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In the current research, a complex microstructure and crystallographic data were developed through quenching and partitioning (Q&P) process to improve tensile properties of commercial pearlitic carbon-silicon steel. Two-stage Q&P process, including full austenitization, quenching at 220 °C, followed by two different partitioning temperatures, was applied to the as-received specimen to generate a complex microstructure composed of tempered martensite, bainite, ultrafine carbides/martensite-austenite/retained austenite particles. Microstructure and crystallographic data were investigated by scanning electron microscopy, electron backscattered diffraction (EBSD), and X-ray diffraction techniques. Then, hardness and tensile properties were evaluated to confirm the improvement of mechanical properties. Dilatation-temperature curves exhibited the kinetics of martensitic and bainitic transformation during quenching and isothermal partitioning stages. The presence of nano-carbide particles inside athermal martensite was confirmed by electron microscopy due to the pre-formed martensite carbon depletion during the partitioning stage coupled with bainitic transformation. The formation of preferential atomic-compact direction in BCC (martensite/bainite) plates characterized by EBSD, could enhance ductility by providing adequate slip systems. Point-to-point misorientation analyses demonstrated a slight dominance of low angle boundaries proportion in bainitic dominance structure in Q&P-220-375 specimen, which could be used in phase characterization. Results revealed that the development of nanoscale carbide dispersed in refined bainite/martensite matrix boosted the yield and ultimate tensile strength by over 100% and 110% compared to the initial pearlitic microstructure. However, ductility reduced to half value in Q&P-220-325 and Q&P-220-375 specimens.

Metastable austenitic stainless steels (MASS) are widely used in various industrial applications due to their exceptional compromise between mechanical properties and corrosion resistance. However, the mechanical properties of these materials can be further enhanced by surface treatments. This paper reviews various surface treatment methodologies used to improve the mechanical properties of MASS, with particular attention to laser treatments. The effects of these surface treatments on the microstructure and chemical composition in the thermal affected zone of the MASS are discussed, and their impact on the material’s mechanical properties, such as hardness, tensile strength, and fatigue life, are investigated in detail. Additionally, the paper highlights the limitations of these surface treatments and points out some areas where further research is needed. The findings presented can be used to guide the selection of appropriate surface treatment techniques for specific applications, ultimately improving the performance and lifespan of MASS in various industrial settings.

This research presents the microstructural and mechanical evolution throughout the welded seam of an austenitic stainless steel (ASS) tube. It was found that the main hardness decrement occurred in the fusion zone (FZ), followed by the heat-affected zone (HAZ) and the base material (BM). Optical microscopy indicated a dendritic structure in FZ and heterogeneous austenitic grain size from the HAZ towards the BM, ranging from 100 µm to 10 µm. The welding process generated an intense texture around the FZ and the HAZ, while the BM still showed an extrusion-like texture. In terms of mechanical behavior, the largest austenite grain size in the FZ led to the lowest strength and ductility of all zones due to the earliest strain localization manifested by heterogeneous strain distribution. However, the strain localization in all zones appeared after 0.4 true strain, indicating an overall good ductility of the seam. These high values were related to two microstructure characteristics: (1) the 10% δ-ferrite after solidification in the FZ favored by the Creq/Nieq=1.67 relationship that delayed the crack propagation along the austenite grains and (2) the heterogeneous microstructure made up of soft austenite and hard martensite in the HAZ and BM producing multiple strain concentrations. Kernel Average Misorientation (KAM) maps obtained by Electron Back-Scattering Diffraction (EBSD) allowed observing higher internal misorientations in the FZ than in the HAZ due to interconnected walls between the δ-ferrite grains. However, the largest KAM values were observed in the BM between γ-austenite and the deformation-induced α’-martensite phases. X-ray diffraction revealed that the residual stresses in the cross-section of the welded seam were compression-type and then switched to tension-type in the outer surface.

Laser surface modification treatments have been applied to samples of a metastable austenitic stainless steel AISI 301LN. The amount of residual stresses and the presence of induced a´-martensite phase on the surface and subsurface of the samples, as a consequence of laser modification, have been investigated in this study. X-ray diffraction has been used to measure these values, which is a non-destructive method. Laser modification has been applied as parallel lines at a distance of 40 µm, using a laser intensity of 4 A and scanning speeds of 1, 3, and 5 bits/ms, frequency had a constant value of 1000 Hz. The results of the phase analysis show an increase in the a´-martensite present in the surface modified by the laser and that the sample corresponding to the lowest laser speed has the highest martensite fraction. Also, high tensile residual stresses were generated, more pronounced for the lowest laser scanning speed.

