Three types of polycrystalline aluminum oxide, with varying amounts of impurities, were tempered and then annealed to study the behavior of residual stress relaxation. Results from microprobe analysis, thermal expansion analysis, annealed strength measurements, and scanning electron fractography clearly indicate stress relaxation at annealing temperatures occurred by elastic creep through crack nucleation and growth in regions under tensile stresses. From the scanning electron fractographs, it was found that intergranular glassy phase played an important role in the crack formation and propagation. At elevated temperature, viscous intergranular glassy phase behaved as an adhesive layer and when under tensile stresses caused de-adhesion of grain-boundaries or the formation of cracks. Fine-grained aluminas, with little or no glassy phase, exhibited both sub-critical and critical crack propagation. Since cracks decrease the load-carrying ability of the aluminas, it was concluded that by minimizing the glassy phase, porosity, grain size and residual stresses one can suppress crack formation. / Master of Science

The effects of thermal cycling on residual stresses are studied in thick-walled 316L stainless steel pipe. A relatively small AT of 280°F is considered. The residual stresses are measured by an x-ray diffraction technique. The initial stress state of the pipe wall shows periodic and random variability in stress from -30 to 20 ksi. The pipe was thermal cycled between a furnace and a quenchant bath. The macro residual stresses became 10 ksi more compressive after the first thermal cycle. Thereafter no significant changes are measured in the macro residual stresses, however, the micro residual stresses continue to increase linearly with the number of cycles. / Master of Science

Power generation in thermal stations typically relies on large steam turbines. The corrosion resistant steel blades used in the last stage of a typical low pressure rotor set are approximately 1m long and experience high centrifugal loading during service. They operate in a wet steam environment, at approximately 60°C while rotating at 3000rpm, and failure modes include high and low cycle fatigue, stress corrosion cracking or corrosion fatigue. The blades are retained by a fir tree root which is normally shot-peened to generate compressive residual stresses that resist crack initiation. Finite element (FE) modelling has indicated that, in the absence of shot-peening, stresses above yield are induced at the fir tree root during operation. In a shot-peened blade these lead to relaxation of the shot peening residual stresses. To date, no systematic information has been obtained on the level of residual stresses induced in the fir tree by shot-peening and their subsequent relaxation during service loading, nor are there any guidelines as to the magnitude of residual stresses necessary to ensure integrity of the turbine over a life span of at least twenty years. At least one of these blades has suffered catastrophic failure in recent years causing severe damage, in excess of €100M, to the turbine-generator set on a South African power station [1]. This thesis reports results from a comprehensive program of residual stress measurements at the shot-peened fir tree roots of service blades, and in specimens that simulate the root conditions, using diffraction data from laboratory and synchrotron X-ray radiation (SXRD). Shot-peening coverage between 75% and 200% was used and stresses were measured over a depth of 5mm into the blades/specimens. Measurements were made in the as-peened condition and after applying cyclic stresses representative of overspeed proof testing and of service operation. The results were used to calibrate FE modelling of residual stresses and as input into fatigue life prediction.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering, 2016 / This research is focused on optimizing the surface integrity of Ti-6Al-4V using advanced carbide inserts and minimum quantity lubrication (MQL). Experiments were designed to machine twenty Ti-6Al-4V blocks under dry and MQL lubricating conditions using innovative cemented carbide inserts produced by Liquid Phase Sintering (LPS) and Spark Plasma Sintering (SPS). The cutting speed, feed rate, and depth of cut, were considered as machining parameters, while the cutting force, temperature, tool wear, surface roughness and residual stress were considered as performance characteristics. The results obtained for surface roughness and residual stress measurements were used to analyze the surface integrity of the machined Ti-6Al-4V samples. The influence of sintering techniques and milling parameters on performance characteristics and surface integrity were investigated from experimental results obtained. Discrepancy in behavioral trends with respect to lubricating conditions was also investigated. The analyses of milling results showed that cutting speed played a major role in the optimization of surface integrity of the Ti-6Al-4V work pieces followed by the depth of cut. The results also showed that the LPS inserts performed better than the SPS inserts due to their better mechanical properties (higher fracture toughness (KIC) and transverse rupture strength). The application of MQL was also observed to significantly reduce milling temperatures resulting in better surface integrity. However, an optimized surface integrity of the Ti-6Al-4V samples was achieved during MQL milling with the 10Co-L insert at a low finishing speed of 75m/minute and a reduced depth of cut of 0.5mm.

