Объектом исследования являлось изучение микроструктуры и механических свойств сталей типа 32Г2ФА. На основе литературных данных и дюрометрических испытаний проведен анализ значений параметра отпуска и определен коэффициент С. Исследована микроструктура сталей после различных режимов отпуска методами электронной и оптической микроскопии. / The object of the study was the study of the microstructure and mechanical properties of steels of the 32G2FA type. On the basis of literature data and durometric tests, an analysis of the values of the tempering parameter was carried out and the coefficient C was determined. The microstructure of steels after various tempering regimes has been studied by electron and optical microscopy.

Este estudo teve como objetivo a determinação dos menores valores de tempo e temperatura para obtenção dos graus 95 e 110, através de projeto de experimento completo fatorial rotacional. Posteriormente foi investigado o comportamento mecânico e estrutural do aço API 5CT. Para os aços bruto, normalizado e seus respectivos graus 95 e 110 nos sentidos transversal e longitudinal. Para isto, foram realizados ensaios de tração, dureza, impacto e metalografia. Os mesmos foram submetidos ao tratamento térmico de têmpera com a temperatura de austenitização de 870°C e resfriados em água. Na sequência, realizou- se o tratamento de revenimento orientado pelo planejamento de experimento. O aço bruto apresentou estrutura perlitica bandeada na matriz ferritica. No aço normalizado foi observada a perlita mais homogeneamente distribuída em toda matriz ferritica. O grau 95 obtido a partir do aço bruto e normalizado apresentou a estrutura martensítica revenida em ripas e Ferrita. O grau 110 partindo dos aços, bruto e normalizado apresentou microestrutura martensítica revenida refinada ou em ilhas e Ferrita. Os módulos de resiliência e tenacidade dos aços bruto e seus graus apresentaram valores em torno de 14% maiores quando comparados ao aço normalizado. De forma similar, os limites de escoamento e tração aumentaram em 10%. A capacidade de encruamento aumentou cerca de 16% e houve um aumento de 5% no coeficiente de resistência. Em ambos os casos, os valores obtidos estão adequados à norma API 5CT. As análises das fractografias das amostras testadas em tração, para as condições bruto e normalizado, revelaram a predominância de fratura dúctil e trincas secundarias provenientes do tratamento térmico. A fratura do ensaio de impacto obtida a 60°C, dos aços bruto e normalizado, apresentaram fraturas dúcteis na região central e fraturas frágeis nas extremidades. Na temperatura de - 196 °C as fraturas foram predominantemente frágeis. Para o aço bruto e seus respectivos graus, a transição dúctil frágil ocorreu a -34°C com 79J de energia absorvida. Para o aço normalizado e os graus correspondentes, a temperatura de transição foi de -48 °C e a energia absorvida de 85J. / The purpose of this study was to determine the lowest time, and temperature values needed to obtain the degrees 95, and 110 of the HSLA steels through a rotational, full factorial experiment design. Subsequently, the mechanical and structural behavior of the API 5CT steel was also investigated. Values for the raw steel samples were normalized transversally and longitudinally. For this purpose, traction, hardness, impact, and metallographic tests were conducted. The aforementioned alloys were subjected to heat treatment by quenching with an austenitizing temperature of 870°C, and then cooled in water. The next step was the treatment of tempering heat as determined by the experimental design. The raw steel showed signs of a pearlite banded structure in the ferritic matrix. The 95 degree sample obtained from the raw and normalized steel displayed martensitic structures tempered in lath and ferrite. The 110 degree sample created from raw and normalized steel exhibited refined tempered standard martensitic structures, in addition to ferrite. The raw steel resilience, and tenacity moduli, along with its temperature values were about 14% greater when compared to the normalized steel results. Similarly, ullage and traction thresholds increased by 10%. The strain hardening capacity increased by about 16% and there was an increase of 5% in the resistance coefficient. In both cases, the values obtained were within API 5CT standards. Sample analyses from fractographies, and traction tests (for raw and normalized steels) revealed the prevalent occurrence of ductile fractures, and secondary cracks caused by the heat treatment. The impact test, at 60°C, for both the raw and normalized samples, caused ductile fractures in the center, and brittle fissures around the edges. At the temperature of -196 °C, fractures were predominantly fragile. For the raw steel samples tested and their respective degrees, the ductile brittle transition occurred at - 34°C with absorbed energy of 79J. For the standard steel and its corresponding degree, the transition temperature was -48°C, and absorbed energy of 85J.

