Microstructure and local mechanical properties of a dissimilar metal welded joint with buttering layer in steam turbine rotor

A novel dissimilar metal welded joint (DMWJ) is demonstrated in a steam turbine rotor application. The integrity of the joint is investigated using the stratified tensile method for the first time and its local tensile mechanical properties are evaluated. The effect of the amount of Cr and Mo, post weld heat treatment, welding heat flow and carbon migration on the microstructure and local mechanical properties of the DMWJ is analyzed by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-hardness and stratified tensile tests. The results indicate that local mechanical properties in three heat affected zones (HAZs) exhibit gradient distributions with a significant mismatch in the boundary regions. The yield strengths of three HAZs varied from 557 to 873, 640 to 700 and 753 to 1035 MPa, and the corresponding ultimate tensile strengths were 690–899, 732–826 and 866–1088 MPa, respectively. The second-phase carbides in base metals and weld regions were mainly M₃C, M₂₃C₆ and M₇C₃. The difference of mechanical properties in weld regions is related to shape of bainite. Drastic changes and mismatch of local mechanical properties in boundary regions are mainly attributed to the shape and quantity of martensite and bainite. Local mechanical properties without size-effect could be obtained by the stretched tensile method, which provides an accurate assessment of the integrity of DMWJs.