Induction Tempering Process Development for 8mm Thick Plate Steel

Rapid tempering has been shown to improve impact toughness when compared to furnace treated material at equivalent hardness values. However, control limitations and physical constraints of industrial induction systems pose challenges to duplicating the desired time-temperature profiles obtained during controlled laboratory experiments, and therefore, to achieving the desired level of tempering. In this research, a series of time-temperature curves were extracted from 2D induction models and used to calculate effective tempering parameters for different heating rates, hold times, and peak temperatures. The effective tempering parameter considers the transient nature of the thermal profile and correlates it to an effective isothermal tempering cycle. The models provided a starting point for the determination of induction system parameters, but the physical limitations of the induction system ultimately determined which time-temperature profiles were achievable. Tempering was performed on a vertical-feed, pilot-scale system with a single-turn, longitudinal coil, and pyrometers for thermal monitoring. Hardness measurements were taken along the width and through-thickness of each plate to determine the level of tempering achieved and to quantify the complex heating profile inherent to induction heating. While hardness does not give insight into the time-temperature profile, it is helpful in understanding the variation inherent to induction tempering treatments.