Microstructural and mechanical properties of service exposed Alloy 625 ammonia cracker tube removed after 100 000 h

The present study examined the evolution of microstructure and mechanical properties of Alloy 625 ammonia cracker tubes of heavy water production plant which had been exposed to temperatures around 600 degrees C for 100 000 h. Detailed investigations revealed considerable modification of microstructure, significant increase of strength and loss of ductility and toughness during service and the extent of degradations were found to be sensitive to service temperature at different locations of the tube. While the increase in strength was due to precipitation of gamma" and Ni-2(Cr,Mo), the loss in ductility resulted from the presence of carbides and needle shaped delta particles at grain boundaries. When the exposed material was tested at elevated temperatures, the material softened with an attendant increase in ductility and toughness. A solution annealing treatment at 1160 degrees C for 2 h was found to restore the microstructural as well as mechanical properties similar to that of virgin material.