Os aços inoxidáveis austeníticos são materiais atrativos para serem utilizados em vários setores industriais que operam sob meios corrosivos, como por exemplo: indústria química, alcooleira, petroquímica, de papel e celulose, na prospecção de petróleo e nas indústrias têxtil e farmacêutica. Contudo, apresentam propriedades tribológicas pobres. No sentido de melhorar essas propriedades, como aumentar a dureza superficial, a resistência ao desgaste e a resistência à fadiga, vários métodos de tratamentos superficiais vêm sendo utilizados. Dentre eles, o mais eficiente é a nitretação por plasma. Este processo é realizado em uma câmara de vácuo sob uma mistura gasosa de hidrogênio e nitrogênio. É aplicada uma diferença de potencial entre o cátodo (porta amostras) e o ânodo (paredes da câmara), acelerando os íons contra a superfície da peça, aquecendo-a e arrancando elétrons de sua superfície. Os íons reagem com espécies da superfície do plasma formando compostos instáveis do tipo FeN que se recombinam para formarem nitretos estáveis. O sucesso deste tratamento se deve à baixa temperatura de operação, ao menor tempo efetivo de tratamento e ao controle da uniformidade da espessura da camada. A camada de nitretos formada durante o tratamento possui uma influência positiva na vida em fadiga de um componente, graças a dois motivos principais. O primeiro é o atraso na nucleação da trinca devido ao aumento da resistência mecânica superficial. O segundo motivo está relacionado com a introdução de tensões residuais compressivas durante o processo de endurecimento da superfície, que retarda a iniciação da trinca e diminui o fator de intensidade de tensão. Os corpos de prova foram nitretados a 400ºC durante 6 horas, com uma pressão de 4,5 mbar e utilizando uma mistura gasosa de 80% vol.H2 e 20%vol.N2. Ocorreu um aumento da resistência mecânica próxima á superfície, devido à camada de nitretos, o que ficou evidente com o sensível aumento no valor do limite de fadiga do material nitretado em relação ao não nitretado. O limite de fadiga do aço AISI 316 não tratado foi de 400MPa e do nitretado foi de 510MPa, enquanto que, para o aço AISI 304, o limite de fadiga do material não tratado foi de 380MPa e o limite para material submetido ao tratamento de nitretação foi de 560MPa. / The austenitic stainless steels are attractive materials to many industrial sectors which work on corrosive environments, as chemical industry alcohol, petrochemical, cellulose industries, in the petroleum prospection and pharmaceutical and textiles industries. However, they present poor tribological properties. In order to improve these properties, like increasing superficial hardness, wear and fatigue resistance superficial heat treatment methods have being used. The most efficient is the plasma nitriding process which occurs in a vacuum container under hydrogen and nitrogen gas mixture. A potential difference is applied between the cathode (samples receptor) and the anode (container walls), accelerating the ions against the piece, heating it and removing electron from the surface of material. These atoms react with the surface plasma species, producing unstable compounds like FeN, which recombine producing stable nitrides. The success of this treatment is due to the low temperature operation, the short effective time of treatment and to the uniformity control of the layer’s thickness. The nitrides layer produced during the treatment have a positive influence in the fatigue life of a component, thanks to two main reasons. The first is the retardation in crack nucleation due to increasing of superficial mechanical strength. The second reason is due to introduction of compressive residual stress during the surface hardening process, which retards de crack initiation process. The specimens were nitriding at 400°C during 6 hours, at a 4,5mbar pressure and using a gas mixture of 80% vol. H2 and 20% vol. N2. The surface mechanical strength increased, due to the nitrides layer, which was evident with the sensitive increase in the fatigue limit of the nitriding specimens, comparing to the untreated ones. The fatigue limit of the AlSl 316 steel in untreated condition was 400 MPa and in nitriding condition was 510 MPa, whereas AlSl 304 steel, the fatigue limit of the untreated condition was 480 MPa and the fatigue limit for the nitrided condition was 560 MPa.

