The relationship between microstructure evolution and impact toughness degeneration of alloy 617B during long term aging

Impact toughness degeneration and microstructure evolution of alloy 617B during 650–725°C long-term aging for up to 12,000 h were investigated in present work. It is found that impact toughness decreases apparently even after just 20 h aging in the temperature range of 650–725°C and keeps decreasing with prolonging aging time. Hence, detailed microstructure observation and mechanical property tests were carried out to reveal the essence of impact toughness degeneration during long-term aging. And the reason is confirmed to be the combination effect of plasticity evolution and grain boundary carbide coarsening. The degeneration of impact toughness with prolonging aging time can be divided into rapid decrease stage, slow decrease stage and stable stage. The decrease of plasticity in the major reason for rapid impact toughness degeneration in the initial stage of aging. Grain boundary carbide coarsens obviously during aging and tends to be continuous. This is the major reason for the slow and continuous decrease of impact toughness in the slow decrease stage. Finally, plasticity and grain boundary carbide change little, resulting in the stable stage of impact toughness evolution. Enough attention should be paid on impact toughness degeneration in the evaluation of long-term stability.