In this study the effects of Sn content and isothermal transformation temperature on the ultimate tensile strength (UTS), elongation and hardness of austempered ductile cast iron (ADI) was investigated. To determine the possible effect of Sn on these properties the Sn content of standard GGG30, GGG40 and GGG50 materials were taken as reference, whose chemical compositions vary from 0,016 to 0,050% in terms of Sn. However the Sn content was increased to a maximum of 0,26% for investigating the effect of Sn on mechanical properties. The test specimens were obtained from standard 1-in Y-blocks and austenitized at 925oC for 1 hour and austempered at 350oC,375oC, 400oC and 420oC for 60 minutes in a salt bath. Microstructural examination of the specimens were also done. To determine the amount of retained austenite of the specimens X-Ray analyses were used. As a result of the study The UTS increases up to 0,1% Sn and then levels off. A similar behavior is observed in the variation of hardness. Percent elongation decreases and falls to a minimum around 0,1% Sn, from this point on, a slight increase is observed. Strength and hardness increase while percent elongation decreases with decreasing temperatures.

Grandes partes dos estudos disponíveis na literatura sobre o comportamento dos materiais com microestruturas martensítica e bainítica relatam que a microestrutura bainítica, comparativamente à martensita revenida, promove melhores propriedades de impacto. No presente trabalho foi proposto o estudo de comportamento ao impacto de aços SAE 4140 e SAE 01 (VND) sob diferentes condições de tratamento térmico. Ao todo foram feitos 57 corpos de prova CHARPY para cada tipo de aço divididos em 3 lotes que foram então: temperados e revenidos, com e sem banho criogênico e austemperados. A dureza final após tratamento térmico foi similar nos diferentes tratamentos térmicos. Os resultados obtidos para os dois tipos de aço mostram que os corpos de provas que passaram pelo tratamento térmico de têmpera, revenido e banho criogênico apresentaram melhor tenacidade ao impacto. / Studies available in the literature about bainitic and martensitic microstructure report that bainitic microstructure compared to tempered martensite has better impact properties. In present work has proposed the study of the impact performance of steel SAE 4140 and SAE 01 (VND) under different conditions of heat treatment. A total 57 CHARPY specimens were divided in 3 sets, which were quenched and tempered, with and without cryogenic bath and austempered. The final hardness after heat treatment was similar in the different heat treatments. The results obtained for the two tipes of steel showed better impact performance for samples submitted to quenching tempering and cryogenie bath.

In this study the effects of different austempering times and temperatures on impact toughness, hardness and fracture toughness properties of 35NiCrMoV12.5 gun barrel steel are investigated. 300 &deg / C, 325 &deg / C and 350 &deg / C were chosen as austempering temperatures. Isothermal holding times at these temperatures were chosen as 1 minute, 10 minutes, 1 hour and 10 hours. It was found that, 350 &deg / C being an exception, austempering temperature and impact toughness has an inverse relationship and impact toughness increases as isothermal holding time increases. However this behavior is valid until some point. Prolonged transformation times causes toughness to decrease. Hardness measurements revealed that, as isothermal holding time increases, hardness decreases. In order to compare the mechanical properties obtained by austempering with that of conventional cooling and tempering, 400 &deg / C was chosen as the tempering temperature and applied to both charpy impact and fracture toughness specimens. It was found that conventional cooling and tempering produced tougher structures. Size of the fracture toughness specimens might have caused an undesired situation such as incomplete transformation to bainite. Optical and scanning electron microscopy was used in order to analyze the microstructures obtained after each treatment. It was observed that the majority of the morphologies occurred is lower bainite. On the other hand, martensitic structures were observed almost at every temperature.

Grandes partes dos estudos disponíveis na literatura sobre o comportamento dos materiais com microestruturas martensítica e bainítica relatam que a microestrutura bainítica, comparativamente à martensita revenida, promove melhores propriedades de impacto. No presente trabalho foi proposto o estudo de comportamento ao impacto de aços SAE 4140 e SAE 01 (VND) sob diferentes condições de tratamento térmico. Ao todo foram feitos 57 corpos de prova CHARPY para cada tipo de aço divididos em 3 lotes que foram então: temperados e revenidos, com e sem banho criogênico e austemperados. A dureza final após tratamento térmico foi similar nos diferentes tratamentos térmicos. Os resultados obtidos para os dois tipos de aço mostram que os corpos de provas que passaram pelo tratamento térmico de têmpera, revenido e banho criogênico apresentaram melhor tenacidade ao impacto. / Studies available in the literature about bainitic and martensitic microstructure report that bainitic microstructure compared to tempered martensite has better impact properties. In present work has proposed the study of the impact performance of steel SAE 4140 and SAE 01 (VND) under different conditions of heat treatment. A total 57 CHARPY specimens were divided in 3 sets, which were quenched and tempered, with and without cryogenic bath and austempered. The final hardness after heat treatment was similar in the different heat treatments. The results obtained for the two tipes of steel showed better impact performance for samples submitted to quenching tempering and cryogenie bath.

