The Effect of Novel Double Pulse Metal Inert Gas Welding on Microstructure and Mechanical Properties of P91 Steel

This study investigates the joining of P91 steel plates using conventional metal inert gas (MIG) and double pulse (DP)‐MIG welding, focusing on the effect of pulse frequency on weld quality by measuring bead geometry, hardness, tensile, and impact strength. The conventional MIG weldment exhibits inferior mechanical properties, with an ultimate tensile strength (UTS) of 270 MPa (44% of base metal strength). Microstructural analysis reveals a coarse columnar structure with large prior austenite grains (PAGs), extensive δ‐ferrite, and severe precipitate coarsening. Energy dispersive spectroscopy confirms significant elemental segregation, resulting in a high peak hardness of ≈480 HV. Electron backscatter diffraction (EBSD) analysis quantifies its poor integrity, showing elevated residual strain and a low fraction of crack‐resistant high‐angle grain boundaries. In contrast, the optimized 8 Hz DP‐MIG process produced a vastly superior weldment with an UTS of 535 MPa (85% of base metal strength), a controlled peak hardness of ≈450 HV, and the highest impact toughness of 20 J. This performance is attributed to exceptional microstructural refinement with fine, equiaxed PAGs and suppressed δ‐ferrite. EBSD analysis validates this superiority, showing minimal residual strain and high HAGB density of 77.6%, which enhances crack propagation resistance.