Neste trabalho foram investigadas as influências de alguns dos principais fatores do torneamento sobre o acabamento da superfície, rugosidade, tensões residuais e microdureza em um conjunto de amostras. Foram analisadas barras de aço ABNT 1045 torneadas e utilizou-se a técnica de planejamento de experimentos (DoE) para formar trinta e duas diferentes combinações de fatores, analisar suas interações e sua influência no acabamento de cada amostra. Os fatores de usinagem investigados foram: (1) avanço, (2) velocidade de corte, (3) raio de quina, (4) ângulo de saída e (5) posição axial. Analises fatoriais foi avaliaram a influência dos cinco fatores na rugosidade média (Ra) e a eficácia de cada conjunto de fatores em obter o acabamento alvo (Ir). Utilizou-se a técnica de difração de raios-X para medir as tensões residuais em sete das dezesseis amostras. As tensões foram medidas na superfície e em profundidades de 5, 10, 20, 50 e 75 μm com auxílio de ataque eletrolítico. Por fim foi realizada análise metalográfica da superfície usinada e perfil de microdureza. Os resultados mostraram que o avanço e o raio de quina possuem elevado efeito padronizado em ambas as análises. Porém, para ambos, enquanto que a aplicação de um menor avanço (ou maior raio de quina) produziu superfícies com baixo Ra, por outro lado também prejudicou a eficácia do torneamento pois contribuiu para obtenção de rugosidade significativamente maior que a teórica. Já o fator velocidade de corte quando teve seu valor elevado foi benéfico ao processo em ambas as análises. Não foi encontrada relevância da região de medição da rugosidade sobre Ra. As tensões residuais sofreram elevada influência dos fatores avanço e velocidade de corte, enquanto o ângulo de saída e o raio de quina mostraram pequeno efeito. A qualidade do acabamento da superfície, regular ou irregular, apresentou relação direta com o mecanismo de geração das tensões residuais. O perfil de microdureza medido mostrou valores de dureza elevados nas amostras com superfície regular e em regiões com maior tensão residual. / This paper investigated the influence of some of the main turning parameters in surface finishing, roughness, residual stress and microhardness of turning samples. Design of experiments (DoE) approach controlled the machining factors of the 0.45%C (ABNT 1045) steel bars samples. DoE also were applied to organize a total of thirty-two parameters combination, analyze its interactions and influences in each sample’s surface finish. Machining parameters investigated were: (1) feed rate, (2) cutting speed, (3) nose radius, (4) rake angle and (5) region of roughness measurement. Factorial analysis evaluated the influence of those five parameters in the average roughness (Ra) and the accuracy of each parameters combination to achieve the expected surface finish (Ir). The X-ray diffraction technique was used to measure the residual stresses in seven out of sixteen samples. The tensions were measured at their surface and at depths of 5, 10, 20, 50 and 75 μm by means of electrochemical material removal. In addition metallographic analysis of the machined surface and microhardness profile were conducted. Results showed that parameters as feed rate and nose radius have strong standardized effect in both factorial analysis. Results shown that reduction of the feed rate (or increase in nose radius) were able to produce lower roughness average (Ra) but, on the other hand, it also contribute to generate a surface roughness significant higher than theoretical roughness. The increase in cutting speed was beneficial to both, achieving a lower roughness average and improving process accuracy. The region of roughness measurement was found not significant to influence Ra. The residual stresses were strongly influenced by the feed rate and cutting speed factors, while the rake angle and nose radius showed small effect. The quality of the surface finish, regular or irregular, was directly related to residual stresses generation mechanism. The microhardness profile shown higher microhardness in samples with regular surface finish and in depths with higher residual stress.

