No description available.

No description available.

Directionally solidified Rene 80 (DS R80) and polycrystalline Inconel 738(IN 738) Superalloys were tested in thermo-mechanical fatigue (TMF) over the temperature range of 500-900°C and plastic strain range from 0.1 to 0.8% using a DSI Gleeble thermal simulator. Thermo-mechanical testing was carried out on the parent material (baseline) in the conventional solution treated and aged condition (STA), as well as gas tungsten arc welded (GTAW) with an IN-738 filler, followed by solution treatment and ageing. Comparison of the baseline alloy microstructure with that of the welded and heat treated alloy showed that varying crack initiation mechanisms, notably oxidation by stress assisted grain boundary oxidation, grain boundary MC carbides fatigue crack initiation, fatigue crack initiation from sample surfaces, crack initiation from weld defects and creep deformation were operating, leading to different “weakest link” and failure initiation points. The observations from this study show that the repaired samples had extra crack initiation sites not present in the baseline, which accounted for their occasional poor fatigue life. These defects include lack of fusion between the weld and the base metal, fusion zone cracking, and heat affected zone microfissures.

Human welders have long been able to monitor a weld pool and adjust welding parameters accordingly. Automated welding robots can provide consistent movement during the welding process, but lack the ability to monitor the weld pool. A vision system attached to the welding robot could provide a way to monitor the weld pool substantially faster than a human being. Previous vision systems to monitor weld pool surfaces have been developed, but their uses are limited since the system is fixed in place. The compact vision system developed in this research attaches directly to the welding torch, which provides no limitations in weld pool monitoring. This system takes advantage of the specular surface of a molten weld pool by reflecting a pattern of laser beams from the weld pool surface. The deformation of the laser beam after it reflects from the weld pool surface can provide clues to the weld pool shape, and thus the penetration of the weld. Image processing techniques and geometric optics are used to reconstruct a weld pool surface shape based on the image captured of the deformed laser pattern.

In the present work, an investigation of the fatigue life benefits emerging from fillet weld geometries optimization has been carried out. At first, an introduction to ageing mechanisms, corrosion and especially fatigue, acting on operating marine structures has been made. Residual stresses at weld toes, stress modes, and types, geometrical factors (weld angle, toe radius, leg length), welding techniques selected, post-welding treatment and plate‟s material are some of the principal factors affecting the fatigue life of a fillet weld joint. Especially, the accuracy of various approaches in fatigue life estimation of specific geometries under pre-set types and levels of stress is studied. It is evident so far that even the notch stress concept is the most accurate method based on S-N curves, the Fracture Mechanics approach can offer more accurate solutions of a crack development through the material. Towards this, a literature review on crack evolution aspects in welded and non-welded plates under bending and tension was performed; substantial parameters were determined and finally implemented in the LEFM model which was used for the simulation purposes of Chapter 6. As far as the crack aspect ratio evolution is concerned, an extensive reference is available in literature since many researchers have investigated its contribution to the determination of geometrical paths, commonly known as “Preferred Propagation Paths”. Their significance is related with our ability to determine accurate SIF solutions leading to precise fatigue life estimations. A typical fillet weld joint 2-D model has been developed in CAE Abaqus software and a Finite Element Analysis of subject T-profile has been carried out. Through this analysis, the fillet weld angle, the weld leg length, the weld toe curvature radio ρ and the carrying load plate thickness are examined for their impacts on the maximum surface stress. Finally, a number of stress mitigating measures are proposed and their effects are analyzed. Undoubtedly, the notch stress concept today is gradually gaining more and more acceptance among other fatigue analysis practices, hence the need for an estimation of the actual surface stresses along fillet weld toes, has become imperative. Towards this, different 2-D geometries are tested against stress concentration factors developed at weld toes, which are calculated on the basis of maximum in-plane principal stresses over nominal stresses in mode I pure bending and pure tension respectively. Moreover, validation with corresponding results from literature is provided. Finally, three different concepts for reducing the maximum surface stresses are presented. The first one proposes grinding of the weld toe area and formulation of an artificial U-notch or a part- circular profile. The second one applies to non-penetrating welds and assumes the existence of a root gap of a specific geometry which is related to the fatigue life and stress concentration factor of the fillet weld joint. Last but not least, the relatively recent concept of the variable radius notch is discussed, even though it is applicable mostly to notched bodies, not weld joints. Afterwards, a Linear Elastic Fracture Mechanics analysis of reference 2D fillet weld model is demonstrated. A number of geometrical parameters considered at previous stage for their impact on surface Stress Concentration levels at the weld toe region, have been correlated to fatigue life benefits in terms of increased number of stress cycles till failure. An extensive analysis of 9 different T-butt weld joint geometries has been provided in order to investigate how positively a possible SCF reduction can affect the fatigue life of a weld joint. Essential geometric variations (weld angle, length, toe radius, root slot) were considered in the 2D model. All calculated benefits both in pure bending and pure tension cases have been reported accordingly. Based on a linear interpolation of the points scatter (SCF, N-cycles) both in banding and tension, it was observed that a surface stress mitigation of 1% could lead to 1,33 up to 2,5% fatigue life benefit in the range of SCF=2 – 2,5. It is evident so far that the geometrical optimization of a weld joint in respect of notch stress mitigation can be a powerful tool both in shipbuilding and maintenance practice in the future. However, technically wise their application may incur high initial costs of improved tools of welding and post welding treatment and robots even though it would consist a cost effective solution in a medium/long term basis. Finally, the above process is followed by a reliability analysis of the most critical geometrical parameters affecting the fatigue life of a fillet weld joint. Reliability assessment results concerning medium, high and low cycle fatigue are provided and a comparative analysis of each factor‟s impact on fatigue life has been carried out.

