Effect of Tempering Mode on the Microstructure and Mechanical Properties of a Lean Alloy Martensitic Steel: Conventional Reheating Versus Induction Reheating

The present study elucidates the effect of the reheating rate and tempering temperature on the type, size, and distribution of carbides/cementite formed within martensite (M)/lower bainite (LB) during the tempering of a high-strength low-alloy (HSLA) steel. A comparison is made between conventional reheating and induction reheating. All the experimental samples were quenched from the austenitization temperature of 920 °C for 30 min. An ultra-high yield strength of 1180 MPa and a good toughness of 94 J at 0 °C were obtained after tempering at 400 °C for 15 min by induction reheating (~ 100 °C/s), while in the case of conventional reheating (~ 2 °C/s) and tempering at 400 °C for 15 min, the steel had a high strength of 1159 MPa, but a poor toughness of 47 J at 0 °C. After induction reheating to 500 °C and tempering for 15 min, a higher toughness was obtained (91 J at 0 °C), but the strength was decreased to 967 MPa. Microstructural characterization revealed that fine needle-like carbides (e-Fe2.4C) were obtained by induction reheating to 400 °C and tempering for 15 min. However, samples tempered at 400 °C by conventional reheating and 500 °C by induction reheating both precipitated coarse θ-Fe3C. In the former, θ-Fe3C precipitated at locations that were previously austenite grain boundaries and led to tempered martensite embrittlement (TME). In the latter, Ostwald ripening occurred, and the precipitates could not maintain high strength. The excellent mechanical properties of induction reheated steel (400 °C, 15 min) are attributed to the following aspects: (a) The selection of appropriate tempering temperature did not lead to decarburization and ensured high strength, and (b) a high reheating rate promoted carbide dispersion and avoided TME in steel.