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

摘要

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 degrees C for 30min. An ultra-high yield strength of 1180MPa and a good toughness of 94J at 0 degrees C were obtained after tempering at 400 degrees C for 15min by induction reheating (similar to 100 degrees C/s), while in the case of conventional reheating (similar to 2 degrees C/s) and tempering at 400 degrees C for 15min, the steel had a high strength of 1159MPa, but a poor toughness of 47J at 0 degrees C. After induction reheating to 500 degrees C and tempering for 15min, a higher toughness was obtained (91J at 0 degrees C), but the strength was decreased to 967MPa. Microstructural characterization revealed that fine needle-like carbides (epsilon-Fe2.4C) were obtained by induction reheating to 400 degrees C and tempering for 15min. However, samples tempered at 400 degrees C by conventional reheating and 500 degrees C by induction reheating both precipitated coarse -Fe3C. In the former, -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 degrees C, 15min) 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.