Microstructure refinement and toughening mechanism of high heat input welding induced CGHAZ in a low carbon Nb-Ti microalloying steel dominated by trace boron

How trace amounts of B positively alter the microstructure and toughness of the coarse-grained heat affected zone (CGHAZ) in a low carbon Nb-Ti microalloying steel subjected to high heat input welding is still unclear but very important. In this attempt, the addition of 0.0011 wt% B to this type of steel significantly led to microstructure refinement and toughness enhancement in its CGHAZ of high heat input welding. The inhibition mechanism of B on grain boundary ferrite (GBF) nucleation was discussed through nucleation kinetics calculations, and the promotion mechanism of acicular ferrite (AF) nucleation inside prior austenite grains (PAGs) was investigated through two-dimensional lattice mismatch model. The results indicated that B segregation at prior austenite grain boundaries (PAGBs) was demonstrated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The reduction in grain boundary (GB) energy induced by B segregation exerted a deleterious effect on GBF nucleation, mainly by impeding grain corner and grain edge nucleation. In addition, the polycrystalline BN phase was determined to be hexagonal structure in the CGHAZ. The two-dimensional lattice mismatch of (002)BN//(01 1)AF was 6.79 %, which facilitated the AF nucleation on the BN phase. On the one hand, B segregation lowered the Ar3 temperature, resulting in a slight increase in M/A constituents and a reduction in DP. This resulted in an increase in crack initiation energy. On the other hand, the combination effect induced by the addition of trace B, including refined PAGs, few GBF, none FSP, and increased AF, which hindered the propagation of microcracks and then elevated crack propagation energy. Trace B increased impact toughness of simulated CGHAZ from 26 J to 133 J. This highlights a novel design of a high heat input weldable steel.