Creep life estimation of Gr.91 based on creep strain analysis

Abstract

The creep deformation behaviour and its effect on creep life have been investigated for Gr.91 by analyzing creep strain data in the NIMS Creep Data Sheet, 450 – 725°C and tr = 11:4 − 68,755 h. The creep and creep rate curves consist of transient region, where the creep rate decreases with time, and of accelerating region, where the creep rate increases with time after reaching a minimum creep rate. In the acceleration region, the logarithm of creep rate increases linearly with strain over a wide range. The creep life tr is inversely proportional to the minimum creep rate min times the increase in creep rate by strain d ln /dε in the acceleration region, as tr = c/[(min) (d ln /dε)]. The constant c is given by 1.5, which corresponds to an inverse of the ratio of duration of acceleration region to the creep life. The creep deformation behaviour in the acceleration region is closely correlated with that in the transient region. The parameter d ln /dε is correlated with the time to minimum creep rate tm and the strain to minimum creep rate εm, which characterize the creep deformation behaviour in the transient region. We obtain tr = 3.7 tm and tr = 2.4/(min/εm). This suggests that the creep life of Gr.91 is reasonably estimated by using the creep deformation parameters tm, min and εm, which can be evaluated by a short-time creep test for up to the end of transient region or for up to the beginning of acceleration region, corresponding to less than 30% of the creep life. It is concluded that the creep life of Gr.91 is reasonably estimated in short times by analyzing creep strain data without any stress extrapolation.