Nuevas formulaciones de aceros ods de alto rendimiento
- Autores: Juan Alberto Meza Manzaneque
- Directores de la Tesis: Mónica Campos Gómez (dir. tes.), María Eugenia Rabanal Jiménez (codir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: José Manuel Torralba Castelló (presid.), Maria Nerea Ordas Mur (secret.), Cornelia Kaden (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
This Ph.D. thesis work, entitled “Nuevas formulaciones de aceros ODS de alto rendimiento” has been developed by Juan Alberto Meza Manzaneque under the supervision of professors Mónica Campos Gómez and María Eugenia Rabanal Jiménez, in the Powders Technology Group belonging to the University Carlos III of Madrid. This thesis work has also been inspected by the Materials’ Science and Engineering doctoral program from the same university.
This investigation has taken place from February 2017 to June 2021, under the projects of Ferro-Ness and AFORMAR by MINECO which funded this research under the National I+D+I program MAT2016-80875-C3-3-R and PID2019-109334RB-C32.
Analogously, this thesis work complies with all the necessary criteria for its mention as an International PhD. Such recognition is specified in the Ordination of Official University Education, as established in Article 15 of the Royal Decree 99/2011 (B.O.E. No. 35 of January 28, 2011, pp. 13909-13926) and as described in the ‘Rules and Regulations’ of PhD studies at Universidad Carlos III de Madrid.
The current world’s energetic situation affronts serious challenges to ensure electricity availability globally while avoiding its undesired catalysing effect over the climate change phenomenon. Because day by day, advanced economies, emerging markets and developing nations are requiring more and more access to electrical power, energy production needs to be able to satisfy this demand. However, nowadays, this energy production is heavily dependent on the use of fossil fuels, which, apart from being limited, are prejudicial to the environment due to their associated emission of greenhouse gases, primarily responsible for global warming.
To assess this problem, renewable energies have risen in the last years, but their complete application is still far from fruition. Thus, nuclear power would be helpful for this energy transition from fossil fuels to sustainable power production, together with the development of high-efficiency, low CO2 generation powerplants.
For these reasons, it is essential to design and improve the materials that will embody these advanced powerplants in terms of efficiency, security, and durability. The oxide dispersion strengthened (ODS) ferritic steels have been selected in this thesis work as the materials that better fulfil these requirements due to their enhanced mechanical behaviour at both room and high temperature (HT), corrosion resistance and irradiation stability.
Since the decade of the 1970s, the development of the ODS ferritic stainless steels (FS) started with the DT and DY alloys (13 %wt. Cr, adding TiO2 or Y2O3 as oxides’ formers respectively) prepared by CEN/SCK in Belgium, and the MA957 (14 %wt. Cr, Y2O3 as oxides’ formers) designed by the International Nickel Company (INCO). These steels incorporated their oxides’ formers by mechanical alloying (MA). Later efforts focused on the analysis of the Ti additions and how this element refined the nanometric Y2O3 oxides and affected the oxide dispersion parameters. Moreover, with the manufacturing of the 12YWT and the discovery of elevated precipitation of nanometric oxides, new studies and investigations have been arising and providing enhanced ODS ferritic steels up to the present day.
Currently, the peak-performance ODS FS include elevated oxides’ dispersion values (1022-1023 ox/m-3) with low nanometric oxides’ sizes (~2-5 nm). These parameters allow them to attain suitable mechanical properties such as balanced high strength/ductility, enhanced fracture toughness, and excellent high-temperature creep behaviour. Traditionally, these features have been achieved by employing processing routes that involve several steps: atomising, MA of the powders, canning, slab by hot isostatic pressing (HIP), hot rolling, additional cold rolling if necessary, and final heat treatments. In view of these complex routes, efforts are made nowadays to reduce the number of manufacturing stages and decrease the cost of the steels.
The fundamentals of oxide dispersion strengthening (ODS) reside mainly in the interactions between the oxide particles and the surrounding dislocations. The existence of extremely small uniformly dispersed particles of a second phase (oxide precipitates) within the ferritic matrix produces a distortion around the particles which, in the end, effectively impede the dislocations’ movement during plastic deformation or heating and thus, strengthen the steels. The smaller the size of the incoherent precipitates, the higher the strengthening as the bowing stress increases with decreasing particle radius. Also, having a higher volume fraction of particles develops into an improved strengthening. Moreover, another reason why a narrow distribution of precipitates is sought is the better swelling resistance achieved. Due to irradiation, He, which is insoluble in metals, will tend to constantly aggregate, forming bubbles in the particle/matrix interface thanks to transmutation reactions. Because of this, a more refined and denser dispersion of nano-oxides will lead to a better distribution of the small He bubbles, and therefore, less agglomeration of He will occur meaning that fewer microvoids will be available for the propagation of the prejudicial cracks. Consequently, the process stages involved in the precipitation, dispersion, and chemical composition of the nanometric oxides must be excelled to achieve the highest precipitates density of the refined particles.
