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Here are 5 reasons to anneal steel.
- To alter the grain structure;
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To develop formability;
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To improve machinability;
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To modify mechanical properties;
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To relieve residual stresses.
The annealing process is a combination of a heating cycle, a
holding period or "soak" at temperature, and a controlled cooling
cycle. Atmospheric controls are generally used to protect the steel
from oxidation.
The temperatures used and the cooling rates are carefully selected
to correspond with each steel grade's chemical composition in order to
produce the results desired.
For bar steels used in our precision machining shops, there are three kinds of annealing that may be encountered:
- Subcritical Anneal
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Solution Anneal
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Spheroidize Anneal
Subcritical Anneal
A subcritical anneal is the metallurgical name for what is termed a process anneal or stress relief anneal in
North American commercial practice. It consists of heating the steel to
a temperature close, but below, the steel's lower critical temperature
or Ac1. This simple anneal reduces stress and hardness in the material
and makes modest changes in its microstructure. Steel mills often
employ this to improve cold shearing or cold forming. This is sometimes
used between cold forming operations to reduce hardness.
Solution Anneal
Lamellar Pearlite photo
Solution annealing is referred to in commercial practice as 'LP Anneal' or Lamellar Pearlite Anneal.
Lamellar pearlite is the microstructure that predominates when doing
this kind of anneal. The cycle for this anneal involves heating the
material above the critical range (Ac3) and holding the steel (soaking)
at that temperature for a length of time followed by slow cooling below
the critical range (Ar1) temperature. This cycle reduces hardness and reprecipitates the carbide phase as lamellar pearlite. Controlling
the time and temperature gives the metallurgist a means to alter the
resulting lamellar pearlite structure, and refine the ferritic (as
rolled) grain size.
LP anneals are usually applied to medium carbon (0.40-0.65 weight %)
plain carbon and alloy steels for precision machining in order to
reduce hardness and improve machinability.
Spheroidize Anneal
Spheroidized Photo
Spheroidize annealing is the term that
describes a thermal process which results in a globular or spheroidal
type of carbide after heating and cooling. There are several types of
spheroidize cycles which we will write about in a future post.
Spheroidized microstructures are desireable for machinability and
improved surface finish when machining higher carbon steels.
Spheroidized microstructures are also preferred when the steel is to be
severely cold worked: cold extruded, cold upsetting, or bent. Most
bearing steels are first spheroidize annealed prior to machining.
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Re: 5 Reasons To Anneal Steel
