Influence of quenching strategy on phase transformation and mechanical properties of low alloy steel

The energy-efficient quenching and nonisothermal partitioning process has been shown to engender an excellent strength-ductility combination in low alloy steels. Conventionally, these steels are quenched to a specific temperature in the martensite region, i.e. between Ms and Mf, followed by slow cooling to room temperature. However, in the present study, an attempt has been made to investigate the behaviour of low alloy steel quenched in the bainite region, i.e. temperature between Bs and Ms, followed by slow cooling to room temperature. For comparison, the same steel was also quenched to two different temperatures in the martensite region as well as direct quenched to room temperature. The XRD results showed the maximum austenite retention for a lower quench temperature in the martensite region. Both the direct quenching and the quenching and nonisothermal partitioning from the martensite region led to a microstructure dominated by the presence of auto-tempered martensitic laths. However, the sample quenched in the bainite region and subsequent slow cooling gave rise to bainite-ferrite laths containing coarse carbides. An interlath precipitation of carbides was observed for the bainite, in contrast to the intralath carbide precipitation in martensite. The austenite remained at the quench temperature decomposed to bainite during slow cooling from the bainite region, whereas M-A constituents were formed during slow cooling from the martensite region. The microstructural constituents were observed to be finest for the direct-quenched sample due to a lower quench temperature. Interestingly, the sample quenched in the bainite region showed a significant improvement in ductility, in contrast to the sample quenched in the martensite region and direct-quenched to room temperature that showed higher strength.