Effect of powder outgazing conditions: On mechanical and microstructural properties of oxides dispersed strengthened steel

Abstract : Oxides Dispersed strengthened (ODS) stainless steels are foreseen for fuel claddingtubes in the coming generation of fission sodium cooled nuclear reactors. In spite of abody-centered matrix, those steels present a convenient creep behavior thanks to veryfine oxides dispersion. Those grades are currently obtained by Powder Metallurgy (PM).After mechanical alloying with the oxide, the powder is commonly consolidated asseamless tube by Hot Extrusion (HE). Before this operation the powder is canned andoutgazed in order to reduce the interaction with nitrogen and oxygen of ambient air. Thecontrol of the mechanical properties after extrusion is a key issue for this graderegarding service conditions. Therefore, effect of processing conditions must be takeninto account. This study focuses on microstructural and mechanical characterization ofthree 14wt%Cr ODS material elaborated with various outgazing conditions before HE.Those materials are tested in term of tenacity (using Charpy impact tests) and microhardness. In the same time microstructural characterization by X-Ray tomography revealthe occurrence of unexpected large phases whose densities is correlated withmechanical properties. EPMA characterizations are driven in order to prospect thecomposition of those phases. Precipitates on Prior Particles Boundaries (PPB) are alsostudied as they play a crucial role on the coalescence of creep damage voids. Finally,effects of outgazing conditions are summarized regarding both microstructural andmechanical properties of consolidated materials.