Els acers inoxidables austenítics metaestables (MASS), comprenen un ampli grup d’acers amb la mateixa microestructura i unes propietats mecàniques úniques. Un elevat límit elàstic i alhora bona ductilitat, proporcionen una gran tenacitat quan es compara amb altres metalls. Per aquest motiu, conformen una nova generació d’acers avançats d’alta resistència (AHSS) que destaca per sobre dels aliatges convencionals. En les últimes dècades el seu ús en la indústria de l’automoció ha augmentat, on és àmpliament utilitzat com a element estructural en els cotxes. Milloren el rendiment del cotxe, el confort i seguretat, alhora que redueixen les emissions de CO2 promovent la sostenibilitat. La seva excepcionalitat recau en l’efecte de la TRansformació Induïda per Plasticitat (TRIP) i el maclat induït per plasticitat (TWIP). Les seves propietats mecàniques han estat profundament estudiades per tal de millorar el seu funcionament en servei, adaptant-se a les diverses aplicacions. Tot i així, l’estudi de la seva microestructura sota certes temperatures dona una valuosa informació sobre els límits de les seves aplicacions i els canvies microestructurals que pateix. En aquest projecte de treball de fi de Màster, l’acer TRIP AISI 301LN, ha estat fatigat tèrmicament a 400, 500, 600 y 800°C, durant varis cicles fins a 106. Els canvis microestructurals i de les propietats mecàniques resultants en l’interior del material són estudiats per mitjà de la difracció de raigs X i mesures de microindentació Vickers. La martensita està transformant a austenita, fent decréixer la duresa de la mostra. Els resultats difereixen de la tendència observada a la superfície per microscòpia òptica, on els anàlisis del percentatge de Martensita induïda per deformació (DIM) mostren com creix amb la temperatura. Tant la superfície com l’interior del material estan influenciades per la difusió i precipitació d’estructures nanomètriques de CrN, en les interfases entre les fases de martensita (α’-) i austenita (ϒ-). Els fins grans d’austenita inversa són formats a l’interior del material mentre redueixen la duresa de l’interior. Alhora, els precipitats de CrN produïts, promouen la martensita induïda per deformació causada pel polit mecànic de les mostres. La velocitat de difusió i precipitació dins d’aquest acer TRIP depenen de la temperatura a la que es realitza la fatiga tèrmica. En aquest context, es realitzen anàlisis de nanoindentació per tal de correlacionar les propietats micromecàniques en termes de duresa amb la microestructura resultant induïda durant el procés de fatiga tèrmica a diferents temperatures. S’ha realitzat un enfocament estadístic per tal d’obtenir la duresa en ambdues fases, comparar-ho amb els resultats de l’OM i poder determinar l’energia d’activació per a la difusió en les fases constitutives. La difusió dels precipitats s’ha estudiat la duresa a diferent profunditat dels grans mitjançant varis cicles de polit i mapes de nanoindentació. Mitjançant la microscòpia electrònica d’escaneig d’emissions de camp (FE-SEM), es realitza el “stitching” del mapa de nanoindentació residual per detectar defectes. Els resultats van evidenciar l’efecte del tractament de fatiga tèrmica a la superfície i interior d’aquest acer TRIP i els canvis induïts en ambdues fases constitutives. Aquesta recerca experimental de treball de fi de Màster proporciona una valuosa informació sobre els efectes tèrmics de la microestructura de l’acer TRIP i estableix un punt de partida per a futures investigacions. Los aceros inoxidables austeníticos metaestables(MASS), comprenden un amplio grupo de aceros con la misma microestructura y unas propiedades mecánicas únicas. Un elevado límite elástico y al mismo tiempo buena ductilidad, proporcionan una gran tenacidad cuando se compara con otros metales. Por este motivo, conforman una nueva generación de aceros avanzados de alta resistencia (AHSS) que destaca sobre las aleaciones convencionales. En las últimas décadas su uso en la industria de la automoción ha aumentado, donde es ampliamente utilizado como elemento estructural en los coches. Mejoran el rendimiento del coche, el confort y seguridad, al tiempo que reducen las emisiones de CO2 promoviendo la sostenibilidad. Su excepcionalidad recae en el efecto de la TRansformación Inducida por Plasticidad (TRIP) y el maclado inducido por plasticidad (TWIP). Sus propiedades mecánicas han sido profundamente estudiadas para mejorar su funcionamiento en servicio, adaptándose a las diversas aplicaciones. Sin embargo, el estudio de su microestructura bajo ciertas temperaturas da una valiosa información sobre los límites de sus aplicaciones y los cambios microestructurales que sufre. En este proyecto de trabajo de fin de Máster, el acero TRIP AISI 301LN, ha sido fatigado térmicamente a 400, 500, 600 y 800°C, durante varios ciclos hasta 106. Los cambios microestructurales y de las propiedades mecánicas resultantes en el interior del material son estudiadas mediante la difracción de rayos X y medidas de microindentación Vickers. La martensita está transformando en austenita, reduciendo la dureza de la muestra. Los resultados difieren de la tendencia observada en la superficie por microscopía óptica, donde los análisis del porcentaje de Martensita Inducida por Deformación (DIM) muestran cómo crece con la temperatura. Tanto la superficie como el interior del material están influenciadas por la difusión y precipitación de estructuras nanométricas de CrN, en las interfases entre las fases de martensita (α'-) y austenita (Υ-). Los finos granos de austenita inversa son formados en el interior del material mientras reducen la dureza. Asimismo, los precipitados de CrN producidos, promueven la DIM causada por el pulido mecánico de las muestras. La velocidad de difusión y precipitación dentro de este acero dependen de la temperatura a la que se realiza la fatiga térmica. En este contexto, se realizan análisis de nanoindentación para correlacionar las propiedades micromecánicas en términos de dureza con la microestructura resultante inducida durante el proceso de fatiga térmica a diferentes temperaturas. Se ha realizado un enfoque estadístico para obtener la dureza en ambas fases, compararlo con los resultados de la OM y poder determinar la energía de activación para la difusión en las fases constitutivas. La difusión de los precipitados se ha estudiado mediante la dureza a diferente profundidad de los granos mediante varios ciclos de pulido y nanoindentación. Con la microscopía electrónica de escaneo de emisiones de campo (FE-SEM), se realiza el "stitching" del mapa de nanoindentación residual para detectar defectos. Los resultados evidenciaron el efecto del tratamiento de fatiga térmica a la superficie e interior de este acero TRIP y los cambios inducidos en ambas fases constitutivas. Esta investigación experimental de trabajo de fin de Máster proporciona una valiosa información sobre los efectos térmicos de la microestructura del acero TRIP y establece un punto de partida para futuras investigaciones. Metastable Austenitic Stainless Steels (MASS) are a wide group of steels with same microstructure and unique mechanical properties. High yield strength and at the same time large elongation yielding as a result of a vast toughness when compared with other metals. This is why they conform a new generation of steels named Advanced High Strenght Steels(AHSS). Because of their good performance, in the last decades it has increased their use in automotive industry where it is already widely implemented as a structural element of the car. They successfully enhance the car performance, comfort and safety at the same time they reduce the CO2 emissions thus enhancing the sustainability. Their exceptionality lies on the Transformation Induced Plasticity (TRIP) effect and the Twinning Induced Plasticity (TWIP) effect. Their mechanical properties have been widely studied in terms of improving their performance in service and adapt to the different applications. Nevertheless, the study of the behavior of the microstructure under certain temperatures gives a valuable information about the limits of their performance, and the microstructural changes they can suffer. In this Master’s thesis project, AISI 301LN TRIP-steel is thermally fatigued at of 400, 500, 600 and 800°C during different number of cycles up to 106 cycles. The resulting microstructural and mechanical changes are spotted in the bulk of the steel by means of X-Ray Diffraction and Vicker’s microindentation measurements. Martensite is reversing to austenite decreasing the hardness of the sample. These results differ on the observed trend at the surface of the material by using optical microscopy (OM) analysis where percentage of Deformation Induced Martensite (DIM) increases with temperature. Both surface and bulk are influenced by the diffusion and precipitation of CrNnanometric clusters on the interphases between the martensitic (α’-) and austenitic (ϒ-) phases. Fine grains of reversed austenite are formed inside of the material while reducing the hardness. And at the same time, CrN precipitation occurs and promotes the DIM on the surface caused by the polishing of the samples. Diffusion and precipitation rate inside the TRIP-steel depends on the thermal fatigue temperature. Within this context, nanoindentation analysis are performed in order to correlate the micromechanical properties in terms of hardness with the resulting microstructure induced during the thermal fatigue process conducted at different temperatures. In this sense, a statistical approach is performed in order to obtain the hardness in both phases, compare with OM results, and be able to determine the activation energy for the diffusion in the constitutive phases. Diffusion of precipitates is spotted by studying the hardness at different depth of the grains by means of cycling polishing and high speed nanoindentation mapping. Using Field Emission Scanning Electron Microscopy (FE-SEM), stitching of the residual nanoindentation map is performed to spot defects on the nanoindentation grid. Results evidenced the effect of thermal fatigue treatment on the surface and bulk of this TRIP-steel and the induced changes in both constitutive phases. This experimental Master’s thesis research project gives a valuable information on the thermal effects of the microstructure of TRIP-steel and sets a starting point for further investigations.

