Abstract: Microstructure and mechanical properties of the 0.6C super bainitic steel were analyzed after isothermal salt bath quenching and quenching-partitioning-tempering (Q-P-T) respectively and then tempering at different temperatures. The results indicate that the reversible temper brittleness appears in the temperature range of 360-500 ℃ after treated by two-step austempering at 230 ℃×10 h and 320 ℃×7 h, while the tempering brittleness temperature range of Q-P-T specimen is 450-470 ℃. Respectively, after Q-P-T process, the tempering brittleness temperature range is obviously narrowed and the mechanical properties of the specimen are improved. The microstructural characterization by SEM, TEM and XRD reveals two main factors resulting in the mechanical properties of the Q-P-T specimen superior to those of the isothermal treatments: after the Q-P-T treatment, the microstructure is more refined, and with the increase of subgrain boundaries, the stability of the microstructure is further enhanced, and the carbide segregation to the grain boundaries is hindered, so it is difficult to connect into a network. Therefore, after the Q-P-T treatment, the reversible temper brittleness temperature range is greatly reduced; after the Q-P-T treatment, the film-like retained austenite increases in the content and is more stable, which is beneficial to weaken the influence of reversible temper brittleness.

Key words: super-bainitic steel, Q-P-T process, reversible temper brittleness, carbide segregation, film-like retained austenite