Behavior of alloy 617 at 650°C in low oxygen pressure environments

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 108-112).The behavior of alloy 617 at 650°C in low oxygen partial pressure environments has been studied under static loading. Of particular interest was the crack growth rate in these conditions. For that, tests were conducted at a constant stress intensity factor of 49.45 MPa'm (45 ksidinch) using a direct current potential drop measurement system to determine crack length. High purity argon gas allowed establishing an oxygen partial pressure as low as 10-22 atm and premixed oxygen/argon gases were used to vary the oxygen potential. To go dee er into the understanding of the phenomena involved, a creep test (constant load of 1.21x10 N in argon environment) and a corrosion experiment (50 ppm oxygen in argon for 500 hours without any load) were also added. The crack growth tests led to a particularly unexpected result: whatever the oxygen potential, the crack growth rate increased with time and the plot for the crack length versus time displayed a recurrent parabolic shape without any change in the environment. No unique crack growth rate could be defined in a given environment and the influence of the environment on the crack growth rate was not clearly visible. Multiple features were found to surround the main crack: secondary cracks parallel to the principal one, intergranular cracking ahead of the crack tip, wedge cracks at grain boundaries and aggregates of Cr-rich carbides near the lips of the crack. Moreover no extensive oxide scale was formed on the surface of the sample exposed to the corrosive environment (50 ppm oxygen in argon) for 500 hours. The non-constant crack growth rates, together with the observed cracking features, were attributed to the competition between creep deformation and mechanical fracture, likely environmentally enhanced. An exponential law was found to fit the data for the crack growth rate as a function of time for a K of 49.45 MPalm (45 ksi'inch). The effects of the environment were overcome by mechanical and thermal processes leading to damage accumulation and so, a reaction of alloy 617 to the external stress and temperature highly dependent on time. This behavior was compared with the one of alloys Haynes 230 and Incoloy 908 in the same conditions.by Fanny Mas.S.M

Growth Morphologies and Primary Solidification Modes in a Dissimilar Weld between a Low-Alloy Steel and an Austenitic Stainless Steel

Dissimilar welds close to the fusion boundary exhibit a variety of solidification microstructures that strongly impact their service behavior. Investigations were therefore undertaken to clarify the origins of the morphological and microstructural evolutions encountered in a 18MND5/309L dissimilar joint produced by submerged arc welding, using a combination of microstructural characterizations, thermodynamic computations, and solidification modelling. An unexpected evolution was observed in the solidification mode, from primary austenite towards primary ferrite with increasing growth rate. Solidification of austenite at the fusion boundary was assigned to its epitaxial growth on the metastable austenitic structure of the base metal resulting from an incipient melting mechanism. The evolution of the solidification mode toward primary ferrite was explained based on computations of the solute built up between austenite cells followed using the so-called “interface response function model”. Analyzing macro- and microstructural characteristic lengths with the published solidification model and data enabled evaluation of local values of the solidification rate, thermal gradient, and cooling rate close to the fusion boundary, thus providing useful data for numerical modelling of the submerged arc-welding process

Solidification et transformations de phases dans une soudure dissimiliaire 18MND5/309l/308L : évolution de la microstructure et des propriétés mécaniques

