Rejuvenation of the microstructure and mechanical properties of a service-exposed IN939 superalloy by heat treatments.

Effect of heat treatment on the recovery of microstructure and mechanical properties of a service-exposed IN939 superalloy was studied. Four-stage heat treatment was performed on the service-exposed alloy. The microstructures of the service-exposed and rejuvenated alloys were examined by optical and scanning electron microscopes. The hardness, tension, and stress-rupture tests were carried out to characterise the mechanical properties. The results showed that the heat treatment could rejuvenate the microstructure of the alloy that was deteriorated during the service at high temperatures. Decomposed MC carbides were transformed to new fine carbides, continuous M23C6 carbides were dissolved and new discontinuous carbides along the grain boundaries were regenerated and, finally, the fine γ׳ particles were reformed. The microstructural recovery resulted in an increase in the hardness and ultimate tensile strength of the alloy. The results also showed that the creep behavior of the alloy at a testing temperature of 850°C could be improved by the heat treatment. [ABSTRACT FROM AUTHOR]