Neste trabalho são estudados os mecanismos de desgaste, atuantes na escala do tamanho de grão (meso-escala), durante ensaios de cavitação vibratória, para diferentes amostras de aços inoxidáveis austeníticos ligados com nitrogênio. Amostras com teores superficiais de nitrogênio de aproximadamente 0, 9 % massa, 1, 4%massa e 20%massa, obtidas a partir do a¸co inoxidável dúplex UNS S31803, foram estudadas. As amostras do a¸co inoxidável duplex UNS S31803, com aproximadamente 0, 9 % N massa, foram obtidas por nitretação gasosa em alta temperatura (temperatura de nitretação entre 1050 e 1200 C) e consistiram em três grupos diferentes: amostras com nitrogênio em solução sólida e solubilizadas, amostras com precipitação de nitretos e amostras com nitrogênio em solução sólida e encruadas. Já as amostras com teor de nitrogênio próximo de 20 % N massa foram processadas por meio de nitretação a plasma na temperatura de 400 C, obtendo-se uma camada superficial de austenita expandida. As amostras de ensaio foram submetidas à caracterização de textura por difração de elétrons retroespalhados, EBSD, e posteriormente à cavitação vibratória em ´agua destilada. Os ensaios de cavitação foram periodicamente interrompidos com o intuito de estudar a deteriora¸cao das amostras por exame das mesmas no microscópio eletrônico de varredura, MEV, e por medidas de perda de massa. Quando comparadas com os aços inoxidáveis austeníticos convencionais (UNS S30403 solubilizado e UNS S31803 como recebido), sem adição de nitrogênio e livre de encruamento, as amostras estudadas apresentaram resistência ao desgaste por cavitação superior, quantificada tanto pelo tempo de incubação do dano com perda de massa quanto pela taxa máxima de perda de massa nos estágios avançados do dano. A taxa máxima de perda de massa para cada tipo de amostra estudada, com relação `a taxa máxima do material de comparação, o aço inoxidável convencional sem adição de nitrogênio e livre de encruamento (UNS S30403) solubilizado, foi de: amostras com precipitação de nitretos (318HTGN+Nit), 6,9 vezes menor; amostras com nitrogênio em solução sólida e solubilizadas (318HTGH+Sol) e laminadas e solubilizadas (318HTGN+Lam+Sol), 26,8 e 25 vezes menor, respectivamente; amostras com nitrogênio em solução sólida e encruadas (318HTGN+Enc) 145 vezes menor; e amostras com camada superficial de austenita expandida (obtidas por nitretação a plasma), (318HTGN+Plas e 304LSol+Plas) 290 e 1,77 e vezes menor respectivamente. O efeito benéfico da adição de nitrogênio na resistência à erosão por cavitação dos aços inoxidáveis austeníticos estudados foi atribuído a: (i) aumento na resistência à deformação plástica; (ii) distribuição mais homogênea da deformação plástica induzida pelas ondas de choque e micro-jatos característicos do processo de cavitação; e (iii) aumento da importância relativa dos mecanismos de perda de massa com elevado consumo de energia de impacto. Nos primeiros estágios do dano erosivo por cavitação se observou clara evidência de deformação plástica, acompanhada de formação de microreelevo superficial e de protrusão de bandas de escorregamento. A perda de massa em nível microscópico (observações no MEV) começa como destacamento de material em microtrincas e micropites. Observou-se que tanto a nucleação do dano como o seu crescimento se apresenta de forma heterogênea na escala do tamanho de grão. Os sítios microestruturais nos quais se iniciou o dano com perda de massa foram preferencialmente protuberâncias nas protrusões de bandas de escorregamento, protuberâncias nos contornos de grão e as interfaces matriznitreto. O incremento do teor de nitrogênio (em solução sólida) na amostra aumentou a importância relativa dos contornos de grão como locais de nucleação do dano, em relação ao dano iniciado no interior dos grãos. Observou-se que o interior dos grãos com planos 100 ou 111 orientados de forma aproximadamente paralela à superfície das amostras são regiões muito suscetíveis à incubação do dano e ao crescimento do mesmo. Já os grãos com planos 101 orientados