Seamless steel pipes for application in the oil and gas industries are manufactured from quenched and tempered steel. Aiming to meet the required characteristics, it is necessary that the microstructure and the mechanical properties are homogeneous along the wall thickness of the pipes, which is even more critical for thick wall pipes. Considering the seamless steel pipe manufacturing process, any chemical segregation along the thickness could affect the phase transformation kinetics during heat treatments and, consequently, microstructure and mechanical properties. In this context, this pioneer work when evaluating the effect of the chemical homogeneity of a seamless pipe on the mechanical properties and phase transformations. The studied pipe was manufactured by a C-Mn steel designed for oil and gas industry applications. Two regions of the pipe wall were analyzed - a region close to the inner surface and another near the outer surface. The steel was subjected to quenching and tempering heat treatments. Scanning electron and optical microscopy techniques were used to characterize the resultant microstructures. Microhardness and Charpy impact tests were performed aiming to analyze the pipe mechanical behavior in the studied regions. In addition, dilatometric tests were performed in order to determine the continuous cooling transformation diagrams of these regions.

Resumo O aço X70 é um material que possui boa relação entre resistência mecânica e tenacidade, sendo um grau bastante utilizado em condução/transporte de óleo e gás. Com o objetivo de avaliar a soldabilidade deste material, características microestruturais e mecânicas foram avaliadas ao longo da seção transversal de juntas obtidas por soldagem pelos processos GTAW (raiz) e GMAW (preenchimento), de tubos sem costura de aço API 5L X70QS. Os segmentos de tubos, usinados em chanfro J, foram soldados com rotação circunferencial de modo a manter a posição de soldagem sempre plana, sendo utilizados neste estudo três valores para o aporte térmico. Corpos de prova foram confeccionados para ensaios de tração, dureza Vickers, impacto e análise química, além de caracterização metalográfica no metal de base e nas juntas soldadas. O grau X70QS obtido e soldado com diferentes aportes térmicos apresentou excelente comportamento mecânico e em especial, baixo dureza na região da ZTA.

Abstract The X70 steel is a material that has a good relation between mechanical strength and toughness, being very used in conduction and transport of oil and gas. With the objective to evaluate the weldability, the microstructural and mechanical characteristics were evaluated along the cross section of joints obtained by GTAW (root) and GMAW (filling) processes of API 5L X70QS seamless steel pipes. The pipe segments machined in J bevel, welded with circumferential rotation to maintain flat welding position with three different heat inputs. Test specimens for tensile tests, Vickers hardness, Charpy impact and chemical analysis, besides metallographic characterization was made for base material and welded joints. The grade X70QS obtained and welded with different heat inputs showed excellent mechanical behavior and particularly low HAZ hardness.

Abstract The corrosion of C-Mn steels in the presence of hydrogen sulfide (H2S) represents a significant challenge to oil production and natural gas treatment facilities. The failure mechanism induced by hydrogen-induced cracking (HIC) in a Inconel 625 coating / C-Mn steel has not been extensively investigated in the past. In the present work, an API 5CT steel was coated with In625 alloy using laser cladding and the HIC resistance of different regions, such as the coating surface, the substrate and HAZ, were evaluated. SEM observations illustrated that all HIC cracks were formed at the hard HAZ after 96h of exposure. No HIC cracks were observed in the substrate and the In625 coating after the same exposure duration. Pitting was recorded in the substrate caused by non-metallic inclusion dissolving.

Resumo Características microestruturais e mecânicas foram avaliadas ao longo da seção transversal de juntas obtidas por soldagem (GMAW), de tubos sem costura de aço API 5L X65Q. Dois tipos de juntas foram obtidas a partir da confecção de chanfro V e ½ V. Dividindo-se as chapas soldadas em 8 camadas espaçadas de 3mm, no sentido transversal, corpos de prova foram confeccionados para ensaios de tração, microdureza Vickers e análise química. Foi realizada ainda caracterização metalográfica no metal base e nas juntas soldadas. Observou-se pequena alteração na composição química do metal de solda entre as camadas da junta V e junta ½ V. A maior taxa de resfriamento promoveu maior fração de ferrita acicular no chanfro ½ V. Alguns corpos de prova de tamanho reduzido não atenderam aos limites mínimos de escoamento e de resistência especificados na Norma API 5L, resultado atribuído à presença de descontinuidades em algumas regiões da solda. A maior microdureza Vickers da junta ½ V foi atribuída à maior presença de ferrita acicular. A adoção de chanfro estreito para aumento de produtividade na soldagem do aço em estudo não impactou negativamente as propriedades da junta, entretanto, aumentou a ocorrência de falta de fusão na face do bisel.

