Interesting relationships for creep deformation and damage and their applicability for 9Cr-1Mo ferritic steel

The paper presents the validity of several interesting relationships examined for better understanding of creep behaviour of 9Cr-1Mo ferritic steel. Creep rate-rupture time relationships of Monkman-Grant type have been found to be valid. Like stress dependence of creep rate and rupture life, both Monkman-Grant and modified Monkman-Grant relations (MGR and MMGR) exhibited distinct constant values of CMG and CMMG, respectively for low and high stress regimes. The validity of MGR and MMGR is a consequence of the creep deformation behaviour of 9Cr-1Mo ferritic steel obeying first order kinetics. On the basis of creep rate-rupture time relationships of Monkman-Grant type, several other relationships involving transient and tertiary creep parameters have been evolved and their applicability have been examined for the steel. Analogous to MGR and MMGR, a relationship involving transient creep parameters and the other involving tertiary creep parameters were found to be valid. Further, 9Cr-1Mo steel obeyed a recently introduced critical damage criterion interrelating time to reach Monkman-Grant ductility with rupture life, and the criterion depends only on creep damage tolerance factor. This unique relationship is evolved based on the seminal concept of time to reach Monkman-Grant ductility as the time at which the useful safe creep life is exhausted and damage attains a critical level. The important implications of this concept have been discussed