Chemical and microstructural evolution on ODS Fe–14CrWTi steel during manufacturing stages

Abstract

Oxide Dispersion Strengthened (ODS) steels are promising candidate materials for fission and fusion applications thanks to their improved properties related to both their fine grained microstructure and high density of Y–Ti–O nanoscale clusters (NCs). The Fe–14Cr–1W–0.3Ti–0.3Y2O3 ODS ferritic steel was produced by powder metallurgy: Iron-base gas atomized powders were mechanically alloyed with 0.3% Y2O3 particles in an attritor. Then, the ODS powders were encapsulated in a soft steel can, consolidated by hot extrusion and cold rolled under the shape of tube cladding.The present work investigates the evolution of the chemical composition and the microstructure after each stage of the fabrication route (i.e. mechanical alloying, extrusion and cold rolling). Chemical analysis indicates a significant increase of the carbon content and a moderate increase of oxygen and nitrogen after mechanical alloying compared to initial atomized powders. After extrusion, the measured oxygen content corresponds mainly to the oxygen coming from yttria addition during MA process. In addition, electron microprobe analyses are performed after hot extrusion to determine the concentration and the distribution of the constitutive elements (Cr, Ti, W, Y, O). The microstructure was investigated by transmission electron microscopy (TEM) and small angle neutron scattering (SANS) in order to characterize the size distribution of Y–Ti–O particles. TEM results reveal a fine microstructure (average grain size of 600nm in the transverse direction) including Y–Ti–O NCs with a mean diameter close to 3nm after extrusion. A slight coarsening of Y–Ti–O NCs is evidenced by SANS after cold rolling and heat treatments.