Annealing

Here are some of the processing options:

Full Annealing - This 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, the material is heated to slightly above the austenitic temperature and held for sufficient time to allow the material to fully form austenite or an 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 left to cool down inside) but in some cases it is 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. 

Sub-critical Annealing - Is annealing carried out slightly below the eutectoid temperature [Ac1 point = eutectoid transformation (1330°F for carbon-steels)].  Sub-critical annealing does not involve the formation of austenite.  The aim of this annealing process is to form an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher.  Spheroidal carbides can be obtained by either heating to a lower temperature for a longer period of time or using a higher temperature for a shorter time. Sub-critical annealing is based on the former, using temperatures as close as possible to, but below, the Ac1 temperature.

Isothermal Annealing - Is used to achieve a more homogeneous microstructure within the steel and is faster and less expensive than full annealing.  A full anneal will require approximately 30 hours, but an isothermal anneal will require approximately four hours, depending on the alloy. 

Spheroidized Annealing - Involves the heat treatment of carbon steels and iron-based alloys at temperatures which are held constant, and slightly below the temperature at which a solid solution is formed.  This process anneals the steel though heating and then cooling very slowly to provide a metallurgical structure that looks like “spheres” or “balls.” These structures allow the metal to be stamped or formed more easily. The main objective of spheroidized annealing is the improvement of the machinability of the steel.    

Graphitized Annealing - This is a heat treatment procedure used to convert carbides to pearlite, ferrite and graphite (free carbon) in gray, malleable and ductile iron castings. 

Normalizing - Is an annealing process applied to ferrous alloys to give the material a uniform fine-grained structure and to avoid excess softening in steel.  It involves heating the steel to 65°F–125°F above its upper critical point, soaking it for a short period at that temperature, and then allowing it to cool in air. Heating the steel just above its upper critical point creates austenitic grains (much smaller than the previous ferritic grains), which during cooling, form new ferritic grains with a further refined grain size. The process produces a tougher, more ductile material, and eliminates columnar grains and dendritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.