STEADY STATE CREEP DEFORMATION BEHAVIOUR OF 9Cr-IMo FERRITIC STEEL FORGING IN QUENCHED AND TEMPERED CONDITION

Constant load isothermal creep tests have been carried out to assess the influence of stress (60-300 MPa) and temperature (773-873 K) on the steady state creep deformation behaviour of 1000 mm diameter and 300 mm thick 9Cr-lMo ferritic steel forging in quenched and tempered condition. Applied stress dependence of steady state creep rate obeyed Nortonʹs power law and exhibited 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 also reflected 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, the two stress exponents and two apparent activation energy values in the two stress regimes have been rationalized using modified Dorn equation. Following Lagneborg and Bergman, the resisting stress has been determined from the creep data. The variation of resisting stress with applied stress and temperature indicated that at low stresses resisting stress is insensitive to test temperature and increases linearly with applied stress with proportionality constant of 0.77. On the other hand, at high stresses the resisting stress remains constant equal to threshold stress and decreases with increasing test temperature. Incorporating resisting stress in both low and high stress regimes into modified Dorn equation, a single slope behaviour with stress exponent of about 4 and a true activation energy of about 250 kJ mol^-1 have been obtained. These observations suggest that the steady state creep deformation in both the stress regimes is climb controlled.