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Normalizing
Normalizing is the process of raising
the temperature to over 60 † C (140 †F), above line
A3 or line ACM fully into the Austenite range. It is
held at this temperature to fully convert the structure
into Austenite, and then removed form the furnace and
cooled at room temperature under natural convection.
This results in a grain structure of fine Pearlite with
excess of Ferrite or Cementite. The resulting material
is soft; the degree of softness depends on the actual
ambient conditions of cooling. This process is considerably
cheaper than full annealing since there is not the added
cost of controlled furnace cooling.
The main difference between full annealing and normalizing
is that fully annealed parts are uniform in softness
(and machinablilty) throughout the entire part; since
the entire part is exposed to the controlled furnace
cooling. In the case of the normalized part, depending
on the part geometry, the cooling is non-uniform resulting
in non-uniform material properties across the part.
This may not be desirable if further machining is desired,
since it makes the machining job somewhat unpredictable.
In such a case it is better to do full annealing. |
Process
Annealing
Process Annealing is used to treat
work-hardened parts made out of low-Carbon steels (<
0.25% Carbon). This allows the parts to be soft enough
to undergo further cold working without fracturing.
Process annealing is done by raising the temperature
to just below the Ferrite-Austenite region, line A1on
the diagram. This temperature is about 727 †C (1341
†F) so heating it to about 700 †C (1292 †F) should suffice.
This is held long enough to allow recrystallization
of the ferrite phase, and then cooled in still air.
Since the material stays in the same phase through out
the process, the only change that occurs is the size,
shape and distribution of the grain structure. This
process is cheaper than either full annealing or normalizing
since the material is not heated to a very high temperature
or cooled in a furnace.
Stress Relief Annealing
Stress Relief Anneal is used to reduce residual stresses
in large castings, welded parts and cold-formed parts.
Such parts tend to have stresses due to thermal cycling
or work hardening. Parts are heated to temperatures
of up to 600 - 650 †C (1112 - 1202 †F), and held for
an extended time (about 1 hour or more) and then slowly
cooled in still air.
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