Efeitos da energia de soldagem e consumíveis sobre a resistência à flexão de juntas "T" soldadas em perfis tubulares

O uso de aços de alta resistência mecânica vem crescendo significativamente, principalmente em situações onde é necessário um material com grande limite elástico para a conformação a frio e boa soldabilidade. Nessas aplicações, a elevada resistência reduz o peso e/ou aumenta a carga que pode ser suportada pelas estruturas produzidas. A introdução dos aços fabricados por processamento termomecânico controlado (TMCP) demandou maiores propriedades mecânicas e metalúrgicas das juntas soldadas sobre eles produzidas. Em função desses fatores, a energia de soldagem é estritamente limitada, e para evitar uma eventual redução da resistência mecânica da zona afetada pelo calor (ZAC), é necessário testar os procedimentos de soldagem próximos daqueles que serão usados na estrutura real. O objetivo deste estudo é obter dados detalhados referentes à eficiência mecânica de juntas soldadas por MAG e com diferentes energias, sobre perfis tubulares quadrados em aço TMCP, formando um conjunto soldado coluna/viga. Seis energias de soldagem e dois metais de adição foram utilizados (AWS ER80S-G e AWS ER120S-G), o primeiro somente com as soldas posicionadas em todo o contorno do perfil, e o segundo com soldas em todo o contorno, transversais e longitudinais em relação à direção de aplicação da carga. Vinte e quatro estruturas foram soldadas, instrumentadas e submetidas à flexão. Comparando as juntas soldadas com mesma energia, observou-se maior resistência à flexão nas soldas longitudinais e em todo contorno, e valores inferiores para soldas transversais. Além disso, pôde ser constatado que 1,2 kJ/mm de energia de soldagem deu os melhores resultados para as juntas soldadas com ambos os consumíveis, uma vez que até esse valor a área da seção resistente das juntas soldadas aumentou significativamente. O uso de energias mais elevadas não foi eficaz para aumentar a resistência à flexão das juntas soldadas ora investigadas, pois o efeito da energia de soldagem sobre a redução da dureza e, consequentemente, resistência mecânica da ZAC, aparentemente se sobrepõe ao aumento da área da seção resistente da junta soldada.The use of high strength steels is growing significantly, especially in situations where it is necessary a material with high strength for cold forming and good weldability. In these applications, the high strength decreases the weight and/or increases the load that can be supported by the structures produced. The introduction of steels made by thermomechanical controlled processing (TMCP) demanded higher mechanical and metallurgical properties of the welded joints produced on them. Given these factors, the heat input is strictly limited, and to avoid any reduction in the mechanical strength of the heat affected zone (HAZ), it is required to test welding procedures close to those that will be used in the actual structure. The aim of this study is to obtain detailed data on the mechanical efficiency of welded joints with GMAW and different heat inputs, on square tubular profiles in TMCP steel, disposed as a column/beam weldment. Six different heat inputs and two filler metals were used (AWS ER80S-G and AWS ER120S-G), the first one with the welds located around the profile contour, and the second one with welds all around the profile, transverse and longitudinal to the axis of the weld bead. Twenty-four welded structures were instrumented and subjected to bending. Comparing the welded joints with the same heat input, it can be noticed that higher bending strength is obtained from all-around contour and longitudinal welds, and lower values for the transverse welds. Moreover, it could be seen of 1.2 kJ/mm heat input gave optimum results to the welded joints with both consumables, since up to this value the area of the resistant section of welded joints increased significantly. The use of higher heat inputs was not effective to increase the bending strength of the welded joints herein investigated, since the effect of heat input on the reduction of the hardness and, consequently, mechanical strength of the HAZ, apparently overrides the increase in the area of the resistant section of the welded joint

Influência da nitretação a plasma sobre a resistência ao desgaste de engrenagens forjadas em aço bainítico de resfriamento contínuo de baixo carbono

