Welding heat input effect on microstructure and toughness of coarse-grained heat-affected zone in V-Ti-N-Nb microalloyed weathering steel

The development of low-alloy steels with high heat input welding performance remains a research focus in engineering, yet welding-induced embrittlement in the coarse-grained heat-affected zone (CGHAZ) presents a persistent challenge to their practical application. Leveraging a V-Ti-N-Nb microalloyed weathering steel system, this study systematically investigates the evolution of CGHAZ microstructural characteristics and cryogenic toughness under varying welding heat inputs via Gleeble-3500 thermal simulation. Results demonstrate a gradient transformation of the dominant matrix microstructure with escalating heat input (10 → 30 → 50–70 kJ/cm): a transition from lath bainitic ferrite to granular bainitic ferrite and eventually to a hybrid microstructure of intragranular acicular ferrite + polygonal ferrite. Concurrently, the size and area fraction of hard martensite/austenite constituents increase. The evolution of microstructure exacerbates local strain concentration and grain coarsening defined by a 15° misorientation tolerance angle. This directly weakens crack initiation resistance while reducing effective barriers to crack propagation paths, ultimately manifesting as a decreasing trend in -40 ℃ impact absorption energy with increased heat input. Notably, under high-heat-input conditions (50–70 kJ/cm), V-rich (Ti,Nb,V)(C,N) composite precipitates effectively mitigate the attenuation of high-angle grain boundary density by promoting heterogeneous ferrite nucleation. Consequently, the CGHAZ subjected to 70 kJ/cm heat input retains a remarkably high mean -40 °C impact energy of 63.4 J.