Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/ cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nanoand submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (delta) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15 was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and lowtemperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.