aproximadamente paralela à superfície das amostras, apresentam regiões com resistência ao dano bem maior. Esses resultados são discutidos, considerando as diferenças de tensão (resultantes da ação de ondas de choque causadas pela implosão de bolhas de cavitação) crítica projetada para cisalhamento de grãos com diferentes orientações. O dano ocorre preferencialmente em contornos de grãos com acentuados gradientes de tensão resolvida para a deformação plástica, onde se desenvolve elevada concentração de tensões. Em particular, os contornos de macla CSL 3 são acentuadamente mais suscetíveis à incubação do dano que os outros tipos de contornos CSL e que os contornos não CSL. / High nitrogen austenitic stainless steels containing 0.9 wt-% N and 20 wt- % N were tested in a ultrasonically induced vibratory cavitation testing device. Incubation times for damage initiation and mass losses were periodically measured during the cavitation-erosion tests. Scanning Electron Microscopy observation of the damaged surfaces allowed identifying the wear mechanisms operating during each step of the cavitation-erosion test. 0.9 wt-% N specimens were obtained through High Temperature Gas Nitriding UNS S31803 duplex stainless steel, at temperatures between 1050 and 1200 oC. Three groups of specimens were obtained: solubilized with all nitrogen in solid solution, solubilized and work hardened specimens and nitride containing specimens. The 20 wt- % N specimens were obtained through Low Temperature Plasma Nitriding the already High Temperature Gas Nitrided specimens and getting an expanded austenite layer at the surface. The specimens were firstly characterized by Electron Backscattered Diffraction - EBSD techniques and then submitted to the cavitation-erosion tests in distilled water. When compared to conventional UNS S30403 lean nitrogen solubilized austenitic stainless steel specimens, greater incubation times and smaller maximum wear rates were observed. The maximum wear rates (compared to those of the solubilized UNS S30403 steel) were: for the nitride containing specimen 6.9 times smaller; for textured and non-textured all nitrogen in solid solution specimens 26.8 and 25 times smaller, respectively; for the solubilized and work hardened specimen 145 times smaller; for the expanded austenite layer, with circa 20 wt- % N, specimens 300 times smaller. The beneficial effect of nitrogen on the cavitation-erosion resistance of the studied specimens was attributed to: (i) an increase in resistance to plastic deformation; (ii) a more homogeneous distribution of the plastic deformation; and (iii) an increase of the relative participation of energy consuming mass loss mechanisms. Plastic deformation accompanied by formation of micro relief at the surface and slip bands protrusions were clearly identified, during the first stages of cavitation erosion. The first evidences of mass loss (detected by SEM observations) were seen as particles detaching from micro cracks and micro pits formed at the grain surface. Nucleation and growth of cavitation damage was heterogeneously distributed at the grain scale. Slip bands protrusions, grain boundary protrusions and nitride matrix interfaces sites were more prone to nucleating the damage. Increasing nitrogen contents in solid solution increased the relative contribution of grain boundary nucleated damage, compared to the total amount of nucleation sites. Grains with 100 and 111 crystallographic planes approximately parallel to the surface were more prone to nucleation and growth of cavitation damage. Grains with 101 planes // surface were much more resistant to cavitation-erosion damage. These results are discussed considering differences of critical resolved shear stresses for grains with different orientations. Cavitation erosion damage occurs preferentially at grain boundaries across which steep stress gradients arise. Particularly, CSL -3 twin boundaries are much more susceptible to cavitation erosion damage incubation than other types of CSL boundaries and non CSL boundaries.

