Effect of Cyclic Stress Change on Creep Behavior in ASME P91 Steel

Effect of cyclic stress changes on creep rupture strength was investigated at 600°C for MGQ and MGS heat of ASME P91 steel. There was no large difference of creep rupture strength among the heats. However, the creep rupture ductility of MGS was lower than that of MGQ in the long-term. The initial creep stress and stress after stress reduction was 120 MPa and 24 MPa to 84 MPa, respectively. The time interval of stress reduction was 6 days for each test. For MGQ heat, no effect of stress reduction to 84 MPa on creep rupture strength was observed. The stress reduction to 60 MPa slightly increased time to rupture as compared to creep test under constant stress. A small amount of decrease in time to rupture was confirmed in case of stress reduction to 36 MPa.