The effect of reheat treatments on recovering the creep behavior of a corrosion-resistant nickel-based superalloy

The present work is an attempt to possibly obtain the most suitable reheat treatment, which could provide the optimal microstructural characteristics for creep-exposed superalloys. After creep at 850 °C/350 MPa for 96 h and 140 h a cast nickel-based superalloy was reheat-treated with different programs, and then the specimens were subjected to creep again at the same condition. Creep tests indicate that during reheat treatment applying aging at lower temperature, 850 °C, for proper time (4 h) before aging at 1050 °C rather than a repeated standard heat treatment is beneficial to restore creep life. Creep-induced changes in the microstructure, such as excessive grain-boundary secondary M23C6 and/or M6C carbides formation, and primary MC carbides decomposition, are noticeably more advanced in the reheat-treated alloys. A quantitative study on the dissolution of σ phases and the formation of cavitations is presented. After the interrupted creep, the γ′ particles at dendrite cores remained cubic-shaped. However, the rafting of cuboidal γ′ precipitates was observed in the same creep condition by the reheat treatment programs. Design of a reheat treatment for recovering the creep properties should emphasize the promotion of ductility rather than strength of the alloy. An equation based on the continuum damage-mechanics theory, was suggested to evaluate the remaining creep life.