Analysis of Monkman-Grant relationships and damage tolerance factor for creep of 25Cr35NiNb micro-alloyed steel

Abstract

Creep behavior of high Cr austenitic stainless steel was investigated at temperatures ranging from 650 to 1050 °C and stress levels in the range 47–120 MPa. Primary, secondary and tertiary creep strains were analyzed in terms of minimum creep rate, constant true stress and temperature. Minimum creep rates, strain to rupture and rupture time were analyzed in terms of Monkman-Grant and modified Monkman-Grant relation. Good correlation between minimum creep rate and rupture time using both Monkman-Grant relationships is observed for the creep data on the steel. Validity of the relationship between time to reach Monkman-Grant ductility tMGD and rupture life tr in terms of damage tolerance factor λ was established for the steel. Effect of temperature on microstructure and carbide content of the steel is presented in this paper. Microstructural studies reveal failure mechanisms by nucleation and growth of cavities around carbide particles at grain boundary regions. Based on the critical continuum damage mechanics approach, the ratio of tMGD to tr express by constant fCDM separated safe and unsafe regimes of creep behavior of the steel.