A pipe consisting of Inconel 600 welded to grade 106-B Carbon-Steel using Inconel 182 weld filler is used to transport heavy water in nuclear reactors. A confidential report concluded that cracking is one of the problems these pipes are currently facing. Before cracking can be fully understood the mechanical properties of the weld must be determined. This thesis analyzed the pipe at various length-scales using optical microscopy, micro-hardness testing, small and large scale tensile testing and digital image correlation (DIC). This thesis successfully achieved it goals of determining the mechanical properties and creating a model of the Inconel dissimilar metal weld. It partially met the goal of observing fracture mechanisms as it was able to observe fracture in tensile samples but was not able to successfully track crack growth.

Directionally solidified Rene 80 (DS R80) and polycrystalline Inconel 738(IN 738) Superalloys were tested in thermo-mechanical fatigue (TMF) over the temperature range of 500-900°C and plastic strain range from 0.1 to 0.8% using a DSI Gleeble thermal simulator. Thermo-mechanical testing was carried out on the parent material (baseline) in the conventional solution treated and aged condition (STA), as well as gas tungsten arc welded (GTAW) with an IN-738 filler, followed by solution treatment and ageing. Comparison of the baseline alloy microstructure with that of the welded and heat treated alloy showed that varying crack initiation mechanisms, notably oxidation by stress assisted grain boundary oxidation, grain boundary MC carbides fatigue crack initiation, fatigue crack initiation from sample surfaces, crack initiation from weld defects and creep deformation were operating, leading to different “weakest link” and failure initiation points. The observations from this study show that the repaired samples had extra crack initiation sites not present in the baseline, which accounted for their occasional poor fatigue life. These defects include lack of fusion between the weld and the base metal, fusion zone cracking, and heat affected zone microfissures.

