Investigation of abnormal high impact toughness in simulated welding CGHAZ of a 8%Ni 980 MPa high strength steel

It is well known that the toughness in the coarse grain heat affected zone (CGHAZ) is of the lowest in the heat affected zone (HAZ) of a HSLA steel welding joint. In the present work although the grain size in the simulated CGHAZ was as large as 100–300 μm and its tensile strength and elongation were lower than those of the fine grain heat affected zone (FGHAZ), however its Charpy V impact toughness at room temperature reached 193 J, which was appreciably higher than 138 J of the simulated FGHAZ with grain size of 30 μm. This abnormal phenomenon was investigated in this paper by analyzing the micromechanism of ductile rupture in associated microstructures and taking account of the effects of stress triaxiality.