Cette étude porte sur l'évolution de la microstructure des aciers inoxydables austénitiques sous irradiation et les conséquences de cette évolution sur leur comportement en milieu REP. Un acier 304L a été irradié aux protons à 360°C à 5 et 10 dpa. Suite à ces irradiations, la sensibilité du matériau à l'IASCC a été étudiée en milieu primaire simulé à 350°C, avec suivi par microextensométrie des champs locaux de déformation. Parallèlement à ce travail, des lames minces ont été irradiées in situ aux ions Ni++ à 500°C jusqu'à 2 dpa avec implantation simultanée d'hélium. Ces expérimentations nous ont permis (i) grâce au couplage microstructure /champs mécaniques /fissuration de mieux comprendre les paramètres responsables de l'amorçage de l'IASCC en milieu réducteur (ii) de définir le rôle joué par l'hélium sur l'évolution des défauts d'irradiation. Il s'avère que, dans les conditions d'étude, l'implantation d'hélium n'a qu'un effet limité sur les populations de boucles de dislocation et de cavités pour des rapports inférieurs à 800 appm He/dpa. Des cavités ont été observées avec et sans implantation d'hélium, y compris dans les joints de grains ce qui pourrait être un facteur de fragilisation. L'ensemble des essais de corrosion sous contrainte ont validé que la densité de fissures augmente avec l'augmentation du taux de déformation et qu'un chargement séquentiel conduit à une plus grande ouverture et propagation en surface des fissures. Ces fissures se propagent en profondeur dans la couche irradiée notamment du fait de la surcontrainte générée par le fort gradient de propriétés entre la zone irradiée et non irradiée du matériau. Les mécanismes de déformation activés sont complexes et du maclage a été observé après 2 et 10% de déformation macroscopique. La déformation après irradiation est fortement localisée sous forme de bandes intragranulaires et autour de certains joints de grains, mais la déformation de ces joints ne semble pas constituer un critère d'amorçage. L'absence de transmission de la déformation de part et d'autre des joints fissurés est par contre systématiquement observée et la connaissance de l'état de contrainte local s'avère indispensable pour décrire l'amorçage de l'IASCC en milieu réducteur. Une méthodologie basée sur l'exploitation des résultats expérimentaux (champs d'orientation cristallographique, champs cinématique) appliquée à une simulation aux éléments finis permet d'estimer l'état local de contrainte, seul à même de discriminer un critère d'ouverture de fissure. / This work deals with the microstructure evolution of austenitic stainless steels under irradiation and the consequences of this evolution on their behavior in PWR environment. 304L steel was proton-irradiated at 360°C to 5 and 10 dpa. Following these irradiations, IASCC was studied in a 350°C simulated primary water, with strain fields measurements using digital image correlation. In parallel, thin foils were irradiated in situ with Ni++ ions at 500°C up to 2 dpa with simultaneous helium implantation. These experiments allowed us (i) to have a better understanding of the key parameters responsible of the IASCC initiation in reducing environment thanks to the coupling between microstructure, mechanical fields and cracking (ii) to define the role of helium on the nucleation and evolution of radiation defects. It turns out that, in the studied conditions, the implantation of helium has only a limited effect on the dislocation loop and cavity populations for ratios lower than 800 appm He/dpa. Cavities were observed with and without helium, including in the grain boundaries which could be a factor of embrittlement. The stress corrosion cracking tests resulted in an increase of the crack density with the increase of the macroscopic deformation and in a bigger opening and on-surface propagation of cracks after a sequential loading. These cracks propagate deeply in the irradiated layer partly because of the overstress generated by the strong gradient of mechanical properties between the irradiated and non-irradiated zones of the material. The activated deformation mechanisms are complex and twinning was observed after 2 and 10% of macroscopic deformation. The deformation after irradiation is strongly localized in transgranular bands and around some grain boundaries, but it appears that the strong deformation around boundaries is not an initiation criterion. Deformation discontinuity on both sides of cracked boundaries is systematically observed and evaluation of the local stress state appears to be essential to describe IASCC initiation. This local stress state could be calculated by finite elements, taking into account the experimental results in terms of crystallographic orientation fields or Kinematics fields strong heterogeneity of local deformation quantified in this work.

