Long-term microstructural stability and mechanical properties of 9Cr3W3Co heat-resistant steel for ultra-supercritical units

9Cr3W3Co heat-resistant steel has become a widely used material for key components in advanced ultra-supercritical thermal power units due to its excellent high-temperature properties and corrosion resistance. In this study, the steel was subjected to aging treatment at different temperatures and times, then the microstructure and mechanical properties were investigated. After long-term aging, Laves phase forms along martensite lath boundaries and prior austenite boundaries. With increasing aging time and temperature, the width of martensite lath and the size of M₂₃C₆ carbides increase. The Laves phase particles grow from 0.210 μm to 0.351 μm, and their morphology changes from regular short-rod shapes to irregular ones. The coarsening rate of Laves phase reaches a maximum at 630 °C. Moreover, hardness exhibits a slight decrease with increasing aging temperature and time. Plasticity, strength, and the growth and aggregation of precipitates all influence the impact properties. After 20,000 h of aging, both yield strength and elongation remain at relatively high levels, with impact energy values all exceeding 20 J. After 30,000 h of aging, the coarsening of precipitates has a more pronounced effect on impact toughness. In summary, 9Cr3W3Co heat-resistant steel maintains high stabilities of microstructure and property after long-term exposure at high temperature.