Effect of Long Term Aging on Fracture Resistance of Modified 9Cr-1Mo (P91) Steel

Tensile and Fracture properties (J1c or fracture toughness and J-R curves) of Modified 9Cr-1Mo steel was evaluated after long term aging in the temperature range 550-650 °C and 5000-20,000 h duration and tested at 400 and 550 °C. Decrease in yield strength (YS) and ultimate tensile strength (UTS) was observed after aging at 650 °C (10,000 h) and tested at 400 °C. Decrease in % Total elongation was observed after 20,000 h aging duration (all aging temperatures) and both test temperatures. The strength levels (YS and UTS) obtained after all aging conditions and test temperatures are higher than the minimum specified tensile properties as per relevant codes, ASME and RCC-MRX. Microstructures observed after aging revealed presence of MX, M23C6 and Laves type of precipitates. It was observed that area fractions of all precipitates combined increased after aging. Decrease in J1c and Tearing Modulus was observed after 10,000 h aging duration (all aging temperatures) and tested at 400 and 550 °C. Fractographic observations revealed decrease in stretch zone width after 20,000 h aging duration (all aging temperatures) and tested at 400 °C. Tensile properties and fracture results are explained based on changes in microstructure.