This study has been conducted to establish a relation between welding quality and strength in various types of steel. This because specification of quality levels higher or lower than required leads to an increase in manufacturing cost. A procedure has been developed to achieve the above objective. In this procedure EN 25817, EN 288-1 and similar welding and nondestructive testing standards have been taken as a basis. Furthermore, effort has been exerted to ensure that welding is performed in a manner that reflects the actual conditions encountered in the industry to the extent possible. The same principles have been pursued in material selection, and the materials have been selected from the low-carbon manufacturing steel types (St37, St44 and St52) that are frequently used in steel construction, boiler manufacturing and similar manufacturing areas. The welded pieces manufactured in accordance with the established procedure have been tested through radiographic and ultrasonic examination methods to check whether they conformed to the welding quality standards set in the procedure. v Quality levels B and C of the EN 25817 standard have been selected for this study. The sizes of potential defects of quality levels have been defined within this standard. In this study, plates with weld seam that has no defect have been used for Quality level B and plates with weld seam having gas pores in sizes specified in the relevant standard have been used for Quality level C. After this stage, the pieces have been subjected to mechanical tests and their strength values have been identified. Thereby, the association between welding quality and strength has been established, enabling us to specify which strength values can be achieved in specific welding quality levels.

Submitted by Ronildo Prado (ronisp@ufscar.br) on 2017-08-10T18:48:04Z No. of bitstreams: 1 TeseVG.pdf: 12758396 bytes, checksum: 0cff4ed1d7ee114e48143bc5de57404f (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-10T18:48:14Z (GMT) No. of bitstreams: 1 TeseVG.pdf: 12758396 bytes, checksum: 0cff4ed1d7ee114e48143bc5de57404f (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-10T18:48:20Z (GMT) No. of bitstreams: 1 TeseVG.pdf: 12758396 bytes, checksum: 0cff4ed1d7ee114e48143bc5de57404f (MD5) / Made available in DSpace on 2017-08-10T18:48:27Z (GMT). No. of bitstreams: 1 TeseVG.pdf: 12758396 bytes, checksum: 0cff4ed1d7ee114e48143bc5de57404f (MD5) Previous issue date: 2016-09-19 / Não recebi financiamento / Circumferential field-welding joints of oil pipelines are susceptible to defects that may lead to failures when in operation. The type of product to be transported is an important factor in the evaluation of environment-induced cracking, especially of welded joints, where fatigue cracks may originate at points of high stress concentration. In Brazil, the construction of a long distance pipeline has been started in 2014, to transport sugarcane ethanol, which is currently widely mixed with motor fuels. In this work a circumferential joint of an API 5L X70 line pipe was evaluated by hardness, tensile and impact tests; the microstructure was observed by optical microscopy in the different regions of the welded joint and the fractured surfaces were analyzed by SEM. The material of the tube was classified according to API 5L specifications and the welded regions showed hardness distribution and microstructures in agreement with the literature. The fracture surface of the welding metal after tensile and impact tests indicated that the presence of welding discontinuities may affect strongly the fracture behavior, leading to embrittlement, especially under impact testing. The susceptibility of welded joints to stress corrosion cracking (SCC) in a simulated fuel-grade ethanol (SFGE) environment was evaluated by slow strain rate (SSR) and fatigue crack growth (FCG) tests. The tests were designed to promote crack propagation through the heat-affected zone (HAZ) or through the weld metal (WM) of a circumferential weld on an API X70 steel pipe. The SSR tests in SFGE showed reduction of elongation to failure and mixed fracture, with clear signs of SCC in both the regions of the weld. The FCG tests demonstrated that the time-dependence of corrosion fatigue interactions may involve alternating periods of increasing and decreasing crack growth rates throughout the test. Acceleration of crack propagation due to SCC predominated in the WM and deceleration predominated in the HAZ. / A construção de dutos utilizados no transporte de combustíveis e derivados de petróleo e gás envolve a soldagem em campo dos tubos que, quando em operação, serão submetidos a esforços mecânicos repetitivos e a meios agressivos, dependendo do produto transportado e das condições de instalação do sistema. Em 2014 iniciou-se no Brasil a construção de um duto de longa distância que fará o transporte de etanol, uma alternativa de fonte renovável de energia com alto potencial brasileiro de produção. O processo de soldagem em campo gera pontos de concentração de tensões e regiões mais susceptíveis a defeitos e à formação de trincas, cujo processo de propagação pode ser afetado pelo contato com o etanol. Neste trabalho uma junta de solda circunferencial de um duto de aço API 5L X70 foi avaliada por meio de ensaios de dureza, tração e impacto; a microestrutura foi observada por microscopia ótica nas distintas regiões da junta e as superfícies fraturadas foram analisadas ao MEV. O efeito da Corrosão sob Tensão (CST), na solda e na zona termicamente afetada (ZTA), foi avaliado em ambiente contendo etanol simulado (SFGE), por meio de ensaios em baixa taxa de deformação (BTD) e de propagação de trinca em fadiga. O material de base do tubo foi classificado dentro das especificações da norma API 5L e a região da solda apresentou distribuição de dureza e microestruturas compatíveis com as encontradas na literatura. A análise da fratura do metal de solda em ensaios de tração e de impacto indicou que as descontinuidades da solda, ainda que qualificada, interferem significativamente no mecanismo de fratura, tendendo à fragilização, especialmente no impacto. Ensaios de baixa taxa de deformação (BTD), com corpos de prova entalhados em contato com o etanol, indicaram perda de ductilidade e fratura mista com evidência de CST na solda e ZTA. Ensaios de propagação de trinca por fadiga em etanol mostraram efeitos da interação corrosão-fadiga dependentes do tempo que podem resultar em períodos de aceleração ou desaceleração no crescimento da trinca, com predominância da aceleração na região da solda e da desaceleração na ZTA.

