Bead-on-plate laser welds were made on an industrially produced DP780 steel to determine the effect of normalized welding heat input on the microstructure and mechanical properties within the weld fusion zone (FZ) and heat affected zone (HAZ) with reference to the base material (BM) mechanical properties. Normalized welding heat input was calculated using an established model from the literature utilizing measurements from the weld cross-section microstructures along with known materials properties. Microhardness profiles and optical microscopy were employed to evaluate materials properties and microstructural changes across the various microstructural zones of each weld. The mechanical properties of the welds were evaluated globally through standard ASTM tensile specimens as well as through a series of specialized mechanical testing sample geometries which examined the properties of individual microstructural zones. These specialized sample geometries included non-standard uniaxial and plain strain tension where effective stress and effective strains were used to compare the mechanical properties across samples. It was determined that there was a good correlation between ASTM standard samples and the specialized sample geometries employed in this study and that the UTS and YS values obtained in both cases were comparable. Sigmoidal decay behaviour was observed in the UTS and YS with increasing heat input for both the FZ and HAZ of all welds. It was found that welds with heat inputs greater than 60 J/mm2 had both a UTS and YS which were significantly depressed in the FZ and HAZ when compared to the base material values. Conversely, welds with heat inputs below 36.3 J/mm2 were found to have a UTS and YS in both the FZ and HAZ microstructural zones which were above the values determined for the BM. When global weld properties were tested, it was found that welds with a heat input greater than 60.0 J/mm2 failed within the HAZ while welds with heat inputs below 36.3 J/mm2 failed within the BM. It has been shown that there is a significant correlation between the heat inputs of laser welded DP steels and both the mechanical properties and microstructural features of the various microstructural zones as well as the location of failure during weld tensile testing. It has also been demonstrated that the mechanical properties of weld microstructural zones can be qualitatively evaluated using specialized tensile testing geometries. / Thesis / Master of Applied Science (MASc)

No description available.

