The properties of steels can be greatly
modified by thermal treatments, which change the internal crystalline
structure of the alloy. Hardening of steel is based on the fact that
iron undergoes a change in crystal structure when heated above its
“critical” temperature.
Above this critical transformation
temperature, the structure is called austenite, a phase capable of
dissolving carbon up to 2%. Below the critical temperature, the steel
transforms to ferrite, in which carbon is insoluble and precipitates
as an iron carbide compound, FeeC (sometimes called cementite).
If a steel is cooled rapidly from above
the critical temperature, the carbon is unable to diffuse to form
cementite, and the austenite transforms instead to an extremely hard
metastable constituent called martensite, in which the carbon is held
in supersaturation. The hardness of the martensite depends
sensitively on the carbon content.
Low-carbon steels (below about 0.20%)
are seldom quenched, while steels above about 0.80% carbon are
brittle and liable to crack on quenching. Plain carbon steels must be
quenched at very fast rates in order to be hardened. Alloying
elements can be added to decrease the necessary cooling rates to
cause hardening; some alloy steels will harden when cooled in air
from above the critical temperature.
It should be noted, however, that it is
the amount of carbon that primarily determines the properties of the
alloy; the alloying elements serve to make the response to heat
treatment possible.
Normalizing is a treatment in
which the steel is heated over the critical temperature and allowed
to cool in still air. The purpose of normalizing is to homogenize the
steel. The carbon in the steel will appear as a fine lamellar product
of cementite and ferrite called pearlite.
Annealing is similar to
normalizing, except the steel is very slowly cooled from above the
critical. The carbides are now coarsely divided and the steel is in
its softest state, as may be desired for cold-forming or machining
operations.
Process annealing is a treatment
carried out below the critical temperature designed to recrystallize
the ferrite following a cold-working operation. Metals become
hardened and embrittled by plastic deformation, but the original
state can be restored if the alloy is heated high enough to cause new
strain-free grains to nucleate and replace the prior strained
structure. This treatment is commonly applied as a final processing
for low-carbon steels where ductility and toughness are important, or
as an intermediate treatment for such products as wire that are
formed by cold working.
Stress-relief annealing is a
thermal treatment carried out at a still lower temperature. No
structural changes take place, but its purpose is to reduce residual
stresses that may have been introduced by previous nonuniform
deformation or heating.
Tempering is a treatment that
always follows a hardening (quenching) treatment. After hardening,
steels are extremely hard, but relatively weak owing to their
brittleness. When reheated to temperatures below the critical, the
martensitic structure is gradually converted to a ferrite-carbide
aggregate that optimizes strength and toughness.
When steels are tempered at about
260°C, a particularly brittle configuration of precipitated carbides
forms; steels should be tempered above or below this range. Another
phenomenon causing embrittlement occurs in steels particularly
containing chromium and manganese that are given a tempering cycle
that includes holding at or cooling through temperatures around 567
to 621°C. Small molybdenum additions retard this effect, called
temper brittleness. It is believed to be caused by a segregation of
trace impurity elements to the grain boundaries.
