Effect of Prior Austenite Grain Size on Bainitic Transformation and Mechanical Properties of 40CrNiMo Steels Above and Below Ms

This study examines the role of prior austenite grain size (PAGS) in bainitic transformation kinetics and the resulting mechanical property response of 40CrNiMo steel. By combining dilatometry, SEM, XRD, EBSD, and the Ravi kinetic model, the effect of PAGS on the bainitic transformation behavior and mechanical properties of 40CrNiMo steel was systematically investigated. The results show that, with increasing PAGS, the transformation rate exhibits no significant variation above Ms, whereas it tends to accelerate below Ms; both the activation energies for grain-boundary nucleation and autocatalytic nucleation decrease with increasing PAGS. In terms of mechanical properties, only minor differences are observed for transformations above Ms, while a decreasing trend is observed with increasing PAGS for transformations below Ms.