FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2025 62 S alt bath (nitrite/nitrate) quench systems are typically used for heat treatment processes to reduce distortion and increase toughness compared to conventional heat treatments such as an oil quench and temper (Q&T) process. These salt quench processes include austempering (AUS) and marquenching (MQ). Figure 1 is a schematic isothermal transformation (I-T) diagram that compares Q&T, MQ, and AUS processes. MARQUENCHING Marquenching (also known as martempering) is a modified Q&T process that was developed to address nonuniform transformation resulting from the surface of a part cooling faster than the core. When the face-centered-cubic (FCC) austenite lattice structure transforms to body-centered-tetragonal (BCT) martensite during the quench portion of a Q&T process, an expansion occurs. This can cause a buildup of tensile residual stresses as the hard, brittle martensitic surface that first forms on cooling will constrain the core from expanding when it transforms to martensite at a later time. The stresses that form can be large enough to distort the part shape or cause quench cracking. Because the magnitude of marten- *Member of ASM International Fig. 1 — This schematic I-T diagram compares three different heat treating processes: austempering (green), quench & tempering (red), and marquenching (purple). All three processes begin by heating to form austenite. sitic expansion increases with increasing carbon content in the steel, hypereutectoid steels are the most prone to distortion and quench cracking during a Q&T process. Marquenching begins like a Q&T process; however, the part is not quenched below the martensite start temperature (Ms). Rather, it is quenched in a salt bath to a temperature above the Ms and held until the cross-section of the part is stabilized at the quench temperature, see Fig. 1. In a MQ process, the purpose of the quench is to cool the part without transforming the austenite. Upon removal from the salt bath, the part will cool in air through the temperature range where martensite forms. Because the component is at a more uniform temperature and air is a slow “quenching” medium, a more uniform trans- formation occurs as the time lag between when the surface and core transform is shorter. As a result, distortion and quench cracking issues are mitigated. For these reasons, MQ is a preferred process for medium to high carbon steels. Note that the final microstructure formed after a MQ process is identical to that from a Q&T process and both processes must undergo a tempering step to acquire the final desired combination of strength and toughness. Marquenched steel is used in applications that require good toughness at a high hardness with minimal distortion. A partial list of components that are marquenched includes gears, shafts, springs, fasteners, mower blades, bearing races and rollers, tool dies, and structural components for heavy equipment. Examples of commonly MQ steel grades include 1080, 1095, 4140, 4340, 5160, and 52100[1]. AUSTEMPERING Austempering is an isothermal heat treatment process applied to ferrous materials to increase strength and toughness. In general, the process consists of three steps: austenitizing, rapidly quenching to a temperature above the Ms, and isothermally transforming to form either bainite (in steel) or ausferrite (in cast iron). During an AUS process, the microstructure transformation occurs uniformly over many minutes or hours in a salt bath at one temperature (Fig. 1), so cracking does not occur. THE BASICS OF SALT BATH QUENCH HEAT TREATMENT PROCESSES: MARQUENCHING AND AUSTEMPERING Marquenching and austempering offer heat treatment solutions for steel that minimize distortion and quench cracking versus a conventional quench and temper process. Kathy Hayrynen, FASM* Aalberts surface technologies, Livonia, Michigan 8
RkJQdWJsaXNoZXIy MTYyMzk3NQ==