O uso do processamento termomecânico utilizando aços bainíticos de resfriamento contínuo é um tema atual de alto potencial para a redução de etapas de processamento, redução do consumo energético e melhoria na qualidade de forjados. Para aplicações do aço bainítico em engrenagens, é necessário elevar sua dureza superficial e resistência ao desgaste. A hipótese desta tese é que a nitretação a plasma por permitir o uso de baixas temperaturas é mais adequada para preservar a microestrutura bainítica e reduzir o consumo energético. O objetivo principal deste estudo é avaliar a potencialidade do aço bainítico forjado e testar o desempenho das superfícies nitretadas a plasma. Neste contexto, amostras e engrenagens forjadas em aço DIN 18MnCrSiMo6-4 foram fabricadas e nitretadas a plasma com diferentes misturas gasosas contendo: 5, 24 e 76% N2. As amostras foram caracterizadas quanto a rugosidade, microestrutura, dureza, composição química, tenacidade a fratura, composição de fases, estado de tensões residuais e testadas por deslizamento recíproco para análise do coeficiente de atrito e desgaste, e por fim avaliadas estatisticamente. As engrenagens foram submetidas à ensaios de fadiga de contato para avaliar a resistência ao desgaste por pitting. Os resultados apresentados demonstram a viabilidade da utilização do aço DIN 18MnCrSiMo6-4 para a redução da rota de processamento de engrenagens forjadas. A nitretação a plasma mostrou-se ser um processo viável para aumentar a dureza superficial e a resistência ao desgaste do aço DIN 18MnCrSiMo6-4. As superfícies nitretadas a plasma com composição de nitrogênio de 24 e 76% N2 formaram uma camada de compostos bifásica de ε-Fe2-3(C)N e γ’-Fe4N. À medida que foi diminuído o percentual de nitrogênio na mistura gasosa, aumentou o percentual de γ’-Fe4N na camada de compostos, mas uma camada de compostos monofásica só foi alcançada com 5% N2. As composições gasosas ricas em nitrogênio (76% N2) apresentaram uma diminuição na tenacidade à fratura da camada de compostos, pois elas têm mais ε-Fe2-3(C)N. A zona de difusão das diferentes superfícies nitretadas a plasma apresentaram tensões residuais compressivas. A distribuição de Weibull revela que o melhor desempenho foi obtido nas engrenagens nitretadas com 24% N2, devido a melhor combinação entre a dureza superficial, tenacidade a fratura, tensões residuais e espessura da camada de compostos. As engrenagens nitretadas com 24% N2 têm uma melhoria de 10,0 vezes sobre as engrenagens forjadas não nitretadas, enquanto as engrenagens nitretadas com 5 e 76% N2 têm uma melhoria de 3,7 e 5,4 vezes.The use of thermomechanical processing using continuous cooling bainitic steels is a current topic of high potential for reducing processing steps, reducing energy consumption, and improving the quality of forged products. For bainitic steel applications in gears, it is necessary to increase their surface hardness and wear resistance. The hypothesis of this thesis is that plasma nitriding for allowing the use of low temperatures is more suitable for preserving the bainitic microstructure and reducing energy consumption. The main objective of this study is to evaluate the potential of forged bainitic steel and test the performance of plasma nitrided surfaces. In this context, samples and forged gears in DIN 18MnCrSiMo6- 4 steel were manufactured e plasma nitrided with different gas mixtures containing: 5, 24 and 76 vol.% N2. Samples were characterized for concerning the roughness, microstructure, hardness, chemical composition, fracture toughness, phase composition, the state of residual stress and tested by reciprocating sliding for friction coefficient and wear analysis and finally evaluated statistically. The gears were subjected to contact fatigue tests to evaluate the pitting wear resistance. The results presented demonstrate the feasibility of using DIN 18MnCrSiMo6-4 steel to reduce the processing path of forged gears. Plasma nitriding proved to be a viable process to increase the surface hardness and wear resistance of DIN 18MnCrSiMo6-4 steel. The plasma nitrided surfaces with nitrogen composition of 24 and 76 vol.% N2 formed a biphasic compound layer of ε-Fe2-3(C)N and γ’-Fe4N. As the percentage of nitrogen in the gas mixture was decreased, the percentage of γ’-Fe4N in the compound layer increased, but a monophasic compound layer was only reached with 5 vol.% N2. The nitrogen rich gas compositions (76 vol.% N2) showed a decrease in fracture toughness of the compound layer, as they have more ε-Fe2-3(C)N. The diffusion zone of the different plasma nitrided surfaces showed residual compressive stresses. The Weibull distribution reveals that the best performance was obtained in the nitrided gears with 24 vol.% N2, due to the better combination between the surface hardness, fracture toughness, residual stresses, and compound layer thickness. The nitrided gears with 24 vol.% N2 have a 10.0 times improvement over the non-nitrided forged gears, while the nitrided gears with 5 and 76 vol.% N2 have an improvement of 3.7 and 5.4 times