[pt] As correlações dos parâmetros de processamento com a microestrutura e propriedades mecânicas resultantes apresentam grande relevância para o controle da qualidade e manutenção do grau API do tubo curvado por indução. Este trabalho enfoca principalmente nas alterações dos mecanismos de endurecimento para avaliar os efeitos do curvamento a quente e do revenimento posterior. Foi observado que além do refino de grão, a precipitação fina dos microligantes na austenita e a densidade de discordâncias são responsáveis pelas propriedades mecânicas do material como recebido. O endurecimento por solução sólida caracterizado para todas as amostras foi aproximadamente igual. O endurecimento devido ao refino de grão dependeu do fator de endurecimento. Nas curvas as transformações de fases de baixas temperaturas foram induzidas pela elevação da temperabilidade em função de maiores temperaturas de curvamento, contudo a extensão das camadas temperadas ficou limitada pelas taxas de resfriamento. A principal alteração promovida pelo curvamento a quente e diretamente associada ao projeto de liga do aço é devido à inibição da precipitação do vanádio e sua manutenção em solução sólida. O endurecimento das curvas a quente devido à precipitação do molibdênio, transformação de fases e densidade de discordâncias não foi suficiente para atingir o limite de escoamento mínimo especificado pela API, para o grau X80, sem a aplicação de revenimento posterior a 600 graus Celsius para obter precipitação fina de vanádio. / [en] The correlation between high frequency hot induction bending parameters with microstructure and mechanical properties is very important in order to keep the bent pipe within the API grade, in according with the API Specification 5L after the induction bending. The measured values of yield strength are a function of hardening mechanisms in both the tangent end and the bent section. The changes imposed by the thermal cycles of hot bending and tempering can modify the contribution of the strengthening mechanisms. This work aims to evaluate the changes of mechanical properties in the tangent end and the bent section from the point of view of the strengthening mechanisms such as phase transformation, dislocation density and precipitation. The results of the microstructural evaluations of the tangent section have shown that the hardening by grain refinement, precipitation in the austenite and the high dislocation density were responsible for high yield strength. The restrict range of cooling rate originated from the hot bending temperature was the most significant parameter on the microstructure, precipitation, dislocation density and hardening of the layers of the bent section. However, in the bend sections the restriction of precipitation of Vanadium significantly decreased the yield strength level. After hot bending the contributions of the strengthening mechanisms such as precipitation, phase transformation and dislocation hardening did not produce the desired minimum value of 552 MPa for the yield strength. Only after the tempering heat treatment at 600 Celsius degrees it was possible to obtain an increase in the yield strength.

A refinement of a minimal model for protein folding originally proposed by Imamura is presented. The representation of the alpha-helix has been improved by adding in explicit modelling of the entire peptide unit. A four-helix bundle consisting of four alpha-helices and three loop regions is generated with the parallel tempering Monte Carlo scheme. Six native states are found for the given sequence, four U-bundle and two Z-bundle states. All six states have energies of E approx -218ε and all appear equally likely to occur in simulation. The highest probability of folding a native state is found to be at a hydrophobic strength of Ch = 0.8 which agrees with the value of Ch = 0.7 used by Imamura in his studies of alpha to beta structural conversions. Two folding stages are observed in the temperature spectrum dependent on the magnitude of the hydrophobic strength parameter. The two stages observed as temperature decreases are 1) the hydrophobic energy causes the random coil to collapse into a compact globule 2) the secondary structure starts forming below a temperature of about T = 0.52ε/kB. The temperature of the first stage, which corresponds to the characteristic collapse temperature Tθ, is highly dependent on the hydrophobic strength. The temperature of the second stage is constant with respect to hydrophobic strength. Attempts to measure the characteristic folding temperature, Tf , from the structural overlap function proved to be difficult due mostly to the presence of six minima and the complications that arose in the parallel tempering Monte Carlo scheme. However, a rough estimate of Tf is obtained at each hydrophobic strength from a native state density analysis. Tf is found to be significantly lower than Tθ.

