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.
Highlights
With inlet temperatures of ultra-supercritical steam turbines being up to 600/620 ◦ C, high temperature component design has become a major issue
The chemical compositions of the bolts were measured by a spark source atomic emission spectrometer (SPECTROMAXx, SPECTRO GmbH, Kleve, Germany), the evolution of the microstructure was characterized by an optical microscope (OM, Axio Oberver.D1m, Carl Zeiss GmbH, Jena, Germany), scanning electron microscope (SEM, Axio Oberver.D1m, FEI Company, Hillsboro, OR, USA), energy dispersive spectroscopy (EDS, EDAX Inc., Mahwah, NJ, USA) and transmission electron microscope (TEM, TECNAI F30, FEI Company, Hillsboro, OR, USA)
The secondary needle β phase was observed within the grain boundary for samples after aging 1000 h at 700 ◦ C (Figure 3e), as well as at 650 ◦ C (Figure 3b)
Summary
With inlet temperatures of ultra-supercritical steam turbines being up to 600/620 ◦ C, high temperature component design has become a major issue. 66.7 months) service time, Han et al [11] designed the experimental parameters for Inconel 783 alloy up to 5000 h (about 6.9 months) at 750 ◦ C, and Tao et al [16] selected Inconel 783 bolt samples with service times of 0, 7 and 60 months (about 43,200 h) in a power plant 660 MW Ultra supercritical unit to study the microstructural evolution and mechanical properties. The bolts of a intermediate pressure main stop which had served 35,000 h (about 48.6 months) in an 1000 MW ultra-supercritical unit were used, and the characteristics of microstructure and mechanical properties were investigated systematically with aging time in 700 ◦ C, compared with the 650 ◦ C aging temperature. This research work is devoted to providing valuable information for the actual service, and to investigate the evolution of performance of Inconel 783 alloy bolts with long term high temperature aging
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