High cycle fatigue and fracture behaviors of CrMoV/NiCrMoV dissimilar rotor welded joint at 280 °C

Abstract

High cycle fatigue (HCF) behaviors were systematically evaluated of a CrMoV/NiCrMoV dissimilar metal welded joint (DMWJ) with buttering layer in steam turbine rotor at 280 °C. Fatigue curves and parameters of the base metals (BMs), weld metal (WM), buttering layer (BL) and heat affected zones (HAZs) in the welded joint were obtained. Characteristic microstructures, carbides, and fatigue fracture were observed using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results show that the fatigue properties of each region in the DMWJ are different. Fine-grained HAZs show a lower slop of fatigue curve than BMs. The fracture locations of HAZs occur at fusion boundaries, and this is related to the ferrite layer and carbon migration at the interface. The interface with ferrite layer is the weakest zone in whole DMWJ. After fatigue, dislocation density increases, and the dislocations are mainly entangled around the carbides, which enhances the fatigue properties of the welded joint. Coarse M7C3-type carbides in BL and WM are mainly concentrated on the subgrain boundary, and this causes fatigue crack propagation along the carbides. A fatigue fracture model associated with carbide is established to explain the cause and process of the fatigue crack propagation along carbides.