Characterization of untransformed ferrite in 10Cr and 12Cr ODS steels

Two new ferrito-martensitic oxide dispersion strengthened (ODS) steels reinforced with (Y, Ti, O) nanoparticles were elaborated using a high-energy attritor. The milled powder was consolidated by hot extrusion at 1050 degrees C. The two types of ODS steels differ by chromium content, with 10 wt% Cr and 12 wt% Cr respectively. According to thermodynamic calculations, those grades are supposed to exhibit an austenitic transformation at high temperatures. X-ray diffraction (XRD) above austenitic temperature transformation reveals the presence of both ferrite and austenite phase. This unexpected ferrite phase is assumed to be untransformed low temperature ferrite. The α → γ phase transformation specific enthalpy is monitored by differential scanning calorimetry (DSC). The untransformed ferrite fraction is calculated using dilatometric data and confirmed by electron backscatter diffraction (EBSD) microstructural analysis. The quenched samples from the austenitic domain give an image of the high-temperature partitioning. EBSD maps reveal two distinct elementary microstructures, one martensitic inherited from austenite and the other corresponds to the untransformed ferrite. This untransformed ferrite keeps the crystallographic alpha-fiber conferred by hot-extrusion. The 10 Cr ODS has equiaxed untransformed ferrite areas. In contrast, the untransformed ferrite into 12 Cr ODS is distributed as elongated areas, parallel to the hot-extrusion direction. Moreover, electron probe micro analyzer (EPMA) mapping exhibits chromium content gradients, consistent with phase partitioning at high temperatures. Creep properties are evaluated at 650 degrees C for both grades. Small-angle X-rays scattering (SAXS) shows a similar size and distribution of the oxide particles in both grades. (authors)