Nenhuma / Para o aprimoramento da qualidade e confiabilidade dos equipamentos considerados de grande responsabilidade, existe a necessidade de promover maior controle das imperfeições que venham a ocorrer durante os diversos processos de fabricação. Dentre eles, observa-se que nos de soldagem, ocorrem fenômenos físicos e metalúrgicos, termicamente ativados, que resultam no aparecimento de dois tipos principais de imperfeições: tensões residuais e distorções geométricas. O estado de tensões residuais distribuídas nas juntas soldadas representa um fator importante na determinação do valor de carga aplicável e na vida útil do equipamento, para cada aplicação e está diretamente alinhado com o procedimento de soldagem envolvido na fabricação. Assim, o monitoramento do estado de integridade do equipamento, em serviço, deve ser realizado por meio da aplicação regular e periódica de ensaios não destrutivos, adequados às características das descontinuidades relevantes. Vários trabalhos demonstram a sensibilidade do ruído magnético Barkhausen como método complementar de caracterização de materiais ferromagnéticos, na verificação de características de sua microestrutura, tais como tamanho de grão e fases envolvidas e, também, para avaliação do teor de carbono, da presença de descontinuidades superficiais, distribuição de tensões mecânicas e características de resistência à fadiga e fratura. O objetivo deste trabalho é apresentar um estudo experimental sobre a utilização do método não destrutivo baseado na análise do Ruído Magnético Barkhausen - RMB para a detecção e medição de tensões residuais do aço ASTM A 508 de uma junta soldada dissimilar. Para tanto, foram estabelecidas medidas e análises do RMB Também, foram realizadas medidas com o furo central. Os resultados obtidos apontam para a viabilidade de aplicação do RMB como método de END na medição de tensões residuais em juntas soldadas de materiais dissimilares. Algumas limitações para a aplicação prática do método foram encontradas e são apresentadas. / With aim to improve the quality and reliability of great responsibility equipment, it is necessary to promote higher control over the flaws and discontinuities that could appear during the fabrication processes. Welding processes involve thermally activated metallurgical and physical phenomena, resulting in two main types of defects: residual stresses and geometric distortions. Distributed residual stresses state on welded joints represents an important factor to the determination of applicable loading and the lifetime of such equipment, for a particular application and is directly aligned with the welding procedure used for its fabrication. So, their integrity condition - in service - should be verified by means of regular and periodic application of nondestructive tests -- NDT, focused into the characteristics of the relevant flaws. Several studies demonstrate sensibility of magnetic Barkhausen noise as a complementary method useful to characterize ferromagnetic materials, to search some of their micro structural properties, such as: grain size and phases, and to evaluate carbon content, surface discontinuity presence, mechanical stresses distribution, and fatigue and fracture resistance characteristics. The objective of this work is to present an experimental study on the usage of magnetic Barkhausen noise MBN as a nondestructive method for the detection of residual stresses in a dissimilar ASTM A 508 welded joint. Correlations were established comparing the stresses values determined by the MBN analysis with those equivalent ones obtained by means of classical central hole drilling method. The achieved results pointed out the viability of the application of MBN to measure residual stresses in dissimilar welded joints. Some limitations for the practical implementation of such method were found and are mentioned.

