Optimization of the Fabrication Route of Ferritic/Martensitic ODS Cladding Tubes: Metallurgical Approach and Pilgering Numerical Modeling

Abstract : Oxide Dispersion Strengthened ferritic/martensitic alloys are developed as prospective cladding materials for future Sodium-Cooled-Fast-Reactors (GEN IV). These advanced alloys present a good resistance to irradiation and a high creep rupture strength due to a reinforcement by the homogeneous dispersion of hard nano-sized particles (such as Y2O3 or YTiO). ODS alloys are elaborated by powder metallurgy, consolidated by hot extrusion and manufactured into cladding tube using the pilger cold-rolling process. ODS alloys present low ductility and high hardness at room temperature which complicate their manufacturing as tubes. Moreover extrusion and rolling processes induce strong crystallographic and morphological anisotropies. The manufacturing by means of cold rolling passes implies intermediate softening heat treatments. Fabrication route optimization is needed to ensure a safe manufacturing and reduce the cladding tube anisotropy. A better understanding of the deformation paths, the behavior laws and the metallurgy of ODS materials is required. This paper presents the studies conducted on the optimization of the fabrication routes of the new ferritic / martensitic ODS tubes fabricated at CEA. Two main ODS alloys are considered, a Fe-9Cr-1W-Ti-Y2O3 ODS martensitic steel and a Fe-14Cr-1W-Ti-Y2O3 ODS ferritic alloy. The numerical simulation of the cold pilgering process leads to the determination of the strain path undegone by a material point. The determination of the ODS constitutive law and the development of the numerical model allow calculating the stress triaxiality during the process. A systematic analysis of all strokes helps defining which stroke could contribute mainly to the oligocyclic fatigue of the materials. The predicted residual stress state results in longitudinal and ortho-radial tensile stresses. Those results are discussed regarding the possible crack initiation and the subsequent propagation in the formed tube. In parallel to the numerical developments conducted to ensure the formability of the material, an optimization of the heat treatments is conducted with the objective of releasing internal stresses and reducing the stored energy. The 9Cr-ODS steel presents a phase transformation from ferrite to austenite which allows reducing the tube hardness. Manufacturing through two different fabrication routes allows comparing the effects of annealing temperature on cold-workability, microstructure evolution and mechanical properties. Microstructure control of 14Cr-ODS alloy, which does not present a phase transformation, is more complex and the control of the recrystallization microstructure appears critical. Different examples are shown and discussed.