Investigation of the microstructure and mechanical properties of the laser welded joint of P22 and P91 steel

Abstract The present manuscript investigates the microstructural aspects and mechanical properties of laser beam welded ASTM A335 Grade P91 and P22 steel joint for power plant applications. Detailed microstructure characterization of the weld metal (WM) and heat-affected zone (HAZ) were carried out in as-welded and post-weld heat treatment (PWHT) conditions. A variation in microstructure was observed along the welded joint. This resulted in inhomogeneity in mechanical properties. The PWHT resulted in the formation of the stabilized microstructure in weldments and reduced heterogeneity in mechanical properties along the weldments. Tensile strength of weld joint in as-welded (AW) and PWHT condition was found slightly higher (617 MPa and 628 MPa, respectively) than the tensile strength of P22 steel (610 MPa). The fracture location for both AW and PWHT was observed in the P22 base region, which indicates that welded joint is stronger than the base material. Maximum tensile strength of the WZ was found 864 MPa in the as-welded condition. The hardness of WM, P91 CGHAZ and P22 CGHAZ were found 376 HV, 420 HV and 302 HV, respectively. After the PWHT, the hardness of the WM, P91 CGHAZ, P91 ICHAZ, P22 CGHAZ and P22 ICHAZ were measured 237 HV, 264 HV, 212 HV, 208 HV and 190 HV, respectively. The reduction in hardness is attributed to the tempering reaction, which results in the formation of the tempered martensite in WM and HAZ of P91 and tempered bainite in P22 HAZ. The impact toughness of the weld metal in as-welded condition was measured 85 J, which was lower than P22 and P91 steel. PWHT resulted in a drastic increase in impact toughness of WM and it was 145 J. The impact toughness of the P22 and P91 HAZ was measured 146 J and 92 J, respectively, for AW and 168 J and 140 J, respectively, after PWHT. The optimum microstructure and mechanical properties of the dissimilar welded joint were obtained after the PWHT.