Microstructure evolution and mechanical properties in multi-phase composite nanobainitic steel via isothermal treatment

This study investigates the effects of different low-temperature austempering temperature and time on the phase fraction, morphology, distribution of microstructures and mechanical properties in 67Si2CrNiAlMnMoCu steel. The results indicate that the specimen austempered at 200 °C exhibits the earliest transformation due to the pre-forming martensite acting as nucleation for bainitic transformation and the lath martensite disappears as the austempering temperature and time increasing. The M/A islands decrease slightly in size and exhibit markedly less surface relief when specimen austempered at 240 °C from 9 h to 21 h. In addition, the product of strength and ductility of austempered specimens are all higher than the oiled specimens, especially the specimen austempered at 200 °C for 21 h is ∼1.5 times higher than the oiled one. The impact toughness of the specimen austempered at 240 °C for 21 h is nearly 12 times higher than that of austempered at 200 °C for 3h.