La France prévoit de construire des réacteurs nucléaires de 4e génération. Ils utiliseraient du sodium liquide comme fluide caloporteur et seraient construits en acier austénitique 316L(N). Afin de garantir un fonctionnement optimal, la tenue de cet acier doit être vérifiée. Pour cela, la corrosion de l’acier 316L(N) par le sodium liquide doit être bien connue.La littérature montre que plusieurs phénomènes de corrosion sont possibles. Pour chacun de ces phénomènes, l’influence de la présence d’oxygène dans le sodium est grande. Nous avons donc étudié la corrosion des aciers austénitiques par le sodium liquide en présence d’oxygène.Les données thermodynamiques permettent de montrer que la formation de chromite de sodium est possible sur les aciers austénitiques immergés dans le sodium contenant de l’ordre de 10 μg.g-1 d’oxygène pour des températures inférieures à 650 °C (conditions réacteurs).L’étude expérimentale montre que la chromite de sodium se forme à 650 °C dans le sodium contenant 200 μg.g-1 d’oxygène. À cette même concentration et à 550 °C, la chromite de sodium est observée avec certitude uniquement pour les longues durées d’immersion (> 5000 h). Les résultats à 450 °C sont moins évidents. Par ailleurs, l’acier est appauvri en chrome dans toutes les conditions étudiées.Ces résultats suggèrent que la chromite de sodium se dissout dans le sodium au fur et à mesure de sa formation. Des modèles de formation de la chromite de sodium, approchéelimitée par la diffusion du chrome dans l’acier (en volume et aux joints de grains) et de dissolution, supposée limitée par le transport dans le métal liquide ont permis de montrer que la formation et la dissolution simultanée de la chromite de sodium est un mécanisme possible pour expliquer nos résultats. / France is planning to construct the 4th generation of nuclear reactors. They will use liquid sodium as heat transfer fluid and will be made of 316L(N) austenitic steel as structural materials. To guarantee optimal operation on the long term, the behavior of this steel must be verified. This is why corrosion phenomena of 316L(N) steel by liquid sodium have to be well-understood.Literature points out that several corrosion phenomena are possible. Dissolved oxygen in sodium definitely influences each of the corrosion phenomenon. Therefore, the austenitic steel corrosion in oxygen-containing sodium is proposed in this study.Thermodynamics data point out that sodium chromite formation on 316L(N) steel is possible in sodium containing roughly 10 μg.g-1 of oxygen for temperature lower than 650 °C (reactor operating conditions).The experimental study shows that sodium chromite is formed at 650 °C in the sodium containing 200 μg.g-1 of oxygen. At the same concentration and at 550 °C, sodium chromite is clearly observed only for long immersion time (> 5000 h). Results at 450 °C are more difficult to interpret. Furthermore, the steel is depleted in chromium in all cases.The results suggest the sodium chromite is dissolved in the sodium at the same time it is formed. Modelling of sodium chromite formation - approached by chromium diffusion in steel (in grain and grain boundaries -, and dissolution - assessed by transport in liquid metal - show that simultaneous formation and dissolution of sodium chromite is a possible mechanism able to explain our results.

A combination of fractography, microstructural analysis and finite element modelling was used to investigate several topics relating to the fatigue of nuclear grades of austenitic stainless steel operating in both air and simulated PWR water environments. The work is broadly separated into four main categories. The first two involved analysing specimens from standard fatigue endurance tests using a wide range of microscopic techniques. The relevance and uses of a modern laser scanning confocal microscope are presented and the benefits of using such a technique are discussed. Methods for the automation of both striation counting procedures and hysteresis data analysis are described and the results are demonstrated. Finite element analyses were performed in order to develop the understanding of fatigue crack growth within standard cylindrical endurance specimens. A variety of different crack tip parameters were used in order to develop expressions for crack growth rates in terms of the strain intensity factor and the J-integral. The derived expressions were compared to the results of striation spacing measurements from multiple endurance specimens that were tested in both air and water environments. The expressions were used to perform back-fitting calculations on standard endurance curves in order to produce alternative curves representing the number of loading cycles to cause the initiation of short cracks with depths in the range of 0.25-0.5 mm. The effects of hold-times on the fatigue life of stainless steel endurance specimens were explored as part of the international AdFaM research programme. Results from the programme partners are presented which demonstrate the beneficial effects of static hold-times on extending the fatigue lifetime of specimens. A range of microstructural analyses were performed on test specimens and results are presented. No significant effects of hold-times on microstructure, crack growth rates or material hardness were found. Analysis of hysteresis data demonstrated an increase in the cyclic hardening and a decrease in the plastic strain range after a hold. From an analysis of the fatigue test results, it was concluded that hold-times affect the earliest stages of fatigue (nucleation and initiation), most likely due to the effects of strain ageing. Several possible explanations for the observed phenomenon of specimen shrinkage during static holds are presented and discussed, however no conclusive explanation was identified. Further work is identified that could lead to future improvements in the understanding of all areas of investigation that have been reported. Overall, the work reported here has helped to develop the understanding of fatigue behaviour and mechanisms in the materials of interest. This was done through investigations using a synergistic combination of microscopy and numerical modelling techniques.

