[pt] Aços inoxidáveis duplex (AIDs) são aços que apresentam boas propriedades mecânicas e de resistência à corrosão, devido a microestrutura composta de, aproximadamente, partes iguais de austenita e ferrita. A exposição dos AIDs a altas temperaturas, durante um processo de soldagem por exemplo, pode resultar na formação de intermetálicos numa faixa de temperatura entre 1000 e 600 C, bem como numa mudança microestrutural na faixa de temperatura de 1200 e 800 C. Estes podem reduzir as propriedades da junta soldada principalmente zona termicamente afetada pelo calor (ZTA), como por exemplo em corrosão em meio cloreto. No entanto, durante processos de fabricação ou de manutenção, tratamentos térmicos ou soldagem, as propriedades do material podem ser alteradas, bem como a resistência à corrosão por pites. O presente estudo tem como objetivo correlacionar as microestruturas obtidas por diferentes taxas de resfriamento, sendo estas microestruturas obtidas por simulação, equivalente aquela obtida em soldagem, a qual corresponde a um regime de não-equilíbrio, com a microestrutura obtida em um regime de equilíbrio e determinar como estas transformações afetam a resistência a corrosão. O estudo foi realizado para dois tipos de AIDs: o UNS S32304 e o UNS S32750. Foram obtidas microestruturas equivalentes a zona termicamente afetada (ZTA) por meio do simulador Gleeble utilizando dois aportes de calor 1,0 e 3,0 KJ/mm, e por tratamentos térmicos realizados a 1000 e 1100 C por 24, 72 e 240 horas seguidas de têmpera em água. A análise microestrutural foi realizada por meio de microscopia ótica (MO) e microscopia eletrônica de varredura (MEV), para caracterização da morfologia de fases, quantificação das fases e caracterização química das fases. Foi determinada a microdureza das fases, a dureza e realizado ensaio de corrosão (ASTM G48). A fração volumétrica da fase austenita das amostras que foram tratadas termicamente reduziu com o aumento do tempo e da temperatura de tratamento, enquanto para as amostras simuladas termicamente diminuiu com a diminuição do aporte térmico para ambos os AIDs. Na avaliação da resistência a corrosão por pites foi observado que o aumento da temperatura de tratamento térmico igualou os valores de Pitting Resistance Equivalent Number (PREN) das fases (austenita e ferrita) devido ao equilíbrio termodinâmico dos elementos nas fases, desta forma a amostra tratada termicamente na temperatura de 1250 C por 24 horas obteve melhor resistência à corrosão por pites para ambos AIDs. Para as amostras simuladas termicamente, o aporte de 3 KJ/mm obteve melhor resultado de resistência à corrosão por pites em ambos os AIDs. / [en] Duplex stainless steels (DSSs) are steels that have good mechanical properties and corrosion resistance due to the microstructure comprised of about equal parts of austenite and ferrite. Exposure of DSSs to high temperatures, during a welding process for example, result in formation of intermetallics in the temperature range of 1000 to 600 C, including a microstructural change between 1200 to 800 C. These can reduce the properties of the welded joint, especially the heat affected zone (HAZ), such as corrosion in chloride environment. However, during manufacturing or maintenance processes, either by heat treatment or welding processes, the properties of the material can be together with the pitting resistance corrosion. The present study aims to correlate the microstructures obtained by different cooling rates, being these microstructures obtained by simulation, equivalent to that obtained in welding, which is equivalent to a non-equilibrium regime, with the microstructure obtained in an equilibrium regime and to determine how these transformations affect the corrosion resistance. The study was conducted for two DSSs: UNS S32304 and UNS S32750. HAZ equivalent microstructures were obtained by the Gleeble simulator for two heat inputs 1.0 and 3.0 KJ/mm, and the heat treatments were performed at 1000, 1100 and 1250 C for 24, 72 and 240 hours followed by quenching in water. Microstructural analysis was performed by optical microscopy and scanning electron microscopy (SEM) to characterize the phase morphology, quantification and chemical composition. The microhardness of the phases, the hardness and corrosion test (ASTM G48) were determined. The volumetric fraction of the austenite phase of the samples that were heat treated decreased with increasing time and temperature treatment, while for the thermally simulated samples it decreased with the decrease of the heat input for both DSSs. The evaluation of the pitting corrosion resistance it was observed that the increase of the heat treatment temperature equaled the phases (austenite and ferrite) Pitting Resistance Equivalent Number (PREN) values due to the thermodynamic balance of the elements in the phases, thus the heat-treated sample at 1250 C for 24 hours showed better pitting corrosion resistance for both AIDs. For the thermally simulated samples, the heat input of 3 KJ/mm obtained better result of pitting corrosion resistance in both AIDs.

