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

Abstract

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°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 γ′′ and Ni₂(Cr,Mo), the loss in ductility resulted from the presence of carbides and needle shaped δ 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°C for 2 h was found to restore the microstructural as well as mechanical properties similar to that of virgin material.

Keywords:

• Alloy 625
• Ammonia cracker tube
• Microstructure
• Tensile properties
• Fracture toughness

Authors are thankful to Heavy Water Board, Mumbai, India, for providing service aged material used in the study. The authors wish to thank Director of Materials Group for his keen interest in this work.