Effect of carbon on the microstructure and stress rupture properties of a new Ni-Cr-W-Fe alloy for advanced ultra-supercritical power plants

This paper studied the effect of carbon on the microstructure and stress rupture properties of a new Ni-Cr-W-Fe alloy. With carbon content increasing from 0.008 wt% to 0.073 wt%, the volume fraction and average size of MC and M23C6 carbides increased significantly. The morphology of MC remained dispersed blocky shape while the morphology of M23C6 changed from continuous thin strip to granular. But no obvious variations were observed in the quantity, size and morphology of γ′ phase. During heat treatment, the hot-rolled grains grew up quickly and the grain growth rate of 0.008% C alloy was almost three times higher than that of 0.073% C alloy. The reason was that the MC carbides effectively inhibited the grain growth. After stress rupture test at 750 °C/300 MPa and 750 °C/400 MPa, the stress rupture life was in parabolic curve relationship with carbon content, 0.034 wt% and 0.045 wt% C alloys had higher stress rupture lives than other alloys. With the increase of carbon content, the ductility increased firstly and then reached relatively stable status, the fracture mode gradually varied from intergranular fracture to intergranular dimple mixed fracture. The change of stress rupture properties with carbon content was closely related to carbides and the grain size, which would be analyzed in this paper.