Understanding the effect of prior bainite/martensite on the formation of carbide-free bainite

The isothermal bainite transformation in low-carbon low-alloy steel under direct and step-quenching conditions have been investigated in the present work. Varying quantities of prior bainite and prior martensite were formed separately by quenching above and below Ms temperature to understand their effects on the extent and kinetics of subsequent bainite transformation in Bs−Ms regime. Formation of prior bainite at higher temperature has been found to stabilise part of remaining austenite against subsequent bainite transformation at lower temperature showing thermal stabilisation of austenite. Hence, the total amount of bainite in two-step treatment is less than that obtained in single-step isothermal bainite transformation at the latter temperature, indicating the inapplicability of ‘additivity rule’ for continuously cooled bainite. A decrease in the bainitic ferrite plate volume due to higher strength of parent austenite and higher nucleation rate, together with limited number of bainite nucleation sites at the latter temperature are believed to be responsible for the thermal stabilisation of austenite. Prior martensite also stabilises part of remaining austenite against further bainite transformation through fragmentation of austenite grains and increased defect density in austenite. The kinetics of bainite transformation in each of the above-mentioned conditions have been analysed quantitatively using an established model and the results are correlated with the microstructures.