On the Influence of Cold Rolling Parameters for 14CrW-ODS Ferritic Steel Claddings

Abstract

Oxide dispersion strengthened (ODS) steels, produced by powder metallurgy, are considered as promising material for high burn up cladding tubes for future Sodium Cooled Fast reactors. They present superior radiation resistance compared with austenitic steels and high creep strength due to reinforcement by the homogeneous dispersion of hard nano-sized particles. While the manufacturing route of 9Cr-based martensitic ODS steels are relatively well mastered thanks to the alpha  gamma phase transformation, the cold processing of ferritic ODS steels is more complicated because the material recovery after amounts of cumulative plastic strain is quite difficult. The aim of this study is to investigate several possible cold rolling routes for a Fe-14Cr-1W-0,3Ti-0,3Y2O3 ODS ferritic grade comparing the effects of annealing temperature on cold-workability, microstructure evolution and mechanical properties. A three-roll type HPTR rolling mill was used to manufacture ODS steel claddings. Cold rolling passes and intermediate annealing were repeated until reaching the final geometry: 10.73mm external diameter and 500µm thick. Depending of the cold rolling routes, different annealing temperatures of 1150°C, 1200°C and 1250°C were applied on the mother tube. Each pass was conducted using cross-section reduction ratio varying from at least 15% up to 25%. In each case, intermediate annealing at 1200°C for 1 hour were applied between one or several passes. The optical and SEM observations, hardness measurements, tensile tests were conducted to characterize the manufactured cladding tubes. The highest annealing temperature used on the mother tube enhances the recovery which leads to the lowest hardness level. The intermediate heat treatments applied in the course of the cold processing induces relatively low decrease of hardness. Microstructure characterization of hot extruded mother tubes shows highly anisotropic structures with equiaxed grains in the transverse direction but with significant elongation in the longitudinal direction. The elongated grain structure produced during hot extrusion is retained during cold rolling processes. Tensile tests are carried out on both longitudinal and circumferential directions by mean of respectively tile and ring tensile specimens for temperatures between 20°C and 700°C. The lowest is the annealing temperature applied on the mother tube the highest is the ultimate strength and the lowest is the uniform elongation. For the lowest annealing temperature, the UTS values measured at room temperature are ~1500MPa and ~1300MPa in the longitudinal direction and the circumferential direction, respectively. UTS values around 1000MPa in the both directions are found in case of lower annealing temperature showing a less pronounced anisotropy. For each test temperature, the uniform elongation values are relatively low compared to values obtained by other authors on 12%Cr-ODS ferritic steels. The lowest values of elongation are measured around 400°C.