A refinement of a minimal model for protein folding originally proposed by Imamura is presented. The representation of the alpha-helix has been improved by adding in explicit modelling of the entire peptide unit. A four-helix bundle consisting of four alpha-helices and three loop regions is generated with the parallel tempering Monte Carlo scheme. Six native states are found for the given sequence, four U-bundle and two Z-bundle states. All six states have energies of E approx -218ε and all appear equally likely to occur in simulation. The highest probability of folding a native state is found to be at a hydrophobic strength of Ch = 0.8 which agrees with the value of Ch = 0.7 used by Imamura in his studies of alpha to beta structural conversions. Two folding stages are observed in the temperature spectrum dependent on the magnitude of the hydrophobic strength parameter. The two stages observed as temperature decreases are 1) the hydrophobic energy causes the random coil to collapse into a compact globule 2) the secondary structure starts forming below a temperature of about T = 0.52ε/kB. The temperature of the first stage, which corresponds to the characteristic collapse temperature Tθ, is highly dependent on the hydrophobic strength. The temperature of the second stage is constant with respect to hydrophobic strength. Attempts to measure the characteristic folding temperature, Tf , from the structural overlap function proved to be difficult due mostly to the presence of six minima and the complications that arose in the parallel tempering Monte Carlo scheme. However, a rough estimate of Tf is obtained at each hydrophobic strength from a native state density analysis. Tf is found to be significantly lower than Tθ.

Made available in DSpace on 2016-03-15T19:36:48Z (GMT). No. of bitstreams: 1 Jefferson Vieitas Fragoso.pdf: 11096922 bytes, checksum: 614fce3b64151fd99da32b8caf716e7f (MD5) Previous issue date: 2015-02-05 / This study aims to evaluate through destructive testing of steel specimens quick tempered subjected to a variation in the number of tempering treatments ranging from 1 to 4 cycles. Every condition of heat treatment at least three specimens were subjected to Charpy impact test on specimens without notch determining the modulus of rupture by three points flexion test. The samples were subjected to chemical and metallographic analyzes. The fracture surfaces were observed visually, by using microscope with increases ranging from 8 to 20 times, and also by scanning electron microscopy to determine the active fracture mechanisms. This experimental technique allowed the characterization in terms of semi quantitative chemical composition of the phases present in the specimens heat treated, namely primary and secondary carbides. The results were discussed in the light of information gathered in the literature review, and one of the main conclusions is that the sequence of tempering in the league studied did not alter significantly the energy absorbed results in impact tests and flexion resistance of bodies evidence submitted to bending test at three points. / O presente trabalho tem como objetivo avaliar, através de ensaios destrutivos, corpos de prova de aço rápido temperados, submetidos a uma variação no número de tratamentos de revenimento variando de 1 a 4 ciclos. A cada condição de tratamento térmico, pelo menos três corpos de prova foram submetidos ao Ensaio de Impacto Charpy em corpos de prova sem entalhe, determinando o módulo de ruptura de ensaio de flexão por três pontos. As amostras foram submetidas a análise química e metalográfica e, testes de dureza. As superfícies de fratura foram observadas a olho nu, através da utilização de estéreo microscópio com aumentos variando de 8 a 20 vezes, e também, por microscopia eletrônica de varredura para determinação dos mecanismos de fratura atuantes. Esta técnica experimental permitiu a caracterização, em termos, de composição química semi-quantitativa, das fases presentes nos corpos de prova tratados termicamente, a saber, os carbonetos primários e secundários. Os resultados foram discutidos à luz das informações colhidas na revisão bibliográfica, onde uma das observações principais é a de que a seqüência de revenimentos na liga estudada não altera de forma significativa os resultados de energia absorvida em ensaios de impacto e resistência à flexão, de corpos de prova submetidos ao ensaio de flexão a três pontos.

