Creep-rupture behavior of forged, thick section 9Cr-1Mo ferritic steel

Abstract

Detailed investigations have been performed to examine the creep-rupture behavior of a 1000-mm diameter and 300-mm-thick tube plate forging of 9Cr-1Mo ferritic steel in quenched and tempered (Q+T), simulated postweld heat treatment (SPWHT), and thermally aged (TA) conditions. Creep tests were conducted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy exhibited well-defined primary, steady-state, and extended tertiary creep stages at all test conditions. At 793 K, no significant difference in the creep-rupture properties was noted between Q+T, SPWHT, and TA conditions. On the other hand, SPWHT specimens exhibited lower creep-rupture strength than that of Q+T specimens at 873 K. Applied stress (σ _a ) dependence of rupture life (t_r ) exhibited two-slope behavior. Both the Monkman-Grant (ε _s×t _r=C _MG) and modified Monkman-Grant (ε _s×t _r /ε _f=C _MMG) relationships were found to be valid for 9Cr-1Mo steel, where ε_s is the steady-state creep rate and ε _f is the strain to failure. The two-slope behavior was also reflected as two constants in the Monkman-Grant relationship (MGR) and modified Monkman-Grant relationship (MMGR) in the two stress regimes. Further, two creep damage tolerance factors (λ=1/C _MMG) of 5 and 10 were also observed in the high and low stress regimes, respectively. The alloy exhibited high creep ductility, which was retained for longer rupture lives at low stresses, and the creep ductility increased with increase in test temperature. The failure mode remained trangranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure. The creep-rupture strength of the forging was found to be lower than that of thin section bars and tubes.