FEATURE ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 41 Martempering involves cooling steel from the austenitizing temperature and rapidly cooling into either specially formulated petroleum oil or a molten salt bath with a specific composition to a temperature slightly above the martensite start (Ms) transformation temperature of the steel, which does not permit martensite formation (Fig. 1a). The holding time at this temperature is selected to permit temperature equalization throughout the part without any structural change (stabilized), after which time the part is cooled in air to ambient temperature. Air cooling (relatively slow) is performed throughout the transformation range because faster cooling through this range would produce residual-stress patterns similar to those produced by a direct quench and negate any advantages of the marquenching process. Therefore, hardening occurs during air cooling and the hardness may sometimes be somewhat lower than that achieved by conventional quenching[2,4]. This process is conducted to minimize residual stress and the potential for cracking, because martensite forms relatively uniformly throughout the part. BASICS OF HEAT TREATING: MARTEMPERING This article, adapted from the latest ASM Handbook on quenching and quenchants, describes suitable steels and variables that influence martempering. 4 When carburized parts are marquenched, they are cooled from the austenitizing temperature to just above the Ms temperature of the case[5]. After marquenching, parts are cooled to room temperature in air. Martempering and modified martempering (Fig. 1b, c) are performed to reduce distortion problems that may be encountered with conventional quenching[1,2]. Herring summarized these problems to include out-of-round, outof-flat, bending, bowing, bucking, tapering, dishing, and closing-in of bores[5]. These distortion problems can arise either during heating or austenitization through the relief of internal stresses from prior operations; by sagging or creep due to inadequate part support during heating; mechanical damage; nonuniform heating; or during quenching, and are the result of an imbalance of the internal residual stresses generated. Contributing factors to these problems include steel composition and hardenability, component part geometry, mechanical handling, types of quenchant, quenchant temperature, condition of quenchant, and quenchant circulation. Fig. 1 — Time-temperature transformation diagrams with superimposed cooling curves showing quenching and tempering. (a) Conventional process. (b) Martempering (marquenching). (c) Modified martempering (hot oil quenching). Adapted from Refs. 1, 2. (d) Schematic illustration of the relationship between the isothermal transformation diagram and cooling curves during austempering. Adapted from Ref. 3. (a) (b) (d) (c)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==