ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation, HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y2O3 powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y-Ti-O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe-14Cr-2W-0.3Ti-0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y-Ti-O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300 degrees C. Specimens HIPped at 900 and 1220 degrees C were heat treated (HT) at temperatures ranging from 1200 to 1320 degrees C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.