Microstructure and mechanical properties of Inconel 625 superalloy

The service-exposed (similar to 60 000 h/873 K) Alloy 625 ammonia cracker t ubes showed higher strength and lower ductility compared to the virgin mate rial in the solution annealed state. Precipitation of intermetallic gamma " and Ni-2(Cr,Mo) phases and the inter and intragranular carbides were found to be responsible for higher strength of the service-exposed alloy. Subjec ting the service-exposed alloy to thermal aging treatments subsequently at 923 It and 1123 It (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 Ni-2(Cr,Mo)phase. The dissolut ion of the Ni-2(Cr,Mo)-phase exhibited significant influence upon yield str ength OCS) but negligible effect on ductility. Prolonged aging of the alloy for 500 h at 923 K resulted in the precipitation of intermetallic F-phase. Post-service aging of the alloy at 1123 K promoted the dissolution of both Ni-2(Cr,Mo) and gamma " formed during service. Longer duration aging at th e same temperature led to the precipitation of the delta -phase with an ass ociated increase in strength and loss in ductility. Re-solution annealing o f the service-exposed alloy at 1423 K caused the dissolution of the strengt hening phases. When the re-solution annealed alloy was subjected to prolong ed exposure at 923 K, the yield stress was found to increase rapidly with a ging time with attendent loss in ductility due to the precipitation of gamm a ". (C) 2001 Elsevier Science B.V. All rights reserved.