Papers by André Barros Cota
The effect of the cooling rate on martensite formation kinetic and its microstructural evolution ... more The effect of the cooling rate on martensite formation kinetic and its microstructural evolution in a ferritic stainless steel with 0.011 %C - 11.1 %Cr - 0.013 N was studied, using the dilatometric technique and optical microscopy. Samples were austenitized at 1000°C during one minute, followed by cooling under various rates to room temperature. The microstructure consists predominantly composed of martensite with a small volumetric fraction of ferrite, and the values of Vickers microhardness vary from 319±4 HV0.2 to 338±4 HV0.2 when the cooling rates increase from 0.5 to 100 °C/s. The results show that the martensite start and finish temperatures decrease when the cooling rate increases. The martensitic transformation rate grows abruptly reaching the maximum value when the martensitic fraction is about 42 % and then it decreases to zero when the martensitic transformation comes to the end. The maximum rate of martensitic transformation grows with increase of cooling rate, being 0.0...
Materials Characterization, 2017
The trapping of hydrogen in nanostructured bainitic steel has been investigated using thermal des... more The trapping of hydrogen in nanostructured bainitic steel has been investigated using thermal desorption analysis, in order to determine the potency of the ferrite-retained austenite (α/γ) interfaces and retained austenite as trapping sites. Thermal desorption data showed that the volume of retained austenite is more effective in trapping hydrogen than the interfaces. There is a close correlation between the quantity of hydrogen and the retained austenite content rather than the density of interfaces. A local equilibrium model was able to reproduce the hydrogen desorption behaviour of saturated and unsaturated samples considering both retained austenite and α/γ interfaces as the trapping sites. A trap binding energy ranging from 47-52 kJ/mol was estimated for retained austenite, suggesting that the observed trapping capacity originates from the austenite lattice sites.
A descricao microestrutural de acos e feita frequentemente por meio das tecnicas de microscopia o... more A descricao microestrutural de acos e feita frequentemente por meio das tecnicas de microscopia optica e eletronica de varredura, que permitem identificar as principais fases e constituintes presentes. A complexidade microestrutural dos constituintes formados em maiores taxas de resfriamento, durante o resfriamento continuo, dificulta a caracterizacao destes. Em especial, as microestruturas bainiticas e martensiticas fornecem caracteristicas morfologicas de dificil visualizacao. A microscopia de forca atomica foi usada visando complementar a microscopia optica e eletronica de varredura na descricao dos produtos do resfriamento continuo de um aco baixo carbono com e sem adicao de Nb. As amostras foram austenitizadas, algumas temperadas, enquanto outras resfriadas ate Ar 3 e Ar 1 e, entao, temperadas. A microscopia de forca atomica permitiu determinar que, para ambos os acos, as amostras temperadas, a partir da temperatura de austenitizacao, apresentaram microestrutura predominantemen...
The full set of experimental data relating to the paper, including dilatometry, differential ther... more The full set of experimental data relating to the paper, including dilatometry, differential thermal analysis, scanning and transmission electron microscopy, X-ray diffraction.
ABM Proceedings, Oct 1, 2018
Resumo Este trabalho apresenta um estudo sobre a influência de diferentes ciclos de tratamentos t... more Resumo Este trabalho apresenta um estudo sobre a influência de diferentes ciclos de tratamentos térmicos sobre a microestrutura e propriedades mecânicas de dois aços (A e B) especificados pela norma API 5CT. O aço B tem um conteúdo de elementos de liga maior que o aço A. Caracterizou-se de forma qualitativa e quantitativa as microestruturas por meio de microscopia óptica e eletrônica de varredura. As propriedades mecânicas foram medidas por meio do ensaio de tração. As amostras do aço A submetidas ao resfriamento ao ar calmo e forçado apresentaram uma maior fração volumétrica do constituinte MA (martensita/austenita retida) em relação ao aço B e o aumento da taxa de resfriamento aumentou a quantidade de MA para os dois aços, o que resultou em um aumento no limite de resistência a tração para os aços A e B. As amostras que foram resfriadas ao ar calmo e ao ar forçado apresentaram menores limites de escoamento que as amostras temperadas e revenidas. Este resultado pode ser explicado pela presença de bainita e do constituinte MA nas amostras resfriadas ao ar calmo e ao ar forçado cuja presença gera uma maior densidade de discordâncias. Palavras-chave: Tratamento térmico; Microestrutura; Propriedades mecânicas; Aço.
