Modélisation du comportement thermo-métallurgique des aciers

Abstract
Mechanical analysis of nuclear components may require consideration of residual stresses resulting from manufacturing processes such as welding or heat treatment. Although correct assessment of the quenching or welding thermal evolutions undergone a delicate problem, it is essential that the structural analysis involve the metallurgical phenomena induced by these operations. For the ferritic steel welding, the most important phenomenon is the structural transformations which modify some thermo-mechanical properties and induced residual stresses. We propose in this paper a model of cooling-induced structural transformation in the form of a thermometallurgical constitutive equation which remains compatible with the usual space scale of computational mechanics. The chosen framework is that of thermodynamics of internal variables. The pilot variable of the model is the temperature. Thus we also take account as variable its first time derivative. The other variables (so-called internal variables) describe the metallurgical state of steel. We choose here the proportions of the metallurgical component present at a given point, the austenitic grain size resulting of the heating and the martensitic starting temperature (Ms). The identification of the model is based of the Continuous Cooling Transformations diagrams (C.C.T.) and the Koïstinen-Marbürger's law of the considered steels. After validation, this model has been introduced in the finite element code of the "Direction des Etudes et Recherches d'E.D.F." (Code Aster). With taking into account the mechanical consequences of the structural transformations, it allows to simulate the development of welding residual stresses such as presented in the application.