Dissimilar welds between low-alloy steel and stainless steel are numerous within the French nuclear power plants where they enable connecting the main components to the primary circuit pipes. The internal cladding (in stainless steel) of the pressure vessel (in bainitic steel) made by submerged arc welding is another case of dissimilar weld whose goal is the protection against corrosion. This PhD work aims at understanding the complex microstructures which form at the interface between both steels during welding together with their evolution during the post-weld heat-treatment at 610°C and their consequences on the mechanical behavior of the welded assembly. Starting from the base metal, one meets successively a thin layer of martensite, a fully austenitic zone and the two-phase δ/γ microstructure of the stainless steel. Microscopy techniques (SEM, EDS, EBSD) combined with thermo-kinetics calculations (Scheil-Gulliver model, dendrite tip undercooling) have allowed explaining the graded microstructure and the reasons for the observed phase transitions. During the post-weld heat-treatment, the large gradient of carbon chemical potential across the fusion line leads to cementite dissolution and grain growth on the low-alloyed side. Carbon diffusion through the interface and Cr-rich carbides precipitation in both the martensitic layer and the austenitic weld have also been observed. An in-depth characterization has been performed at different scales (from the millimeter to the atomic level) to quantify the extent of carbon diffusion and carbides precipitation during the phase transformations. A mesoscopic thermodynamic and kinetic model based on Calphad databases has been developed to fully couple long-range diffusion in a multi-component system with precipitates nucleation and growth (Numerical Kampmann-Wagner approach). It allowed a prediction of the carbon content, volume fraction and size distribution of the precipitates at any distance from the fusion line. The consequences of the high variability of microstructures on the local mechanical behavior have been examined in the last part of this work, in particular the localization of deformation and the ductile failure. Elasto-plastic constitutive laws were determined for each region of the dissimilar weld in the heat-treated state. Ductile failure mechanisms in the weak zones of the weld, namely the decarburized base metal and the stainless steel cladding layers, were investigated through in-situ observations and damage modeling.Les liaisons bimétalliques entre acier faiblement allié et acier inoxydable sont nombreuses au sein des réacteurs nucléaires français où elles assurent la connexion entre les principaux composants et les tuyauteries du circuit primaire. Le revêtement interne de cuve réalisé par soudage feuillard-flux est un autre cas de soudure dissimilaire dont le rôle est d'assurer une bonne protection contre la corrosion. Ce travail de thèse a notamment pour but de comprendre la genèse des microstructures complexes se formant à l'interface entre les deux aciers pendant le soudage. Il étudie d'autre part leur évolution durant le traitement thermique post-soudage à 610°C ainsi que les conséquences de ces transformations sur le comportement mécanique du joint soudé. Partant du métal de base, on rencontre successivement une fine bande de martensite, une zone purement austénitique puis la microstructure biphasée δ/γ de l'acier inoxydable. Des techniques de microscopie (MEB, EDS, EBSD) combinées à des calculs thermo-cinétiques (modèle de Scheil-Gulliver, surfusion en pointe de dendrite) ont permis d'expliquer le gradient de microstructure et les raisons des transitions de phases observées. Au cours du traitement thermique de détensionnement à 610°C, le gradient de potentiel chimique du carbone à travers l'interface de fusion cause la dissolution de la cémentite et la croissance des grains du côté faiblement allié. On observe également la diffusion du carbone à travers l'interface et la précipitation de carbures riches en Cr dans le liseré martensitique et la zone austénitique. Une caractérisation détaillée des profils de composition et de la précipitation a été réalisée à différentes échelles (depuis le millimètre jusqu'au niveau atomique). Un modèle mésoscopique, s'appuyant sur des bases de données thermodynamiques et cinétiques, a été développé pour coupler la diffusion à longue distance dans un milieu multi-constitué à la germination et croissance des précipités (approche de type Kampmann-Wagner). Il a permis de prévoir la teneur en carbone ainsi que la fraction volumique et la distribution de taille des précipités en fonction de la distance à l'interface. Les conséquences de la forte variabilité de microstructure sur le comportement mécanique local ont été analysées dans la dernière partie de ce travail, en particulier les aspects de déformation localisée et de rupture ductile. Des lois de comportement élasto-plastique ont été déterminées pour chacune des régions de l'assemblage à l'état détensionné. L'étude des mécanismes de rupture ductile dans les zones les plus faibles de la soudure, c'est-à-dire le métal de base décarburé et les couches de revêtement austénitique a donné lieu à des observations in-situ et une modélisation de l'endommagement

Development of a plane strain tensile test to characterize the formability of 5xxx and 6xxx aluminium alloys