Response of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel to different plasma nitriding gas mixtures

Continuous cooling bainitic steels has been widely used in industrial processes owing to its excellent mechanical properties and toughness. Although the surface properties of them are acceptable for many purposes, for their use in mechanical components like gears, it is necessary to improve their surface properties. Plasma nitriding treatments was carried out of a DIN 18MnCrSiMo6-4 steel at 500 °C, with three different nitrogen gas composition: 76, 24 and 5 vol.% nitrogen in hydrogen, for 3, 6 and 9 hours. The surfaces were characterized concerning the microstructure, microhardness, fracture toughness, nitrogen concentration and carbon composition, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different nitrogen gas composition was determined. The carbon profiles of samples indicate that there was decarburization during the plasma nitriding. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples with 76 and 24 vol.% nitrogen, while the nitrided samples with 5 vol.% nitrogen indicated the formation of monophasic compound layer. The diffusion zone presented compressive residual stresses with highest values near the surface

Response of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel to plasma nitriding with a nitrogen rich gas composition

The use of continuous cooling bainitic steels can provide a more energy efficient manufacturing route. However, for their use in mechanical components like gears, it is necessary to improve their surface properties without impacting the core properties to guarantee reliable mechanical performance. The effect of temperature and time on the plasma nitriding response of a DIN 18MnCrSiMo6-4 steel was investigated. The plasma nitriding was performed for 3, 6 and 9 hours, at 400, 450, 500 and 550 °C, using a gas mixture composed of 76 vol.% nitrogen and 24 vol.% hydrogen. Samples were characterized before and after plasma nitriding concerning the microstructure, hardness and microhardness, fracture toughness, phase composition and residual stress states. Based on the results presented, layer growth constants (k) for different temperatures was determined. Moreover, it could be found that 500 °C gave the best results investigated here, as higher temperature took to core and surface hardness decrease. The nitrided samples with thicker compound layers presented a fracture behavior dominated by the formation of Palmqvist cracks. X-ray phase analysis indicated the formation of biphasic compound layer on the surface of all nitrided samples. The diffusion zone presented compressive residual stresses with highest values near the surface

Otimização dos processos de nitretação e Sinterização a plasma utilizando tela ativa

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Surface processing to improve the fatigue strength of bainitic steels : an overview

Currently, one of the major challenges for automotive industries is to reduce the weight and energy consumption of vehicles by using stronger and advanced low-cost materials. Conventional solutions using quenched and tempered steels not always fulfill the desired technical, economic and environmental requirements. Modern continuous cooling bainitic steels can provide a good combination of mechanical strength and toughness, being considered an excellent alternative to replace quenched and tempered martensitic steels in the manufacture of forged components. To meet the desired industry standards in highly loaded components, properties like surface hardness, fatigue strength, wear and friction resistance of these steels can be further improved by subsequent mechanical and thermochemical treatments. Therefore, this paper presents the state of the art in the use of continuous cooling bainitic steels for forging and low energy consumption surface improvement techniques such as: deep rolling and plasma nitriding. Finally, case studies are presented, and conclusions drawn on the current trends and reported practices. Surface modification techniques must be carefully controlled and combined with the material of interest to ensure that undesirable characteristics are not introduced during the manufacturing of the components. The development of processes based on the use of forged continuous cooling bainitic steels can be an excellent alternative to replace the conventional quenching and tempering treatment with considerable reduction of the energy consumption