Abstract Microstructural and mechanical characteristics were evaluated along the cross section of joints obtained by (GMAW) of seamless API 5L X65Q steel pipes. Two types of joints were obtained from the bevel V and ½ V. By the division of the welded plates into 8 layers spaced 3 mm in the transverse direction, specimens were prepared for tensile tests, Vickers microhardness and chemical analysis. In addition, metallographic characterization was performed on the base metal samples and welded joints. It was observed a small change in the chemical composition of the weld metal between the 1st and 8th layer of the V joint and ½ V joint. The higher cooling rate of the ½ V bevel promoted higher acicular ferrite content. Some specimens of reduced size did not meet the minimum tensile properties limits specified in API 5L, a result attributed to the presence of discontinuities in some regions of the weld. The highest Vickers microhardness of the ½ V joint was attributed to the greater presence of the acicular ferrite. The adoption of narrow groove to improve the welding productivity did not negatively impact the properties of the joint, however, increased the occurrence of lack of fusion in the bevel face.

This research presents the influence of manufacturing processes and heat treatments on the resulting microstructures and mechanical properties of an API 5L PSL2 seamless steel pipe. Three different conditions are considered - as rolled, normalized and quenched and tempered - to obtain the grades X42R, X42N and X70Q, respectively. Scanning electron microscopy techniques was used to characterize the resultant microstructures. Tensile, hardness, impact, fracture toughness (J integral) and fatigue crack growth tests (da/dN x ΔK) were used to study the materials mechanical behavior. The results show the possibility of achieving API grades for a seamless pipeline steel, through suitable heat treatments. The microstructural modifications and mechanical properties changes observed showed a remarkable structure-properties relationship of the steel, and attempt to a proper selection as required by the structural design. The quenching and tempering process increased tensile mechanical properties and fracture toughness, but combined to a significant decrease in fatigue crack growth resistance.

Abstract High-strength steels are commonly used in the automobile industry in order to reduce the weight of the vehicles. However, a technical difficulty appears due to the need of hot stamping of the components, which leads to oxidation. Therefore, the application of a coating on the substrate to avoid high-temperature oxidation is used. In this work, experimental analysis and computer thermodynamic calculation were used to describe the phase transformations within an Al-Si coating on a quenchable high strength steel. The Al-Si coating was deposited by hot dipping and its characterization was done using SEM and XRD techniques. Computer thermodynamics calculations were done using the commercial software FactSage using the Calphad methodology. It demonstrated a good relationship between the experimental results and the computer calculations of phase stabilities for the as-deposited condition and after diffusion experiment at 920ºC for 7 minutes, which simulates the thermal cycle of hot stamping of the quenchable steel used.

Threaded and coupled connections generally present a non-uniform stress distribution, which is related to the higher stiffness of the box when compared to the pin. The non-uniform stress distribution can contribute to fatigue cracks and other failure modes in high pressure oil wells. An API 5CT P110 steel obtained from a seamless pipe was mechanically characterized. FEA models were carried out to investigate and compare two API casing connections under make-up torque and tensile efforts. A parametric study was performed using numerical models to determine the influence of some geometric features on the behavior of the API Buttress threaded connection. The API Buttress connection supported higher tensile loads than the API Short Round, however both standard connections showed high stress concentration in the last engaged thread and a non-uniform stress distribution. The manufacturing of grooves at the first and the last engaged threads proved to be an efficient way to reduce the stress concentration of Buttress casing connection and could be an alternative to the development of new products.

