Austempering vs. Q&P: Comparative effects on microstructure and mechanical properties of complex-alloyed TRIP-assisted steel

This study compares the effect of austempering (AT) and two-step quenching and partitioning (Q&P) treatments on the microstructure and mechanical properties of a 0.2 wt% C Mn-Si-Cr-Mo-V-Nb TRIP-assisted steel. Both treatments included austenitization (770 °C or 900 °C) followed by quenching and isothermal holding (20 min). AT used a 350 °C isothermal hold, while Q&P involved quenching to 200 °C or 230 °C, followed by partitioning at 350 °C. Microstructural analysis (OM, SEM, XRD, EPMA, EBSD) revealed ferrite and (Nb,V)C in both treatments, with bainite (AT) or tempered martensite/bainite (Q&P) and varying retained austenite (RA) content depending on the austenitization temperature. Austenitization at 770 °C resulted in Cr- and Mo-promoted cementite precipitation, reducing the RA volume fraction and ductility. In contrast, 900 °C austenitization resulted in a cementite-free structure with 7–9 vol% RA exhibiting a clear TRIP effect. The ultimate tensile strength (UTS) increased with austenitization temperature (820–880 MPa to 1040–1090 MPa) due to a decreased proeutectoid ferrite, grain refinement, and enhanced TRIP effect, though at some expense of ductility/toughness. Under the same austenitization parameters, Q&P yielded slightly higher UTS (∼ 50 MPa higher), and much greater impact toughness (by 2 times at 770 °C and 1.4 times at 900 °C). Due to a superior strength-ductility-toughness balance, Q&P is identified as a more promising heat treatment for this TRIP-assisted steel.