Evolution in Microstructure and Mechanical Properties of Inconel 783 Alloy Bolts after Long Term High-Temperature Aging at 700 °C

Abstract

This paper described systematically the changes in microstructure and mechanical properties of Inconel 783 alloy after a considerably long time (equivalently 55,000 h, about 76.4 months) of thermal exposure. Based on the Inconel 783 alloy bolts of an intermediate pressure main stop valve used in a 1000 MW ultra-supercritical unit, the evolution of microstructures and mechanical properties were studied after 700 °C aging temperature with different aging times (1000 h, 3000 h and 20,000 h, corresponding to about 1.4 months, 4.2 months and 27.8 months, respectively), using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffractometer (XRD), a universal tensile testing machine and impact testing machine. The results indicated that the bolts aged for 1000 h in two temperatures, showing the second needle β phase, of which the quantity and size obviously increased with aging time. Meanwhile, the characteristics in quantity and shape of the primary β phase changed obviously with the aging time, which transformed to strip the Ni5Al3 and Laves-Nb-rich brittle phase in the matrix after aging for 20,000 h. The size of the γ’ phase grew bigger with aging time, and orientation distributions have been observed obviously at 3000 h aging in 700 °C. Compared with the 650 °C aging temperature, the coarsening of γ’ precipitates and second needle β, the orientation distributions of γ’ were more obvious at the 700 °C aging temperature with aging time, which resulted in the rapid decline in yield strength and tensile strength and obvious increase in the brittleness for Inconel 783 alloy bolts.