Annealing is the process of heating the steel to a particular temperature in the austenite region and cooling down the steel very slowly. There are many derivatives of the annealing process, but generally the process is a slow cool process.

Another derivative of the annealing process is known as sub-critical anneal. This process involves soaking at a temperature below the lower transformation line, in the region of 1,200xF to 1,300xF, until the steel has equalized across its cross-section in temperature, followed by a slow cool. Slow cooling can mean a cooling rate between 5xF per hour up to 50xF per hour.

As can be imagined, the cooling period can be a considerable amount of time. It should be noted that the nickel alloyed steels and the A series tool steels should be cooled very slowly, as nickel will cause an air-hardening effect.

Other Types of Annealing:

1) Bright Anneal. This method is a method of annealing which uses a protective atmosphere to prevent the steel surface from oxidation.

2) Process Anneal. This procedure is done at a temperature close to the lower critical line on the iron carbon diagram. Sometimes confused with sub-critical annealing, it is used when considerable cold working is to follow.

3) Recrystallization Anneal. Once again, this is a process often mistaken for subcritical annealing. It is used after cold working to produce a specific grain structure.

4) Sub-Critical Anneal. This method is used on cold-worked steel and is carried out below the lower critical line on the iron carbon equilibrium diagram. It is sometimes applied to tool steels that have been over tempered and require annealing before hardening and tempering.

5) Spheroidize Anneal. This process is a controlled heating and cooling procedure to produce spheroidal or globular cementite particles. It is usually applied on high carbon steels for good machining characteristics such as high alloy steels and tool steels.

6) Isothermal Annealing. The process temperature of this procedure is determined by knowledge of the steel’s carbon content. The steel is then taken to that temperature and cooled down to a holding temperature that allows the steel to transform isothermally.

7) Full Anneal. This is a process that involves raising the steel’s temperature up to the sustenite region followed by a slow cool.