To explain the causes that promote the precipitation of the nano-oxides inside the steels, one of the most accepted theories argues that when oxide-forming elements (such as Y, Ti, Zr) are added by means of a compound, then during the MA the elements are dissolved in the ferritic matrix and, later during consolidation, these precipitate as nanometric oxides taking part of the oxygen dissolved in the ferritic matrix. Usually, the best ODS ferritic steels contain oxides with sizes comprehended between 1-5 nm, while other ODS steels with precipitates sizes around 10-30 nm decrease their strengthening. Once the nanometric oxides have started to precipitate, they will continue to grow and coarse as both the consolidation’s temperature and time increase. Thus, the ideal scenario would be the one where the sintering process takes place in a swift way, with low sintering times and moderated temperatures to avoid excessive precipitates’ coarsening. However, in ODS ferritic steels developed by a PM route, the high sintering temperature plays a huge role in the decrease of the associated porosity. Taking this into account, the use of the spark plasma sintering (SPS) technique allows for the consolidation of the steels at high temperatures but with low sintering times that allow the achievement of high densifications, nonetheless, attaining this ideal scenario mentioned previously.
Of course, the composition of the oxides also plays a strong influence on the final size of the nanometric oxides. Traditionally, Y2O3 has been used as the precursor for the oxides’ precipitation due to its high melting point and low solubility into the ferritic matrix, and together with Ti additions, enable an increased refinement of the precipitated oxides (due to the lower interfacial energy between the oxide particles and the ferritic matrix).
However, the presence of Al in the ferritic matrix (between 3-5 wt.%), which enhances the corrosion behaviour, also partially decreases the effectiveness of the precipitates’ strengthening due to the formation of coarser Y-Al-O oxides which may potentially coarsen when the ODS steels’ components are exposed to the operating conditions. Thus, to counter this, Zr is usually introduced to promote the formation of smaller Y-Zr-O complex oxides instead of Y-Al-O particles, achieving superior high-temperature strength and excellent oxidation/corrosion resistance by controlling excess oxygen content.
Because these characteristics of the ODS steels are provided mainly through its stainless behaviour and through the homogeneous finely dispersed nanometric oxides incorporated into the ferritic matrix, which improve the mechanical properties by precipitates strengthening; then it is important to achieve an adequate composition of the steels and to attain a suitable manufacturing process, thus, obtaining a tailored microstructure able to provide a promising performance as candidates in future energy production.
From a compositional point of view, the selection of the alloying elements and their proportion has to be adequate so that all of them fulfil their purpose. The inclusion of Cr (typically between 12-16 %wt.) induces the formation of a Cr2O3 protective layer against corrosion and oxidation phenomena at HT, in addition, to enable the ferritic nature of the alloy. To further improve the corrosion resistance, Al is added to develop an extra Al2O3 layer; however, as explained before, it is important to control the amount of this element because it also forms undesired coarse nano-precipitates that do not strengthen the steels as effectively as the other more refined nanometric oxides. To adequately strengthen the steels by solid solution, promote their high-temperature strength, and also preserve the ferritic phase, W is introduced in the steels in a weight percentage comprehended in the range of 1-3 %wt. Besides, it is critical to remark the influence of the elements responsible for the precipitate’s formation because the composition of these will have a strong influence on their size and dispersion, thus altering the final strengthening by precipitates. The Y2O3 and the Ti will be used as the main precursors of the nanometric oxides, where Ti is in charge of refining the oxides’ size by promoting the precipitation of smaller Y-Ti-O type oxides. Furthermore, to address the issue of the coarser Y-Al-O precipitates formation, Zr is added to form preferentially fine Y-Zr-O precipitates and to refine the Al-containing oxides too Furthermore, the inclusion of these oxide formers as a whole unique compound, labelled as Y-Ti-Zr-O, has been investigated in this thesis work. Adding these elements in this way allows for more precise control on the oxides’ composition compared with the addition of these elements as pure.
The effect of B on the steels has also been studied. This element can segregate at grain boundaries and improve the creep properties of the steels, so although its detection is primarily difficult, it is possible to observe the effect that it produces in the mechanical performance.
