Effect of Multi-layer and Multi-pass Welding on the Microstructure and Properties of V-N-Cr Weathering Steel
摘要
The microstructure, toughness, and variation of the critical reheating coarse-grain heat-affected zone of V-N-Cr weathering steel were investigated using a multi-pass welding thermal simulation test machine, MMS-300. The results indicate that when Tp2 is set at 750 °C, the microstructure primarily consists of polygonal ferrite, lath bainite, and a small quantity of granular bainite. As Tp2 increases to 900 and 1050 °C, the proportion of lath bainite decreases, while the proportions of polygonal ferrite and acicular ferrite increase, accompanied by a significant presence of M/A islands. At 1050 °C, the presence of fine polygonal ferrite and acicular ferrite markedly enhances the balance between hardness and toughness. However, identifying the critical reheat in the coarse-grain heat-affected zone along the original austenite grain boundary may be challenging due to the extensive size and distribution of the M/A island chain. The widespread dispersion of V(C,N) phase precipitates offers favorable nucleation sites for the formation of fine-grained ferrite, which in turn facilitates the development of ultrafine ferrite crystals at high-angle grain boundaries, in contrast to the low-angle grain boundaries typical of granular bainite, thereby increasing resistance to crack propagation. Additionally, the high density of V(C,N) precipitates depletes nitrogen and carbon in the austenite, resulting in a reduction in the size and hardness of the M/A islands due to low carbon enrichment.