Metastable austenitic stainless steels are distinguished by their susceptibility to the Transformation Induced Plasticity (TRIP) effect, where austenite (γ) to martensite (α’- and/or ε) transformation is triggered mechanically. They present an optimal relationship between mechanical and physical properties. Surface modification of TRIP steels, and particulaly an AISI 301LN metastable austenitic stainless steel specimens, was performed in order to study the possibility of enhancing their tribological and mechanical properties, by means of a parametric ground polishing analysis, used as a Surface Mechanical Modification Technique. To the best knowledge of the author, this technique has not been studied yet for this application, according to the performed literature review. Microstructural and mechanical properties are studied with the help of advanced characterisation techniques, e.g. Field-Emission Scanning Electron Microscopy (FESEM), Focus Ion Beam (FIB), etc. Experimental results demonstrate that an increase in grinding abrasion induces phase transformation and nano-structuring of the materials’ subsurface, by generation of compressive residual stresses, which contribute to an increase in the overall hardness of the material, which has been noticeable up to the micro-range. In addition, a qualitative analysis of the tribological images obtained leads to demonstrating an evolution in the surface deformation mechanisms, from shear bands to mechanisms related to the grain boundary rotation, as the grinding treatment intensifies. The path opened by this investigation could allow future studies to focus on a more exhaustive analysis of the benefits produced by this surface modification technique, by studying wear rates, the evolution of the friction coefficient and also the fatigue properties.