Este trabalho apresenta um estudo dos mecanismos de fadiga em mola para válvula automotiva, realizado através da análise do comportamento mecânico correlacionado ao aspecto microestrutural. As amostras foram fabricadas em aço de alta resistência Si-Cr, submetidas a tratamentos térmicos convencionais (têmpera e revenimento) e não convencionais (austêmpera), obtendo um mesmo nível de dureza (aproximadamente 610 HV). Os corpos de prova foram submetidos a ensaios de bancada com diversos níveis de carregamentos, controlados através de variação da amplitude de tensão cisalhante. Esta variação promoveu diferentes regimes de fadiga, conhecidos como alto e ultra-alto ciclo, para maiores e menores níveis de amplitudes de tensões, respectivamente. O desempenho foi medido através do cálculo da vida total em fadiga, probabilidade de sobrevivência, além da verificação do relaxamento da força elástica e da tensão residual. As amostras foram caracterizadas através de métodos metalográficos, determinação da composição química, ensaios de dureza e resistência à tração, além de técnicas difratométricas de raios-X. As superfícies de fratura foram investigadas utilizando técnicas de microscopia eletrônica de varredura Os mecanismos de início e propagação das trincas, de ambas as amostras, foram semelhantes, apesar do tratamento térmico proposto de austêmpera produzir uma microestrutura mista, com predominância de bainita inferior, contendo regiões esparsas de austenita retida. A microestrutura bainítica, resultante da austêmpera, quando comparada com as amostras martensíticas, geradas pela têmpera e revenimento, propiciou um aumento da vida total em fadiga nos maiores níveis de amplitudes das tensões, tendendo à equalização com a diminuição desta amplitude, além de apresentarem um maior relaxamento elástico, gerando uma discreta diminuição de magnitude da tensão média. Por outro lado, não houve relaxamento das tensões residuais, em ambas as amostras, após uma quantidade significativa de ciclos, portanto este fenômeno não influenciou em sua vida total. / This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

Este trabalho apresenta um estudo dos mecanismos de fadiga em mola para válvula automotiva, realizado através da análise do comportamento mecânico correlacionado ao aspecto microestrutural. As amostras foram fabricadas em aço de alta resistência Si-Cr, submetidas a tratamentos térmicos convencionais (têmpera e revenimento) e não convencionais (austêmpera), obtendo um mesmo nível de dureza (aproximadamente 610 HV). Os corpos de prova foram submetidos a ensaios de bancada com diversos níveis de carregamentos, controlados através de variação da amplitude de tensão cisalhante. Esta variação promoveu diferentes regimes de fadiga, conhecidos como alto e ultra-alto ciclo, para maiores e menores níveis de amplitudes de tensões, respectivamente. O desempenho foi medido através do cálculo da vida total em fadiga, probabilidade de sobrevivência, além da verificação do relaxamento da força elástica e da tensão residual. As amostras foram caracterizadas através de métodos metalográficos, determinação da composição química, ensaios de dureza e resistência à tração, além de técnicas difratométricas de raios-X. As superfícies de fratura foram investigadas utilizando técnicas de microscopia eletrônica de varredura Os mecanismos de início e propagação das trincas, de ambas as amostras, foram semelhantes, apesar do tratamento térmico proposto de austêmpera produzir uma microestrutura mista, com predominância de bainita inferior, contendo regiões esparsas de austenita retida. A microestrutura bainítica, resultante da austêmpera, quando comparada com as amostras martensíticas, geradas pela têmpera e revenimento, propiciou um aumento da vida total em fadiga nos maiores níveis de amplitudes das tensões, tendendo à equalização com a diminuição desta amplitude, além de apresentarem um maior relaxamento elástico, gerando uma discreta diminuição de magnitude da tensão média. Por outro lado, não houve relaxamento das tensões residuais, em ambas as amostras, após uma quantidade significativa de ciclos, portanto este fenômeno não influenciou em sua vida total. / This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

The thesis ,,The Influence of Transformation dwell on the Structure and Mechanical Properties of ADI´´ deals with problems of optimal properties of the unalloyed nodular cast iron, which are obtained by the heat treatment. The heat treatment consists of austenitization and then rapidly quenching to the transformation temperature, isothermal transformation and water cooling in the end. The aim of the thesis is focused on estimating the influence of isothermal transformation on the content of stabilized austenite, static mechanical properties and on the fatigue properties in high-cycle fatigue region for selected samples. The theoretical part of this work analyses the production of the nodular cast iron and its possibilities of the heat treatment, final technological and mechanical properties. Furthermore, it examines the conditions of austempering (temperature and time of isothermal dwell), which has a fundamental influence on mechanical properties of ADI. Problems of fatigue properties are described at the end of this part. The experimental part contains a chemical composition of studied samples and their metallographic analysis. Among others the basic mechanical static properties were determined as well as the Wöhler curves including values of fatigue limits for ten on seventh cycles. Wöhler curves were obtained by a mathematical processing using data reached by fatigue tests.