SharedIt link: https://rdcu.be/du0UfInternational audienceAccessible at SharedIt link: https://rdcu.be/du0Uf.This article presents the development of a plane strain tensile test aiming at an easy classification of aluminium automotive alloys according to their formability in prototyping steps. A parametric study with finite element method is performed on three different designs inspired by literature. It is found that, due to plastic anisotropy, specimens designed for steel are not suited for aluminium alloys. One optimized specimen geometry, ensuring near plane strain state on a large zone all along the deformation range up to failure, is selected. On this geometry, tensile tests instrumented by Digital Image Correlation are performed for five different aluminium alloys (5xxx and 6xxx) in three different directions of the metal sheet (rolling, diagonal and transverse). From Digital Image Correlation analysis, necking limits are evaluated and their relevance for the ranking of alloys according to their formability is discussed in comparison with a standard formability test, namely the Limiting Dome Height test

Effect of Minor Alloying Elements (Ag, Mg, Zn) and Temperature on Natural Ageing Kinetics of Al-Cu-Li Alloys

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Carbon quantification in a dissimilar steel weld

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Growth Morphologies and Primary Solidification Modes in a Dissimilar Weld between a Low-Alloy Steel and an Austenitic Stainless Steel

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Some new applications of a second-gradient model for porous ductile materials

International audienceA second-gradient model for porous ductile materials extending the standard GTN first-gradient model (Gurson, 1977; Tvergaard, 1981; Tvergaard and Needleman, 1984) was proposed by Gologanu et al. (1997), with the aim of solving the problem of potentially unlimited lo-calization of strain and damage resulting in mesh sensitivity in finite element computations. An efficient numerical implementation of Gologanu et al. (1997)'s model has been proposed by Bergheau et al. (2014), using an innovative procedure of elimination of the additional nodal degrees of freedom representing the strains ("nodal strains"). The aim of this paper is to present some new applications of the model and associated numerical algorithm. The first, relatively simple application consists of 2D numerical simulations of an experiment of ductile rupture of some pre-notched and pre-cracked CT specimen. The goal here is essentially to illustrate one major advantage of the procedure of elimination of the nodal strains, the possibility of easily mixing elements obeying first-and second-gradient models, and thus using the latter type of model only in those limited zones where it is really needed. The second, more complex application , concerns the 3D numerical simulation of crack propagation over a long distance in a multiphase material. The aim here is to illustrate the possibility of using the model, in spite of its sophistication, for the study of complex fracture problems of practical, industrial interest

Microstructures formation and phase transformations around the interface of welds between dissimilar steels

The goal of this study is to understand the complex microstructures which form at the interface between a low-alloy steel and a stainless steel during welding and subsequent heat-treatment at 610 ∘C. A dual approach with both experimental measurements and thermodynamic and kinetic modeling has been chosen for both solidification and solid-state phase transformations. A very good agreement has been obtained for the carbon profile across the interface and the nature of phases which form

Heterogeneities in local plastic flow behavior in a dissimilar weld between low-alloy steel and stainless steel

In dissimilar welds between low-alloy steel and stainless steel, the post-weld heat-treatment results in a high variety of microstructures coexisting around the fusion line, due to carbon diffusion and carbides dissolution/precipitation. The local constitutive laws in the vicinity of the fusion zone were identified by micro tensile specimens for the sub-millimeter sized zones, equivalent bulk materials representing the decarburized layer using both wet H2 atmosphere and diffusion couple, and nano-indentation for the carburized regions (i.e. the martensitic band and the austenitic region). The decarburized zone presents only 50% of the yield strength of the low-alloy steel heat affected zone and a ductility doubled. The carburized zones have a yield strength 3–5 times higher than that of the low-alloy steel heat affected zone and have almost no strain hardening capacity. These properties result in heterogeneous plastic deformation happening over only millimeters when the weld is loaded perpendicularly to the weld line, affecting its overall behavior. The constitutive laws experimentally identified were introduced as inputs into a finite elements model of the transverse tensile test performed on the whole dissimilar weld. A good agreement between experiments and simulations was achieved on the global stressstrain curve. The model also well predicts the local strain field measured by microscale DIC. A large out-of-plane deformation due to the hard carburized regions has also been identified