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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 …