MODELING OF THE THERMAL AND METALLURGICAL BEHAVIOR OF STEELS

Mechanical analysis of nuclear components may require consideration of residual stresses resulting from manufacturing processes such as weld ing or heat treatment. Although correct assessment of the quenching or welding thermal evolutions undergone a delicate problem, it is essent ial that the structural analysis involve the metallurgical phenomena i nduced by these operations. For the ferritic steel welding, the most i mportant phenomenon is the structural transformations which modify som e thermo-mechanical properties and induced residual stresses. We propo se in this paper a model of cooling-induced structural transformation in the form of a thermo-metallurgical constitutive equation which rema ins 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 t he model is based of the Continuous Cooling Transformations diagrams ( C.C.T.) and the Koistinen-Marburger's law of the considered steels. Af ter validation, this model has been introduced in the finite element c ode of the ''Direction des Etudes et Recherches d'E.D.F.'' (Code Aster ). With taking into account the mechanical consequences of the structu ral transformations, it allows to simulate the development of welding residual stresses such as presented in the application.