Type 304 stainless steel is used in various applications where corrosion resistance is required. This material is selected for weldability and corrosion resistance, but it can suffer stress corrosion cracking (scc), corrosion fatigue, pitting and crevice corrosion in chloride environments. The aim of this project was to produce Type 304 containers with intentional stress corrosion cracks, to serve as test samples for future weld repair trials. A test rig was constructed which used thermal stress to crack Type 304 tube samples; a central Type 310 stainless steel bar contained a heating element, so that the bar serve as a heat source and a stressing element. The rig was filled or half filled with magnesium chloride solution. The elastic strain in the tube sample was directly related to the temperature difference between the central bar and the sample. The thermal stress was sufficient to cause stress corrosion cracking. The tests were terminated when the first crack extended through the wall thickness of the 304 tube sample. The distribution and depth of cracks were determined after the tests. The test procedure caused the formation of multiple cracks in the tube sample. The temperature controller caused cyclical variation in the bar temperature and hence in the thermal stress. However, the temperature variation did not have an effect on cracking; corrosion fatigue did not contribute to cracking, and the cracks had the classic branched transgranular morphology of chloride cracking. Copyright / Dissertation (MSc)--University of Pretoria, 2009. / Materials Science and Metallurgical Engineering / unrestricted

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<p>The high temperature microstructure of the solid phases within the electric arc furnace (EAF) slag has a large effect on the process features such as foamability of the slag, chromium recovery, consumption of the ferroalloys and the wear rate of the refractory. The knowledge of the microstructural and compositional evolution of the slag phases during the EAF process stages is necessary for a good slag praxis.</p><p>In <strong>supplement 1</strong>, an investigation of the typical characteristics of EAF slags in the production of the AISI 304L stainless steel was carried out. In addition, compositional and microstructural evolution of the slag during the different EAF process stages was also investigated. Computational thermodynamics was also used as a tool to predict the equilibrium phases in the top slag as well as the amount of these phases at the process temperatures. Furthermore, the influence of different parameters (MgO wt%, Cr₂O₃ wt%, temperature and the top slag basicity) on the amount of the spinel phase in the slag was studied. In <strong>supplement 2</strong>, a novel study to characterize the electric arc furnace (EAF) slags in the production of duplex stainless steel at the process temperatures was performed. The investigation was focused on determining the microstructural and compositional evolution of the EAF slag during and at the end of the refining period.</p><p>Slag samples were collected from 14 heats of AISI 304L steel (2 slag samples per heat) and 7 heats of duplex steel (3 slags sample per heat). Simultaneously with each slag sampling, the temperature of the slag was measured. The selected slag samples were studied both using SEM-EDS and LOM. In some cases (<strong>supplement 2</strong>), X-ray diffraction (XRD) analyses were also performed on fine-powdered samples to confirm the existence of the observed phases.</p><p>It was observed that at the process temperature and at all process stages, the stainless steel EAF slag consists mainly of liquid oxides, magnesiochromite spinel particles and metallic droplets. Under normal operation and at the final stages of the EAF, 304L steelmaking slag contains 2-6 wt% magnesiochromite spinel crystals. It was also found that, within the compositional range of the slag samples, the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content. Petrographical investigation of the EAF duplex stainless steelmaking showed that, before FeSi-addition, the slag samples contain large amounts of undissolved particles and the apparent viscosity of the slag is higher, relative to the subsequent stages. In this stage, the slag also includes solid stoichiometric calcium chromite. It was also found that, after FeSi-injection into the EAF and during the refining period, the composition and the basicity of the slag in the EAF duplex steelmaking and EAF stainless steelmaking are fairly similar. This indicates that, during the refining period, the basic condition for the utilization of an EAF foaming-slag praxis, in both austenitic and duplex stainless steel cases, is the same. Depending on the slag basicity, the slag may contain perovskite and/or dicalcium silicate too. More specifically, the duplex stainless steel slag samples with a higher basicity than 1.55 found to contain perovskite crystals.</p>

