Effect of Grain-Boundary Fe₂Nb Phase on Stress-Assisted Grain-Boundary Oxidation Behavior in Novel Austenitic Heat-Resistant Steel of Fe-20Cr-35Ni-2.5Nb

Many Ni-base superalloys are susceptible to intergranular brittle fracture during holding under high stresses at relatively low temperatures, known as stress-assisted grain boundary oxidation (SAGBO). These superalloys are strengthened by geometrically close-packed precipitates such as Ni₃Al (L1₂) and Ni₃Nb (D0₂₂) within grain interiors, whereas role of grain-boundary precipitates in strengthening of the alloys is limited. It is thus interesting to know how the presence of grain-boundary precipitates affects the SAGBO behavior. Recently Takeyama et al. has developed a novel austenitic heat-resistant steel Fe-20Cr-35Ni-2.5Nb (at.%) strengthened by intermetallic phases of topologically close-packed Fe₂Nb Laves precipitated at grain boundaries, together with Ni₃Nb within grain interiors. The creep rupture strength of the steel at 800 °C is comparable to Ni-base alloys, and the higher the area fraction ρ of the grain boundaries covered by the Laves phase, the lower the creep rate, thereby leading to the superior strength. This strengthening mechanism is called “Grain-boundary Precipitation Strengthening (GBPS)”. In this study, the relationship between the GBPS and SAGBO behavior has been investigated using the novel steels based on Fe-20Cr-35Ni-2.5Nb. Two specimens with similar strength but different ρ values were prepared. Constant-load tests were conducted at 90% of the yield stress and 600 °C in air. Rupture time and rupture elongation become larger in high ρ specimen. Ductile morphologies were observed on the fracture surfaces in the high ρ specimen, whereas intergranular surfaces were more obvious in the low ρ specimen. The results clearly demonstrate that GBPS is effective in suppressing susceptibility to SAGBO under high load condition.