A full anneal typically
results in the second most ductile state a metal can assume for metal alloy.
Its purpose is to originate a uniform and stable microstructure that most
closely resembles the metal's phase diagram equilibrium microstructure, thus
letting the metal attain relatively low levels of hardness, yield strength and
ultimate strength with high plasticity and toughness. To perform a full anneal on a steel for example, steel is heated
to slightly above the austenitic temperature and held for sufficient time to
allow the material to fully form austenite or austenite-cementite grain
structure. The material is then allowed to cool very slowly so that the
equilibrium microstructure is obtained. In most cases this means the material
is allowed to furnace cool (the furnace is turned off and the steel is let cool
down inside) but in some cases it's air cooled. The cooling rate of the steel
has to be
sufficiently slow so as to not let the austenite transform into
bainite or martensite, but rather have it completely transform to pearlite and
ferrite or cementite. This means that steels that are very hardenable (i.e.
tend to form martensite under moderately low cooling rates) have to be furnace
cooled. The details of the process depend on the type of metal and the precise
alloy involved. In any case the result is a more ductile material but a lower
yield strength and a lower tensile strength. This process is also called LP
annealing for lamellar pearlite in the steel industry as opposed to a process
anneal, which does not specify a microstructure and only has the goal of
softening the material. Often the material to be machined is annealed, and then
subject to further heat treatment to achieve the final desired properties.
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