Os aÃos inoxidÃveis austenÃticos (AIAs) sÃo largamente aplicados nas indÃstrias de alimentos, transportes nuclear, petroquÃmica devido à adequada combinaÃÃo de resistÃncia mecÃnica, conformabilidade e resistÃncia à corrosÃo. Dentre estes tipos de aÃo, destaca-se o AISI 301 por sua resistÃncia mecÃnica superior. Entretanto, este tipo de aÃo apresenta um dos piores desempenhos em termos de corrosÃo. Toda a resistÃncia à corrosÃo dos AIAs se baseia em sua camada de filme passivo contendo Cr203 que pode facilmente ser destruÃda em ambientes contendo cloreto. Neste trabalho, estabeleceu-se a meta de estudar os aÃos AISI 301 LN e 316L em respeito Ãs mudanÃas na microestrutura por imposiÃÃo de deformaÃÃo e seu efeito na corrosÃo por pites, visto que o AISI 301LN foi escolhido como material base dos vagÃes que servirÃo o sistema de transporte metropolitano de Fortaleza. Empregou-se difraÃÃo de raios âX, metalografia quantitativa, microscopias Ãtica, eletrÃnica de varredura e de forÃa atÃmica para caracterizar a microestrutura, textura cristalogrÃfica, caracterizaÃÃo magnÃtica, microdureza e ensaio de imersÃo em FeCl3 6H2O para caracterizar o comportamento dos dois aÃos em corrosÃo por pites. A deformaÃÃo provocou a formaÃÃo de martensita â no aÃo 301LN e encruamento da austenita. Isto provocou o mais baixo desempenho em corrosÃo por pites. A textura cristalogrÃfica forneceu indÃcios para inferir que a transformaÃÃo austenita-ferrita se deu obedecendo a relaÃÃo de KURDJUMOV-SACHS.

Global increase in energy consumption and global warming require more energy production but less CO2emission. Increase in efficiency of energy production is an effective way for this purpose. This can be reached by increasing boiler temperature and pressure in a biomass power plant. By increasing material temperature 50°C, the efficiency in biomass power plants can be increased significantly and the CO2emission can be greatly reduced. However, the materials used for future biomass power plants with higher temperature require improved properties. Austenitic stainless steels are used in most biomass power plants. In austenitic stainless steels a phenomenon called dynamic strain aging (DSA), can occur in the operating temperature range for biomass power plants. DSA is an effect of interaction between moving dislocations and solute atoms and occurs during deformation at certain temperatures. An investigation of DSA influences on ductility in austenitic stainless steels and nickel base alloys have been done. Tensile tests at room temperature up to 700°C and scanning electron microscope investigations have been used. Tensile tests revealed that ductility increases with increased temperature for some materials when for others the ductility decreases. This is, probably due to formation of twins. Increased stacking fault energy (SFE) gives increased amount of twins and high nickel content gives a higher SFE. Deformation mechanisms observed in the microstructure are glide bands (or deformations band), twins, dislocation cells and shear bands. Damage due to DSA can probably be related to intersection between glide bands or twins, see figure 6 a). Broken particles and voids are damage mechanisms observed in the microstructure.

The austenitic stainless steel 316L has been compared to duplex stainless steels to be able to highlight a choice of material for manufacturing of spare tanks used for cooling water at nuclear power stations on the Swedish west coast. In this report 316L and the duplex stainless steels 2205, 2304 and LDX 2404 have been compared according to corrosion resistance, strength, manufacturing aspects and prices. The steels arranged by increasing corrosion resistance: 316L < 2304 < LDX 2404 < 2205. The steels arranged by increasing strength (considering the thickness of the plates needed for construction): 316L < 2304 < LDX 2404 and 2205. The steels arranged by increasing price/tank: 2304 < LDX 2404 < 2205 < 316L. One of the duplex stainless steels is recommended rather than the austenitic stainless steel 316L. In terms of price 2304 is preferable to 2205 and LDX 2404. When it comes to corrosion resistance 2205 is superior to 2304 and can sometimes be considered as unnecessary good and therefore not relevant for this application.