ABM Proceedings, 2018
Resumo Este trabalho teve como objetivo o estudo do efeito da taxa de resfriamento sobre a cinéti... more Resumo Este trabalho teve como objetivo o estudo do efeito da taxa de resfriamento sobre a cinética de transformação martensítica do aço USISAR 80T, estrutural, microligado, de boa temperabilidade e considerado de boa soldabilidade. Corpos de prova dilatométricos foram aquecidos a 3°C/s até a temperatura de 920°C, mantidos nessa temperatura por 180 segundos e posteriormente resfriados com diferentes taxas de resfriamento (25°C/s, 50°C/s, 75°C/s e 100°C/s). O software Matcalc foi utilizado de forma a prever a estabilidade termodinâmica das fases presentes em função da temperatura. Observou-se que as temperaturas de início e fim de transformação martensítica, e , respectivamente, diminuíram com o aumento da taxa de resfriamento, enquanto a taxa de transformação e a microdureza aumentaram. Os resultados mostram que o aumento da taxa de resfriamento conduziu a um refinamento da microestrutura.
Acta Crystallographica Section C Crystal Structure Communications, 1990
There has been an increasing focus on social and emotional development in educational programmes ... more There has been an increasing focus on social and emotional development in educational programmes in early childhood as both variables are believed to influence behavioural outcomes in the classroom. However, relationships between social and emotional development and behaviour in early childhood have rarely been explored. This article sets out to investigate the conceptualisation of these variables and their interrelationships. Structural equation models were used to assess whether differences exist between boys and girls in relation to social and emotional competences, which could affect the relative success of such programmes. This article is based on cross-sectional data collected from 749 four-to six-year-olds and their teachers. The findings generally supported the hypothesised relationships between social and emotional development variables and prosocial behaviour (including internalising behaviour) for boys and girls. However, some gender differences were noted in externalising behaviour, which teachers often consider to be most significant due to its potentially disruptive nature in the classroom.
Proceedings of the 23rd ABCM International Congress of Mechanical Engineering, 2015
Scripta Materialia, 2004
The continuous cooling kinetics of the nucleation and growth of the ferritic phase was studied on... more The continuous cooling kinetics of the nucleation and growth of the ferritic phase was studied on a Nb microalloyed steel. Johnson-Mehl-Avrami equations were found for three austenitizing conditions and a direct relation between the time exponent and the transformation time was revealed.
Microscopy and Microanalysis, 2005
Materials Characterization, 2000
HSLA low-carbon bainitic steel containing B was submitted to torsion tests to simulate controlled... more HSLA low-carbon bainitic steel containing B was submitted to torsion tests to simulate controlled rolling, followed by interrupted accelerated cooling. Microstructural characteristics and the mechanisms for the refinement of structure were evaluated using light microscopy, scanning electron microscopy, transmission electron microscopy, and Vickers hardness testing. The final microstructure was found to contain complex mixture of granular bainite, small islands of MA constituent, bainitic ferrite, and polygonal ferrite. Increasing the cooling rate or decreasing the finish cooling temperature resulted in a decrease in the volume fraction and average size of the MA islands and the polygonal ferrite. A finish cooling temperature of 400 Њ C produced a microstructure consisting of fine laths of bainitic ferrite with an interlath MA constituent. A quantitative relationship between the accelerated cooling variables and the ferrite grain size was developed.
Materials Characterization, 2007
The kinetics and microstructural evolution of austenite formation in a low carbon steel, with ini... more The kinetics and microstructural evolution of austenite formation in a low carbon steel, with initial microstructure composed of ferrite and pearlite, were studied during continuous heating, by using dilatometric analysis and measurements of microstructural parameters. The formation of austenite was observed to occur in two stages: (a) pearlite dissolution and (b) ferrite to austenite transformation. The critical temperatures of austenite formation in continuous heating increase with increasing heating rate, with greater influence on the finishing temperature of austenite formation. For both the 1°C/s and 0.1°C/s heating rates, the formation rate of austenite reaches a maximum at approximately the finishing temperature of pearlite dissolution, and the formation rate of austenite as a function of the temperature is greater at the higher heating rate.
