Isothermal Oxidation Behaviour of NiCoCrAlYTa Coatings Produced by HVOF Spraying and Tribomet™ Process

Protective NiCoCrAlYTa coatings are used on gas turbine single crystal superalloy blades to provide environmental resistance. They can be deposited by several processes. In this study, isothermal oxidation behaviour of NiCoCrAlYTa coatings produced by HVOF spraying and Tribomet TM process and deposited on single crystal nickel-based superalloy CMSX-4 were compared between 950 and 1,150°C for several exposure durations. Microstructure and chemical composition of both coatings were examined before and after oxidation testing and quite similar observations were made for both coating processes. The combination of phase and chemical analyses allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation conditions

High temperature creep behavior and damage of thin-walled single crystal superalloys : influence of thermal cycling and MCrAlY coating

Les pales de turbine Haute Pression des turbines d'hélicoptères sont soumises à des sollicitations cycliques, thermiques et mécaniques, complexes. L'objectif de ce travail est de caractériser l'influence du cyclage thermique sur le comportement et l'endommagement en fluage de systèmes MCrAlY/superalliage à base de nickel et d'étudier les interactions entre le substrat et le revêtement à haute température. Dans un premier temps, des essais de vieillissement et de fluage isothermes ont été menés afin d'évaluer les dégradations environnementales et mécaniques de systèmes superalliage monocristallin CMSX-4/revêtement NiCoCrAlYTa, déposé selon différents procédés (codéposition électrolytique, projection HVOF). Ces essais de vieillissement isotherme et l'analyse microstructurale qui a été réalisée ont notamment permis d'évaluer le pouvoir protecteur des revêtements entre 950 et 1150°C. Des diagrammes d’occurrence de phases représentatifs de l'état de dégradation de ces revêtements ont été construits et ont montré une équivalence des deux procédés vis-à-vis de la protection du superalliage. Les essais de fluage isotherme ont ensuite confirmé cette équivalence et ont constitué une référence pour les essais de fluage cyclés thermiquement. La comparaison des essais de vieillissement et fluage isothermes a également montré que l’épaisseur de la zone d’interdiffusion entre le superalliage et le revêtement était indépendante de l’application d’une contrainte. Dans un second temps, une étude approfondie de l'influence du cyclage thermique sur le comportement en fluage à 1150°C des superalliages a été menée tout d'abord sur le superalliage MC2, pour évaluer l'effet des paramètres qui caractérisent les cycles thermiques (durée du palier à haute température, vitesses de refroidissement, effet de la température basse du cycle). L'effet du cyclage sur la vitesse de déformation et sur la durée de vie à rupture a été confirmé, et le rôle de la répétition des cycles thermiques sur l'accélération prématurée de l'endommagement de la structure du substrat et la déformation accrue a été mis en évidence. Enfin, le comportement en fluage cyclé thermiquement du système CMSX-4/NiCoCrAlYTa déposé par codéposition électrolytique a été caractérisé aux températures 1050 et 1150°C. Pour ces différentes conditions, particulièrement à 1150°C, il a de nouveau été montré sur ce matériau que les séquences rapides et successives de dissolution - re-précipitation de la phase γ' induisaient une accélération et une généralisation dans le substrat de la déstabilisation de la morphologie en radeaux. Dans une moindre mesure, il a également été montré que le cyclage thermique affecte la zone d'interdiffusion plus en profondeur dans le substrat.Turbine blades of helicopters are subjected to complex thermal and mechanical cycles. The purpose of this work is to characterize the influence of the thermal cycling on high temperature creep behaviour and damage of MCrAlY coating / single crystal nickel-based superalloys systems, and to study the interactions between the superalloy and the coating. Firstly oxidation and isothermal creep tests have been performed to measure the environmental and mechanical resistance of systems made of CMSX-4 superalloy / NiCoCrAlYTa coating deposited by several processes (Tribomet process and HVOF spraying). Microstructure and chemical composition of both coatings were examined after oxidation and creep testing and quite similar observations were made for both coating processes. The combination of phases and chemical analysis after oxidation testing allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation and creep conditions. Moreover the growth of the inderdiffusion zone between coating and superalloy is independent of the stress. Secondly the influence of thermal cycling parameters on creep properties of the MC2 single crystal nickel-based superalloy has been studied at 1150°C and 80MPa.We have demonstrated that thermal cycling creep was more damaging than isothermal creep in terms of creep lifetime and rates. Furthermore, the number of low temperature incursions and low cooling and heating rates have severe detrimental effect on creep resistance, whereas the low temperature value of the thermal cycle is not significant. These results specify the role of re-precipitation and dissolution of γ’ particles during thermal cycling creep. Finally the effect of thermal cycling on high temperature creep of thin-walled and coated CMSX-4 single crystal superalloy was evaluated by performing thermal cycling and isothermal creep tests at 1050°C and 1150°C. The deleterious effect of thermal cycling on the creep behavior and lifetime has been confirmed, particularly for the condition at 1150°C and 80 MPa. Furthermore, the number of low temperature incursions has severe detrimental effect on creep resistance on account of the rafted microstructure destabilization induced by these sequences. These results confirm the role of re-precipitation and dissolution of γ’ particles during thermal cycling creep. Thermal cycling infers also on destabilization of subcoating zone in the superalloy but this effect seems to be secondary