Comparative analysis of the friction and microstructural properties of WC-10Co-4Cr and Cr3C2-25NiCr coatings sprayed by high-velocity oxy-fuel (HVOF)

In this study, coatings WC-10Co-4Cr and Cr3C2-25NiCr were deposited on the AISI H13 steel by oxy-fuel Thermal spraying (HVOF). This coating increases the wear resistance of surfaces subjected to severe conditions, such as: abrasive wear, thermal fatigue and plastic deformation. The coatings microstructure, hardness and wear resistance are investigated through friction and wear tests performed through a pin-on-disc type tribometer following the procedures defined in ASTM G99-04. It was verified that both materials used in the spraying have high resistance to wear, however, in the sample coated with Cr3C2-25NiCr there was a greater removal of material during the test

Microstructure and wear properties of a low carbon bainitic steel on plasma nitriding at different N2-H2 gas mixtures

For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2 -H2 gas mixtures, containing 5, 24, and 76 vol.% N2 . Samples were characterized concerning the resulting roughness, microstructure, compound layer’s phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ’-Fe4 N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ’-Fe4 N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ’-Fe4 N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N

Influence of plasma nitriding with a nitrogen rich gas composition on the reciprocating sliding wear of a DIN 18MnCrSiMo6-4 steel

In this study, the sliding wear of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel plasma nitrided with a nitrogen rich gas composition was investigated. To evaluate the influence of processing time and temperature on mechanical and microstructural characteristics of nitrided layer, the samples were nitrided at 400, 450, 500 and 550 °C for 3, 6 and 9 h. The produced nitrided layers were characterized concerning the microstructure, phase composition, microhardness and surface roughness. The samples were tested by ball-on-flat reciprocating dry sliding for friction coefficient and wear analysis. The tests were stopped after a given damage criteria involving the rapid growth of the friction coefficients and wear. The correlation of the different treatment parameters and resulting case depths and surface hardness with sliding distance at the time of microcracks formation or delamination of the surface layer was evaluated statistically by the analysis of variance (ANOVA). The plasma nitrided samples at 550 °C showed better wear performances in the ball-on-flat tests than the other groups investigated, since these samples have a thicker compound layer and diffusion zone higher than the other conditions investigated. In general, the beginning wear is slower because of the hardest region of the compound layer

Microestrutura e Propriedades de Desgaste do Aço Bainítico de Resfriamento Contínuo DIN 18MnCrSiMo6-4 Nitretado a Plasma em Diferentes Temperaturas

Os novos aços bainíticos de resfriamento contínuo (CCBS) são uma excelente alternativa quando se requer elevada dureza, tenacidade e resistência à fadiga. Contudo, estima-se que a taxa de desgaste dos CCBS seja insuficiente para aplicação em componentes com alta solicitação mecânica, sendo necessário aprimorar suas propriedades de superfície. O objetivo deste estudo é o de analisar os efeitos da nitretação a plasma sobre à microestrutura e propriedades de desgaste do aço bainítico de resfriamento contínuo DIN18MnCrSiMo6-4. Portanto, tratamentos com duração de 6 horas foram conduzidos com pressão de 3 mbar, mistura gasosa composta por 76% N2 + 24% H2, e temperaturas de 400, 450, 500 e 550 °C, respectivamente. As amostras foram analisadas através de microscopia eletrônica de varredura, difração de raios-X, microdureza e ensaios de desgaste por deslizamento recíproco. Nas condições investigadas, foi possível obter uma camada de compostos e atingir um aumento de até 393% na dureza superficial, entretanto, o uso de temperaturas mais elevadas durante os tratamentos favoreceu a formação de camadas mais profundas. Em todas as amostras nitretadas, constatou-se a formação predominante da fase de nitretos ε-Fe2-3N e, em menor quantidade, da fase de nitretos γ’-Fe4N. Nos ensaios de deslizamento recíproco houve diminuição do desgaste das amostras nitretadas na temperatura de 550°C. Palavras-chave: Aços Bainíticos de Resfriamento Contínuo, Nitretação a Plasma, Microestrutura e Propriedades de Desgaste