The current US fleet of nuclear reactors has been in service for three decades. Over this period, existing welds in stainless steel (SS) shrouds have sustained stress corrosion cracking (SCC) and are in need of repair. Additionally, helium has formed interstitially as a byproduct of proton bombardment. Current repair technology, such as TIG welding, puts extreme amounts of heat into the material and allows for interstitial helium atoms to aggregate and form bubbles/voids at grain boundaries. This significantly weakens the material, proving to be a very counterproductive and ineffective repair technique. Much study has been done on friction stir processing (FSP), but none has explored it as an enabling repair technology for use in nuclear applications. Because of its relatively low energy input as a solid state joining technology, it is proposed that FSP could effectively heal SCCs in these existing welds without the negative side effect of helium bubble formation. A spread of speeds and feeds were initially tested using a PCBN-W-Re tool on 304L SS. Six of these parameter sets were selected as representations of high, medium, and low temperature-per-power outputs for this research: 2 IPM-80 RPM, 2 IPM-150 RPM, 4 IPM-150 RPM, 4 IPM-250 RPM, 6 IPM-125 RPM, and 6 IPM-175 RPM. These varied parameter sets were tested for their tensile, micro-hardness, and corrosion resistant properties. In general, the lower IPM and RPM values resulted in higher ultimate tensile strengths (UTS). Higher IPM and RPM values resulted in tunnel, pin hole, and surface void defects. These defects caused premature failure in tensile tests and could often be identified through microscopy. Micro-hardness testing demonstrated a strong correlation per the Hall-Petch relationship – finer grain sizes resulted in higher yield strength (hardness values) of the material. The tool temperature during FSP was a good indicator of the expected hardness – lower temperatures resulted in higher hardness values. Corrosion testing was performed with a 1000-hour alternate immersion test in a room temperature 3.5% NaCl solution. With these testing parameters, the results demonstrated that FSP had no effect on the corrosion resistance of 304L SS under these conditions.

Continuing an investigation on using FSP to heal stress corrosion cracks (SCC) in welds on nuclear reactors, this study seeks to use AM in addition to FSP to aid crack repair. Previous studies address that current repair technology on nuclear reactors involves the use of TIG welding which can allow helium atoms to aggregate and form voids at the grain boundaries. This weakens the material and renders the repair ineffective. Another previous study evaluated the effectiveness of FSP alone in repairing SCC which did have defects depending on the parameters used during FSP. This study evaluated the use of AM in addition to FSP. Literature is available on FSP and AM separately and literature is available on technologies that used both them together. However, the current processes that are available that use both AM and FSP can be expensive and may be impractical for some purposes. This study shows a new process that is both less expensive and more practical in SCC repair. Initial proof of concept trials was performed on 1018 mild steel using both wire fed additive and insert additive technologies. A slot would be removed and filled in with an additive process and processed using FSP. Because of poor repeatability, substantial distortion, and voids present this study went forward using insert technologies in further experiments rather than wire wed additive technologies. In addition, the depth and width of the insert or area where the added material would be placed was varied in initial trails. Tensile testing was performed on initial steel trials and the stainless steel experiments and it demonstrated a correlation between depth of the added material and the tensile strength. Micro-hardness mapping performed on initial steel trials also showed hardening in the FSP stir zone. Three-point bend tests were performed to show that an existing crack underneath the FSP zone would not propagate through the nugget. All evaluations supported a substantial increase in yield strength increased after FSP.

