Effects of prior austenite grain size and fast cooling rate on lower bainite kinetics of high-strength low-alloy steel

Abstract

Bainite transformation has long been a major research focus in materials science, particularly the development of quantitative models for its transformation kinetics. In this study, the morphological evolution, kinetics, effects of prior austenite grain size (PAGS), and the influence of fast cooling rate on lower bainite (LB) transformation kinetics of a high-strength low-alloy steel were investigated. Laser scanning confocal microscopy, thermal simulation, and electron backscattered diffraction experiments were utilized. A specific kinetic equation for LB transformation was established. The results demonstrated that: (i) the kinetic constant was approximately proportional to the logarithmic volumetric PAGS and the square of cooling rate; (ii) the nucleation number per unit volume decreased with increasing PAGS; and (iii) the overall LB packet growth rate increased with both PAGS and cooling rate. The key factor in obtaining fine LB packets was to increase the nucleation number per unit volume of LB, which could be achieved through two approaches: reducing PAGS and modulating the cooling rate. However, reducing PAGS had a much stronger effect compared to modulating the cooling rate.

Keywords

lower bainite / lath-like bainite / kinetic equation / phase transformation rate / nucleation and growth

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