Annealing services is a combination of heating, holding and cooling cycles. In general, atmospheric controls are used in order to keep the steel free of oxidation. The temperatures and cooling rates are chosen carefully according to the chemical makeup of each steel grade to achieve the desired results.
There are three different types of annealing for the bar steels that are used in precision machining shops. These processes include subcritical annealing, solution annealing and spheroidize annealing.
Subcritical Annealing
Subcritical annealing services are also known as stress relief annealing. This process involves heating steel to a temperature that is close to critical temperature, but still below it. It is a simple annealing process that lowers the hardness and stress of the material, making modest alterations to the microstructure.
Often times, steel mills will use this to enhance cold forming or cold shearing. In addition, it is also used sometimes between cold forming operations in order to lessen the hardness.
Solution Annealing
In commercial practice, this type of anneal is referred to as Lamellar Pearlite Anneal or LP Anneal. With this type of annealing services, lamellar pearlite is the microstructure that prevails.
The cycle in this annealing method consists of the material being heated higher than critical range and keeping it at that temperature for a certain length of time. Then, the material is cooled slowly to below critical range temperature. Hardness is reduced and the carbide phase is reprecipitated as lamellar pearlite. By controlling the temperature and time, the metallurgist is given the chance to alter the structure of the lamellar pearlite that results, refining the ferritic grain size.
LP anneals are typically used with medium carbon, alloy steels and plain carbon for precision machining. The reason for this is to improve machinability and decrease hardness.
Spheroidize Annealing
Spheroidize annealing is a thermal process that produces a globular carbide following the heating and cooling cycles. Spheroidized microstructures are necessary to improve the surface finish and machinability of higher carbon steel. It is also favored for severely cold worked steel, such as cold upsetting, cold extruded or bent. The majority of bearing steels are put through this process before being machined.
The primary reasons to choose annealing services are to change the grain structure, grow formability, enhance machinability, alter mechanical properties and release residual stresses. All of these are vital to the success of the work done by precision machining shops.