Les cermets, nommés M(p)-CMC(s), « Ceramic Matrix Composite dispersed with Metallic Particles », sont prometteurs dans les applications à haute température. Un cermet modèle (304L-YSZ) constitué de 40 % vol. de particules d'acier inoxydable 304L dispersées dans une matrice de zircone partiellement stabilisée à l’yttrium a été préparé par métallurgie des poudres. Les cermets sont homogènes et possèdent une densité voisine de 97 % par rapport à la densité théorique. Une étude cinétique basée sur l’hypothèse de l’étape limitante, a été réalisée afin de proposer un mécanisme et un modèle d’oxydation du cermet. Dans un premier temps, l’oxydation de la poudre d’acier inoxydable a été étudiée sous oxygène à 800 °C. Les expériences d'oxydation sous He (80 %)-O2 (20 %) ont été conduites par thermogravimétrie. Conformément à la littérature, la cinétique d’oxydation de la poudre suit un régime de diffusion. Une couche d’oxyde à base de chrome à croissance externe est observée.En revanche, la cinétique d’oxydation du cermet modèle est différente de celle de la poudre de 304L. Un fort gain de masse est observé dès l’introduction de l’oxygène au début du palier isotherme. Pendant le palier isobare, la prise de masse est lente et n’est pas régie par la diffusion. La caractérisation des cermets oxydés montre que des nodules riches en chrome apparaissent tout d’abord à l’intérieur du métal. Puis une couche d’oxyde à base de chrome et de fer se forme par croissance externe. La matrice de zircone ne limite pas la diffusion de l’oxygène vers le métal. Elle se fissure sous l’action des contraintes mécaniques induites par le changement de volume des particules de 304L pendant l’oxydation. / Ceramic matrix composite dispersed with metallic particles, called M(p)-CMC(s), are being developed for optimizing several functions of industrial components in severe atmospheres at high temperature.The corrosion of a model M(p)-CMC(s), based on 304L stainless steel and yttrium doped zirconia (304L(p)-ZrO2(s); 40%/60% in volume) is studied and compared with the oxidation behaviour of the stainless steel powders. The oxidation behaviour of the model 304L(p)-ZrO2(s) composite produced by powder metallurgy, studied by means of the thermogravimetry under 20 % O2 in helium at 800 °C, is presented. Oxidation curves show a fast increase in mass gain followed by slow one for the composite material. SEM observations and Auger spectroscopy measurements of the oxidized composite indicate an outward complex Cr and Fe rich oxide layer whereas Cr2O3 nodules are observed to nucleate and develop inward. For 304L stainless steel powder, the shape of the mass gain curve is parabolic, in agreement with a diffusion controlled oxidation. SEM observations of oxidised powder and in situ XRD measurements at 800 °C under oxygen show an external growth of Cr2O3 oxide layer.The low resistance to oxidation of the composite (compared to the powder) in the initial period seems to be due to the properties of the zirconia/metallic particles interface obtained after the sintering process. Under reducing conditions, the initial Cr2O3 layer reacts with zirconia matrix. TEM observation of the “as sintered” interface between the metallic particles and the ceramic shows no chromia layer. Sudden changes in oxygen partial pressure during experiments reveal an accelerating effect of the oxygen pressure.

La recristallisation, qui peut se produire de façon dynamique ou statique, est un important phénomène qui transforme la microstructure des matériaux métalliques déformés modifiant ainsi ses propriétés mécaniques. Malgré l'existence de travaux approfondis sur la modélisation numérique du phénomène de recristallisation, la littérature scientifique manque de modèles précis capables de prédire l'évolution microstructurale dans des conditions de mise en forme multi passes. Bien que des efforts aient été réalisés dans cette direction, la plupart des modèles existants dans la littérature présentent soit un manque de validation expérimentale, soit ne fournissent que des accords qualitatifs entre les résultats numériques et expérimentaux dans des conditions de déformations connues et sélectionnées. De plus, les relations entre la recristallisation statique (SRX), la recristallisation dynamique (DRX), la recristallisation post-dynamique (PDRX) et la croissance de grains (GG) sont généralement trop simplistes. Par ailleurs, la plupart de ces modèles ne sont pas conçus pour la réalisation de simulations avec des conditions thermiques et/ou mécaniques variables et limite par conséquent leur utilisation pour des applications industrielles. Pour cette étude, un modèle à champ moyen 2 sites a été développé afin de décrire l'évolution microstructurale de l'acier 304L. L'originalité de ce modèle réside dans : (a) l'interaction de chaque grain avec deux milieux homogènes équivalents, avec respectivement une densité de dislocation élevée et faible; (b) le poids relatif de chaque milieu est lié à leur fraction volumique ; (c) la