The evolution of microstructures and the variation of impact toughness of Grade 92 steel aged 200,500, 800, 1000 hours at 700 °C were studied in this paper. The results show that the impact energy of Grade 92 steel was reduced significantly with the aging time increase. After 200 h aging, the value of impact energy decreased about 10%; then the impact energy variation trends to be stable with 500 h aging. When the aging time increase to 800 h, the impacts energy decreased about 26% compared with the original sample; and then the variation trends stable with 1000 h aging. The precipitation and growth of M23C6 Phases at the prior austenite grain boundaries and martensitie lath boundaries during aging are major reasons of the impact energy decrease. 200 h aging, M23C6 carbides precipitate at the grain boundaries. Laves phase was discovered after 500 h aging, then the M23C6 and Laves phase coarsening with the aging time. This made the impact toughness degradation more.

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