Here are 5 reasons to anneal steel.
To alter the grain structure;
To develop formability;
To improve machinability;
To modify mechanical properties;
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:
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 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 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.