FEATURE ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 43 6 EFFECT OF MASS The limitation of section thickness or mass must be considered in martempering. With a given severity of quench, there is a limit to bar size beyond which the center of the bar will not cool fast enough to transform entirely to martensite. This is shown in Fig. 3, which compares the maximum diameter of a bar that can be hardened by martempering, oil quenching, and water quenching for 1045 steel and five alloy steels of various hardenabilities. For some applications, a fully martensitic structure is unnecessary, and a center hardness 10 HRC units lower than the maximum obtainable value for a given carbon content may be acceptable. By this criterion, the maximum bar diameter is 25 to 300% greater than the maximum diameter that can be made fully martensitic (lower graph, Fig. 3). Nonmartensitic transformation products (pearlite, ferrite, and bainite) were observed at the positions on endquenched bars corresponding to this reduced hardness value. The influence of mixed structures such as these on the mechanical properties of the steel would have to be determined for each application. CONTROL OF PROCESS VARIABLES The success of martempering depends on close control of variables throughout the process. It is important that the prior structure of the material being austenitized be uniform. Also, use of a protective atmosphere (or salt) in austenitizing is required because oxide or scale will act as a barrier to uniform quenching in hot oil or salt. The process variables that must be controlled in martempering include austenitizing temperature, temperature of martempering bath, time in martempering bath, salt contamination, water additions to salt, agitation, and rate of cooling from the martempering bath. Steels selected for martempering must be judged on hardenability and section size. To form the same amount of martensite for a given section size, the carbon content, alloy content, or both must be somewhat higher for martempering than for conventional (uninterrupted) quenching. Austenitizing temperature is important in establishing martempering procedures because it controls austenitic grain size, degree of homogenization, and carbide solution, and because it affects the Ms temperature. Temperature control during austenitizing is the same for martempering as for conventional quenching: a tolerance of 8°C is common. In most instances, austenitizing temperatures for martempering are the same as those for conventional oil quenching. Occasionally, however, medium-carbon steels are austenitized at higher temperatures prior to martempering to increase as-quenched hardness. For carburized parts, low austenitizing temperatures usually yield better size control during martempering. To obtain minimum dimensional changes, the lowest austenitizing temperature that yields satisfactory core properties should be used. The ratio of case depth to core, as well as the prior processing history of the steel (such as forging, rolling, or drawing), can also be controlling factors, particularly for critical section shapes and sizes. The temperature of the martempering bath varies considerably, depending on the composition of the workpieces, the austenitizing temperature, and the desired results. In establishing procedures for new applications, Fig. 3 — Approximate maximum diameters of bars that are hardenable by martempering, oil quenching, and water quenching. LEARN MORE This article is adapted from Martempering and Austempering, by Lauralice C.F. Canale, Xinmin Luo, and George E. Totten, and will be published in ASM Handbook, Volume 4F, Quenchants and Quenching Technology. Volume 4F is the latest volume in the ASM Handbook series on heat treatment and is a practical guide to quenching, its implementation, and its effects on the microstructure and properties of cast irons and steels. Volume 4F is available for preorder from the ASM International bookstore: bit.ly/3y5OGfE and online-first articles are available now in the ASM Digital Library.
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