germination et la disparition des grains rendent la microstructure variable au cours du temps ; (d) les paramètres dépendent de la température et de la vitesse de déformation mais pas de la taille des grains dans les conditions DRX, et uniquement de la témpérature dans les conditions statiques (SRX/PDRX/GG); (e) des accords quantitatifs avec les résultats expérimentaux sont obtenus en fonction de (i) la cinétique de recristallisation, (ii) la courbe contrainte-déformation, (iii) taille de grain après recristallisation, et (f) le modèle a été développé pour être utilisé en conditions multi- passes, avec des valeurs variables de température et de vitesse de déformation. Afin de vérifier et valider le modèle, plusieurs essais de tractions ont été réalisés sous de nombreuses conditions différentes de température et de vitesse de déformation, afin de caractériser le mode DRX. Pour la vérification de la SRX et PDRX, des traitements de recuit ont été réalisés après la déformation plastique, respectivement à froid et à chaud. Les paramètres du modèle ont premièrement été estimés à partir des donnés expérimentales ou présentes dans la littérature, et ont ensuite été établis par analyse inverse. Il a été constaté que tous les paramètres du modèle évoluent de manières physiquement cohérentes en fonction de la température et de la vitesse de déformation. Les résultats obtenus à partir de la simulation de la DRX, SRX/PDEX/GG ont été analysés, en prenant en compte les effets de la température de déformation, la vitesse de déformation, la déformation appliquée ainsi que la taille de grain initiale. Un bon accord entre les résultats numériques et expérimentaux a été observé pour les différents types de recristallisation, ce qui ouvre la voie à la modélisation de la mise en forme en conditions multi passes pour des applications industrielles. Finalement, des traitements thermiques avec analyse in situ ont été réalisés afin d'obtenir une meilleure compréhension des mécanismes de SRX/PDRX/GG. Le rôle du maclage pendant le traitement de recuit a été discuté : il semble favoriser à la fois la germination et la migration des joints de grains.

The thesis is focused on the technology of manufacturing of helical flute in the frame servomotor of austenitic chromium-nickel steel with improved machinability. One part of this thesis is a theoretical analysis of stainless steel and more detailed analysis of machined steel SANMAC 304L. The thesis also analyzes the current way of machining in the company VUES Brno s.r.o. and suggests new innovative ways of manufacturing helical flute. Innovative proposals include a technological procedure and a choice of instruments and tools for turning and milling. The means of manufacturing are evaluated from an economic and technical point of view. Conclusion of this thesis contains the assessment of the current way of machining with innovative designs.

The high temperature microstructure of the solid phases within the electric arc furnace (EAF) slag has a large effect on the process features such as foamability of the slag, chromium recovery, consumption of the ferroalloys and the wear rate of the refractory. The knowledge of the microstructural and compositional evolution of the slag phases during the EAF process stages is necessary for a good slag praxis. In supplement 1, an investigation of the typical characteristics of EAF slags in the production of the AISI 304L stainless steel was carried out. In addition, compositional and microstructural evolution of the slag during the different EAF process stages was also investigated. Computational thermodynamics was also used as a tool to predict the equilibrium phases in the top slag as well as the amount of these phases at the process temperatures. Furthermore, the influence of different parameters (MgO wt%, Cr2O3 wt%, temperature and the top slag basicity) on the amount of the spinel phase in the slag was studied. In supplement 2, a novel study to characterize the electric arc furnace (EAF) slags in the production of duplex stainless steel at the process temperatures was performed. The investigation was focused on determining the microstructural and compositional evolution of the EAF slag during and at the end of the refining period. Slag samples were collected from 14 heats of AISI 304L steel (2 slag samples per heat) and 7 heats of duplex steel (3 slags sample per heat). Simultaneously with each slag sampling, the temperature of the slag was measured. The selected slag samples were studied both using SEM-EDS and LOM. In some cases (supplement 2), X-ray diffraction (XRD) analyses were also performed on fine-powdered samples to confirm the existence of the observed phases. It was observed that at the process temperature and at all process stages, the stainless steel EAF slag consists mainly of liquid oxides, magnesiochromite spinel particles and metallic droplets. Under normal operation and at the final stages of the EAF, 304L steelmaking slag contains 2-6 wt% magnesiochromite spinel