Heat treatment is a way to significantly change materials properties. When presented with materials that lack certain mechanical properties, it is possible to change its chemical properties and microstructures by applying heat. This can help achieve better yield strength, ductility and toughness. This project discusses the effects of multiple distinct heat treatment methods for several materials in order to improve ductility and elongation without diminishing strength. The materials in question are High Aluminum Steel and Strenx 700MC steel, the first being under development and the second being a commercially available steel. These steels show promise to be used as high ductility, high strength, and 3rd generation steels. The heat treatments can change the mechanical proprieties of the base materials in order to optimize these steels for applications in vertical access solutions. The heat treatments in this project were Quenching and Partitioning (QP), Quenching and Tempering (QT), Austempering (AUST), Intercritical Heat Treatment (IHT) and other usual heat treatments such as Double normalizing (D-Norm). First, the most beneficial type of the above mentioned heat treatments was selected for each steel and series of heat treatments were performed in order to identify and optimize the best method for each steel. Then, heat treated samples underwent a series of tests to numerically quantify their properties and compare them to the existing steels in Alimak’s applications. The results show that Quenching and Partitioning is the most promising heat treatment for optimizing strength and ductility in High Aluminum Steel, with elongation values up to 19% together with yield strengths of 700 MPa. For Strenx 700MC a combination of temperature and time was found that gave an elongation of above 25% with a yield strength of 450 MPa. The explanation for the good properties was partly grain refinement and phase transformations during heat treatments.

O ADI constitui-se no desenvolvimento mais recente na família dos ferros fundidos nodulares. Com o tratamento de austêmpera, consegue-se produzir uma microestrutura única, constituída de ferrita acicular e austenita estável rica em carbono, a ausferrita, proporcionando alta resistência mecânica aliada à ductilidade e tenacidade, além de boa resistência à fadiga e ao desgaste. Neste trabalho estudou-se o efeito dos elementos de liga Cu, Ni e Mo nas propriedades mecânicas e austemperabilidade do ADI. Foram produzidas barras cilíndricas de ferros fundidos nodulares nos diâmetros de Ø2, Ø3 e Ø4 ligadas com Cu, Cu-Ni, Cu-Ni- Mo respectivamente. Os corpos de prova utilizados nos ensaios de tração, impacto e microdesgaste foram retirados a meio-raio das barras. Posteriormente, cada conjunto de corpo de prova foi austenitizado a 890°C durante 2 horas, sendo em seguida submetido a uma condição de tratamento de austêmpera específica, combinando temperaturas de austêmpera de 300 e 360°C e tempos de 1, 2, 3 e 4 horas. Realizou-se análises microestruturais ópticas e por microscopia eletrônica de varredura para correlacionar propriedades mecânicas com a microestrutura. Na análise de austemperabilidade, as barras cilíndricas foram austemperadas para verificar a variação de microestrutura em função do raio da barra. Verificou-se que a temperatura de austêmpera exerce forte influência na microestrutura do ADI e consequentemente nas propriedades de tração, ductilidade, tenacidade e resistência ao desgaste. Nos tratamentos a 300°C obteve-se uma microestrutura mais refinada, com maior quantidade de ferrita acicular, responsável pela maior resistência mecânica e resistência ao desgaste; enquanto que no tratamento realizado a 360°C obteve-se uma microestrutura mais grosseira, com maior quantidade de austenita estável, responsável pela melhor ductilidade e tenacidade. Nos tempos de austêmpera analisados, não ocorreram variações significativas nas microestruturas e propriedades mecânicas. A resistência mecânica e a tenacidade decresceram com a adição de Mo, provavelmente devido à segregação deste elemento, mas a resistência ao desgaste a seco e a austemperabilidade foram mais efetivas em comparação com ligas contendo Cu e Cu-Ni. No ensaio de desgaste usando-se lubrificante, o ADI ligado com Cu-Ni austemperado a 360°C apresentou uma resistência ao desgaste um pouco inferior a de um aço 17CrNiMo6 cementado. / The ADI is the most recent development in the nodular iron family. With the austempering treatment, a unique microstructure, consisting of acicular bainite ferrite and stable austenite rich in carbon, named ausferrite is produced. This microstructure provides high mechanical strength combined with ductility, toughness and good fatigue and wear resistances. In this work, the effect of alloying elements Cu, Ni and Mo on the mechanical properties and austemperability of the ADI were studied. To conduct these studies, cylindrical bars with diameters of Ø2\", Ø3\" and Ø4\" were cast with Cu, Cu-Ni and Cu-Ni-Mo alloying, respectively. The samples were then manufactured by removing specimens from the bar midradius position. Subsequently, each set of the samples was austenitized at 890°C for 2 hours, and then subjected different conditions of austempering treatment. These treatments were developed by combining austempering temperatures of 300 and 360°C with austempering times of 1, 2, 3 and 4 hours. Microstructural analysis was carried out using optical and scanning electron microscopy to correlate mechanical properties with the microstructure. In the austemperability analysis, cylindrical bars were austempered (at 360°C for 3 hours) to correlate microstructure and radius. The experimental results showed that the austempering temperature exerts strong influence on the ADI´s microstructure and consequently on the mechanical properties. The austempering temperature of 300°C produced the best results, a refined microstructure, with a greater amount of acicular ferrite, responsible for greater strength and wear resistance. The austempering temperature of 360°C produced a coarse microstructure, with larger amount of austenite and responsible for better ductility and toughness. The austempering times used in this work did not produce significant variations in the properties and microstructures. Regarding alloying elements, the mechanical strength and toughness decreased with the addition of Mo, probably due to the segregation of this element, but the wear resistance and austemperability were more effective when compared with the alloys containing Cu and Cu-Ni. In the lubricated wear test, the ADI alloyed with Cu-Ni and austempered at 360°C presented a wear resistance only a fraction lower than a cemented 17CrNiMo6 steel.

