Creep deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel in the temperature range 773-873 K

Creep tests have been performed to examine the creep deformation and ruptur e behaviour of forged thick section 9Cr-1Mo ferritic steel in quenched and tempered (Q+T), simulated post weld heat treatment (SPWHT) and thermally ag ed (TA) conditions in a wide range of stresses (50-300 MPa) and temperature s (773-873 K). Applied stress dependence of steady state creep rate obeyed Norton's power law and exhibited two slope behaviour with stress exponents of similar to 5-6 at low stresses and 10.2 at high stresses. This is also r eflected in two apparent activation energy values of 266 and 468 kJ mol(-1) in the low and high stress regimes, respectively. Invoking the concept of resisting stress into Dorn type creep relationship, a single slope behaviou r with a true stress exponent of similar to 4 and a true activation energy of 250 kJ mol(-1) have been obtained. These observations suggest that the c reep deformation in both the stress regimes is climb controlled. Stress dep endence of rupture life exhibited two slope behaviour. This is also reflect ed in the two constants in the Monkman-Grant and modified Monkman-Grant rel ationships and two creep damage tolerance factors in the two stress regimes . The high creep ductility of the alloy increased with increase in temperat ure. The failure mode remained transgranular. At 793 K, no significant diff erence in the creep-rupture properties was noticed between Q+T, SPWHT and T A conditions, while at 873 K, SPWHT specimens exhibited inferior creep-rupt ure strength than that of Q+T specimens. The extensive tertiary creep obser ved in the alloy has been attributed to microstructural degradation associa ted with precipitates and dislocation substructure. The creep-rupture stren gth of the forging has been found to be lower than that of thin section bar s and tubes.