Microstructure and creep fracture behavior in HR3C/T92 dissimilar steel welds

Abstract This paper investigated the microstructure and creep fracture characteristic of the HR3C/T92 dissimilar steel welded joint using Ni-based filler metal. After crept for 2415.75 h, the joint preferentially fractured at the FGHAZ of T92 side in welded joint, not at the interface between T92 steel and Ni-based WM, which exhibited type IV cracking. The type IV fracture was triggered by the accumulation of cavity damage, the number of cavities was relatively few within the 0.4 tf in the FGHAZ of T92 side, but increased significantly from the 0.6 tf to 0.8 tf. After 0.8 tf, the creep cavities rapidly coalesced into cracks and growth, leading to brittle fracture. In the CGHAZ of T92 side close to the fusion line, the formation of blocky δ-ferrite phases was observed, which is responsible for the diffusion of carbon atom from the CGHAZ to WM/CGHAZ interface during welding. The hardness of δ-ferrite is much lower than that of martensitic matrix of CGHAZ, however, the creep damage preferentially takes place in the FGHAZ instead of the CGHAZ with formation of blocky δ-ferrite. It can be recognized that the CGHAZ with a few isolated δ-ferrite blocks near the interface is not the weakest zone for the early failure of the HR3C/T92 dissimilar steel welded joint compared with the FGHAZ.