Nueva ruta pulvimetalúrgica de producción de aceros inoxidables ferríticos de baja activación endurecidos por dispersión de óxidos (ODS-RAF) para su aplicación en futuros reactores de fusión nuclear

Resumo

Oxide dispersion strengthened (ODS) ferritic stainless steel alloys are being considered for structural components within future generation nuclear power reactors (fusion reactors and generation IV fission reactors (GEN IV)) and Ultra Supercritical Coal-Fired Reactors (USC). These alloys have demonstrated superior high temperature strength and creep resistance, as well as high resistance to swelling, helium embrittlement and irradiation creep. Low additions of yttrium form dispersoids with high temperature stability, whereas titanium additions promote further refinement of the resulting oxide-dispersoid size. Moreove,r W is a solid-solution strengthener for the iron-chromium matrix phase. The improved mechanical properties are a direct result of a very fine grain size, which can be developed through a series of thermo mechanical treatments. This grain size is stabilized due to the formation of finely spaced nanometric oxide precipitates. All this prevents movements of dislocations. Thus, the functionality of an ODS alloy relies on developing a microstructure that contains a distribution of finely spaced and highly stable nanometric oxide particles. Conventional route of these ferritic ODS steels consists on the mechanical alloying of the elemental powders or gas atomized prealloyed powders where Y2O3 or yttrium elemental or alloyed with other elements is introduced in order to generate precursor oxide dispersion forming particulate. This is followed by encapsulation, hot isostatic pressing (HIP) or hot extrusion consolidation and different thermo-mechanical treatments for the formation of the fine grain structure and very fine Y-Ti-O nanoclusters dispersion. However, mechanical alloying step involves several drawbacks such as contamination from grinding media and jars, resulting in the increase of interstitials elements (O, N, C) associated to long milling times and grinding atmosphere, and batch to batch heterogeneities. In order to avoid this step, the so-called Gas Atomization Reaction Synthesis (GARS) method was developed at AMES Laboratory (USA). This process is a simplified way to obtain steel powders which will generate oxide dispersion during further steps of the manufacturing route. In this thesis the feasibility of a new processing route of ferritic ODS steels inspired by GARS method is shown. Ferritic stainless steel powders already containing the oxide-dispersion formers (Fe-14Cr-2W-(0.3-0.56)Ti-(0.18-0.37)Y) were produced by inert gas atomization. In this way, mechanical alloying step is avoided. The atomization process and their parameters influence on powder characteristics are deeply studied. ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) is used to determine the exact composition of powders, especially titanium and yttrium contents. The retention of these elements turn out to be highly dependent on atomizer vacuum level. Low temperature oxidation of atomized steel powders (100-375 °C) was applied to adjust oxygen content to value equivalent to Y2O3 additions and the optimal parameters are selected to achieve this target. It is verified that the oxidation with these parameters follow a logarithmic rate, which enables robust adjustment of the required oxygen values. This step yielded the growth of an ultrathin metaestable Fe-rich oxide layer on the particles surfaces that acts as O reservoir for the formation of Y-Ti-O nanoclusters during subsequent stages of processing course. XPS technique (X-Ray Photoelectron Spectroscopy) allows the study of the superficial oxide evolution during oxidation process and the identification of each oxide. Moreover, the superficial and internal microstructure of the as-atomized and oxidized powder was observed with FEG-SEM (Field-Emission Gun Scanning Electron Microscope) and TEM (Transmission Electron Microscopy) to identify differences between them. On the other hand, oxidation kinetics studies revealed a logarithmic rate law. After oxidation, HIPping was carried out at different temperatures to select the optimum temperature for dissociating oxides located at PPBs (Prior Particle Boundaries). If HIPping carried out at elevated temperatures, Y-Ti-O nanoclusters were formed by dissociation of the oxide layer. Oxygen from this layer become free and diffuses from PPBs to the internal prior particle (PP) matrix where react with yttrium and titanium. Heat treatment was performed to as-HIPped material in order to study the evolution of precipitates on PPBs and the interior of PP. FEG-SEM and TEM were used to evaluate microstructural features after consolidation by HIP and post-HIP thermal treatments. Besides, EBSD (Electron Backscatter Diffraction) technique was applied to study changes in the grain size with temperature. Moreover, yttrium and titanium evolution, from atomization to post-HIP heat treatments, was studied by X-Ray Absorption Spectroscopy at Latvia University. Finally, hot rolling was carried out (at KIT (Karlsruhe Institute of Technology)), to refine grain size and improve nanoparticle dispersion achieving better mechanical properties than for previous step material. Tensile tests of machined samples of hot rolled material were performed at temperatures between 20-700 °C so as to study strength and ductility of final material. These properties provide a deep insight into the feasibility of applying this material on different type of reactors. It has been shown the viability to obtain ferritic ODS steels by proposed new processing route, without mechanical alloying.