Resumo Neste trabalho, investigaram-se efeitos do fenômeno de envelhecimento por deformação em juntas obtidas por soldagem a arco elétrico com proteção gasosa (GMAW), de tubos sem costura de aço API5L X65Q. Corpos de prova (CPs) obtidos a partir das juntas soldadas foram submetidos à uma deformação plástica a frio de 3% e posteriormente envelhecidas por 1h a 250°C, de forma a se induzir envelhecimento estático. Buscando avaliar o envelhecimento dinâmico, os CPs foram deformados até 3% e o carregamento foi mantido durante 1h a 250°C. Os CPs envelhecidos foram submetidos a ensaios de tração e amostras representativas foram analisadas ao microscópio óptico e microscópio eletrônico de varredura. Observou-se que o fenômeno modificou a fração volumétrica de constituintes secundários no metal de solda e na ZTA, com aumento na proporção de agregados ferrita-carbonetos. Com relação às propriedades mecânicas, constatou-se que o fenômeno reduziu a razão elástica das juntas soldadas devido a um aumento no limite de resistência, além de aumentar o alongamento total, não prejudicando, portanto, as propriedades mecânicas em tração das juntas soldadas. Ao se comparar o envelhecimento estático com o envelhecimento dinâmico, observou-se que o aumento na capacidade de deformação plástica das juntas soldadas foi maior após envelhecimento estático por deformação.

Abstract In this work was investigated the influence of the strain ageing on joints obtained by gas-metal arc welding (GMAW) of API 5L X65Q seamless pipes. Specimens were machined from the welded joints, plastically strained to 3% and aged at 250°C for 1h posteriorly, in order to characterize effects of static strain ageing. For the dynamic strain ageing characterization, the specimens were simultaneously strained and aged. Then, the aged specimens were submitted to tensile tests and representative samples were analyzed at a light optical microscope and a scanning electron microscope (SEM). From the results, it was observed that the strain ageing phenomenon changed the volumetric fraction of secondary constituents in the weld metal and in the heat-affected zone, increasing the proportion of ferrite-carbide aggregates. Regarding the mechanical properties, it was noticed that the phenomenon led to higher tensile mechanical properties such as ultimate tensile strength and total elongation and lower elastic ratio, and, therefore, not impaired the mechanical properties under tensile of the welded joints. Comparing the static strain ageing with dynamic strain ageing it was observed that the effect on the plastic straining ability of welds was higher after static strain ageing.

ResumoAços que atendem à norma API 5L são aplicados na construção de dutos e há grande interesse naqueles de maior resistência, como o grau API 5L X70Q. Ainda são escassos os estudos relacionados a tubos sem costura. Durante a montagem, os tubos podem ser soldados por eletrodos revestidos (SMAW) e eventualmente sofrer aquecimento indutivo para adequação à topografia do terreno, processos que podem alterar a microestrutura das juntas. Neste trabalho foram obtidas juntas soldadas pelo processo SMAW, a partir de tubo sem costura grau API 5L X70Q. As juntas foram submetidas a tratamentos térmicos pós-soldagem nas temperaturas 920 °C e 600 °C. Foram realizados ensaios de tração, impacto Charpy e microdureza, além de caracterização metalográfica nas amostras de metal base e nas juntas como soldadas e após os tratamentos térmicos. O tratamento térmico a 920 °C promoveu a formação de estrutura ferrita/perlita, com consequente redução na resistência mecânica das juntas a valores inferiores aos especificados para o grau X70Q. Porém, o tratamento térmico a 600 °C mostrou-se benéfico devido à redução da fração volumétrica dos constituintes martensita/austenita e martensita-austenita-bainita, sem reduzir de forma significativa a resistência mecânica e com relativo ganho na tenacidade ao impacto na ZTA e na ZF.

