FEATURE ADVANCED MATERIALS & PROCESSES | MAY/JUJNE 2024 42 5 MINIMIZING DISTORTION Martempering is used primarily to minimize distortion if steel hardenability is adequate. This occurs because fewer harmful residual stresses are developed in martempering relative to conventional quenching. The greatest thermal variations occur while the steel is in the relatively plastic austenitic phase; additionally, final transformation and thermal changes occur throughout the part at approximately the same time[3]. In many instances, martempering has eliminated the need for quenching fixtures that were required to minimize distortion during conventional quenching, thus reducing the cost of tooling and handling. In some cases, it is possible to incorporate a straightening operation before the part has fully transformed to martensite (fully hardened). However, when changing from conventional quenching to marquenching, note that the dimensional variations may be different for the two processes, which must be considered in establishing preheat conditions[1,2]. In some cases, a modified (hot oil) martempering operation may be specified, which involves quenching at a martempering oil bath temperature just below the Ms temperature to approximately 95°C but above the martensite transformation finishing temperature (Mf), which will produce faster cooling rates than in the standard martempering process, as shown in Fig. 1c. Hot oil quenching has been described specifically to involve quenching at oil temperatures in the 105° to 190°C range and may be performed in either batch or continuous atmosphere or vacuum-sealed quench furnaces[3]. This process may be necessary for lower-hardenability steels that require faster cooling rates to obtain the desired depth of hardening. Hot oils are generally applied to high-precision engineering components requiring critical dimensional control, such as thin-section bearing races and transmission gears and shafts. Another advantage of the modified martempering process is that the lower bath temperatures permit the use of simpler quenching equipment, thus lowering equipment and process costs. Tempering of martempered or modified martempered steels to the desired hardness and tensile strength is performed identically to that in typical quench and temper operations. SUITABILITY OF STEELS Alloy steels generally are more adaptable than carbon steels to martempering[6]. In general, any steel that is normally quenched in oil can be martempered. Some carbon steels that are normally water quenched can be martempered at 205°C in sections thinner than 5 mm, using vigorous agitation of the martempering medium. In addition, thousands of gray cast iron parts are martempered on a routine basis. The grades of steel that are commonly martempered to full hardness include 1090, 4130, 4140, 4150, 4340, 300M (4340M), 4640, 5140, 6150, 8630, 8640, 8740, 8745, SAE 1141, and SAE 52100. Carburizing grades such as 3312, 4620, 5120, 8620, and 9310 also are commonly martempered after carburizing. Occasionally, higher-alloy steels such as type 410 stainless are martempered, but this is not a common practice. Success in martempering is based on a knowledge of the transformation characteristics (time-temperature- transformation, or TTT, curves) of the steel being considered. The temperature range in which martensite forms is especially important. Figure 2 shows the martensite temperature ranges for 14 carbon and low-alloy steels. Two trends may be observed in these data: as carbon content increases, the martensite range widens, and the martensite transformation temperature is lowered. The martensite range of a triple-alloy (Ni-Cr-Mo) steel is usually lower than that of either a single-alloy or a double-alloy steel of similar carbon content. Any steel that is to be martempered successfully must contain sufficient carbon or alloying additions to move the nose of the TTT curve to the right, thus permitting sufficient time for quenching of workpieces past the nose of the TTT curve. Fig. 2 — Temperature ranges of martensite formation in 14 carbon and low-alloy steels. Ms, martensite start.
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