The requirement for lighter, safer and fuel efficient cars has created a major stir in the steel research society to develop advanced automotive steels. Since there is a trade off between strength and ductility, most of the conventional high strength steels do not address the strength-formability combination. With the realization of the TRIP phenomenon first in austenitic stainless steels, a new generation of advanced steels called TRIP steels were realised with an inexpensive and easier to process C-Mn-Si chemistry. TRIP or TRransformation Induced Plasticity is a phenomenon where the timely strain induced transformation of Retained Austenite (RA) to Martensite locally strengthens the steel at the point of plastic instability, causing failure by necking to be postponed and shifted elsewhere along the steel. This phenomenon repeated over and over again allows increased levels of strength and ductility, prior to fracture. / In current TRIP grades, the retained austenite particles present have to posses certain characteristics such as, optimum carbon concentration, optimum grain size and morphology etc. in order to account toward mechanical properties. Such limiting characteristics in turn minimize the processing window and make TRIP processing expensive and difficult to control. In this work, it is suggested that Tempered Martensite Assisted Steels (TMAS) obtained from TRIP steels via subcritical annealing of cold rolled TRIP steels may potentially replace TRIP steels. Relationship between the retained austenite volume fractions and mechanical properties was developed for TRIP steels. The effect of variation of retained austenite on tempered martensite volume fraction in TMAS, which in turn affect the mechanical properties was also investigated in depth. Results indicate that tempered martensite particles in TMAS do not have any limiting factors as in the case of RA in TRIP steels, in order to contribute toward enhancement of mechanical properties. Results also indicate that TMAS offers better strength levels compared to TRIP steels for same the level of formability. / Retained austenite volume fractions in TRIP steels were measured through XRD. Cold rolling of the samples was done in a laboratory scale rolling machine. The microstructures were analysed using conventional and color etching techniques. A new color etching technique for viewing all the four major phases in TRIP steel was developed in this work. The mechanical properties of both TRIP and TMAS were assessed by shear punch testing. And finally, the relationship between tempered martensite volume fraction and TMAS properties was developed and was compared to TRIP properties.

The aim of this thesis is to investigate the effects of Mn content and alloying additions such as Cr and Mo, and various heat treatment procedures on both microstructure and mechanical properties of austenitic manganese (Hadfield) steels. For this purpose, steels with two different Mn content were considered (12-14 Mn, 16-18 Mn). First, five different heat treatment procedures were applied to the as-cast 12-14 Mn specimens to decide the procedure resulting the optimum tensile properties. Then, the specimens having various amounts of Mn, Cr and Mo were cast and heat-treated to investigate the effect of alloy modifications on austenitic manganese steels. Optical and scanning electron microscopies were used for microstructural investigation. To determine the mechanical properties, tensile tests and hardness tests were carried out. In addition to correlation between microstructure and mechanical properties, ultrasonic velocity measurements were also done. The results show that both composition and heat treatment affect the performance of hadfield steels extensively, and these changes also affect the propogation velocity of the ultrasonic waves.

Austenitic stainless steels have a major problem during solution annealing or welding in the temperature range of 500-800 &deg / C due to the formation of chromium carbide, which causes chromium depleted areas along grain boundaries. This means that the structure has become sensitized to intergranular corrosion. Susceptibility to intergranular corrosion can be determined by means of destructive acid tests or by nondestructive electrochemical potentiokinetic reactivation (EPR) tests. The EPR test, which provides quantitative measurements, can be practiced as single loop or double loop. Single loop EPR method for AISI 304 and 304L type stainless steels was standardized / however double loop EPR (DLEPR) method has not been validated yet. In this study, the degree of sensitization was examined in AISI 304L and 316L type steels by DLEPR method whose experiments have been carried out on sensitive and nonsensitive steels to examine and determine the detailed parameters / solution temperature, concentration and scan rate of the DLEPR method. In order to determine the degree of sensitization, oxalic acid, Huey and Streicher tests were carried out and revealed microstructures and measurements of weight loss by the acid tests were then correlated with DLEPR method results, as a first step towards standardization of DLEPR method for 316L steels. Best agreement was provided with test parameters which are 1M H2SO4 + 0.005M KSCN at 3 V/hr scan rate with 30 &deg / C solution temperature. It was concluded that specimens can be classified as step, dual and ditch, if the Ir:Ia ratios were obtained to be between 0 to 0.15, 0.15 to 4.0 and 4.0 to higher respectively.

Bulk nanocrystalline 304L and 316L austenitic stainless steels (SS) were produced by equal channel angular pressing(ECAP) at elevated temperature. The average grain size achieved in 316L and 304 L SS is ~ 100 nm, and grain refinement occurs more rapid in 316 L SS than that in 304L. Also the structures are shown to retain a predominant austenite phase. Hardness increases by a factor of about 2.5 in both steels due largely to grain refinement and an introduction of a high density of dislocations. Tensile strength of nanocrystalline steels exceeds 1 GPa with good ductility in both systems. Mechanical properties of ECAPed 316L are also shown to have less dependence on strain rate than ECAPed 304L. ECAPed steels were shown to exhibit thermal stability up to 600oC as indicated by retention of high hardness in annealed specimens. Furthermore, there is an increased tolerance to radiation-induced hardening in the nanocrystalline equiaxed materials subjected to 100 keV He ions at an average dose of 3-4 displacement-per-atom level at room temperature. The large volume fraction of high angle grain boundaries may be vital for enhanced radiation tolerance. These nanocrystalline SSs show promise for further research in radiation resistant structural materials for next-generation nuclear reactor systems.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Dr. Richard W. Neu; Committee Member: Dr. David L. McDowell; Committee Member: Dr. Itzhak Green.