Crystallographic study on microstructure and impact toughness of coarse grained heat affected zone of ultra-high strength steel

Abstract

The significant effect of welding heat input on microstructure evolution and toughness of coarse grained heat affected zone (CGHAZ) in ultra-high strength steel was studied from the aspect of crystallography. Charpy impact test confirmed the optimum toughness of CGHAZ was obtained at the heat input of 20 kJ/cm, which was related to the cumulative contribution of its well-refined bainitic structure and the highest density of high angle grain boundaries (HAGBs). Analysis on crystallography shows that the weakest variant selection occurs at the nose temperature of bainite transformation curve, inducing the staggered configuration of different Bain groups, and thus obtaining the highest density of HAGBs, especially the block boundary dominated by V1/V2 variant pair. From the establishment of the relationship between impact toughness and crystallographic structure, the variation in toughness of CGHAZ is primarily correlated to the block size.