Two austenitic stainless steels of strongly different stacking fault energies (SFEs) and correspondingly different stabilities of the austenite phase were studied with respect to their very high cycle fatigue (VHCF) behavior. The metastable austenitic stainless steel 304L shows a very pronounced transient behavior and a fatigue limit in the VHCF regime. The higher SFE of the 316L steel results in a less pronounced transient cyclic deformation behavior. The plastic shear is more localized, and the formation of deep intrusions leads to microcrack initiation. However, the propagation of such microcracks is impeded by α'-martensite formed very localized within the shear bands. A comprehensive description of the microstructural changes governing the cyclic deformation including the transient resonant behavior was developed and transferred into a mechanism-based model. Simulation results were correlated with the observed deformation evolution and the change of the resonant behavior of specimens during VHCF loading providing a profound understanding of the VHCF-specific deformation behavior.

<p>  </p> <p>Stress corrosion cracking (SCC) is a longstanding critical materials challenge in austenitic stainless steels (AuSS). Recently, there has been mounting concern regarding the potential for Chloride-induced stress corrosion cracking (CISCC) along arc weld seams on austenitic stainless-steel canisters used as spent nuclear fuel (SNF) dry storage containers, due to the residual stress from the welding process and exposure to chloride-rich coastal air at storage sites. To ensure the safety of the SNF storage, fundamental understanding and mitigation methods of CISCC are critical in both engineering design and maintenance of the storage canisters before and after their deployment. With the recent development of high-resolution characterization and analysis techniques, a more robust and comprehensive understanding of the fundamental TGCISCC mechanism starts to be more accessible. In this thesis, comprehensive state-of-the-art techniques, including SEM, EBSD, HREBSD, FIB, ATEM, TKD, potential dynamic measurement, XRD, and nanoindentation will be used to further understand the mechanistic mechanism of TGCISCC in AuSS from macroscopic scale down to atomistic scale. </p>

Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular. / <p>QC 20160905</p>

The environmentally-assisted fatigue crack growth behaviour of austenitic stainless steels in deoxygenated high temperature water at low strain rates has been reported to be greatly affected by the sulphur (S) content of the specimen, with high S specimens exhibiting significant reduced crack growth rates (retardation) when compared to low S specimens. To further the understanding of the mechanistic behaviour, fatigue crack growth experiments have been performed on high and low sulphur Type 316 austenitic stainless steel specimens tested in high temperature water and evaluated via microstructural characterisation techniques. At high strain rates the enhanced crack growth for both specimens appeared to be crystallographic and associated with slip localization. Furthermore, matching fracture surface analysis indicated discrepancy of the slip steps and micro-cleavage cracks between the matching surfaces, suggesting that slip steps and micro-cleavage cracking occurred after the crack-tip had advanced. It was also postulated that their formation may involve cathodically-produced hydrogen and shear deformation on the fracture surface. However, when the loading frequency was decreased, the high S specimens retarded the crack growth and the path was no longer crystallographic. Significant differences in the crack-tip opening displacements were observed in both materials, with blunt crack-tips in the high sulphur specimen and sharp tips in the low sulphur specimen when the strain rate was low. EBSD analysis at the crack-tips of both specimens showed that the strain was more localised at the crack-tip of the low sulphur specimen whist the strain ahead of the high sulphur specimen was more homogenous. It is thus postulated that retardation occurs when slip localisation is no longer the dominant factor. The localised deformation during enhancement is believed to have been caused by hydrogen enhanced localised plasticity (HELP) which causes the crack-tip to sharpen. The diffused strain distribution during crack growth retardation is believed to have been caused by hydrogen enhanced creep (HEC) which causes the crack-tip to blunt. It is also believed that both enhancement and retardation mechanisms are associated with contrasting effects deriving from hydrogen enhanced plasticity. Oxide induced crack closure was excluded as a mechanism responsible for retardation of fatigue crack growth when the stress ratio is high. Effects of hydrogen induced alpha' and ε martensite phases on oxidation behaviour of austenitic stainless steels in deoxygenated high temperature water have also been studied. Microstructural characterisation shows that hydrogen induced alpha' martensite enhances oxidation of austenitic stainless steels in deoxygenated high temperature water. The implications of this finding on environmentally assisted cracking of austenitic stainless steels in deoxygenated high temperature water is discussed.

