Kinetics of bainitic transformation and transformation plasticity in a high strength quenched and tempered structural steel

Dilatometric analyses were made on a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe–0.16C–0.2Si–0.87Mn–0.33Cr–0.21Mo (wt%)) structural steel during continuous cooling under different mechanical loading conditions. Bainitic transformation kinetics were calculated from the dilatation curves. In addition, the effects of the applied stress on the interface migration velocity, the transformation plasticity and the mechanical driving force for the bainitic transformation were calculated.The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation or the interface migration velocity. The observed scatter in the interface migration velocity decreases with increasing applied stress due to the transformation induced inhomogeneous plastic deformation. The transformation plasticity increases with increase in the applied tensile stress and the coefficient of transformation plasticity is independent of load. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. For a lower starting temperature, more nuclei form during cooling, resulting in a smaller grain size of the bainite phase with increasing tensile stress.