In this work, stress relaxation technique was employed to analyze the softening-precipitation interaction in an ASTM F-1586 austenitic stainless steel, an alloy used in orthopedic implants. Hot compression tests were performed at temperature range of 800–1100¿°C. Pre-strain of 5.0%, strain rate of 0.1 s-1, and loading time exposure of 40¿min were employed. The experiments were carried out in a dilatometer equipment. The stress- time (s vs. log (t)) curves were plotted, and the microstructure of the specimens was characterized. The acquired results permitted to estimate the start (ts) and the finish (tf) times of the effective interaction between recovery and precipitation. The data revealed three distinct stages in the stress versus log time curves: (i) reduction in the stress levels due to static recovery; (ii) delay in the stress relaxation rate leading to a stress plateau, and (iii) sharp decrease of the stress levels associated with the finish of the precipitation. These results enabled the creation of Precipitation-Temperature-Time (PTT) curves for the present material. It was found that the onset time for the strain-induced Z-phase (CrNbN) is around 120¿s and the precipitates delay the kinetics of the softening.

This research work studies the different strength contributions of a sheet-shaped heterogeneous austenitic stainless steel, after having been processed at room temperature by one ECASE pass. A significant hardness increase was found throughout its thickness, showing higher values near the edges while the middle area presents the smallest gains. The material heterogeneity gives rise to a plastic gradient deformation between the soft and hard areas of the microstructure. As a consequence, a more significant amount of geometrically necessary dislocations concentrates on the interfaces of these areas, which helps the material to maintain a reliable strength-ductility ratio. After the dislocations contribution to the material strength, the second contribution comes from the back stress mechanism, followed by grain size contributions. All types of contributions were higher in the edge neighborhoods than in the middle zone. Unlike the contributions from dislocations and grain size that decrease as they approach the interfaces between the hard and soft areas, the back stress contribution does the opposite, showing high increments in these zones.

