Constitutive equation describing tertiary creep of a modified 9Cr-1Mo steel

Abstract

Creep tests has been conducted of a Mod.9C-1Mo steel to describe the creep behavior on a basis of the omega method. The test conditions were in the range from 107.9 to 196.1 MPa and from 873 to 923 K. The stress and temperature dependence of the Ω value was expressed in the form: Ω=AΩ·σΩ-nΩ·exp(QΩ/RT), where AΩ is the constant independent of temperature and stress, σo is the initial stress, nΩ is the stress exponent for Ω. QΩ is the parameter describing the temperature dependence of Ω, respectively. The magnitudes of AΩ, nΩ, QΩ are 0.15/s, 1.46 and 97.8 kJ/mol, respectively. The studies on the effect of prestrain showed that the magnitude of Ω was substantially independent of the amount of prestrain. The measured creep life was compared with the predicted one based on the omega method. It was revealed that the calculated creep life was longer than the measured one by a factor of 1.5 to 3.1 with stress and temperature. This discrepancy was ascribed to the fact that the fraction of primary creep was relatively large in the whole creep life of a Mod.9Cr-1Mo steel. However, the calculated creep life coincides with the measured one when the specimens were prestrained up to 4%. This was due to the decrease of the fraction of primary creep with increasing strain.