Comportement et endommagement en fluage à haute température de parois minces en superalliages monocristallins (influence du cyclage thermique et du revêtement McrAlY)

Les pales de turbine Haute Pression des turbines d'hélicoptères sont soumises à des sollicitations cycliques, thermiques et mécaniques, complexes. L'objectif de ce travail est de caractériser l'influence du cyclage thermique sur le comportement et l'endommagement en fluage de systèmes MCrAlY/superalliage à base de nickel et d'étudier les interactions entre le substrat et le revêtement à haute température. Dans un premier temps, des essais de vieillissement et de fluage isothermes ont été menés afin d'évaluer les dégradations environnementales et mécaniques de systèmes superalliage monocristallin CMSX-4/revêtement NiCoCrAlYTa, déposé selon différents procédés (codéposition électrolytique, projection HVOF). Ces essais de vieillissement isotherme et l'analyse microstructurale qui a été réalisée ont notamment permis d'évaluer le pouvoir protecteur des revêtements entre 950 et 1150C. Des diagrammes d occurrence de phases représentatifs de l'état de dégradation de ces revêtements ont été construits et ont montré une équivalence des deux procédés vis-à-vis de la protection du superalliage. Les essais de fluage isotherme ont ensuite confirmé cette équivalence et ont constitué une référence pour les essais de fluage cyclés thermiquement. La comparaison des essais de vieillissement et fluage isothermes a également montré que l épaisseur de la zone d interdiffusion entre le superalliage et le revêtement était indépendante de l application d une contrainte. Dans un second temps, une étude approfondie de l'influence du cyclage thermique sur le comportement en fluage à 1150C des superalliages a été menée tout d'abord sur le superalliage MC2, pour évaluer l'effet des paramètres qui caractérisent les cycles thermiques (durée du palier à haute température, vitesses de refroidissement, effet de la température basse du cycle). L'effet du cyclage sur la vitesse de déformation et sur la durée de vie à rupture a été confirmé, et le rôle de la répétition des cycles thermiques sur l'accélération prématurée de l'endommagement de la structure du substrat et la déformation accrue a été mis en évidence. Enfin, le comportement en fluage cyclé thermiquement du système CMSX-4/NiCoCrAlYTa déposé par codéposition électrolytique a été caractérisé aux températures 1050 et 1150C. Pour ces différentes conditions, particulièrement à 1150C, il a de nouveau été montré sur ce matériau que les séquences rapides et successives de dissolution - re-précipitation de la phase g' induisaient une accélération et une généralisation dans le substrat de la déstabilisation de la morphologie en radeaux. Dans une moindre mesure, il a également été montré que le cyclage thermique affecte la zone d'interdiffusion plus en profondeur dans le substrat.Turbine blades of helicopters are subjected to complex thermal and mechanical cycles. The purpose of this work is to characterize the influence of the thermal cycling on high temperature creep behaviour and damage of MCrAlY coating / single crystal nickel-based superalloys systems, and to study the interactions between the superalloy and the coating. Firstly oxidation and isothermal creep tests have been performed to measure the environmental and mechanical resistance of systems made of CMSX-4 superalloy / NiCoCrAlYTa coating deposited by several processes (Tribomet process and HVOF spraying). Microstructure and chemical composition of both coatings were examined after oxidation and creep testing and quite similar observations were made for both coating processes. The combination of phases and chemical analysis after oxidation testing allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation and creep conditions. Moreover the growth of the inderdiffusion zone between coating and superalloy is independent of the stress. Secondly the influence of thermal cycling parameters on creep properties of the MC2 single crystal nickel-based superalloy has been studied at 1150C and 80MPa.We have demonstrated that thermal cycling creep was more damaging than isothermal creep in terms of creep lifetime and rates. Furthermore, the number of low temperature incursions and low cooling and heating rates have severe detrimental effect on creep resistance, whereas the low temperature value of the thermal cycle is not significant. These results specify the role of re-precipitation and dissolution of g particles during thermal cycling creep. Finally the effect of thermal cycling on high temperature creep of thin-walled and coated CMSX-4 single crystal superalloy was evaluated by performing thermal cycling and isothermal creep tests at 1050C and 1150C. The deleterious effect of thermal cycling on the creep behavior and lifetime has been confirmed, particularly for the condition at 1150C and 80 MPa. Furthermore, the number of low temperature incursions has severe detrimental effect on creep resistance on account of the rafted microstructure destabilization induced by these sequences. These results confirm the role of re-precipitation and dissolution of g particles during thermal cycling creep. Thermal cycling infers also on destabilization of subcoating zone in the superalloy but this effect seems to be secondary.TOULOUSE-INP (315552154) / SudocSudocFranceF