Este estudo teve como objetivo a determinação dos menores valores de tempo e temperatura para obtenção dos graus 95 e 110, através de projeto de experimento completo fatorial rotacional. Posteriormente foi investigado o comportamento mecânico e estrutural do aço API 5CT. Para os aços bruto, normalizado e seus respectivos graus 95 e 110 nos sentidos transversal e longitudinal. Para isto, foram realizados ensaios de tração, dureza, impacto e metalografia. Os mesmos foram submetidos ao tratamento térmico de têmpera com a temperatura de austenitização de 870°C e resfriados em água. Na sequência, realizou- se o tratamento de revenimento orientado pelo planejamento de experimento. O aço bruto apresentou estrutura perlitica bandeada na matriz ferritica. No aço normalizado foi observada a perlita mais homogeneamente distribuída em toda matriz ferritica. O grau 95 obtido a partir do aço bruto e normalizado apresentou a estrutura martensítica revenida em ripas e Ferrita. O grau 110 partindo dos aços, bruto e normalizado apresentou microestrutura martensítica revenida refinada ou em ilhas e Ferrita. Os módulos de resiliência e tenacidade dos aços bruto e seus graus apresentaram valores em torno de 14% maiores quando comparados ao aço normalizado. De forma similar, os limites de escoamento e tração aumentaram em 10%. A capacidade de encruamento aumentou cerca de 16% e houve um aumento de 5% no coeficiente de resistência. Em ambos os casos, os valores obtidos estão adequados à norma API 5CT. As análises das fractografias das amostras testadas em tração, para as condições bruto e normalizado, revelaram a predominância de fratura dúctil e trincas secundarias provenientes do tratamento térmico. A fratura do ensaio de impacto obtida a 60°C, dos aços bruto e normalizado, apresentaram fraturas dúcteis na região central e fraturas frágeis nas extremidades. Na temperatura de - 196 °C as fraturas foram predominantemente frágeis. Para o aço bruto e seus respectivos graus, a transição dúctil frágil ocorreu a -34°C com 79J de energia absorvida. Para o aço normalizado e os graus correspondentes, a temperatura de transição foi de -48 °C e a energia absorvida de 85J. / The purpose of this study was to determine the lowest time, and temperature values needed to obtain the degrees 95, and 110 of the HSLA steels through a rotational, full factorial experiment design. Subsequently, the mechanical and structural behavior of the API 5CT steel was also investigated. Values for the raw steel samples were normalized transversally and longitudinally. For this purpose, traction, hardness, impact, and metallographic tests were conducted. The aforementioned alloys were subjected to heat treatment by quenching with an austenitizing temperature of 870°C, and then cooled in water. The next step was the treatment of tempering heat as determined by the experimental design. The raw steel showed signs of a pearlite banded structure in the ferritic matrix. The 95 degree sample obtained from the raw and normalized steel displayed martensitic structures tempered in lath and ferrite. The 110 degree sample created from raw and normalized steel exhibited refined tempered standard martensitic structures, in addition to ferrite. The raw steel resilience, and tenacity moduli, along with its temperature values were about 14% greater when compared to the normalized steel results. Similarly, ullage and traction thresholds increased by 10%. The strain hardening capacity increased by about 16% and there was an increase of 5% in the resistance coefficient. In both cases, the values obtained were within API 5CT standards. Sample analyses from fractographies, and traction tests (for raw and normalized steels) revealed the prevalent occurrence of ductile fractures, and secondary cracks caused by the heat treatment. The impact test, at 60°C, for both the raw and normalized samples, caused ductile fractures in the center, and brittle fissures around the edges. At the temperature of -196 °C, fractures were predominantly fragile. For the raw steel samples tested and their respective degrees, the ductile brittle transition occurred at - 34°C with absorbed energy of 79J. For the standard steel and its corresponding degree, the transition temperature was -48°C, and absorbed energy of 85J.

Wear and friction are among the major problems faced in several industries such as mining industry. This creates challenges to select better materials with good wear behavior in order to improve the service life of the components. In the present project, three steel wire grades OH 70, OH 75 and OH 101 have been heat treated by quenching and partitioning heat treatment and tested using three wear testing methods. The wear tests performed were a pin-on-disc test, dry-pot test and slurry pot-test, and the results were compared with the conventional quenched and tempered steel. Tensile tests, hardness, impact Charpy tests, scanning electron microscope, optical microscope, X-ray diffraction and magnetic measurements were applied to characterize mechanical properties and microstructure of the steels before wear tests. The results showed that the quenched and partitioned steels with considerable amount of retained austenite had higher ductility and good impact toughness than the quenched and tempered steels. After the pin on disc tests, OH 75 grade showed the highest wear resistance, while the lowest wear resistance was obtained by OH 70 grade. The damage mechanisms identified after pin-on-disc were abrasion and oxidative wear. During erosive wear, almost no measurable wear was recorded under the dry pot conditions, while the slurry pot test owned significant wear mass loss. The main modes of the worn surfaces after erosive tests were ploughing and cutting. In addition, cracks were also observed.

