Resumo O presente trabalho tem como objetivo estudar o comportamento de juntas soldadas obtidas pelo processo GMAW com eletrodo rotativo (GMAW-RE), de modo a permitir uma análise comparativa de produtividade e propriedades mecânicas com o processo GMAW convencional (GMAW). Foram realizadas soldagens multipasse pelos dois processos GMAW com preaquecimento de 100 °C em chapas de dimensões 500 × 150 × 10 mm em aço ASTM A36, na posição plana e energia de soldagem média de 2,30 kJ/mm. Após a soldagem, foram realizados ensaios de impacto Charpy-V, microdureza e metalográficos em corpos-de-prova removidos transversalmente ao cordão de solda. Adicionalmente, foi realizada uma análise comparativa de produtividade entre os processos. Os resultados mostram que o processo GMAW-RE propicia um aumento de produtividade e redução de custos significativos em relação ao processo GMAW. Adicionalmente, foram observados valores equivalentes de tenacidade ao impacto para o metal de solda, mas uma redução nesta propriedade na zona termicamente afetada. Embora a literatura disponível destaque o elevado potencial deste processo para substituição do processo GMAW convencional em aplicações industriais, verifica-se a necessidade de mais evidências experimentais para um maior suporte a esta indicação.
AISI 430 is a non-stabilized ferritic stainless steel grade with carbon content lower than 0.12%.After hot and cold rolling this material is annealed. The slow cooling after soaking at temperatures between 900 o C and 1000 o C promotes the formation of a high quantity of carbides and nitrides, while the rapid cooling partially suppresses the formation of these precipitates, but introduces martensite in the microstructure. Intergranular martensite can also be produced in the weld metal and in the heat affected zone (HAZ) of welds of nonstabilized ferritic stainless steels. In this work, several heat treatments between 900 o C and 1000 o C, with different cooling rates, were performed in a commercial sheet of AISI 430 grade. Also, an autogenous welding was produced with GTAW process, and post weld heat treatment at 700°C was carried out. The different microstructures produced were analyzed by optical and scanning electron microscopy (SEM). The degree of sensitization was measured by double loop electrochemical potentiodynamic tests (DL-EPR). The pitting corrosion resistance was evaluated by cyclic polarization tests in 3.5%NaCl solution. Hardness and toughness tests were also performed in selected heat treatment conditions. The results indicate that the slow cooling results in a higher degree of sensitization than observed in the material rapid cooled from the annealing temperature. The ferritic martensitic structure produced by water cooling has higher pitting potential and lower degree of sensitization, but is brittle at room temperature. A subsequent tempering treatment between 600 and 800 o C can increase the toughness, but the corrosion resistance may decrease due to carbides precipitation.The heat affected zone of AISI 430 welds contains intergranular martensite, which is brittle and susceptible to corrosion attack. Post weld heat treatment at 700 o C decomposed the martensite into ferrite and carbides and improved the corrosion resistance.Key-words: ferritic stainless steels, microstructure, DL-EPR test. ________________________________________________________________________________ INTRODUCTIONAISI 430 steel is one of the most popular ferritic stainless steels. Although more modern ferritic stainless steels have been developed, the production of AISI 430 is still elevated due to its low cost and good corrosion properties.The influence of heat treatments on microstructure, corrosion resistance and mechanical properties of stainless steels is a key issue. Depending on the final heat treatment, the mechanical properties and corrosion properties may vary significantly. Frequently, the best heat treatment for corrosion resistance is not the best for the desired mechanical properties.On the other hand, welding always produces important changes on the microstructure of weld metal and heat affected zone (HAZ) which affects corrosion resistance and mechanical properties. In the case of ferritic stainless steels, the main change produced in fusion welding processes is the pronounced grain growth in the weld...
List of symbols ∆ σ Residual stress reduction level (%) σ (b) Stress measured before the vibratory motion σ (a) Stress measured after the vibratory motion σ − S σ , −S σ , (1 − α) × 100 % confidence interval for σ 2 b Variance of the random variable σ (b) 2 a Variance of the random variable σ (a) S (b) σ Half-bandwidth of the confidence interval for the measured stress before the vibratory motion S (a) σ Half-bandwidth of the confidence interval for the measured stress after the vibratory motion π(z) Pdf of the random variable z z ∼ N µ, 2 Z follows a Gaussian distribution with mean μ and variance 2 RS Residual stress HAZ Heat-affected zone PAW Plasma arc welding GTAW Gas tungsten arc welding FZ Fused zone Abstract Stress relief treatment based on mechanical vibrations is a technology that stands out for being an alternative to conventional thermal treatments and also due to its effectiveness for materials with heterogeneous structures. In this context, the application of this technique in welded joints has great potential for reducing residual stresses. This research presents some analyses of the effectiveness of surface residual stress relief treatment by mechanical vibrations using random excitations applied on plasma welded joints. Residual stresses were analysed by the X-ray diffraction technique with the sin 2 ψ method. Measurement uncertainties are taken into account in the computation of the reduction of residual stresses via Monte Carlo simulation analyses. A significant reduction in the residual stresses magnitudes, around 40 %, in the longitudinal direction was observed after the application of mechanical vibration treatments stress. In the transversal direction, most of the test specimens presented reduction levels around 20 %.
The use of structural steel in the industry is increasing every day, and the study of stress state after welding has been shown to be of great importance. Nondestructive techniques become quite appropriate to be performed before and during the service component of welded, and thus ensure its integrity. The magnetic technique to be nondestructive, and easy to apply in the field, has potential to be an inspection tool for measuring residual stresses and other microstructural parameters. In this work it was possible to analyze the state of residual stresses through nondestructive techniques, Magnetic Barkhausen Noise and X-ray Diffraction, as well as the semi-destructive technique, high speed hole drilling method, and thereby determine the residual stresses in ASTM A36 steel plate welded by MAG (Metal Active Gas) process.
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