An austenitic stainless steel was processed at room temperature by equal channel angular sheet extrusion (ECASE). The microstructure evolution was heterogeneous across the sheet thickness with a higher martensitic transformation in the zones closer to the edges than in the sheet core due to the deformation-induced martensite (DIM) phenomena. The material heterogeneity induced by one ECASE pass gave a good strength-ductility relationship. Fil: Muñoz Bolaños, Jairo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Higuera Cobos, Oscar Fabián. Universidad del Atlántico; Colombia Fil: M'Doihoma, Rose. Clermont Auvergne University School of Engineering; Francia Fil: Avalos, Martina Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Bolmaro, Raul Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina

A sheet-shaped austenitic stainless steel (ASS) was processed by one ECASE pass. In this way, larger deformations were introduced close to the sheet edge vicinities, while the middle zone remained less affected by the deformation. The heterogeneous deformation produced larger amounts of deformation induced martensite (DIM) at the sheet edges, resulting in a heterogeneous structure along the sheet thickness. Tensile tests revealed that after the deformation process the material increased its yield strength more than 3 times from 200 MPa in its initial state to ~690 MPa after one ECASE pass. Tensile tests for the processed material in different zones of sheet thickness revealed the existence of two types of materials, one more resistant and less ductile (near the edges) and other more ductile, but less resistant (in the middle zone). Because of the heterogeneity, higher dislocation densities were found in the edges than in the middle of the sheet, giving rise to a plastic deformation gradient in which the region near the edge is capable of withstanding greater plastic deformations by generating a plastic instability as soon as the deformation starts. Through a rule of mixtures, it was found that the greatest strength contribution of the heterogeneous material came from the dislocations on the bcc martensite developed near the sheet edges.

This paper presents a numerical and theoretical study from an ongoing research program focused on the structural behavior of austenitic stainless steel frames. Four different rectangular hollow sections (RHS) subjected to combined symmetrically concentrated vertical and lateral loads were studied. Taking into account material non-linearity and geometrical non-linearity, the influence of global sway imperfection (d) of the frame and local bow imperfection (d) of the member on the structural behavior of frames was analyzed. Based on global plastic analysis, second order effects under different loading cases of altering the loading position of vertical loads were studied. The finite element (FE) analysis software Abaqus was employed to simulate the in-plane structural behavior of all frames.

This Master Thesis consists of the development of an experimental program related to the cross-sectional behaviour of austenitic stainless steel beams and stub columns. The tests are performed on both compact and slender sections under pure bending and pure compression. Current design codes will be evaluated with the results obtained. The tests are framed within a vaster study related to the structural behaviour of austenitic stainless steel elements at the frames, columns, beams and material levels.

The fatigue crack growth behavior of an austenitic metastable stainless steel AISI 301LN in the Paris region is investigated in this work. The fatigue crack growth rate curves are evaluated in terms of different parameters such as the range of stress intensity factor Delta K, the effective stress intensity factor Delta K-eff, and the two driving force parameter proposed by Kujawski K*.; The finite element method is used to calculate the stress intensity factor of the specimens used in this investigation. The new stress intensity factor solution has been proved to be an alternative to explain contradictory results found in the literature.; Fatigue crack propagation tests have been carried out on thin sheets with two different microstructural conditions and different load ratios. The influence of microstructural and mechanical variables has been analyzed using different mechanisms proposed in the literature. The influence of the compressive residual stress induced by the martensitic transformation is determined by using a model based on the proposal of McMeeking et al. The analyses demonstrate the necessity of including K-max as a true driving force for the fatigue crack growth. A combined parameter is proposed to explain the effects of different variables on the fatigue crack growth rate curves. It is found that along with residual stresses, the microcracks and microvoids are other factor affecting the fatigue crack growth rate in the steel studied. (C) 2015 Elsevier Ltd. All rights reserved.

