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

Detailed investigations have been performed to examine the creep-rupture be havior 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 tre atment (SPWHT), and thermally aged (TA) conditions. Creep tests were conduc ted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy ex hibited well-defined primary, steady-state, and extended tertiary creep sta ges at all test conditions. At 793 K, no significant difference in the cree p-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 (sigma(a)) dependence of rupture life (t(r)) exhibited two-slope behavior. Both the Monkman-Grant (( epsilon) over dot(s). t(r) = C-MG) and modified Monkman-Grant ((epsilon) ov er dot(s). t(r)/epsilon(f) = C-MMG) relationships were found to be valid fo r 9Cr-1Mo steel, where (epsilon) over dot(s) is the steady-state creep rate and gis the strain to failure. The two-slope behavior was also reflected a s two constants in the Monkman-Grant relationship (MGR) and modified Monkma n-Grant relationship (MMGR) in the two stress regimes. Further, two creep d amage tolerance factors (lambda = 1/C-MMG) Of 5 and 10 were also observed i n the high and low stress regimes, respectively The alloy exhibited high cr eep ductility, which was retained for longer rupture Lives at low stresses, and the creep ductility increased with increase in test temperature. The f ailure mode remained trangranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradat ion associated with precipitates and dislocation substructure. The creep-ru pture strength of the forging was found to be lower than that of thin secti on bars and tubes.