Martempering of Steel
Martempering is a term used to describe an interrupted quench from the austenitizing temperature of certain alloy, cast, tool, and stainless steels. The purpose is to delay the cooling just above the martensitic transformation for a length of time to equalize the temperature throughout the piece.
This will minimize the distortion, cracking, and residual stress. The term martempering is somewhat misleading and is better described as marquenching. The microstructure after martempering is essentially primary martensitic that is untempered and brittle. Martempering of steel (and of cast iron) consists of : Quenching from the austenitizing temperature into a hot fluid medium (hot oil, molten salt, molten metal, or a fluidized particle bed) at a temperature usually above the martensite range (Ms point) Holding in the quenching medium until the temperature throughout the steel is substantially uniform Cooling (usually in air) at a moderate rate to prevent large differences in temperature between the outside and the center of the section Formation of martensite occurs fairly uniformly throughout the workpiece during cooling to room temperature, thereby avoiding formation of excessive amounts of residual stress.
Martempering is a heat treatment for steel involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the martensite start (Ms) point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimises distortion.
Martempering is used to produce martensite without developing the high stresses that usually accompany its formation. It is similar to conventional hardening except that distortion is minimized. Nevertheless, the characteristic brittleness of the martensite 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.
Straightening or forming is also easily accomplished upon removal from the marquenching bath while the part is still hot. The piece will hold its shape upon subsequent cooling in fixturing or in air cooling after removal from the forming die. The marquenching can be accomplished in a variety of baths including hot oil, molten salt, molten metal, or a fluidized particle bed.
Martempered parts are tempered in the same manner as conventional quenched parts. The time lapse before tempering is not as critical because the stress is greatly reduced. Iron and its alloys experience a variety of crystalline structure changes as they are heated. These state changes are achieved by controlling the temperature to which the metal is heated and its rate of cooling. These crystalline states include austenite, martensite, pearlite, and bainite; each possesses particular working characteristics. Martensite is a crystalline state characterized by extreme hardness which, although a desirable characteristic, is generally accompanied by brittleness, distortion, and the inclination to crack. To offset these negative factors while retaining a degree of the latent hardness of martensite, the process of martempering is applied to manipulate the crystalline structure of the metal. Martempering is a metallurgical production process intended to control martensite characteristics in steel and alloys. Martensite is one of the crystalline states induced in metals by thermal manipulation and one which renders the metal exceptionally hard.