A strain ε vs. time t equation taking account of tertiary as well as primary and secondary creep stages, that is, ε=ε₀+A(1-e^-αt)+B(e^αt-1) was proposed and applied to the creep data of a 1Cr-1Mo-1/4V steel. Here, ε₀, A, B and α are constants which vary with stress and temperature. This equation described sufficiently well the creep curves up to the tertiary stage.
The constants in the equation were determined by the least square regression method and their stress and temperature dependences were examined. The constants relating to strain, ε₀, A and B, were invariable with temperature, suggesting that their values obtained experimentally can be adopted directly in long-term creep curve prediction at lower temperatures under the same stress. The time constant α was found to be given as a simple function of minimum creep rate whose stress and temperature dependences have been well established, so that the α value can be extrapolated to longer time with high accuracy.
The equation proposed in this study was able to represent creep curves up to the tertiary stage, and furthermore the stress and temperature dependences of the constants in it were very favorable for long-term creep curve prediction. Thus, the equation is considered to be more useful than any other equation proposed so far for this purpose.