A comparison of the precipitation kinetics of γ″ particles in virgin and re-solutioned alloy 625

Abstract

Alloy 625 is used as superheater tubes and ammonia cracker tubes in many ammonia plants and in chemical industries. The components, made of this alloy are expected to have a life of about 100,000 hours at an operating temperature in the range of 450°C to 700°C. Since the availability of nickel is scarce and the alloy is very expensive, attempts are being made to resolution the material which has seen its service life and use it again. The data about the ageing behaviour of the resolutionised material under service conditions is necessary to determine the expected life of the component. The results of a detailed investigation on the kinetics of precipitation of intermetallic phases in the resolutioned material as well as virgin alloy are reported in this paper. The ageing behaviour of virgin and resolutioned material was found to be quite different. The distribution of γ″ precipitates was non-uniform and the precipitation was found to occur heterogeneously on dislocations in the resolutioned specimens. These observations are discussed in the light of redistribution of different alloying elements during service and their effect on subsequent precipitation kinetics of resolutioned material.