The technical usage of steel is for one reason based on the fact that a various number of possible heat treatments lead to different properties of one certain material. These different properties are adjustable due to allotropic modification of the steel lattice.
Steels without a α/γ-transformation like austenitic of ferritic steels are neither hardenable nor able to normalize. The various heat treatments can be divided into annealing, hardening and quenching/tempering.
The reason for annealing a material is to create a certain pro-cessability (for example: cold forming, cutting properties). The changing of material properties is done by the following:
-Transformation of structures
-Transformation of grain size, arrangement,
The different ways of annealing are seperated in heating up to a certain temperature (set-point), holding at this temperature and cooling down to the room temperature. During the heating of a material the appearance of an increasing tension between outer and inner areas of the material might cause problems. Therefore the thickness of the material as well as the thermal conductivity should be noticed to avoid for example stress cracking. The same could appear during the cooling process of the material.
Removal of Non-Uniform Structures: Diffusion Annealing, Normalizing
Increasing of Workability: Coarse-Grain Annealin, Soft Annealing, Recrystallization Annealing, Crystal Recovery Annealing
Reducing of Stresse: Stress-Relief Annealin, Stress-Relieving Treatment
Transformation of Structure: Tempering
Special Cases: Controlled Cooling:Quenchin, Bainitic Transformation
The purpose of hardening is to obtain a hard structure. There are various processes of hardening as normal or surface hardening which all rely on the material.
Normal Hardening: Hardening from Hot-Forming Heat, Broken Hardening, Martempering
Surface Hardening: Flame Hardening, Induction Hardening, Dip Hardening, Case Hardening, Nitriding, Gas-Nitro Carbonising.
The characteristic of a hardened material is the hardness which on the other hand leads to a reduced toughness. To get a hard but tough material another heat treatment has to be done which is called quenching and tempering.