Transformation of carbides and mechanisms for cracking in Co-based alloy surfacing layer during thermal fatigue

Abstract

A fantastic metallurgical bond of Stellite 6# Co-based alloy/FB2 steel joint was achieved using gas tungsten arc welding, and thermal cyclic tests were employed to investigate the initiation and propagation of cracks, and the microstructure evolution in the weldments. Multiple cracks with the length over 10 μm were found after 300 cycles of thermal fatigue (TF), and the detail microstructure of Stellite 6# layer before and after TF was investigated. Results showed that the thermal cycling led to the accumulation of thermal cyclic stress, coarsening of eutectic carbides, generation of laves phase and transformation of Co phase, which influenced the initiation and propagation of cracks. More specifically, the initiation of the cracks was attributed to newly generated W-rich laves phase in addition the with the accumulation of thermal cyclic stress, while Cr atoms, together with O and W atoms, were distributed near the cracks, and acted as the crack propagation path. Finally, the martensitic transformation of γ-Co to ε-Co, driven by the plastic deformation at the crack tip, could also in turn help to crack propagation.