The average size of hot-work tools has gradually increased over the past years.This affects the effective temperature cycle tools experience during hardening,as large dimensions prevent uniform and rapid cooling, and thereby the resulting microstructures and properties. In order to avoid the formation of coarse structures or cracking during heat treatment it has become common practise to lower the austenitising temperature below that recommended by the steel manufacturer.In this work, therefore, the effects of austenitising at temperatures lower thancommonly recommended are investigated. Three 5% Cr hot-work tool steelsalloyed with Mo and V were heat treated, resulting microstructures andtempering carbides were studied and transformation characteristics determined for different austenitising temperatures and different cooling rates. The temperatures and cooling rates have been chosen to be representative for heat treatments of different sizes of tools. Bainite rather than martensite formed during slow cooling regardless of austenitising temperature. A lowered austenitising temperature produced largeramounts of both bainite and retained austenite while a higher caused graingrowth. Carbon partitioning during the bainitic transformation resulted in anincrease of the carbon content in the retained austenite of at least 0.3 wt.%. The austenitising temperature influences also the type and amount of tempering carbides that precipitate, which affects the properties of the steel. Higher austenitising temperatures favour the precipitation of MC carbides during tempering. The Mo rich M2C type carbides were proven to be more prone to coarsening during service at 560°C-600°C, while V rich MC carbides preserve their fine distribution. A best practice heat treatment needs to balance the increase of grain size with increasing austenitising temperatures, with the possibility to form more tempering carbides. Higher austenitising temperatures also give less retained austenite, which can affect dimensional stability and toughness negatively after tempering

Nowdays biofuels are becoming a good alternative for petroleum fuels due to environmental issues like high carbon dioxide emission and increasing vehicles population, together with the high price and fast depletion of petroleum oils. This project aims to investigate the corrosive effects of wood Pyrolysis oil on a special grade of nitrogen alloyed tooling steels to be used for injector nozzles in Diesel engines, where high stress and strain encounter high acidity and corrosivity of the Pyrolysis oil and cause breakdown over short periods. Vanax 35 and Vanax 75 manufactured in Uddeholm are two types of powder metallurgy high nitrogen alloyed martensitic stainless steel with a high combination of hardness (over 56HRC), low friction properties, wear resistance, anti-galling and corrosion properties. In this work, the newly developed Vanax material together with the tool steels Elmax and AISI O1 were tempered at various temperatures from 200°Cto 500°C. The tempered steels were then exposed in pyrolysis oil at 4 different temperatures, 20°C, 70°C, 95°Cand 130°C. The materials were investigated by means of corrosion rate measurements, microscopy (LOM, SEM, confocal) and Thermo-Calc calculations. The corrosion rate measurement proved that Vanax tempered at lower ranges (200°C, 400°Cand 450°C) showed the best corrosion resistance while higher tempering temperatures such as 500°C, Elmax and AISI O1 tempered at 200°Csuffered a great deal of general corrosion attack. Thermo-Calc calculations showed the formation of a hard phase, VN as primary nitrides instead of primary chromium carbides at austenizing temperature for the Vanax group. Higher amount of chromium is dissolved in solid solution in Vanax at austenizing temperature hence the martensite matrix has, after quenching, a higher chromium content that helps passivation. The loss in corrosion properties at higher tempering temperatures was due to the formation of CrN secondary phase at around 400˚C which reduces the chromium content of the martensite matrix. The results of light optical and confocal microscopy showed the presence of pits when tempering at 400˚C and 450˚C. No pits were observed at 200˚C. Elmax was not passivated at all which resulted in general corrosion attacks, due to a high chromium loss from the austenite solid solution at the austenizing procedure temperature and also the tempering temperatures. The chromium depletion from the austenite can be explained by a high carbon and a low nitrogen content in the composition which resulted in formation of a high amount of Cr7C3.