crystals. It was also found that, within the compositional range of the slag samples, the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content. Petrographical investigation of the EAF duplex stainless steelmaking showed that, before FeSi-addition, the slag samples contain large amounts of undissolved particles and the apparent viscosity of the slag is higher, relative to the subsequent stages. In this stage, the slag also includes solid stoichiometric calcium chromite. It was also found that, after FeSi-injection into the EAF and during the refining period, the composition and the basicity of the slag in the EAF duplex steelmaking and EAF stainless steelmaking are fairly similar. This indicates that, during the refining period, the basic condition for the utilization of an EAF foaming-slag praxis, in both austenitic and duplex stainless steel cases, is the same. Depending on the slag basicity, the slag may contain perovskite and/or dicalcium silicate too. More specifically, the duplex stainless steel slag samples with a higher basicity than 1.55 found to contain perovskite crystals. / QC 20110413

Les matériaux métalliques sont utilisés dans des conditions de plus en plus sévères et doivent présenter une parfaite intégrité sur des périodes de plus en plus longues. L’objectif de ce travail de thèse est d’évaluer le potentiel d'un traitement de refusion laser pour améliorer la résistance à la corrosion d'un acier inoxydable de type 304L ; l’utilisation du laser dans le domaine des traitements de surface constituant un procédé en pleine évolution à cause des changements récents dans la technologie des lasers. Dans le cadre de ce travail, le choix du laser s’est porté sur un laser nano-impulsionnel à fibre dopée ytterbium dont les caractéristiques permettent la fusion quasi-instantanée sur quelques microns de la surface traitée, immédiatement suivie d'une solidification ultra-rapide avec des vitesses de refroidissement pouvant atteindre 1011 K/s. La combinaison de ces processus favorise l'élimination des défauts surfaciques, la formation de phases hors équilibre, la ségrégation d’éléments chimiques et la formation d’une nouvelle couche d’oxyde dont les propriétés sont gouvernées par les paramètres laser. Afin de les corréler avec la réactivité électrochimique de la surface, l’influence de deux paramètres laser sur les propriétés physico-chimiques de la surface a été étudiée : la puissance du laser et le taux de recouvrement des impacts laser. Pour clarifier ces relations, la résistance à la corrosion par piqûration des surfaces traitées a été déterminée par des tests électrochimiques. Pour des paramètres laser spécifiques, le potentiel de piqûration d'un acier inoxydable de type 304L augmente de plus de 500 mV traduisant ainsi une meilleure tenue à la corrosion localisée en milieu chloruré. L’interdépendance des différents phénomènes résultant du traitement laser a rendu complexe la hiérarchisation de leur effet sur la sensibilité de l’alliage testé. Cependant, il a été montré que la nature de l’oxyde thermique formé au cours de la refusion laser et ses défauts sont du premier ordre pour l’amorçage des piqûres. / Metallic materials are more and more used in severe conditions with particularly strong request for improving their behavior in aggressive environment and especially over long periods. The objective of this PhD work is to estimate the potentiality of a laser surface melting treatment on the improvement of the stainless steel 304L corrosion resistance, surface treatments by laser can be revisited on the basis of a recent change in the laser technology. In the frame of this work, a nano-pulsed laser fiber was chosen : it allows the treated surface to be melted for few microns in depth, followed by an ultra-fast solidification occuring with cooling rates up to 1011 K/s. The combination of these processes leads to the elimination of the surface defects, the formation (trapping) of metastable phases, the segregation of chemical elements and the growth of a new oxide layer which properties are governed by the laser parameters. To correlate these latter to the electrochemical reactivity of the surface, the influence of two laser parameters on the physico-chemical properties of the surface was studied : the laser power and the overlap of the laser impacts. To support this approach, the pitting corrosion resistance of the samples was determined by standard electrochemical tests. For specific laser parameters, the pitting potential of a 304L stainless steel was increased by more than 500 mV corresponding to an important enhancement in localized corrosion resistance in chloride environment. The interdependence of the different phenomena resulting from the laser treatment lead to a quite complex prioritization of their role on the sensibility of the 304L. However, it was demonstrated that the nature of the thermal oxide formed during the laser surface melting and the induced defects are first-order parameters for the initiation of pits.