Influence of intermetallic phase precipitation during prolonged service in alloy 625 on its properties

Abstract

The results of a microstructural investigation on Alloy 625 which has undergone prolonged service at temperatures close to but less than 600°C are reported in this paper. A uniform distribution of precipitates of an intermetallic phase, Ni2(Cr,Mo) with Pt2Mo type structure has been observed in addition to that of the γ″ phase for service periods exceeding 28,000 h. The Ni2(Cr,Mo) phase has been found to dissolve on ageing at temperatures above 600°C. The dissolution of this phase has been noticed to result in a reduction in hardness and impact properties of the alloy. The occurrence of Ni2(Cr,Mo) phase in this alloy has been discussed by taking the alloy chemistry into consideration. Apart from these intermetallic phases, precipitation of M6C carbide particles within the austenite matrix and the formation of a near continuous film comprising discrete M6C/M23C6 carbide precipitates at grain boundaries have been noticed in the alloy after prolonged service. Energy Dispersive Spectroscopy(EDS) analysis has revealed that the majority of the carbides precipitating at the grain boundaries at temperatures around 600°C are of M23C6 type. The influence of the intermetallic and the carbide phase precipitates on the performance, properties and life of Alloy 625 is also discussed in this paper.