A simulação no tratamento térmico tem-se tornado de vital importância, principalmente na fase de projeto de produto e planejamento e controle produção, reduzindo significativamente o tempo e o custo antes dedicados a estas tarefas. Esta tecnologia quando combinada à medição das curvas de resfriamento, torna-se uma poderosa e confiável ferramenta para a predição das propriedades mecânicas e metalúrgicas em peças tratadas termicamente. A modelagem do processo de têmpera traz como resultados, a exeqüibilidade do processo, a constituição microestrutural final e a distribuição das distorções e tensões residuais da peça. O presente trabalho visa simular a transferência de calor, através do cálculo dos coeficientes de transferência de calor, e das tensões residuais térmicas e distorções em corpos de prova de aço AISI 5160 e de uma sonda de Inconel 600, com a análise posterior dos parâmetros obtidos. / The simulation of heat treating is becoming of vital importance, specially during design of product and production planning and control, reducing significantly costs and time that used to be wasted on these tasks. This technology when combined to measured cooling curves, become a powerful and trustful tool for predicting mechanical and metallurgical properties of heat treated components. Modeling the quenching process shows the feasibility of the process, the final microstructure and the distribution of residual stresses and distortions on workpiece. The present work aims at simulate the heat transfer, the thermal residual stresses and distortions in a workpiece of AISI 5160 steel and of a probe of Inconel 600, with posterior analysis of the obtained parameters.

Trefilação é um dos processos de fabricação para a obtenção de arames, tubos e barras. Este processo consiste na redução da área de seção transversal de um material cilíndrico, fazendo-o atravessar uma matriz cônica de menor seção de área transversal, chamada fieira. Destacam-se o excelente acabamento superficial, a boa precisão dimensional e o aumento na resistência mecânica do material como vantagens e, como desvantagens, deformações heterogêneas ocorrem quando atravessam a ferramenta de trefilação (fieira). As deformações heterogêneas provocam retorno elástico também heterogêneo das camadas da barra trefilada, o que induz o aparecimento das tensões residuais relacionadas às distorções de forma, empenamentos e variações dimensionais, causando desperdício de material e energia, tanto no caso de geração de refugo, quanto em tentativas de correção das não conformidades. Desta forma, neste trabalho estudou-se a influência da geometria da fieira no valor da força necessária para realizar a trefilação e no gradiente das tensões residuais em barras de aço, através de experimentos físicos e computacionais. Após a caracterização de um processo de trefilação de um aço SAE 1045 foram desenvolvidos modelos computacionais no software Simufact.forming GP ®, e obtidos os resultados para: a força de trefilação, as tensões residuais das direções principais, as deformações plásticas, o limite de escoamento e o retorno elástico do material após a realização do processo para uma determinada redução. Os modelos foram validados através da comparação dos resultados simulados com experimentos físicos e equações encontradas na literatura. Após a validação, o modelo computacional foi utilizado para simular a variação da geometria da fieira: número de regiões de trabalho, raios de convergência entre as diferentes regiões da ferramenta, diferentes materiais e áreas de seção transversal. Os resultados das simulações computacionais foram correlacionados e utilizados para projetar e fabricar uma fieira em escala reduzida e um dispositivo mecânico para a simulação física do processo de trefilação em uma máquina universal de ensaios Emic ®. Duas diferentes geometrias de fieira foram testadas experimentalmente F1: a fieira convencional, e F4, a fieira modificada a partir das sugestões obtidas a partir da metodologia escolhida para correlacionar os resultados das simulações computacionais. As novas barras trefiladas foram caracterizadas mecanicamente através de ensaios de microdureza e rugosidade para análise das propriedades mecânicas, difração de raios-X para obtenção das tensões residuais superficiais e força de trefilação, que foi medida durante a realização do ensaio. Este trabalho contribuiu cientificamente com uma melhor compreensão do comportamento do escoamento de um material trefilado e através da análise de uma nova geometria de fieira que permitiu uma redução no valor das tensões residuais, um pequeno aumento na força de trefilação, mantendo as propriedades mecânicas do material trefilado quando comparadas com o processo original objeto desta pesquisa. / The wire drawing is a manufacturing process used to produce wires, tubes and rods. It consists in a reduction in cross sectional area by plastic deformation where the raw material (wire rod) is pulled through a die. Some of the main features of the wire drawing process are the achievement of an excellent surface finishing and good