Insight on High Temperature Hydrogen Attack initiation and morphology on case studies - 3D FIB-SEM and TEM analyses for fine microstructural characterization of attacked low carbon steels

International audienceHigh temperature hydrogen attack (HTHA) is a phenomenon which affects low carbon ferrito-pearlitic steels constituting pressure vessels under temperature (T>250°C) and hydrogen pressure [1,3]. It is characterized by the formation of methane bubbles at grain boundaries due to the reaction between hydrogen and carbon atoms present in solid solution and in carbides. It leads to a surface decarburization of the steel. As ageing in hydrogen atmosphere progresses the bubbles grow and coalesce until cracking occurs leading to mechanical properties loss. A special attention is paid to this well-known phenomenon in petroleum refining and petrochemistry but it will also have to be predicted in the new power-to-gas reactors and in biomass energy recovery processes. The identification of the elements at the origin of bubbles creation is a key parameter to prevent irreversible deterioration of pressure vessel steels.The objective of this work is thus to investigate the origin of HTHA in low carbon ferrito-pearlitic steels still used in petroleum facilities. To this purpose different pieces of steels, i.e. base metal and weld, coming from several petroleum refineries and which were submitted to high temperature (between 250 and 400 °C) and hydrogen pressure (ranging from 18 to 136 bars) after tens of years in powerplant have been studied. Their observation allows to describe HTHA after different stages of attack. Advanced characterizations of the steel damage microstructure have been performed by means of optical and electron microscopy as well as X-ray tomography. More specifically a focused ion beam - scanning electron microscopy (FIB-SEM) allowing 3D reconstruction of about 10 µm3 volume of materials and transmission electron microscopy (TEM) have been used to examine the microstructure of the hydrogen attacked steels from micron to nanometer scale.A careful attention has been paid to the location of initiation and the morphology evolution of defects (bubbles/voids, cracks) on the different case studies of this work. On a much-attacked C-steel vessel coming from a CO-methanation tube, observations at micron scale by optical microscopy and SEM reveal a particular attack profile with orthoradial intergranular cracks near the inner surface submitted to hydrogen atmosphere and radial near the outer surface. Pearlite grains, which are constituted of packets of cementite carbides plates distributed in a same ferrite grain, are preferentially attacked since their amount decreases approaching inner surface (Figure 1). This induces a surface decarburization and a weakening of the steel from the inner surface. On a much less attacked vessel coming from a hydrofinishing reactor, SEM observations from inner surface show that voids decorate some pearlite grains. They are located at the interface between cementite and ferrite as well as at grain boundaries. No crack has been noted for this stage of attack in this microstructure. 3D FIB-SEM technique has allowed to visualize in three dimensions the link between voids, carbon phases and inclusions distributed along particular planes which can cross several ferrite and pearlite grains (Figure 2). When a plane rich in inclusions intercepts a grain of pearlite or a grain boundary, voids are detected. Associated to TEM observations, it has been shown that the small inclusions observed both close to voids and carbon phases are mainly nanometric AlN precipitates. They could be a catalyst of voids nucleation. It shows that the advanced characterization techniques used in this study are very powerful for understanding the early hydrogen attack stage and as far as our knowledge, only very few articles [4] describe such combination of fine characterizations in order to highlight the trigger of HTHA

Insight on High Temperature Hydrogen Attack initiation and morphology on case studies - 3D FIB-SEM and TEM analyses for fine microstructural characterization of attacked low carbon steels

