EM - Escola de Minas
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6
Notícias
A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.
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Item Effect of thermal aging on the microstructure and mechanical properties of stainless steel UNS S31803.(2020) Costa, Junia Maria Gândara; Lacerda, José Carlos de; Godefroid, Leonardo Barbosa; Cândido, Luiz CláudioDuplex stainless steel UNS S31803 exhibits high mechanical strength with high corrosion properties, due to its microstructure composed of ferrite and austenite phases, in equal proportion. When the UNS S31803 steel is submitted to high temperatures, some precipitations can occur, such as nitrites, carbides, and third phases (e.g. sigma phase -σ, and alpha prime - α'). These phases are deleterious in relation to the mechanical properties and corrosion resistance, and their effects are analyzed regarding the properties of the steel. In order to evaluate the precipitation of this deleterious phase, isothermal treatment was done at 500°C for 144 hours (α' phase) and at 850ºC for 80 minutes (σ phase). The results were obtained through the microstructural analysis and tensile tests. The presence of sigma phase was verified in the grain boundary, an increase in the mechanical resistance with a loss of toughness. There was as well as an increase in mechanical resistance with the precipitation of α', with less loss of ductility than that observed in the experiments involving the presence of sigma phase.Item Effect of niobium on microstructure and mechanical properties of a hypereutectoid steel.(2022) Pessoa, Ricardo Amorim; Porcaro, Rodrigo Rangel; Cândido, Luiz Cláudio; Pereda, Beatriz; Lopez, BeatrizHigh-carbon steels have been used to high-strength steel wire and strands for prestressing concrete. The necessity of high-strength levels at increasingly larger diameters of wire rods is a technological challenge. Two steels with and without Nb were obtained in a steel mill, submitted to detailed microstructural (previous austenitic grain size, pearlite interlamellar spacing and colony size) and mechanical characterization through tensile tests and hardness. Hot torsion and dilatometry tests were performed to simulate steels processing and to verify the influence of Nb on phase transformation. Adding Nb to steel resulted in a refinement in austenitic grain size and pearlite colonies but had no effect on pearlitic interlamellar spacing. There was a decrease at the start transformation temperature austenite/pearlite and therefore an increase in the hardenability of the Nb-added steel. Finally, Nb addition proved to be a technical and economical way to increase tensile strength and to reduce the variability of the mechanical properties.Item Failure analysis of a steel slide ring of a tubular ball mill used in an iron ore mining plant.(2020) Godefroid, Leonardo Barbosa; Cândido, Luiz Cláudio; Guimarães, Claudinei Roberto; Araújo, Sidney CardosoThis work identified the root cause of an early failure occurring near welded joints that connect the slide ring with the head of a ball mill used in an iron mining plant. The ASTM-A516-G60 steel specified for use at this location had a lower carbon content than required. The microstructure presented by this steel consisted of a totally heterogeneous distribution of ferrite grains and pearlite colonies, with variations in grain size and volume fraction. Moreover, the union between the slide ring and the mill head created a significant geometric stress concentration and the welding process used, although meeting the specification, contributed to the microstructural heterogeneity, due to the thermal input required for this operation. These microstructural and geometrical characteristics led to the development and growth of fatigue cracks near the welded joints, that crossed the thickness of the slide ring. As a consequence, the iron ore pulp was contaminated with the lubricating oil, decreasing the adequate oil level and generating excessive wear on bearings and loss of pressure from the lubrication system. All these facts made impracticable the operation of the mill. Three different steels that meet the specification for application in pressure vessels were analyzed and compared with the steel of the failed mill by means of tension, Rockwell hardness, Charpy impact, fracture toughness (J-R curves) and fatigue tests (σa × Nf curves and da/dN × ΔK curves). It was concluded that the material selection for the mill component needs to consider a suitable chemical composition and microstructure of the steel for that application, to ensure adequate values of mechanical properties compatible with its use.Item Desenvolvimento de dispositivo de baixo custo para mecanização da soldagem GTAW e sua aplicação em estudo de solidificação de aço ABNT 430.(2020) Tavares, Tairine Berbert; Porcaro, Rodrigo Rangel; Cândido, Luiz Cláudio; Faria, Geraldo Lúcio de; Dâmaso, Heitor VieiraLaboratórios de pesquisa em metalurgia da soldagem requerem controle dos principais parâmetros da soldagem ao arco elétrico para obter resultados satisfatórios. Aços inoxidáveis ferríticos são muito utilizados devido ao custo relativamente baixo e sua resistência à corrosão atmosférica, no entanto, a ausência de transformação de fases no estado sólido durante a solidificação oferece certa limitação à soldabilidade. Neste contexto, um dispositivo de baixo custo para mecanização da soldagem GTAW foi desenvolvido e validado, permitindo condições adequadas de controle, repetibilidade e foi aplicado para avaliar a influência de parâmetros de soldagem na solidificação de um aço ABNT 430. Menores velocidades de soldagem, para uma mesma corrente elétrica, resultaram em grãos grosseiros e alongados no centro do cordão e paralelos à direção de soldagem. O aumento da velocidade e da corrente originou subestruturas dendríticas no interior dos grãos, devido à maior instabilidade da poça. Ainda, avaliou-se aplicação de vibração mecânica por sistema de martelete e pré-aquecimento com relação ao refinamento dos grãos e aumento da uniformidade da estrutura no centro do cordão. A aplicação de vibração mecânica gerou uma estrutura mais refinada e homogênea na zona fundida. Palavras-chave: Soldagem TIG; Aço inoxidável ferrítico; Metalurgia da soldagem; Solidificação na soldagem.Item Microstructure and mechanical properties of a flash butt welded pearlitic rail.