Microstructure and Properties of Coarse Grain Zone in Single-Pass Welding of V-N Microalloyed Steel

The single-pass welding thermal simulation test of V-N micro-alloyed steel was carried out by MMS-300 thermal simulation testing machine, and combined with an optical microscope (OM), electron backscattering diffraction, impact test, and other technologies, to study the organization, toughness, and its change rule of critical reheating coarse-grained heat-affected zone under different welding conditions. The results show that the microstructure of single-pass simulated CGHAZ is composed of polygonal ferrite, acicular ferrite, bainite beam, and a small amount of lath martensite at low and medium linear energies. The precipitates in the heat-affected zone of coarse grain in single-pass welding are mainly VN or V(C, N). The microhardness and high-angle grain boundary fraction (> 15°) of CGHAZ decrease with the increase in welding heat input. When t8/5 is 10 s, the microstructure is dominated by fine lath bainite, and a large number of high-angle grain boundaries exist between the lath strands in different microstructure, which can effectively hinder crack propagation. With the increase in t8/5, that is, the increase in welding line energy, the lamellar structure decreases obviously, and the proportion of low-angle grain boundary is higher. Meanwhile, the M/A island hard phase inside the granular bainite reduces the free slip path of the plastic ferrite phase during deformation, which is difficult to prevent crack extension and deteriorates the toughness, and the weld thermal cycling process destroys the strength and toughness equilibrium of the V-N micro-alloyed high-strength candidate steel.