November/December AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 6 4 *Member of ASM International INDUCTION HEATING: EVERYTHING YOU WANTED TO KNOW, BUT WERE AFRAID TO ASK As a regular contributor to the HTPro eNewsletter, Professor Induction answers a wide variety of questions regarding induction heating and heat treating. Valery Rudnev, FASM,* IFHTSE Fellow Inductoheat Inc., Madison Heights, Mich. I nduction heating is a multifaceted phenomenon com- posed of complex interactions involving electromagnet- ics, heat transfer, materials science, metallurgy, and cir- cuit analysis—with applications across multiple industries. Figure 1 shows a small portion of a virtually endless variety of workpieces where electromagnetic induction heating is used to develop an attractive blend of microstructures and properties at a competitive cost. What is the difference between auto-tempering and self-tempering? Answer: Sometimes the terms auto-tempering and self-tempering are incorrectly used interchangeably. Here we will explore the differences in what these two terms really mean [1] . Auto-tempering. Martensitic transformation occurs over a temperature range between the M s (martensite start) and M f (martensite finish) temperatures. The range depends on the steel’s chemical composition and, from a practical perspective, cannot be changed by varying the quench se- verity. In plain carbon steels, the M s and M f temperature range is directly related to the carbon content. For plain car- bon steels with a carbon content of 0.2% to 0.5%C range, M s temperatures are within about a 300 o C/572 o F to 450 o C/842 o F range. Thus, freshly formed martensite will be immediately exposed to tempering temperatures and can be potential- ly softened. This phenomenon is commonly referred to as auto-tempering. The degree of auto-tempering becomes more noticeable with a reduction in quench severity, an in- crease of M s temperatures, and the mass of the heated ma- terial (for example, through heating vs. surface heating), as well as whether an interrupted quenching is used or not [1] . Alloy steels typically exhibit auto-tempering to a lesser de- gree compared to plain carbon steels. Self-tempering. The principle of self-tempering (also referred to as slack-quenching) can be illustrated using the example shown in Fig. 2, which shows the results of numer- ical computer modeling of induction surface hardening of a medium carbon steel solid shaft (50 mm/2-in. diameter) in a normalized condition using a frequency of 16 kHz [1] . The re- quired nominal case depth is 2.5 mm. During the initial stage of induction heating, intensive heating of the surface and near-surface takes place. After 3.5 sec of heating, the surface and 2.5-mm-thick subsurface layer required to be hardened have reached suitable tem- peratures for austenitization, taking into consideration the non-equilibrium phase transforma- tion caused by rapid heating. A short dwell (0.5 sec) is applied to reduce the thermal shock during the initial stage of quenching and promote suf- ficient homogenization of austenite. The temperature at the center (core) of the shaft does not increase significantly (less than 100 o C) during the heating and dwell cycles. Several reasons are responsible for that, in- cluding pronounced skin effect, high power density, and short heating time, which do not permit the great- er amount of heat to be conducted from the surface towards the core. During the initial quenching stage, the high temperature of the surface layer begins to fall rapidly. Fig. 1 — Induction heating applications across a variety of industries. 12