martempering

Example Sentences

• "Atwood Vacuum Machine Co.'s experience with modified martemperings has been very satisfactory. It has now been adopted as standard procedure for most all of the oil hardening […]"
• "11.6.1 Martempering / The most frequent type of interrupted quenching process is called martempering, shown in Fig. 11.25. In martempering, the part is quenched into a molten salt bath at a temperature just above the Mₛ temperature. When the part reaches thermal equilibrium, but before bainite starts to form, it is removed from the salt bath and allowed to air cool past the Mₛ temperature, thus forming martensite. The objective is to delay the cooling just above the martensitic transformation for long enough to equalize the temperature throughout the piece and minimize distortion, cracking, and residual stresses. It should be noted that even though this process is called martempering, it is not a tempering process; instead, it is a hardening process. The martensite produced by martempering still requires a subsequent tempering process. In fact, this process is sometimes more correctly referred to as marquenching. […] The advantage of martempering is the reduced thermal gradient between the surface and center as the part is quenched to the isothermal temperature and then air cooled to room temperature. Residual stresses developed during martempering are lower than those developed during conventional quenching, because the greatest thermal variations occur while the steel is in the relatively plastic austenitic condition, and because final transformation and thermal changes occur throughout the part at approximately the same time. Martempering also reduces or eliminates susceptibility to cracking. Martempering can be accomplished in a variety of baths, including hot oil, molten salt, molten metal, or a fluidized particle bed. Molten salt and hot oil are both widely used for martempering. Oils are used for martempering at temperatures up to 205 °C (400 °F), while molten salt is used in the range of 160 to 400 °C (320 to 750 °F). Martempering is conducted on certain alloy, cast, tool, and stainless steels. Alloy steels generally are more adaptable than carbon steels to martempering. In general, any steel that is normally quenched in oil can be martempered. […] Martempering is especially appropriate for carburized parts, such as splined shafts, cams, and gears, because these parts generally are more difficult to grind, [are] more costly to fabricate, and are manufactured with closer tolerances than are parts made of through-hardening steels. The limitation of section thickness or mass must be considered when martempering."