steel (and other metals), as will presently
be explained.
Obstruction Principle.
Pearlite is naturally harder than ferrite. The numerous
layers of strong hard cementite, between the
layers of ferrite in a pearlite grain, cause the slip
planes in the ferrite to be very short and frequently interrupted.
The plates of cementite also have a re-enforcing
or supporting effect on the surrounding ferrite.
The presence of grains of pearlite in hypo-eutectoid
steels, has a similar hardening action. The pearlite
grain interferes with the progress of slip planes
through the surrounding ferrite grains, and, being
stronger and harder than the ferrite grains, give them