Microstructure and mechanical properties of Inconel 625 superalloy

Abstract The service-exposed (∼60 000 h/873 K) Alloy 625 ammonia cracker tubes showed higher strength and lower ductility compared to the virgin material in the solution annealed state. Precipitation of intermetallic γ″ and Ni2(Cr,Mo) phases and the inter and intragranular carbides were found to be responsible for higher strength of the service-exposed alloy. Subjecting the service-exposed alloy to thermal aging treatments subsequently at 923 K and 1123 K (above the service temperature of the exposed alloy) led to the dissolution of the intermetallic phases that in turn increased the ductility of the alloy. Post-service aging of the alloy at 923 K for short durations resulted in the dissolution of the Ni2(Cr,Mo)-phase. The dissolution of the Ni2(Cr,Mo)-phase exhibited significant influence upon yield strength (YS) but negligible effect on ductility. Prolonged aging of the alloy for 500 h at 923 K resulted in the precipitation of intermetallic δ-phase. Post-service aging of the alloy at 1123 K promoted the dissolution of both Ni2(Cr,Mo) and γ″ formed during service. Longer duration aging at the same temperature led to the precipitation of the δ-phase with an associated increase in strength and loss in ductility. Re-solution annealing of the service-exposed alloy at 1423 K caused the dissolution of the strengthening phases. When the re-solution annealed alloy was subjected to prolonged exposure at 923 K, the yield stress was found to increase rapidly with aging time with attendent loss in ductility due to the precipitation of γ″.