dimensional accuracies and increase in mechanical strength. However, this process generates inhomogeneous deformation due to the different flowing of the material into the drawing tool (die). The inhomogeneous deformation generates residual stress, stresses present in a material free from the action of external loads and temperature gradients. Residual stresses are commonly related to distortions of shape and dimensional variations of the final products, appearing after heat treatments which are related to shape distortions, warping and dimensional variations, causing material and energy waste, in the case of waste generation and in attempts to correct nonconformities. Thus, the influence of die geometry to reducing the residual stresses and the drawing force of bars manufactured by a cold-drawing process was studied. After the characterization of a drawing process of an SAE 1045 steel, computer models were developed in the software Simufact.forming GP ®, and the results were obtained for: drawing force, residual stresses of the main directions, plastic deformations, flow and the elastic return of the material after the process was carried out. The models were validated by comparing simulated results with physical experiments and equations found in the literature. Different die geometries were simulated as: number of tool regions, fillets between different tool regions, different materials and cross-sectional areas. The results of the computational simulations were correlated and used to design and manufacture a scale die and a mechanical device to develop the physical drawing tests in an Emic ® testing machine. Two different die geometries were experimentally tested F1: the conventional die, and F4, the modified from suggestions obtained in the computational simulations correlation F. The mechanical characterization of the new drawn bars were carried out by Vickers microhardness and roughness tests, X-ray diffraction to obtain surface residual stresses and the drawing force was taken during the test. This work contributes scientifically with a better understanding of the behavior of the flow of a drawn material by the analysis of a new die geometry that allowed a reduction in the value of the residual stresses, an increase in the drawing force and maintaining the mechanical properties of the drawn material when compared to the original process object of this research.

La nitruration gazeuse est un traitement thermochimique permettant une meilleure résistance à la fatigue grâce à l’apport de propriétés mécaniques importantes telles que l’augmentation de la dureté et les contraintes résiduelles de compressions. Cette étude est réalisée sur un acier 33CrMoV12-9 utilisé dans l’industrie aéronautique. Un modèle complet adapté à cette nuance permet de quantifier les profils de contraintes résiduelles et de dureté en fonction des paramètres de nitruration. La prise en compte de l’apport de ces propriétés mécaniques est possible grâce à l’utilisation de critère de fatigue de type Crossland. Ce type de critère est intégré dans une méthodologie, afin de calculer la limite en fatigue dans une couche nitrurée et prédire la résistance de la pièce, ainsi que la potentielle zone d’initiation de rupture en fonction d’un chargement, d’une durée de vie et d’une probabilité à rupture donnés. L’utilisation combinée du modèle de calcul des propriétés mécaniques (dureté et contraintes résiduelles) et de la prédiction de l’initiation de rupture dans un matériau nitruré, rend possible une méthode inverse permettant de calculer les paramètres de nitruration adaptés à un chargement subi. Cette approche se justifie dans le cas de nitruration gazeuse, car il a été démontré lors de cette étude, que ce traitement reste robuste face au chargement subi par la pièce. / Gaseous nitriding is a thermochemical treatment that improves fatigue strength by providing important mechanical properties such as increased hardness and residual compressive stresses. This study is carried out on a 33CrMoV12-9 steel used in the aeronautical industry. A complete model adapted to this grade allows to quantify the residual stress and hardness profiles as a function of the nitriding parameters. Taking into account the contribution of these mechanical properties is possible thanks to the use of fatigue criterion such as Crossland. This type of criterion is integrated in a methodology in order to calculate the fatigue limit in a nitrided layer and to predict the resistance of the workpiece, as well as the potential initiation zone of rupture according to a load, a lifetime And a given break probability. The combination of the model for the calculation of mechanical properties (hardness and residual stresses) and the prediction of the initiation of rupture in a nitrided material, makes possible an inverse method allowing to calculate the nitriding parameters adapted to a load suffered. This approach is justified in the case of gaseous nitriding because it has been demonstrated in this study that this treatment remains robust against the loading undergone by the workpiece.