(2019) Porcaro, Rodrigo Rangel; Faria, Geraldo Lúcio de; Godefroid, Leonardo Barbosa; Apolonio, Gabriela Ribeiro; Cândido, Luiz Cláudio; Pinto, Elisângela SilvaThe structural changes resulting from the Flash Butt Welding (FBW) of pearlitic rails have been associated with wear/premature failures, despite this, there are no studies applying dilatometry to correlate the welding thermal cycles with the microstructural development of such material. The microstructural evolution of the heat affected zone is clarified with the aid of dilatometry. The increase in the steel hardenability associated with a larger austenitic grain size promotes the austenite-pearlite transformation at lower temperatures in the grain growth region. This explains why this region has larger pearlite colony size but smaller interlamellar pearlite spacing and higher hardness than the grain refined region. Partial cementite spheroidization in the heat affected zone is responsible for significant decrease in hardness and tensile strength and is correlated to localized dipping, rolling contact fatigue and failures. A dilatometry based methodology is proposed to define a process window and control the post-weld cooling rate at the rail head in order to improve the weld performance due to a better hardness profile, without increasing costs or welding time. For the steel evaluated, a 20% increase in the hardness of the softened area at the HAZ was obtained by dilatometric simulation of a safe accelerated cooling (5 °C/s).Item Effect of the chemical homogeneity of a quenched and tempered C-Mn steel pipe on the mechanical 3 properties and phase transformations.(2019) Lima, Ana Paula da Silva; Faria, Geraldo Lúcio de; Trindade Filho, Vicente Braz da; Cândido, Luiz CláudioSeamless 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.Item Effect of chemical composition and microstructure on the fatigue crack growth resistance of pearlitic steels for railroad application.(2019) Godefroid, Leonardo Barbosa; Moreira, Luiza Pessoa; Vilela, Tamara Caroline Guimarães; Faria, Geraldo Lúcio de; Cândido, Luiz Cláudio; Pinto, Elisângela SilvaThis work characterized the microstructure and evaluated the mechanical behavior of two pearlitic steels used in Brazilian railroads, a C-Mn-Si steel and a V-microalloyed steel. The microstructures were observed by light optical, scanning electron and atomic force microscopy. Prior austenite grain size, pearlite colony size and pearlite interlamelar spacing were measured. Continuous cooling transformation diagrams for both steels were obtained. The mechanical behavior was evaluated by tensile tests, hardness tests, fracture toughness tests (linear elastic fracture mechanics - KIC), and fatigue crack propagation tests (da/dN×ΔK). The fatigue tests were performed with different R-ratios (R=0.1 and 0.7) applying two recent empirical models proposed by Rubaie et al. (2008) and Jones et al. (2012) to predict the material behavior. Different loading histories including overloads in function of the crack size were also applied. The fracture surfaces of all tested specimens were analyzed by scanning electron and atomic force microscopy. The presence of vanadium in steel can certainly provide a more refined microstructure, changing the mechanical properties. However, an appropriate range of simple chemical composition and an adequate thermomechanical processing can provide a steel in accordance to standard specifications for railroad application without the need of microalloying element additions that could make the final product more expensive. In the specific case of fatigue, both steels presented a dependence to fatigue R-ratio. The two empirical models were satisfactory aiming to fit the experimental data, although the equation of Jones et al. is simpler to be manipulated. The studied steels also had a dependence to fatigue overloads that can represent a positive effect related to crack growth retardation with increasing fatigue overloads, but also a negative effect, because increasing the crack size for which the overload is applied and increasing the number of overloads, the intensity of retardation effect decreases. These fatigue results are important to predict the actual behavior of steels used in the railway sector and to perform a proper maintenance control, avoiding a premature failure and a consequent catastrophic accident.Item Evaluation of hydrogen-Induced cracking resistance of the In625 laser coating system on a C-Mn steel substrate.(2017) Trindade, Vicente Braz; Almeida, Natália Chaves; Cândido, Luiz Cláudio; Faria, Geraldo Lúcio de; Lima, Milton Sérgio Fernandes deThe 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.Item Evaluation of the influence of post welding heat treatments on microstructure and mechanical properties of API 5L X70Q weld joints.(2016) Araújo, Lívia Carla Silva de; Cândido, Luiz Cláudio; Trindade, Vicente Braz; Porcaro, Rodrigo RangelSteels that meet the API 5L standard are employed in pipeline construction, and there is considerable interest in those of higher strength, such as grade API 5L X70Q. There have nevertheless been few studies relating to seamless pipes. During installation, the pipes may be welded with covered electrodes (SMAW) and may optionally undergo inductive heating for adjustment to the topography of the terrain, processes that may alter the microstructure of the joints. In this work, welded joints were obtained by the SMAW process, from seamless pipe of grade API 5L X70Q. The joints were submitted to post-weld heat treatments at temperatures of 920 and 600 °C. Tensile, Charpy impact and microhardness tests were carried out, besides metallographic characterization in specimens of base metal and in the joints, as-welded and after the heat treatments. Heat treatment at 920 °C promoted formation of a ferrite/pearlite structure, with consequent reduction in the mechanical strength of the joints to values below those specified for grade X70Q. However, heat treatment at 600 °C proved to be beneficial owing to the reduction of the volume fraction of the martensite/austenite and martensite-austenitebainite constituents, without significantly reducing the mechanical strength and with a relative gain in impact toughness in the heat-affected zone and in the fusion zone (FZ).Item Evaluation of standard API casing connections and parametric API buttress improvement by finite element analysis.(2017) Porcaro, Rodrigo Rangel; Cândido, Luiz Cláudio; Trindade, Vicente Braz; Faria, Geraldo Lúcio de; Godefroid, Leonardo BarbosaThreaded 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.