L’acer inoxidable austenític metastable forma part de la família dels acers d’alta resistència que s’utilitzen actualment en el desenvolupament de vehicles més lleugers. Es caracteritza per ser susceptible a la transformació martensítica sota l’acció d’una deformació plàstica, fet que li confereix un seguit de propietats mecàniques molt adequades per al sector de l’automoció. Tot i així, el grau en el que es produeixi la transformació martensítica pot afectar directament a la capacitat de deformar plàsticament, així com el seu comportament a fatiga, fent que en molts casos sigui difícil predir la seva resposta mecànica. La gran majoria d’estudis que es troben en la bibliografia estan dedicats a analitzar l’efecte de la presència de fase martensítica en acers sotmesos a tracció i/o fatiga a baix i alt nombre de cicles però hi ha molt poca informació relacionada amb la repercussió enfront al desgast. Des d’aquest punt de vista, en aquest projecte s’ha analitzat i avaluat la resposta al desgast de l’acer inoxidable austenític metastable EN 1.4318 (AISI 301 LN), en quatre condicions de partida diferents; una en estat de recuit i tres en estat de laminat en fred amb diferents percentatges de martensita. Els assajos s’han realitzat amb un tribòmetre per a distàncies entre 100 i 1000 m, a diferents càrregues per a una mateixa velocitat de lliscament. S’ha determinat la cinètica de desgast per a cada condició de material mitjançant dos mètodes diferents: mètode de la pèrdua de pes i mètode del volum desgastat. S’han analitzat les petjades de desgast mitjançant un rugosímetre i avaluat el mecanisme de desgast a partir de la tècnica de microscòpia electrònica de rastreig. D’altra banda, l’avaluació de la transformació martensítica originada al llarg dels assaigs de desgast s’ha dut a terme a partir de la tècnica de difracció de raig-X i del perfil de dureses en la secció transversal sota la petjada de desgast. Els resultats mostren que a mesura que augmenta la quantitat inicial de fase martensítica disminueix la resistència al desgast de l’acer. No obstant, s’observa que la velocitat de desgast va disminuint al llarg del temps i s’estabilitza a un valor que és més elevat quan més gran sigui el percentatge inicial de martensita. L‘anàlisis de les petjades de desgast posa de manifest que el mecanisme de desgast predominant, en les condicions de desgast estudiades, és la tribocorrosió i aquest es desenvolupa de forma similar indistintament del contingut inicial de martensita.

Los aceros del grado AISI 301 LN son aceros de baja aleación de tipo austenítico metaestable, es decir la estructura austenítica es susceptible de transformarse en martensita bajo solicitación mecánica. En este proyecto se ha estudiado un acero AISI 301 LN C1050, el cual ha sufrido un 25% de reducción por laminación en frío, lo que hace que contenga un porcentaje de martensita de 28%. El proyecto tiene un doble objetivo: - Primero se ha investigado el tratamiento de reversión óptimo para obtener un acero recristalizado con un tamaño de grano fino de austenita, eliminando al mismo tiempo la presencia de martensita inducida por la laminación anterior. Se ha escogido en base a trabajos previos una temperatura de tratamiento de reversión de 750°C. La investigación se realiza con dos tiempos 10 y 15 minutos para dos desarrollos experimentales. - El segundo objetivo ha sido evaluar el efecto del refinamiento de grano en la vida a fatiga. Se han realizado ensayos a dos millones de ciclos con un relación de esfuerzos R = 0,1 del material tratado y sin tratar. Se ha observado también el efecto del acabado de los bordes de las probetas sobre el límite a fatiga. La caracterización microestructural y mecánica del acero en sus diferentes estados se ha efectuado mediante las técnicas de: - Microscopía óptica (en modo confocal) y difractometría de rayos X para estudiar la microestructura. - Rugosidad para analizar el estado de las probetas y el efecto del pulido sobre el límite a fatiga. - Dureza, tracción y fatiga de alto número de ciclos para evaluar el efecto del afino del grano, comparando las propiedades antes y después del tratamiento térmico de reversión. Los resultados de las investigaciones efectuadas en este proyecto mostraron que el tratamiento de reversión permite obtener una microestructura totalmente austenítica, con un tamaño de grano promedio de 3 μm, frente a las casi 12 μm del estado laminado, probándose así la eficacia de la reversión de la fase martensítica. En lo que se refiere a las propiedades mecánicas, el acero tratado reveló valores más elevados en comparación al acero recocido, como era de esperar debido al afino del grano austenítico. La sensibilidad a fatiga del material tratado de microestructura austenítica alcanzó valores bastante similares al acero laminado (C1050) y supera al acero recocido (2B). Se demostró también que la sensibilidad fue más alta cuando los bordes presentan una rugosidad similar a la de la superficie. Incoming

