I. Maraging Heat Treatment:
Firstly the steel is annealed at approximately 850 degree Celsius and according to hardening thickness time span of heating is decided.
For thin section 15 to 20 minutes are enough while for heavy section 1 hour is enough. Then it is air cooled until it reaches the room temperature and forms the martensite.
Now as it reaches the room temperature it is again put for aging process. It is heated for 3 hours at 480 degree Celsius to 500 degree Celsius.
Then after heating it is again air cooled and again and we get maraged steel. The whole process is explained graphically below.
Austempering is a heat treating process for medium-to-high carbon ferrous metals which produces a metallurgical structure called bainite.
It is used to increase strength, toughness, and reduce distortion.
Parts are heated to the hardening temperature, then cooled rapidly enough to a temperature above the martensite start (Ms) temperature and held for a time sufficient to produce the desired bainite microstructure.
Ductility, hardness and toughness increases here, also shock resistance increases.
The steel is heated from 710 degree Celsius to 900 degree Celsius that is austenizing temperature range.
Then it is quenched in a bath of molten salt or oil maintained at temperature 230 degree Celsius to 370 degree Celsius. And further it is air cooled and we get bainite crystal structure.
The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Ordinarily, annealing is carried out to
(1) Relieve stresses
(2) Increase softness, ductility, and toughness; and/or
(3) Produce a specific microstructure.
A variety of annealing heat treatments are possible, they are characterized by the changes that are induced, which many times are micro-structural and are responsible for the alteration of the mechanical properties. Any annealing process consists of three stages:
(1) heating to the desired temperature,
(2) holding or “soaking” at that temperature, and
(3) cooling, usually to room temperature.
Time is an important parameter in these procedures. During heating and cooling, there exist temperature gradients between the outside and interior portions of the piece; their magnitudes depend on the size and geometry of the piece.
If the rate of temperature change is too great, temperature gradients and internal stresses may be induced that may lead to warping or even cracking.
Also, the actual annealing time must be long enough to allow for any necessary transformation reactions.
Annealing temperature is also an important consideration; annealing may be accelerated by increasing the temperature, since diffusional processes are normally involved.