AbstractPipelines for oil and gas conduction are manufactured of steel tubes that meet the API 5L requirements. Currently, much of the research related to the materials used in this industry is target to steels with higher tensile strength such as API 5L X70Q. But studies related to seamless pipes remains rare. During the manufacture of the pipelines, the pipes are usually welded by the SMAW process and may be heated by the inducted bending process to fit the topography of the land, processes that can change the proportion or morphology of the constituents in the heat affected zone (HAZ) and in the fused metal. The present study used an API 5L X70Q seamless steel manufactured by the Mannesmann Process to produce weld joints by the SMAW process. The joints were subjected to post weld heat treatments in the temperatures of 920 °C and 600 °C. Mechanical tests which included tensile test, Charpy impact and microhardness were applied, in addition to microstructure evaluation in the joints as welded and after heat treatments. The results showed that the first heat treatment (920 °C) promoted the formation of structure ferrite/pearlite, which reduced the mechanical properties to values under the specified for materials API grade X70Q. The second heat treatment (600 °C) was feasible in reducing the volume fraction of the Martensite-Austenite and Martensite-Austenite-Bainite constituents, without significant reduction in mechanical strength of the welded joints and with significant gain in impact toughness in the HAZ and in the weld metal.

Uma previsão razoável da durabilidade de estruturas ou equipamentos que operam em temperaturas elevadas e atmosferas agressivas requer um completo entendimento dos mecanismos de degradação dos materiais causados por cargas mecânicas e corrosão. O objetivo geral desse estudo é simular processos de corrosão em temperaturas elevadas em condições próximas àquelas encontradas na realidade. Isto requer um modelo termodinâmico para prever a estabilidade das fases e um modelamento matemático que descreva os processos cinéticos, por exemplo, difusão no estado sólido - difusão no interior do grão e ao longo dos contornos de grão. Um programa computacional foi desenvolvido para descrever a oxidação interna, nitretação e sulfetação de várias ligas comerciais. O subprograma termodinâmico ChemApp foi integrado em um modelo numérico que usa o método de diferenças finitas. O modelo desenvolvido simula processos de corrosão interna que são controlados por difusão no estado sólido e que envolvam precipitação de várias fases. Formação de zona de corrosão interna, incluindo corrosão intercristalina em aços contendo baixo teor de Cr, é também tratada pelo modelo computacional desenvolvido. Nesse artigo, são consideradas soluções diluídas e monofásicas.

A reasonable prediction of the service life of structures or equipment operating at high-temperatures in aggressive atmospheres requires a full understanding of the degradation mechanisms of the material due to mechanical loading and corrosion. The overall objective of this study is to simulate high-temperature corrosion processes under near-service conditions, which require both, a thermodynamic model to predict phase stabilities for given conditions and a mathematical description of the kinetic process, i.e., solid state diffusion. A computer program was developed in which the thermodynamic program library ChemApp is integrated into a numerical finite-difference diffusion calculation to treat internal oxidation, nitridation and sulfidization processes in various commercial alloys. The model is capable of simulating multi-phase internal corrosion processes controlled by solid-state diffusion into the bulk metal as well as intergranular corrosion occurring in low-alloy steels by fast inward oxygen transport along the grain boundaries of the substrate. In this article, dilute and monophase solutions are considered.

O objetivo deste artigo é introduzir uma ferramenta para simulação computacional, capaz de prever a vida útil de componentes que operam em condições corrosivas tais como oxidação, nitretação e carbonetação em temperaturas elevadas. Diferentes classes de materiais (aços ferríticos, aços austeníticos e ligas de níquel) foram expostas a diferentes atmosferas e temperaturas. O software desenvolvido para simulação de corrosão interna em temperaturas elevadas utiliza a técnica numérica de diferenças finitas para descrever a cinética de corrosão e o conceito de equilíbrio termodinâmico local para tratar a estabilidade das fases. Para isto, um link entre o ambiente MATLAB e a sub-rotina termoquímica ChemApp foi criado. Usando esta ferramenta, cinéticas de oxidação, nitretação e carbonetação foram calculadas levando em consideração a microestrutura das ligas, distinguindo precipitação no interior do grão e no contorno do grão. Foi observada uma excelente correlação entre os resultados experimentais e aqueles obtidos através de simulação computacional.

