Steady state creep deformation behaviour of 9Cr-1Mo ferritic steel forgingin quenched and tempered condition

Constant load isothermal creep tests have been carried out to assess the in fluence of stress (60-300 MPa) and temperature (773-873 K) on the steady st ate creep deformation behaviour of 1000 mm diameter and 300 mm thick 9Cr-1M o ferritic steel forging in quenched and tempered condition. Applied stress dependence of steady state creep rate obeyed Norton's power law and exhibi ted two slope behaviour with stress exponents of about 5-6 at low stresses and 10.2 at high stresses at all temperatures. The two slope behaviour is a lso reflected in two apparent activation energy values of 266 and 468 kJ mo l(-1) in the low and high stress regimes, respectively. Invoking the concep t of resisting stress, the two stress exponents and two apparent activation energy values in the two stress regimes have been rationalized using modif ied Dorn equation. Following Lagneborg and Bergman, the resisting stress ha s been determined from the creep data. The Variation of resisting stress wi th applied stress and temperature indicated that at low stresses resisting stress is insensitive to test temperature and increases linearly with appli ed stress with proportionality constant of 0.77. On the other hand, at high stresses the resisting stress remains constant equal to threshold stress a nd decreases with increasing test temperature. Incorporating resisting stre ss in both low and high stress regimes into modified Dorn equation, a singl e slope behaviour with stress exponent of about 4 and a true activation ene rgy of about 250 kJ mol(-1) have been obtained.. These observations suggest that the steady stare creep deformation in both the stress regimes is clim b controlled.