Microstructural characterization of ODS ferritic steels at different processing stages

Nanostructured Oxide Dispersion Strengthened Reduced Activation Ferritic Stainless Steels (ODS RAF) are promising structural materials for fusion reactors, due to their ultrafine microstructure and the presence of a dispersion of Y-Ti-O nanoclusters that provide excellent creep strength at high temperatures (up to 750 degrees C). The traditional powder metallurgical route to produce these steels is based on Gas Atomization (GA) + Mechanical Alloying (MA) + HIP + ThermoMechanical Treatments (TMTs). Recently, alternative methods have arisen to avoid the MA step. In line with this new approach, ferritic stainless steel powders were produced by gas atomization and HIPped, after adjusting their oxygen, Y and Ti contents to form Y-Ti-O nanoclusters during subsequent heat treatments. The microstructure of as-HIPped steels mainly consists of ferrite grains, Y-Ti precipitates, carbides and oxides on Prior Particle Boundaries (PPBs). Post-HIP heat treatments performed at high temperatures (1270 and 1300 degrees C) evaluated the feasibility of achieving a complete dissolution of the oxides on PPBs and a precipitation of ultrafine Ti- and Y-rich oxides in the Fe14Cr2W matrix. FEG-SEM with extensive EDS analysis was used to characterize the microstructure of the atomized powders and the ODS-RAF specimens after HIP consolidation and post-HIP heat treatments. A deeper characterization of atomized powder was carried out by TEM. (C) 2015 Elsevier B.V. All rights reserved.