El control de la microestructura necesita la optimización de las condiciones del conformado en caliente. La recristalización dinámica es el fenómeno protagonista durante un proceso termomecánico, por lo que tiene un interés industrial, permitiendo mejorar las propiedades mecánicas de los aceros inoxidables mediante el afinamiento de grano, y al mismo tiempo aumentar la resistencia a la fragilización y a la corrosión. El objetivo principal de este trabajo es estudiar el comportamiento termomecánico de cuatro aceros inoxidables austeniticos: dos comerciales AISI 304 relativamente altos de elementos de aleación y dos aceros de laboratorio, de alta pureza y libres de intersticiales. La primera parte del estudio se centrará en los mecanismos elementales que intervienen y afectan a la recristalización dinámica (endurecimiento, restauración dinámica, nucleación y crecimiento de los nuevos granos). La segunda parte, modelizará las curvas de fluencia en diferentes grados de temperatura y diferentes velocidades de deformación mediante la cuantificación de dichos fenómenos. Las curvas de fluencia anteriormente mencionadas se han obtenido mediante ensayos de compresión uniaxial. Y finalmente un estudio de la evolución de la microestructura, mediante el microscopio óptico y MEB-EBSD en las tres principales etapas que caracterizan las curvas de fluencia en dos casos: en el de afinamiento de grano (bajas temperaturas y altas velocidades de deformación) y en el de su crecimiento (altas temperaturas y bajas velocidades de deformación). El uso de la técnica EBSD, permite un estudio amplio de microestructura y al mismo tiempo ofrece datos sobre las orientaciones y la evolución de la microtextura durante el proceso de deformación.El efecto del grado de pureza será discutido y una comparación de diferentes parámetros de los cuatro materiales permitirá concretar el efecto del soluto en general y del carbono en particular, en diferentes etapas de deformación y según las condiciones de deformación. Para el acero de alta pureza y ultra alta pureza se estudiará el efecto del tamaño de grano inicial sobre la etapa del endurecimiento y la cinética de la recristalización dinámica. Finalmente se realizó un estudio sobre el tamaño de grano recristalizado dinámicamente que se considera el cambio microestructural más importante y beneficioso del proceso de conformado en caliente en su totalidad.

This Master Thesis consists of the development of an experimental program related to the flexural buckling behaviour of austenitic stainless steel columns. The tests are performed with rotational constraints in both minor and major axes. Current design codes will be evaluated with the results obtained. The tests are framed within a vaster study related to the structural behaviour of austenitic stainless steel elements at the frames, columns, beams and material levels.

The fatigue crack growth behavior of an austenitic metastable stainless steel AISI 301LN in the Paris region is investigated in this work. The fatigue crack growth rate curves are evaluated in terms of different parameters such as the range of stress intensity factor Delta K, the effective stress intensity factor Delta K-eff, and the two driving force parameter proposed by Kujawski K*.; The finite element method is used to calculate the stress intensity factor of the specimens used in this investigation. The new stress intensity factor solution has been proved to be an alternative to explain contradictory results found in the literature.; Fatigue crack propagation tests have been carried out on thin sheets with two different microstructural conditions and different load ratios. The influence of microstructural and mechanical variables has been analyzed using different mechanisms proposed in the literature. The influence of the compressive residual stress induced by the martensitic transformation is determined by using a model based on the proposal of McMeeking et al. The analyses demonstrate the necessity of including K-max as a true driving force for the fatigue crack growth. A combined parameter is proposed to explain the effects of different variables on the fatigue crack growth rate curves. It is found that along with residual stresses, the microcracks and microvoids are other factor affecting the fatigue crack growth rate in the steel studied. (C) 2015 Elsevier Ltd. All rights reserved.

High ductility, durability, strain hardening and good fire resistance make stainless steel an excellent construction material. In recent years, a considerable amount of research has been devoted to the understanding of the structural performance of single isolated stainless steel members. Notwithstanding, advances related to the analysis of more complex stainless steel structures, such as frames, are scarce. An extensive experimental program on austenitic stainless steel frames is being carried out at Universitat Politècnica de Catalunya. This programme is aimed at assessing the performance of stainless steel frames subjected to static loading. It comprises several subprograms in which the performance of these structures at different levels –material, cross-sections, members and frames– is investigated. The objective of this paper is to present tests results on austenitic stainless steel stub columns, beams and columns comprising the frames to determine cross-section and member resistances, to asses the currents design rules and facilitate the future analysis of the experimental tests on the stainless steel frames. The paper presents experimental results as well as the comparison of traditional measurement techniques with advanced Digital Image Correlation (DIC) solutions.