Annealing is an umbrella term used in heat treating to refer to the process of raising the temperature and controlled cooling of a material to make it more ductile, reduce its hardness and generally make it more amenable to machining and other manufacturing processes. This happens by heating steel or other metals to below, in between or above a critical temperature where the constituent grains recrystallize and carbide is redistributed throughout the material.
Some types of annealing are also sometimes referred to synonymously with “stress relieving,” since it also reduces some of the residual stress incurred by a material during the manufacturing process, although the vagueness of this term can cause some confusion. Annealing is the process that underpins many heat treating services and the principles behind it are applied to a number of specific procedures.
Subcategories of annealing depend on the maximum temperature and the method used to cool it. Holding the steel at a temperature sufficient to form austenite, and then cooling for an extended period of time in a procedure known as full annealing, for instance, will lead to an exceptionally soft steel that’s ideal for machining with most materials. Annealing is often accompanied by a slow quench to preserve the softness that is key for machinability.
The specific type of annealing procedure used for steel is heavily dependent upon the type of material being annealed. Here are the different types of anneals and the types of materials they’re normally conducted on:
- Full anneal – Full anneals involve heating the material to a temperature where it becomes fully austenitized. This type of anneal is conducted on some stainless steels and superalloys, among a few others.
- Inter-critical anneal – This level of anneal works between the lower critical temperature (where austenite starts to form) and the upper critical temperature (where austenite is fully formed) of a material. It is typically conducted on materials including carbon steels, alloy steels and engineering alloys.
- Sub-critical anneal – The temperature with this type of annealing process fully remains below the critical temperature where a material begins to form austenite. It can also be conducted on carbon steels, alloy steel and engineering alloys, but is typically conducted as an intermediate step between fabrication sequences.
As noted above, the main differences between these procedures concern the maximum temperature and rate of cooling used in each. In many cases, the annealing process is conducted as a part of the overall manufacturing process.