<p>Duplex stainless steels are a very attractive alternative to austenitic grades due to their higher strength and good corrosion performance. The austenitic grades can often be welded autogenously, while the duplex grades normally require addition of filler metal. This is to counteract segregation of important alloying elements and to give sufficient austenite formation to prevent precipitation of chromium nitrides that could have a negative effect on impact toughness and pitting resistance. The corrosion performance of the recently-developed lean duplex stainless steel LDX 2101 is higher than that of 304 and can reach the level of 316. This thesis summarises pitting resistance tests performed on laser and gas tungsten arc (GTA) welded LDX 2101. It is shown here that this material can be autogenously welded, but additions of filler metal, nitrogen in the shielding gas and use of hybrid methods increases the austenite formation and the pitting resistance by further suppressing formation of chromium nitride precipitates in the weld metal. If the weld metal austenite formation is sufficient, the chromium nitride precipitates in the heat-affected zone (HAZ) could cause local pitting, however, this was not seen in this work. Instead, pitting occurred 1–3 mm from the fusion line, in the parent metal rather than in the high temperature HAZ (HTHAZ). This is suggested here to be controlled by the heat tint, and the effect of residual weld oxides on the pitting resistance is studied. The composition and the thickness of weld oxide formed on LDX 2101 and 2304 were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the 300 series. A new approach on heat tint formation is consequently presented. Evaporation of material from the weld metal and subsequent deposition on the weld oxide are suggested to contribute to weld oxide formation. This is supported by element loss in LDX 2101 weld metal, and nitrogen additions to the GTA shielding gas further increase the evaporation.</p><p> </p>

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.

Duplex stainless steels are a very attractive alternative to austenitic grades due to their higher strength and good corrosion performance. The austenitic grades can often be welded autogenously, while the duplex grades normally require addition of filler metal. This is to counteract segregation of important alloying elements and to give sufficient austenite formation to prevent precipitation of chromium nitrides that could have a negative effect on impact toughness and pitting resistance. The corrosion performance of the recently-developed lean duplex stainless steel LDX 2101 is higher than that of 304 and can reach the level of 316. This thesis summarises pitting resistance tests performed on laser and gas tungsten arc (GTA) welded LDX 2101. It is shown here that this material can be autogenously welded, but additions of filler metal, nitrogen in the shielding gas and use of hybrid methods increases the austenite formation and the pitting resistance by further suppressing formation of chromium nitride precipitates in the weld metal. If the weld metal austenite formation is sufficient, the chromium nitride precipitates in the heat-affected zone (HAZ) could cause local pitting, however, this was not seen in this work. Instead, pitting occurred 1–3 mm from the fusion line, in the parent metal rather than in the high temperature HAZ (HTHAZ). This is suggested here to be controlled by the heat tint, and the effect of residual weld oxides on the pitting resistance is studied. The composition and the thickness of weld oxide formed on LDX 2101 and 2304 were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the 300 series. A new approach on heat tint formation is consequently presented. Evaporation of material from the weld metal and subsequent deposition on the weld oxide are suggested to contribute to weld oxide formation. This is supported by element loss in LDX 2101 weld metal, and nitrogen additions to the GTA shielding gas further increase the evaporation. / QC 20101126