Characterization of microstructure and mechanical performance in fiber laser welded modified 9Cr–1Mo steel for power plant applications

Abstract

The current study involved butt welding 9Cr–1Mo creep strength enhanced ferritic steel, which is microalloyed with vanadium, niobium, and molybdenum, utilizing a fiber laser welding process. The main contribution is the successful application of fiber laser welding to achieve full penetration welds with minimized heat-affected zone. This study aims to assess the room temperature microstructural features and mechanical attributes of fully penetrated fiber laser welds, with an emphasis on their applicability in power plant settings. A key finding is the attainment of complete penetration welds with a notably low heat input of 75 J/mm, accompanied by a minimum top bead width of 1.2 mm and a minimal heat-affected zone measuring 0.363 mm in width. The development of an untempered lath martensite structure, accompanied by the total dissolution of precipitates resulting from the autogenous characteristics of laser welding, has demonstrated an average hardness of 502 HV_0.3 within the fusion zone. The process of post weld heat treatment has led to the formation of M₂₃C₆ and MX-type precipitates, resulting in a hardness value of 252 HV_0.3 and a notable decrease in internal grain strain. The evaluation of root and face bend tests demonstrated outstanding ductility in the weld joints, confirming their structural integrity. The specimens, both as-welded and post-weld tempered, demonstrated average tensile strength and impact toughness values of 773.6 MPa and 64.66 J, as well as 661 MPa and 104.33 J, respectively. These results exceed the minimum specified criteria of 585 MPa and 47 J necessary for use in power plants.