Martempering Marquenching
Martempering Marquenching is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typically in an oil or brine solution at a temperature right above the "martensitic start temperature". At this temperature, residual stress within the material are relieved and some bainite may be formed from the retained ferritic which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperature equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stress and stress cracks, but it also increases the impact resistance
Martempering is used to produce martensite without developing the high stress that usually accompany its formation. It is similar to conventional hardening except that distortion is minimized. Nevertheless, the characteristic brittleness of the martensitic remains in a gray iron casting after martempering, and martempered castings are almost always tempered. The casting is quenched from above the transformation range in a salt, oil, or lead bath: held in the bath at a temperature slightly above the range at which martensite forms (200 to 260°C or 400 to 500°F. for unalloyed irons) only until the casting has reached the bath temperature; and then cooled to room temperature.
If a wholly martensitic structure is desired, the casting must be held in the hot quench bath only long enough to permit ii to reach the temperature of the bath. Thus, the size and shape of the casting dictate the duration of martempering.
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