Creep constitutive equations for a 0.5Cr–0.5Mo–0.25V ferritic steel in the temperature range 600–675°C

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Abstract
Constitutive equations have been developed to describe the creep behaviour of a 0.5Cr-0.5Mo-0.25V ferritic steel under uniaxial and multi-axial states of stress, as well as over the temperature range of 600–675°C. This material has widespread application as the parent steam pipe in the power generation industry. An accurate description of creep behaviour has been achieved by formulating constitutive equations that describe the physical mechanisms of deformation and rupture. To this end, two state variables were introduced to model intergranular creep cavitation and the coarsening of carbide precipitates respectively. The influence of temperature on the constitutive equations is also considered and activation energies are identified for each of the material constants that show a notable temperature dependence. Numerical optimization methods are discussed for the determination of the material constants and of the corresponding activation energies. The constitutive equations provide a good representation of the experimental creep data. The limitations on the range of validity of the equations are explained with reference to the mechanisms of material deformation and rupture. The description of the creep behaviour is completed by the determination of the constant that defines the multi-axial stress rupture criterion of the material. It is argued that the constitutive equation set is suitable for the analysis of engineering components.

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