Abstract: In order to ensure the safety of the piping and equipment used in the thermal system, the creep rupture test is carried out at 570 ℃ on the P91 specimens with different hardness. The test results are fitted by the logarithmic functionfor safety evaluation and lifespan prediction of the P91 pipe, such that the overestimation on long-term creep rupture caused by isothermal extrapolation method can be avoided. The study results show that for low hardness P91 pipe fittings, the creep rupture of 570 ℃/1×10⁵ h tends to drop with the decrease of hardness. Especially when the hardness goes down below 165 HBW, the creep rupture strength of P91 steel will decrease below the material allowable stress, which means its long-term safe operation cannot be ensured. The P91 pipes in operation with low parameter settings have a larger strength margin, and its hardness is allowed to be lower to some extents. However, for the main steam pipeline operated at 570 ℃, especially without thickening designed bend, the strength margin is very low. Therefore, the safety evaluation and lifespan prediction is compulsory.

Key words: thermal power, thermal system, low hardness P91 pipe fittings, extrapolation of logarithmic function, tissue stability, safety evaluation, lifespan prediction