Made available in DSpace on 2017-07-21T20:43:49Z (GMT). No. of bitstreams: 1 Jamil M Guimaraes Junior.pdf: 6099500 bytes, checksum: a74509c2e4fa6af13be50e78a82c9a13 (MD5) Previous issue date: 2016-07-08 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Stainless steels with properties which allow high conformability and satisfactory mechanical resistance due to phase transformations are becoming prominently active for industries and metallurgical research centers. Therefore, the phenomena related to the work hardening must be understood and mastered, once the strain has an extremely important function on the phase transformation of these materials. For the investigations, are conducted different kinds of strains on the stainless steels by filing, compression and cold rolling at 12%, 31%, 50%, 70%, 80% and 90% reduction levels. In order to compare the materials, are conducted the following technics: optical microscopy, EDS, EBSD, X ray diffraction, hardness measurements and ferritoscope. The high hardness presented by these as received materials is attributed to the fact that they were not annealed. Low rolled steels showed high transformation into ε-martensite, and high rolled levels (like 90%) showed high grain elongation and very high α’- martensite fractions. Filling and compression also showed high α’- martensite fractions for the AISI 304. However, the AISI 316 didn’t show a significant fraction of α’- martensite after the compression test. Therefore, the high hardness showed by AISI 316 steel after the compression test is certainly due to classic work hardening mechanisms, not due to a α’- martensite transformation, which depends on the Stacking Fault Energy (SFE). / Aços inoxidáveis com propriedades que permitam alta conformabilidade aliada a resistência mecânica satisfatória devido a transformações de fase vêm sendo destaque nas indústrias e centros de pesquisa de metalurgia. Para tanto, os fenômenos relacionados ao encruamento devem ser compreendidos e dominados, pois os processos de deformação desempenham uma função extremamente importante na transformação de fases desses materiais. As investigações são conduzidas com deformações plásticas nos aços inoxidáveis por meio de limagem, compressão a 60% de redução da altura e laminação a níveis de 12%, 31%, 50%, 70%, 80% e 90% de redução. Para fins comparativos, são conduzidas análises microestruturais por meio de: microscopia óptica, EDS, EBSD, difração de raios X, medidas de dureza e ferritoscopia. Percebeu-se uma dureza acima do esperado nos materiais inicialmente caracterizados devido ao fato de os mesmos não estarem na condição de recozidos. Baixos níveis de redução na espessura promoveram elevados índices de transformação da austenita para martensita ε, enquanto que altos níveis de redução resultaram em elevado alongamento dos grãos e as maiores frações de martensita α’ entre todas as condições de deformação. Deformação por meio de limagem e por meio de compressão promoveram altos índices de transformação da austenita em martensita α’ para o aço inoxidável AISI 304, porém quando submetido à compressão, o aço AISI 316 não apresentou consideráveis frações de martensita na microestrutura. Sendo assim, conclui-se que o aumento de dureza do aço AISI 316 se deu por mecanismos clássicos de endurecimento, e não pela formação de martensita induzida por deformação. Isso porque a tendência em formar martensita induzida por deformação é dependente da EDE, e consequentemente, da composição química do material.

