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, ε=ε₀ +ε_T [(1-exp(-r×t)] +ε_s .t, where ε₀ is the initial strain at time t=0, ε_T is the limiting transient creep strain, r is the rate of exhaustion of transient creep and ε_s is the steady state creep rate. The stress dependences of ε_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 ε_i to ε_s (i.e. ε_i=β× ε_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 ε_s , ε_T and time for onset of steady state creep t_os of the form ε_s^α×t_os / ε_T=constant was found to be valid; the value of αwas equal to unity.