Transient Creep Behaviour of Forged Thick Section 9Cr-1Mo Ferritic Steel

Abstract

The results of constant load creep tests on quenched and tempered 9Cr-1Mo ferritic steel in the temperature range 773-873 K and in the stress range 60-275 MPa are analysed in the framework of first order kinetics for transient and steady state creep. The strain-time data was adequately described by the Garofalo relation, e=e 0 +e T [(1-exp(-r×t)] +e s .t, where e 0 is the initial strain at time t=0, e T is the limiting transient creep strain, r is the rate of exhaustion of transient creep and e s is the steady state creep rate. The stress dependences of e s and r exhibited two slope behaviour. The results in both the stress regimes obeyed first order kinetics for transient creep, but with different values of β= 15 and 4.1 for low and high stress regimes respectively, where β is a constant and is the ratio of initial creep rate e i to e s (i.e. e i =β× e s ). Separate master creep curves relating transient and steady state creep were obtained in the respective stress regimes. Further, analogous to Monkman-Grant and modified Monkman-Grant relationships, the relationship between e s , e T and time for onset of steady state creep t os of the form e s α ×t os / e T =constant was found to be valid; the value of αwas equal to unity.