Au cours d'une irradiation neutronique à long-terme dans les Réacteurs à Eau Pressurisée (REPs), une modification importante du comportement mécanique des matériaux utilisés dans les internes de cuve (composés des aciers inoxydables austénitiques de la série 300) est observée, y compris un durcissement et un adoucissement induit par irradiation, une perte de la ductilité et de la ténacité. Jusqu'à présent, beaucoup efforts ont été contribués pour identifier les effets d'irradiation sur l'évolution microstructurale du matériau (dislocations, boucles de Frank, cavités, ségrégation, etc.). Le gonflement induit par irradiation, considéré comme un facteur limitant la durée de fonctionnement des réacteurs, pourrait modifier les propriétés mécaniques des matériaux (plasticité, ténacité, etc), même conduire à une distorsion des structures du fait des modifications dimensionnelles entre les différentes composantes.L'objectif principal de ce travail de thèse est d'étudier qualitativement l'influence de l'effet du gonflement sur le comportement mécanique des matériaux irradiés. Un modèle micromécanique constitutif en grandes déformations basé sur les évolutions de la densité de dislocations et de défauts d'irradiation (boucles de Frank) est développé et implémenté dans les codes de calcul éléments finis ZéBuLoN et Cast3M. Les simulations numériques sont réalisées pour calculer les propriétés mécaniques d'un agrégat polycristallin. Par ailleurs, la technique d'homogénéisation est appliquée pour développer un modèle de type Gurson. Les simulations d'une cellule poreuse sont utilisés pour étudier le comportement mécanique des monocristaux poreux, en tenant compte des différents effets de la triaxialité, de la porosité et de l'orientation cristallographique, afin d'étudier l'effet de la présence des cavités sur la plasticité et la rugosité du matériau irradié à l'échelle polycristallin. / During long-time neutron irradiation occurred in Pressurized Water Reactors (PWRs), significant changes of the mechanical behavior of materials used in reactor core internals (made of 300 series austenitic stainless steels) are observed, including irradiation-induced hardening and softening, loss of ductility and toughness. So far, much effect has been made to identify radiation effects on material microstructure evolution (dislocations, Frank loops, cavities, segregation, etc.). The irradiation-induced cavity swelling, considered as a potential factor limiting the reactor lifetime, could change the mechanical properties of materials (plasticity, toughness, etc.), even lead to a structure distortion because of the dimensional modifications between different components.The principal aim of the present PhD work is to study qualitatively the influence of cavity swelling on the mechanical behaviors of irradiated materials. A micromechanical constitutive model based on dislocation and irradiation defect (Frank loops) density evolution has been developed and implemented into ZéBuLoN and Cast3M finite element codes to adapt the large deformation framework. 3D FE analysis is performed to compute the mechanical properties of a polycrystalline aggregate. Furthermore, homogenization technique is applied to develop a Gurson-type model. Unit cell simulations are used to study the mechanical behavior of porous single crystals, by accounting for various effects of stress triaxiality, of void volume fraction and of crystallographic orientation, in order to study void effect on the irradiated material plasticity and roughness at polycrystalline scale.

Les aciers inoxydables austénitiques de type 304L et 316L sont largement employés dans le circuit primaire des centrales nucléaires à Réacteurs à Eau sous Pression (REP). Le retour d'expérience indique la présence d'un nombre limité de fissures intergranulaires dues à la corrosion sous contrainte (CSC) sur des composants en acier inoxydable écroui. Il a été démontré qu'une pré-déformation importante associée à un chargement cyclique favorise la propagation des fissures de CSC. L'objectif de l'étude est d'améliorer la compréhension du rôle de la pré-déformation par traction ou par laminage sur les mécanismes d'amorçage de la CSC pour les aciers inoxydables austénitiques. Le comportement mécanique des matériaux écrouis a été caractérisé et des essais d'amorçage en milieu primaire simulé ont été réalisés sur des éprouvettes entaillées. L'ensemble des essais d'amorçage réalisés a confirmé un fort effet du trajet de chargement sur la sensibilité à l'amorçage des matériaux étudiés, quel que soit le niveau de pré-déformation. Un critère global a été proposé pour réunir les deux aspects de l'amorçage de la fissuration que sont la densité de fissures et leur profondeur. Ce critère est utile pour caractériser l'amorçage tandis que la profondeur maximale de fissure est le paramètre pertinent pour définir la transition entre propagation lente et propagation rapide. Des cartes de sensibilité à l'amorçage vrai ont été établies. Une profondeur critique de fissure de 10 à 20 µm a été déterminée pour les aciers 316L A et B pré-déformés par laminage. Elle est comprise entre 20 µm et 50 µm pour les matériaux pré-déformés par traction. Une ébauche de modèle d'ingénierie applicable aux aciers