Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of P91 Heat-Resistant Steel Coating on Mild Steel

P91heat-resistant steel was deposited on mild steel using cold metal transfer (CMT) arc cladding to enhance oxidation resistance and extend service temperature. After cladding, the Widmanstatten phase was formed in the heated affect zone (HAZ) of the substrate, which severely degraded the mechanical properties of the substrate, and the cladding layer was almost entirely composed of quenched martensite with very low toughness. In order to improve the microstructure and enhance the toughness of the cladding specimen, post-weld heat treatment (PWHT) was conducted on the cladding specimens, and the effect of PWHT temperature on microstructure and mechanical properties of P91 heat-resistant steel coating on mild steel was investigated. The results of investigation shows that the Widmanstatten phase was eliminated by PWHT, and precipitates, including M₂₃C₆ and MX phases, were generated in the deposited layer. A carbon-enriched zone was formed in the deposited layer adjacent to the fusion zone, and a carbon-depleted layer was formed in the HAZ of the substrate due to carbon migration. The grain in the carbon-depleted zone is clearly coarsened due to the dissolution of carbides in the grain boundary. The hardness of the coating was reduced with the increasing of heat treatment temperature and reached its minimum value of 215 HV after heat treated by 780 °C × 2 h, which decreases by up to 55% compare to the maximum hardness of as welded condition, and the toughness increases to about 44.5 J. The shear strength initially decreases with the rise of PWHT temperature and then gradually increasing to a minimum value of 350 MPa after heat treated by 760 °C × 2 h. The optimal PWHT parameter is determined to be 740 °C  × 2 h.