Effect of a quenching-long partitioning treatment on the microstructure and mechanical properties of a 0.2C% bainitic steel

A quenching-long partitioning (Q-LP) heat treatment has been proposed for large-scale industrial production of steel components. Compared to the conventional quenching-tempering (Q-T) and quenching-partitioning (Q-P) process, the steel treated by air-quenching from austenitizing temperature and 60 min-partitioning at 200 °C exhibits the best combination of strength and toughness. With a nearly unchanged product of strength and elongation of around 18 GPa%, the room-temperature impact toughness has been significantly improved from 84 J/cm2 for the air Q-T treatment to 104 J/cm2 for the Q-LP treatment, which is attributed to the more retained austenite and the effective relief of the microstresses. The multi-phase microstructure of the Q-LP treated steel contains martensite, bainite and retained austenite exhibiting two different morphologies, i.e., one is the inter-lath plates, and the other is the austenite films within the bainite plates.