The aim of this paper is to introduce a computer simulation tool, which is designed for prediction of service-life of components operating under corrosive conditions such as carburization, oxidation and nitridation at high-temperature. Different classes of materials (low-Cr ferritic steels, austenitic steels and Ni-base superalloys) were exposed at temperatures between 550 ºC and 1000 ºC to different corrosive atmospheres (carburizing, oxidizing and nitriding). The tool for the prediction of internal/inwards corrosion processes at high-temperatures makes use of the numerical finite-difference technique to treat diffusional kinetics on the one hand and the concept of local equilibrium thermodynamics on the other hand. For this purpose, a link has been established between the numerical environment of MATLAB and the thermodynamic library ChemApp. Using this new tool, the kinetics of nitridation, oxidation and carburization processes were predicted numerically taking the material’s microstructure into consideration by distinguishing between precipitation along the grain boundaries and within the grain interior. Excellent agreement between experimental observations and simulation results revealed the high potential of the computer modeling for application to complex corrosion processes.

Metastable austenitic stainless steels are known to undergo a partial transformation of austenite to martensite as a consequence of plastic deformation. In the case of cyclic loading, a certain level of plastic strain must be exceeded, and phase formation takes place after an incubation period, during which the necessary amount of plastic deformation is accumulated. The susceptibility of the austenitic phase to deformation-induced martensite formation is strongly affected by the temperature of loading and the stability of austenite, which itself depends on the chemical composition. A key element in this regard is carbon which stabilizes the austenitic phase. It is shown in this study that the carbon concentration can be analysed systematically and reproducible by means of annealing treatments, if the parameters of these treatments are carefully defined on the basis of advanced theoretical thermodynamic and kinetic considerations. First results on the effect of carbon concentration and temperature of fatigue testing on the austenite/martensite transformation are presented, in order to illustrate the significance of these parameters on the martensite formation rate.

The global pressure for recycling and ecological energy production increases steadily in combination with the demand of cost-effective application of materials. However, some severe corrosion problems, associated with the high internal/intergranular corrosion rates in boiler components need to be avoid. Some commercial boiler materials contain a Cr content of 0.55 (wt. (%)) - 2.25 (wt. (%)). This Cr concentration in the alloys is not sufficient for the formation of a complete dense Cr2O3 scale. Hence, high oxidation kinetics may result. In this study, pure Fe and the steel 2.25Cr1Mo were oxidized in laboratory air at 550 °C using a thermobalance system. The surface as well as the cross section of oxidized specimens were analysed using scanning electron microscopy in order to quantify several factors (e.g. surface finishing, cold working and grain size) on the overall oxidation kinetics. For alloys with low Cr content, a decreasing in the grain size leads to an acceleration of the oxidation rate by facilitating the oxygen diffusion along alloy grain boundaries leading to an inward oxide layer formation. The study of effects of surface finish and cold working yielded results revealing that the oxidation process is complex and comparison of results from different laboratories is difficult and should be done.

Generally, oxide scales formed on high Cr steels are multi-layered and the kinetics are strongly influenced by the alloy grain boundaries. In the present study, the oxidation behaviour of an austenite steel TP347 with different grain sizes was studied to identify the role of grain-boundaries in the oxidation process. Heat treatment in an inert gas atmosphere at 1050 °C was applied to modify the grain size of the steel TP347. The mass gain during subsequent oxidation was measured using a microbalance with a resolution of 10-5 g. The scale morphology was examined using SEM in combination with energy-dispersive X-ray spectroscopy (EDS). Oxidation of TP347 with a grain size of 4 µm at 750 °C in air follows a parabolic rate law. For a larger grain size (65 µm), complex kinetics is observed with a fast initial oxidation followed by several different parabolic oxidation stages. SEM examinations indicated that the scale formed on specimens with smaller grain size was predominantly Cr2O3, with some FeCr2O4 at localized sites. For specimens with larger grain size the main oxide is iron oxide. It can be concluded that protective Cr2O3 formation is promoted by a high density of fast grain-boundary diffusion paths which is the case for fine-grained materials.