Competitive growth of martensite/austenite constituent and degenerated pearlite and the impact toughness in the coarse-grained heat-affected zone of a low carbon microalloying steel

Abstract

The microstructure evolution of the martensite/austenite (M/A) constituent and degenerated pearlite (DP), as well as the impact properties in the simulated coarse-grained heat-affected zone (CGHAZ) of a low carbon microalloying steel, have been investigated using the Gleeble 3800 thermal simulation machine. A comprehensive investigation of the competitive growth behaviour and fracture mechanism of M/A constituent and DP during continuous cooling transformation was carried out. In this study, as the welding heat input (E j ) elevated, the decreased cooling rate contributed to the diffusion of C atoms during continuous cooling, which established the dominance of DP in competitive growth. It was evidenced by a decrease and even complete disappearance of the M/A constituent, accompanied by an increase in the area fraction and size of the DP. Compared to M/A constituent, DP was the preferred nucleation site for microvoids. The formation of microvoids on DP was achieved through the separation of cementite (θ) particles from the surrounding degenerated pearlite ferrite (DPF). As the E j elevated, the prior austenite grains (PAGs) of CGHAZ coarsened, the density of HAGB per unit area decreased, the distance of propagation path was extended. It has been demonstrated that the elevated E j had a detrimental effect on the resulting toughness of CGHAZ. For the low carbon microalloying steels, the reduction in E j and the increased cooling rate facilitated the reduction in the formation of DP, which enhanced the impact toughness of the CGHAZ.