Regarding the processing of these materials, the high energy milling stage of the starting powders, where the mechanical alloying takes place, is one of the keys to attain the final microstructures responsible for high-performance and competitive ODS steels. Usually, high energy ball mills are selected to fulfil this purpose, such as the attritor mills, where their rotor blades rotate, and the milling balls produce intense collisions between the initial powders and the grinding media. During this phase of the process, the powders endure heavy plastic strain due to the mechanical energy induced by the high energy mill and the balls inside it. In this stage and depending on the hardness and stiffness of the raw powdered material, the kinetic energy transmitted from the balls to the powders’ particles is substantial enough to plastically deform them into flat morphologies or fracture them. As milling time progress, those particles can mechanically weld themselves developing particles with larger sizes with a sandwich-like structure or with small fragments of harder particles trapped between more ductile flakes forming welded particles. As balls collisions increase, these particles harden becoming more brittle and thus, breaking into smaller particles with irregular shapes until an equilibrium is reached between the deformation, welding and fracture phenomena. At this point, the milling stage could be satisfactory and can be stopped. Thanks to this, the dislocations density is greatly increased strengthening the powders [94,95], while at the same time, the oxide formers are finely distributed through all the milled powders creating enriched environments which in the end will precipitate and form the nanometric oxides during the consolidation stage. Afterwards, the next step is to consolidate them. The consolidation/sintering stage is one of the most important stages in any powder metallurgy route, as it has a huge impact on the final density obtained in the processed components and, thus, the proper mechanical behaviour of these. The consolidation of the powders by SPS bases its operation in the simultaneous application of pressure and an electrical field responsible for the heating necessary to densify the powders. Taking advantage of the heating produced by Joule’s effect once the pulsed DC (direct current) passes through the powders, the particles sinter densifying the final material in the process while limiting excessive grain growth during the process.
The present thesis work had established as main objectives the processing of ODS steels through a PM route, and their examination in terms of microstructure, mechanical properties at RT and HT and surface stability. By varying the composition of the steels, 4 ODS ferritic steels have successfully been manufactured applying a MA on the powders followed by an SPS consolidation. The effect of the Y-Ti-Zr-O and/or B additions on the steels has been studied and discussed, showing remarkable variations that, overall, have proved to be beneficial for the development of next-generation ODS FS.
Regarding the starting powders and the new formulations of the ODS steels’ microstructure: The nanometric Y-Ti-Zr-O compound has been correctly synthesised through a co-precipitation method in water, with high segregation of the oxide formers in the complex oxide and without C contamination, generating a proper powder that has been incorporated in the MA.
All the developed ODS steels have been fully densified, have shown a bimodal grain distribution, and have developed a fine distribution of nanometric oxides thanks to the favourable processing parameters selected in the PM route, which have proved to be less complex than other manufacturing routes.
The inclusion of B in the steels has affected the morphology of the carbides formed in the grain boundaries by making them rounder, which has enhanced the mechanical behaviour at RT and has also improved the creep properties.
The steels with Y-Ti-Zr-O additions have attained increased nano oxides densities (from 1.5·1022 to 5.5·1022 ox/m3) thanks mainly to the presence of Ti and Zr, being this last element the principal responsible for refining the nanometric oxides (down to 3-10 nm). However, the formation of Y-Al-O type precipitates could not be completely avoided with the Zr inclusions.
An assessment of the thermal stability of the nanometric precipitates has been developed by in-situ TEM explorations. These have evinced that after a 550 °C annealing, no changes have occurred in the position, morphology, and size of the oxides, dislocations, or grains. Thus, the steels have shown their ability to properly behave at working temperatures.
Concerning the mechanical performance of the ODS steels: The achieved microstructures in the ODS steels have positively affected their final mechanical performance. The microhardness and UTS values at RT have been incremented in the compositions with the Y-Ti-Zr-O compound (14Al-X-ODS and 14Al-X-ODS-B), mainly due to the enhanced nano oxides densities.
Likewise, the SP tests have shown the remarkable performance of the Y-Ti-Zr-O-containing steels, especially at HT when these steels have proved to equal or even surpass the behaviour of other commercial steels like the GETMAT alloy.
Besides, an enhancement of the strength/toughness ratio has been achieved as a result of the attained bimodal grain distribution, delivering ODS steels with increased ductility. This feature is particularly notable when contrasting it with the general poor ductility attained in other commercial ODS steels processed in the industry.
The most remarkable feature of the ODS steels manufactured in this investigation has been their creep resistance. The SPCT performed at 650 °C with different loads have revealed the excellent behaviour against creep in the ODS steels that incorporated both the Y-Ti-Zr-O and the B, achieving outstanding times to rupture (> 1300 h with an applied load of 250 N) and diminished deflection rates compared with other commercial steels.
These solid results in the mechanical behaviour have also demonstrated the benefits of their development through a PM route by saving processing stages while achieving mechanical properties improved at the same time.
About the surface stability of the steels that have been studied: Due to the presence of adequate proportions of Cr and Al in the ferritic matrix, the ODS steels have disclosed a superior resistance to oxidation at HT. Even after the tests have been carried out for 600 h at 900 °C, the alumina scale formed on their surface has proved its effectiveness at protecting the steels.
Analogously, the steels have also resisted at moderate temperatures the surface degradation and liquid metal embrittlement (LME) provoked by the metallic Pb and LBE alloy coolants, in agreement with the wetting angle measurements performed.
