MARTEMPERING, Suitability of Steels for Martempering
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. Figure 1(a and b) shows the significant difference between conventional quenching and martempering. 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. 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.