Combined effects of phase transformation–induced PFZ and stress corrosion cracking on the failure of an Inconel 625 boiler tube

This study investigates the failure mechanism of an Inconel 625 alloy boiler tube that had been in service for approximately 18 months before failure. Numerous through-thickness cracks were observed in the weld heat-affected zone (HAZ). Microstructural analysis revealed that long-term exposure at elevated temperature induced phase transformations, which in turn altered the microstructure. Fine γ″ and Ni 2 (Cr,Mo) precipitates formed within the matrix and contributed to matrix strengthening, whereas a large number of Cr-rich carbides were observed along the grain boundaries. These boundary precipitates produced precipitation-free zones adjacent to the grain boundaries and created conditions similar to those associated with stress relaxation cracking. In addition, chloride was detected in the corrosion oxides present along both the main cracks and secondary branch cracks. The results indicate a failure mechanism in which precipitation-free zones formation caused by phase transformation acts in combination with stress corrosion cracking.