Foram produzidas camadas de carbonetos por meio do tratamento termorreativo de difusão (TRD) em quatro ferros fundidos nodulares: um sem elemento de liga e outros três ligados com Cu, Cu-Ni e Cu-Ni-Mo, respectivamente; e um ferro fundido vermicular. O banho era composto por borato de sódio, alumínio, ferro-nióbio e/ou ferro-vanádio, a 1000 °C com tempo de permanência de tratamento de 2 horas. Realizou-se austêmpera, a partir dessa temperatura, em banho de sal fundido a 300 °C. As amostras foram caracterizadas por meio de microscopias óptica e MEV, EDS, DRX, ensaios de durezas Brinell e Vickers, Daimler-Benz Rockwell-C, ensaios de desgaste microadesivo, do tipo esfera fixa, e de corrosão por meio de ensaios de polarização potenciodinâmica. Os tratamentos TRD foram altamente eficientes para a produção de camadas de carbonetos uniformes e com boas aderências aos substratos, em todos os ferros fundidos em estudo. O ferro fundido nodular não ligado apresentou os menores valores de espessuras para as camadas produzidas de carbonetos simples de nióbio e de vanádio, tendo ocorrido um aumento dessas espessuras com a adição dos elementos de liga Cu e Cu-Ni, com exceção do Mo. As camadas apresentaram durezas acima de 2300, 2500 e 3050 HV para os casos dos carbonetos de Nb, V e Nb-V, respectivamente. As resistências ao desgaste das amostras foram aumentadas de 5 a 35x em relação aos substratos austemperados. As camadas apresentaram resistências a corrosão muito superiores as dos substratos, indicando a grande eficácia do tratamento TRD para todos os materiais estudados. Os compostos produzidos, constituídos por um substrato otimizado e de camadas de elevados desempenhos tribológicos, tornam-no altamente indicado para as condições severas de uso. / Carbide layers were produced by thermo-diffusion treatment (TRD) in four nodular cast irons: one without alloying element and the other three bonded with Cu, Cu-Ni and Cu-Ni-Mo, respectively; and a vermicular cast iron. The bath was composed of sodium borate, aluminum, iron-niobium and/or iron-vanadium, at 1000 °C with treatment time of 2 hours. The austempering treatment was carried out at 300 °C for 4 hours in a molten salt bath. The samples were characterized by optical microscopy and SEM, EDX, XRD, Brinell and Vickers hardness tests, Daimler-Benz Rockwell-C, microadhesive wear test (fixed ball type) and corrosion tests using potentiodynamic polarization. The TRD treatments were highly efficient for the production of uniform carbide layers with good adhesion to the substrates in all cast irons under study. Non-alloyed nodular cast iron presented the lowest thickness values for the layers produced from simple niobium and vanadium carbides, with an increase in these thicknesses with the addition of Cu and Cu-Ni alloys, with the exception of Mo. The layers achieved hardness above 2,300, 2,500 and 3,050 HV for the carbides of Nb, V and Nb-V, respectively. The wear resistance of the samples was increased from 5 to 35x in relation to the austempered substrates. The layers presented corrosion resistance far superior if compared to those of the substrates, indicating the great effectiveness of the TRD treatment for all the studied materials. The compounds, consisting of an optimized substrate and layers of high tribological performances, makes it highly suitable for severe conditions of use.