What is Annealing? Steel Foundations
Annealing is a heat treatment that alters the metal’s microstructure to increase its ductility and make it softer and more “workable.”
This is necessary, because cold work processes, such as cold rolling and forming work harden the metal, which increases the strength of the metal, but it also increases its brittleness.
The annealing process is typically performed in batch or bell anneal furnaces, where multiple metal coils, bundles of wire, parts, etc. are all heated at the same time. During the annealing process, the furnace heats the metal items above their recrystallization temperature, which allows them to soften or “temper” by relieving their microstructures of the internal stresses created during prior cold working processes.
The microstructure of the metal goes through a series of steps during the annealing process to become fully annealed: recovery, recrystallization, and grain growth.
The metal in the furnace is heated in a controlled gas atmosphere, typically consisting of pure hydrogen, but other gas mixtures, such as nitrogen and argon can be used. This avoids re-oxidation of the metal’s surface, which would result in scale.
Due to carbon in steel and stainless-steel alloys, there is an anneal process that is specific to steel called spheroidization. This process breaks up the pearlite (carbon) plates that are formed during the hot rolling process and changes them to spheroids (round globs). Spheroidized steel is the softest and most formable state of steel.