Changes in Monkman-Grant relation among four creep regions of modified 9Cr-1Mo steel

The relation of creep rupture life to minimum creep rate, namely Monkman-Grant (MG) relation is examined by using a creep database of modified 9Cr-1Mo steel. The database covers wide ranges of creep rupture life up to 1.2 × 10 ⁵ h and of minimum creep rate down to 7.6 × 10 ⁻⁸ h ⁻¹ . The MG plot exhibits a low overall value of MG exponent (p = 0.85) together with wide scatter of data points at long rupture life. Causes of the low p value and the wide scatter are discussed in the present study paying attention to change in creep curve shape with creep rupture life. There are four regions H, M, L ₁ and L ₂ showing different creep behavior in the steel. Regional MG relations are examined also in each region. Values of p are less than unity and change among the four regions. The p value is especially low (0.62) in long-term region L ₂ . The regional p values less than unity result in the low overall p value of 0.85. The especially low regional value of p in the long-term region causes the wide scatter of data points in the overall MG plot. A modified Monkman-Grant equation (Eq. (29)) is proposed taking account of creep deceleration rate A (=−1/(dln ε˙/dε), where ε˙ and ε are the creep strain rate and the creep strain) in primary creep and creep acceleration rate B (=1/(dln ε˙/dε)) in tertiary creep. The rates characterize shape of a creep curve. The modified equation can explain the difference of p values among the four regions.