Improvement of HAZ toughness for high heat input welding by using boron diffusion from weld metal

In high heat input welding, such as 1-pass electrogas arc welding (EGW) of thick steel plates, HAZ toughness is deteriorated because HAZ microstructure coarsens significantly. TiN precipitates dispersed in steel plates are effective to suppress coarsening HAZ microstructure by pinning effect. However TiN precipitates resolve in high temperature region near the fusion line, where deterioration of HAZ toughness is unavoidable near the fusion line. As a solution of this problem, the authors paid attention to boron in the weld metal. Boron is known to be effective to prevent coarse ferrite formation at prior austenite grain boundaries, and to diffuse rapidly in steel. In this research, boron diffusion from weld metal to HAZ during EGW process was confirmed by SIMS analysis. It was certified that diffused boron in HAZ improved HAZ toughness because boron suppressed microstructure coarsening, and fixed free nitrogen originated by TiN resolution as BN near the fusion line. These research results proposed a new HAZ structure control technology considering combination of high boron bearing weld metal and TiN-treated steel plates.