Correlation of microstructure and mechanical properties with ultrasonic velocity in the Ni-based superalloy Inconel 625

Abstract Inconel 625 tubes are used extensively in ammonia cracker units of heavy-water plants. During service, the alloy is exposed to temperature close to 873 K for a prolonged period (about 60 000 h), leading to substantial decrease in ductility and toughness of the alloy due to heavy intragranular and intergranular precipitation. Service-exposed Inconel 625 material (873 K for about 60 000 h) was given post-service ageing treatments at different temperatures (923, 1023 and 1123K) up to 500 h. These heat treatments altered the microstructure, which in turn had an influence upon both the tensile properties and the ultrasonic velocity. The alloy that had seen service was solution annealed at 1423 K for 0.5 h followed by ageing at different temperatures (923 and 1123 K) and the influence of these heat treatments on changes in microstructures and in turn their effect on room-temperature tensile properties and ultrasonic velocity have been studied. The present study aims at establishing the correlation between room-temperature tensile properties and ultrasonic velocity with the microstructural changes that occurred during ageing treatments in Ni-based superalloy Inconel 625. For the first time, the present authors have demonstrated the influence of various precipitates, such as intermetallic phases γ″, Ni2(Cr, Mo) and δ, and grain-boundary carbides, on the correlation of yield strength and ultrasonic velocity.