Bainitic Austempering Incorporating Quenching and Partitioning for Favorable Tuning of Microstructure and Mechanical Properties

A novel aspect of combining bainitic austempering with quenching and the partitioning process is addressed herein to obtain a favorable microstructure with desired mechanical properties. In this regard, Mn-Si-Cr-Al steel is first intercritically annealed at 840 degrees C and quenched to 140 degrees C after isothermal treatment at 360 degrees C for 15 min and then partitioned at 360 degrees C for 30 min. Multiphase microstructure containing ferrite, bainite, martensite, and retained austenite (RA) is obtained. Uniform elongation and total elongation increase appreciably from 11.5% to 20.5% and 21.0% to 28.3%, respectively, while maintaining near-identical levels of tensile strength, compared with traditional bainitic austempering. The significant enhancement of tensile properties is attributed to the increase in volume fraction and mechanical stability of RA.