inoxydables austénitiques a été proposée : l'effet de la température est négligeable dans la gamme 290°C-360°C et l'impact de la contrainte sur le temps pour obtenir la transition varie comme (max/Rp0.2, T°C)11,5. L'effet du trajet de chargement ainsi que de l'écrouissage de surface dû à l'usinage sont intégrés indirectement à l'indice de contrainte, à ce stade du modèle. L'effet « matériau » observé dans cette étude tient principalement à l'effet du trajet de déformation. La puissance élevée de la dépendance à la contrainte traduit l'intégration de différents paramètres favorisant la localisation de la déformation. Pour cette raison, il sera nécessaire de définir le champ des contraintes locales pour parvenir à une modélisation plus physique. / Austenitic stainless steels are widely used in primary circuits of Pressurized Water Reactors (PWR) plants. However, a limited number of cases of Intergranular Stress Corrosion Cracking (IGSCC) has been detected in cold-worked (CW) areas of non-sensitized austenitic stainless steel components in French PWRs. A previous program launched in the early 2000's identified the required conditions for SCC of cold-worked stainless steels. It was found that a high strain hardening coupled with a cyclic loading favoured SCC. The present study aims at better understanding the role of pre-straining on crack initiation and at developing an engineering model for IGSCC initiation of 304L and 316L stainless steels in primary water. Such model will be based on SCC initiation tests on notched (not pre-cracked) specimens under “trapezoidal” cyclic loading. The effects of pre-straining (tensile versus cold rolling), cold-work level and strain path on the SCC mechanisms are investigated. Experimental results demonstrate the dominating effect of strain path on SCC susceptibility for all pre-straining levels. Initiation can be understood as crack density and crack depth. A global criterion has been proposed to integrate both aspects of initiation. Maps of SCC initiation susceptibility have been proposed. A critical crack depth between 10 and 20 µm has been demonstrated to define transition between slow propagation and fast propagation for rolled materials. For tensile pre-straining, the critical crack depth is in the range 20 - 50 µm. Experimental evidences support the notion of a KISCC threshold, whose value depends on materials, pre-straining ant load applied. The initiation time has been found to depend on the applied loading as a function of (max/YV)11,5. The effect of both strain path and surface hardening is indirectly taken into account via the yield stress. In this study, material differences rely on strain path effect on mechanical properties. As a result, a stress high exponent has been identified which includes all micro-scale mechanisms leading to strain localisation at initiation sites.

Abstract The effects of restoration and certain elements, nitrogen, sulphur, calcium and Misch metal, on the hot ductility of austenitic, high-alloyed austenitic and duplex stainless steels have been investigated by means of hot rolling, hot tensile, hot bending and stress relaxation tests. The results of these different testing methods indicated that hot rolling experiments using stepped specimens is the most effective way to investigate the relationship between the softening and cracking phenomena under hot rolling conditions. For as-cast, high-alloyed and duplex stainless steels with a low impurity level, the cracking tendency was observed to increase with increasing pass strain and temperature, being minimal for the small strain of 0.1. No cracking occurred in these steels when rolled in the wrought condition. It could be concluded that the cracking problems are only exhibited by the cast structure with the hot ductility of even partially recrystallised steel being perfectly adequate. However, the recrystallisation kinetics of the high-alloyed austenitic stainless steels, determined by stress relaxation and double-pass rolling tests, were found to be so slow that only partial softening can be expected to occur between roughing passes under normal rolling conditions. In the duplex steel, the restoration is fairly fast so that complete softening can occur within typical interpass times in hot rolling, while certain changes in the phase structure take place as well. Sulphur was found to be an extremely harmful element in duplex stainless steel with regard to their hot ductility so that severe cracking can take place with sulphur content above 30 ppm. However, the effect of sulphur can be eliminated by reducing its content and by calcium or Misch metal treatments that significantly increase the number and decrease the average size of the inclusions. It seems that the desulphurisation capacity of an element is the most important property for assessing its usefulness in reducing the detrimental influence of sulphur. The hot ductility of type 316L stainless steel determined by tensile tests was found to be better for nitrogen content of 0.05 wt-% than 0.02%, while in double-hit tensile tests the hot ductility values were identical. The mechanism whereby nitrogen affects hot ductility remains unclear but a retarding effect on static recrystallisation was observed.