Effects of Rapid Induction Heating on Transformations in 0.6% C Steels

Rapid induction surface hardening of high-carbon steel with an initial martensitic microstructure can attain the very high-strength levels needed for lightweighting components such as shafts, gears, and bearings that are subjected to high operating stresses. Specimens of pearlitic and martensitic 0.6 wt pct C steels were heat-treated using a dilatometer to investigate the effects of prior microstructure, heating rates (5 to 500 °C/s), and austenitizing temperatures (850 to 1050 °C) on the transformation to austenite and subsequent transformation to martensite during helium quenching. Specimens were quenched from selected temperatures during heating to assess the initial cementite precipitates formed in martensite before transformation to austenite. Other specimens were isothermally held at the austenitizing temperature to assess cementite dissolution rates and austenite grain evolution. Higher heating rates increased the Ac₁ and Ac₃ temperatures and lowered the M_s temperatures. Martensitic prior microstructures resulted in lower M_s temperatures and lower hardness as compared to a pearlitic prior microstructure. Alloy content and the austenitizing temperature (900 or 1150 °C) utilized to provide the initial martensitic microstructure also influenced the transformation temperatures and the amount of retained austenite.