Journal Of The Brazilian Chemical Society, 1991
Journal de Physique IV (Proceedings), 2003
Pericarditis and hemopericardium causing cardiac tamponade, caused by large and transmural ventri... more Pericarditis and hemopericardium causing cardiac tamponade, caused by large and transmural ventricular infarction with subsequent rupture into the pericardium, is a rare complication post acute myocardial infarction. This is frequently fatal with rapid clinical course. We reported a 65-year-old female who presented with persistent diaphoresis and dyspnea for several hours followed by syncope. Acute myocardial infarction was diagnosed with primary percutaneous coronary intervention performed though persistent hypotension was observed. Moderate amount of hemopericardium leading to tamponade was found. It was relieved after pericardiocentesis. Real-time three-dimensional echocardiography revealed blood clot with specific whirling pattern around the infracted ventricular wall with suspected leak.
Acta Crystallographica Section E Structure Reports Online, 2003
The title compound, C 31 H 48 O 3 , was obtained by methylation of 3-oxoolean-12-en-20-oic acid, ... more The title compound, C 31 H 48 O 3 , was obtained by methylation of 3-oxoolean-12-en-20-oic acid, isolated from the root bark of Austroplenckia populnea (Celastraceae). Rings A, B, D and E are in chair conformations and ring C is in an envelope conformation. Comment Considerable attention has arisen from the investigation of chemical constituents of plants of the Celastraceae family, on account of the antitumor and antileukemic activities found in maytansinoids and quinonemethide triterpenes isolated from plants of that family (Marine-Bettolo, 1974). In previous papers, we described the chemical constitution of the root bark (Sousa et al., 1988), bark wood (Sousa, Silva, Pedersoli & Alves, 1990) and root wood (Sousa, Silva & Pedersoli, 1990) of Austroplenckia populnea (Reiss) Lundell, a specimen of the Celastraceae family. The plant is popularly known in the state of Minas Gerais (Brazil) as`mangabeira-brava', and the decoction of its branches is used as a traditional antidysenteric medicine (Corre à a, 1978).
ABM Proceedings, 2018
Este trabalho apresenta uma avaliação da temperabilidade, microestrutura e tenacidade de dois tub... more Este trabalho apresenta uma avaliação da temperabilidade, microestrutura e tenacidade de dois tubos de aços (A e B) especificados pela norma API 5CT. O aço B tem um teor de elementos de liga maior que o aço A. As amostras foram temperadas em água a 29ºC e revenidas a 700ºC/15min. Avaliou-se a temperabilidade dos aços por meio do ensaio Jominy e a tenacidade por meio da energia absorvida no ensaio de impacto Charpy. As propriedades mecânicas foram medidas pelo ensaio de tração. A caracterização da microestrutura foi realizada de forma qualitativa por meio de microscopia óptica e eletrônica de varredura. O aço B com maior teor de carbono e de elementos de liga tem maior temperabilidade do que aço A. Os aços A e B apresentam uma microestrutura constituída de martensita revenida e pequena quantidade de bainita. As amostras do aço B apresentam maior energia absorvida, maior limite de resistência e menor ductilidade que o aço A. A presença de Mo em maior quantidade no aço B faz com que sua tenacidade seja superior ao aço A.
Resumo A fosfatização é um pré-tratamento superficial que tem como principais funções: a lubrific... more Resumo A fosfatização é um pré-tratamento superficial que tem como principais funções: a lubrificação, proteção contra corrosão e proteção contra abrasão. Os metais fosfatizantes mais comumente utilizados são o zinco, manganês e o ferro, sendo o zinco o mais aplicado nas indústrias siderúrgicas, automobilísticas e de linha branca. Atualmente, grande parte das pesquisas relacionadas aos processos de fosfatização é referente ao fosfato de zinco, sendo escasso o estudo sobre fosfato de manganês. O objetivo deste trabalho foi estudar a cinética de formação da camada de fosfato de manganês em aços baixo carbono, analisando a influência do substrato na formação da camada fosfatizada. As análises foram baseadas na obtenção das curvas de ganho de massa, sendo caracterizadas por microscopia eletrônica de varredura. Os resultados indicaram que para o substrato constituído essencialmente de martensita revenida, uma maior quantidade de carbonetos resultou em uma menor taxa de ganho de massa, pois estes funcionam como sítios ativos no processo de fosfatização, fazendo com que exista uma maior densidade de núcleos.
Uploads
Papers by André Barros Cota