International audienceHigh temperature hydrogen attack (HTHA) is a phenomenon which affects low carbon ferrito-pearlitic steels constituting pressure vessels under temperature (T>250°C) and hydrogen pressure [1,3]. It is characterized by the formation of methane bubbles at grain boundaries due to the reaction between hydrogen and carbon atoms present in solid solution and in carbides. It leads to a surface decarburization of the steel. As ageing in hydrogen atmosphere progresses the bubbles grow and coalesce until cracking occurs leading to mechanical properties loss. A special attention is paid to this well-known phenomenon in petroleum refining and petrochemistry but it will also have to be predicted in the new power-to-gas reactors and in biomass energy recovery processes. The identification of the elements at the origin of bubbles creation is a key parameter to prevent irreversible deterioration of pressure vessel steels.The objective of this work is thus to investigate the origin of HTHA in low carbon ferrito-pearlitic steels still used in petroleum facilities. To this purpose different pieces of steels, i.e. base metal and weld, coming from several petroleum refineries and which were submitted to high temperature (between 250 and 400 °C) and hydrogen pressure (ranging from 18 to 136 bars) after tens of years in powerplant have been studied. Their observation allows to describe HTHA after different stages of attack. Advanced characterizations of the steel damage microstructure have been performed by means of optical and electron microscopy as well as X-ray tomography. More specifically a focused ion beam - scanning electron microscopy (FIB-SEM) allowing 3D reconstruction of about 10 µm3 volume of materials and transmission electron microscopy (TEM) have been used to examine the microstructure of the hydrogen attacked steels from micron to nanometer scale.A careful attention has been paid to the location of initiation and the morphology evolution of defects (bubbles/voids, cracks) on the different case studies of this work. On a much-attacked C-steel vessel coming from a CO-methanation tube, observations at micron scale by optical microscopy and SEM reveal a particular attack profile with orthoradial intergranular cracks near the inner surface submitted to hydrogen atmosphere and radial near the outer surface. Pearlite grains, which are constituted of packets of cementite carbides plates distributed in a same ferrite grain, are preferentially attacked since their amount decreases approaching inner surface (Figure 1). This induces a surface decarburization and a weakening of the steel from the inner surface. On a much less attacked vessel coming from a hydrofinishing reactor, SEM observations from inner surface show that voids decorate some pearlite grains. They are located at the interface between cementite and ferrite as well as at grain boundaries. No crack has been noted for this stage of attack in this microstructure. 3D FIB-SEM technique has allowed to visualize in three dimensions the link between voids, carbon phases and inclusions distributed along particular planes which can cross several ferrite and pearlite grains (Figure 2). When a plane rich in inclusions intercepts a grain of pearlite or a grain boundary, voids are detected. Associated to TEM observations, it has been shown that the small inclusions observed both close to voids and carbon phases are mainly nanometric AlN precipitates. They could be a catalyst of voids nucleation. It shows that the advanced characterization techniques used in this study are very powerful for understanding the early hydrogen attack stage and as far as our knowledge, only very few articles [4] describe such combination of fine characterizations in order to highlight the trigger of HTHA

Caractérisation de la température de transition et de la ténacité sur prélèvements quasi non destructifs

International audienceLes propriétés de ténacité des matériaux métalliques sont souvent nécessaires pour toutes les analyses d’aptitude au service.La détermination de ces caractéristiques peut se faire soit par l’utilisation de données de la littérature, de corrélations établies à partir d’autres caractéristiques mécaniques mesurables de manière non destructive ou par mesure des caractéristiques réelles des matériaux. La dernière méthode permet à la fois de prendre en compte des mécanismes de vieillissement/fragilisation, mais aussi d’éviter de trop grands conservatismes pouvant amener à des coupes (arrêt d’exploitation) ou réparations non nécessaires, entrainant des coûts élevés et des contraintes opérationnelles non négligeables. Jusqu’ici la plupart des méthodes de détermination de la ténacité sont destructives et reposent sur la réalisation d’essais de résilience ou mieux CTOD sur éprouvettes standardisées. Ces éprouvettes utilisées pour ces types d’essais doivent répondre à des critères dimensionnels très précis et nécessitent des prélèvements de matière d’épaisseurs conséquentes de par leur géométrie standard. De ce fait les opérations de prélèvement génèrent un arrêt des opérations (coupe d’un échantillon de matière de l’équipement) ou dans le meilleur des cas une réparation de l’équipement lorsque cela est possible, ce qui n’est pas toujours le cas notamment si la réparation nécessite un traitement thermique après soudage. Ces opérations induisent dans les deux cas des conséquences techniquement et économiquement importantes en termes d’immobilisation et de perte d’exploitation.Confronté à plusieurs de ces situations, un travail de recherche et développement a été conduit pour développer des méthodes d’évaluation de la température de transition et de la ténacité à partir de prélèvements de dimensions suffisamment limitées pour s’affranchir de réparation. Dans la plupart des cas cela conduit à pouvoir tester des prélèvements d’épaisseur maximale de 3 mm - correspondant généralement aux sur-épaisseurs de corrosion prévues lors de la conception - jusqu’à moins d’un mm d’épaisseur pour des cas très spécifiques. Ce papier présente différentes méthodes de prélèvements testées et évaluées en termes d’affectation du prélèvement et de l’équipement, ainsi que la mise au point de méthodologies spécifiques permettant une caractérisation des propriétés de mécanique de la rupture des prélèvements. Des exemples de comparaison de caractérisations « classiques » et sur petits prélèvements sont présentés, et les perspectives d’évolutions méthodologiques et normatives sont développées