Resisting stress of a low-alloy ferritic steel after creep exposure in service

Creep data generated on the materials in the service exposed and the virgin states of a 1 Cr-0.3 Mo-0.25 V steel and also on materials with a wide range of microstructures prepared from the virgin state, have been analysed to determine the resisting stress. The analysis shows that for a given microstructure, as the applied stress increases and reaches a critical value called the transition stress, the creep rate increases sharply from a region of low stress sensitivity to a region of high stress sensitivity. Correspondingly, the resisting stress which increases with applied stress, reaches a threshold value. The results show that, as expected, the transition stress and also the resisting stress in the high stress sensitivity region, depend on the microstructure. However, a remarkable observation is that the resisting stress in the low stress sensitivity region is practically independent of the initial microstructural states investigated. Also the threshold stress is observed to be about 0.5 times the hot yield strength. Based on these observations, a simple method for creep life prediction is proposed.