Evolutions on Microstructure and Impact Toughness of G115 Steel after Long-Term Aging at 700 °C

The microstructure and impact toughness evolution of G115 steel after long-term (ranging from 500 h to 10,000 h) aging at 700 degrees C were investigated in this study. The results showed that the microstructure of the G115 steel evolved from a finer-grained matrix with minor precipitates to a coarse-grained matrix with more precipitate with aging time, presenting a decrease in the local deformation degree in the matrix. The impact toughness of the steel decreased with aging time, presenting the largest decline at the initial aging times. The decrease in impact toughness was attributed to the coarsening of precipitates (M23C6 and Laves phase) in the steel matrix. The stable impact toughness during the whole aging process (from 500 h to 10,000 h) should be related to the comprehensive effects, including the precipitation of the Laves phase, the increase in high-angle grain boundaries, and the softening of the metal matrix.