O objetivo deste trabalho é analisar a influência dos parâmetros de um sistema mecanizado de soldagem, composto por uma tocha TIG e outra MAG em tandem, sobre a geometria do cordão de solda resultante. Os resultados obtidos foram comparados com soldas realizadas pelos processos TIG e MAG operando isoladamente. Os testes foram executados por meio de simples deposição de cordões sobre chapas de aço ao carbono AISI 1010 com espessura de 6,3 mm. Os parâmetros analisados foram: corrente de soldagem e distância da ponta do eletrodo até a peça no processo TIG, velocidade de alimentação de arame e tensão do arco no processo MAG, além de velocidade de soldagem e distância entre as tochas. Os cordões de solda foram executados em um dispositivo de soldagem mecanizado, que permite a variação da velocidade de soldagem, distância da ponta do eletrodo TIG e a distância entre as tochas. Os ajustes de corrente e tensão de soldagem foram feitos em duas fontes de potência separadas, ambas do tipo transformador-retificador, uma para cada tocha, devido às diferentes curvas características de cada processo. Através do projeto de experimento fatorial fracionado, foram avaliados os efeitos de dois níveis, um mínimo e outro máximo, mais quatro réplicas dos seus níveis intermediários, resultando em 20 experimentos para cada processo de soldagem. Os valores dos resultados finais indicam uma diferença ao utilizar uma tocha TIG para pré-aquecer o material base, porém, estatisticamente, essa diferença não é significativa entre os processos TIG e MAG em tandem e MAG convencional para a faixa de parâmetros de soldagem adotada. / This study aims to investigate the influence of process parameters of a mechanized welding system, comprising a GTAW torch and a GMAW torch in tandem configuration, on the geometry of the resulting weld bead, performed using GTAW and GMAW welding processes in tandem. The results were compared with weld beads performed by conventional GTAW and GMAW welding processes. The tests were conducted performing bead-on-plate deposits over AISI 1010 6,3 mm thick carbon steel plates. GTAW current, GMAW wire feed speed, GTAW arc length, GMAW voltage, welding speed and the distance between GTAW and GMAW torches were the analyzed parameters. The weld beads were performed in a mechanized welding bench, which allows the variation of speed, distance between torches and arc length of the GTAW torch. Current and voltage adjustments were made in two separate transformer-rectifier power sources, one for each welding torch, due to their different characteristic curves of each process. Through the fractional factorial design of experiment, the effects of two different levels for each parameter were analyzed, plus four replicates of the average values of these levels, resulting in 20 experiments for each welding process. The results show a difference on the weld bead geometry using a GTAW torch to preheat the base metal, however, this difference is not statistically significant between GTAW and GMAW welding process in tandem and GMAW welding process for the adopted range of welding parameters.