Termisk modellring av hårdhet genom beräkning och simulering av den värmepåverkade zonen i en lasersvetsad stålplatta är en omfattande process. Dock är analysen viktig då mikrostrukturella fastransformationer förorsakade av svetsningen kan ge oönskade hårdhetsnivåer av den värmepåverkade zonen jämfört med hårdeheten i basmaterialet. I denna avhandling har analytiska ekvationer implementerats och testats för validitet mot simuleringar gjorda av andra författare och mot experimentella värden.Eftersom termisk modellering av svetsar är ett omfattande område var avhandlingen tvungen att smalnas av för att göra analysen mer fokuserad. Begränsningar gjordes för den matematiska modelleringen genom att endast titta på två-dimensionellt värmeflöde i svetsade plattor där endast den analytiska lösningen är av intresse. Arbetet har också inriktats mot stål då detta material är vida använt över hela världen. Då lasersvetsning är en snabb och kostnadseffektiv process så är hårdhetsanalysen av största vikt. Avhandlingen är uppdelad i tre övergripande delar; den första är att ta fram och förstå arbetet som gjorts inom termisk modellering av svetsar, alltså förstå matematiken bakom problemet. Modelleringen är till för att producera diagram parametrar från en termisk cykel, för att kunna fortgå med korrekt hårdhets analys. För det andra så sätts den matematiska modelleringen på prov i ett antal situationer som var och en simulerar olika förutsättningar. Detta gjordes i ett grafiskt användargränssnitt av ren bekvämlighet. Detta gör att ingenjörer lätt kan implementera olika egenskaper för materialet och få fram diagram och kurvor.Sist, ett liknande grafisk användargränssnitt för att simulera hårdheten i valfri punkt i den värmepåverkade zonen programmerades och därigenom implementerades ekvationerna som denna avhandling handlar om i grund och botten. En teoretisk bakgrund till fasomvandlingen är också inkluderad som förklaring till grundproblemet med oönskad hårdhet i den värmepåverkade zonen i lasersvetsat stål.Huvudslutsatser i avhandlingen:•Matematisk modellering av värmeöverföring i svetsar genomförd av Rosenthal är fortfarande applicerbar på modern lasersvetsningsapparatur. •Den empiriska modellen från Ion et al. (1984) är ej applicerbar med godkänt resultat för hårdhetsuppskattning.•Ekvationerna från Ion (2005) är statistiskt godkända för att simulera hårdhet.•Den analytiska lösningen är överlägsen den numeriska när det gäller snabb och enkel implementering för att simulera termiska cykler och hårdhet, medan den numeriska lösningen kan ta i beaktning mera avancerade egenskaper.•Förvärming av stålet innan svetsning kan vara mycket fördelaktigt för hårdheten i den värme-påverkade zonen, speciellt vid högre kolekvivalent. / Thermal modelling of hardness in the heat-affected zone (HAZ) in a laser welded steel plate is a cumbersome process both in calculation and simulation. The analysis is however important as the microstructural phase transformations induced by welding may cause unwanted hardness levels in the HAZ compared with that of the parent material. In this thesis analytical equations have been implemented and checked for validity against simulations made by other authors and against experimental values.With such a large field as thermal modelling, the thesis had to be narrowed down to make the analysis more subject focused. Limitations made were for mathematical modelling only looking at a two-dimensional heat flow in welded plates; in this thesis only the analytical solution to the heat flow is considered. The work was also directed towards steel; such a material as used largely all over the globe. As laser welding is a fast and cost-effective process, an analysis of hardness is of great importance. Work was divided into three overlapping parts; the first was to derive and understand the work done in the field of thermal modelling of welds, thus understanding the mathematics behind the basic problem. This modelling provides a number of curves and parameters from a thermal cycle, thus enabling one to do the hardness analysis correctly. Secondly, this mathematical modelling was applied to a number of cases, simulating different circumstances. This was done using self-programmed Graphical User Interfaces (GUI) for convenience. This enables engineers to easily plug in the materials and processing properties and thus simulate the required parameters and curves for further analysis.Lastly, a GUI for simulating the hardness of any point in the HAZ was programmed and used, thus implementing and validating the equations. A theoretical introduction of the phases induced in the HAZ is also included, in order of understanding the problems of unwanted hardness in the HAZ of laser-welded steel.Main conclusions of this thesis:•Mathematical modelling of heat transfer in welds by Rosenthal (1946) is still applicable for modern laser welding apparatus.•The empirical model presented by Ion et al. (1984) is not applicable with experimental results of hardness in the HAZ of the steels investigated here.•Equations by Ion (2005) are accurate for simulating the hardness.•The analytical solutions investigated are superior to numerical solutions with regard to quick, simple simulations of thermal cycles and hardness. Numerical solutions allows for more advanced modelling, which can be lengthy.•Preheating the steel prior to welding is favourable in reducing hardness levels, especially with steel of higher carbon equivalent.

<p>Advanced high strength steels, such as dual-phase and martensitic steels, are increasingly being used by automakers to decrease the thickness of steel sheet used in parts without sacrificing part strength. When welded, the martensite within the dual-phase and martensitic steel microstructures tempers, reducing the heat-affected zone (HAZ) hardness compared to the base material, locally reducing strength. This process is known as HAZ softening. HAZ softening has been well studied; however, the kinetics of this process has not been quantified and the processes responsible for HAZ softening have not been examined. This thesis investigated both of these topics.</p> <p>HAZ softening was modelled using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. As the thermal profile during welding is non-isothermal, the effects of temperature and time on steel tempering kinetics could not be separated by examining post-welded properties. The effects of tempering temperature and time were separated through a series rapid isothermal tempering experiments. Hardness data from these experiments allowed the HAZ softening rate to be empirically quantified through fitting the JMAK equation. This material model was then validated by predicting HAZ softening in laser and resistance spot welds. Although the fitted JMAK constants could be used to predict post-weld HAZ hardness, they did not agree with the classic literature values associated with martensite tempering.</p> <p>To understand why the JMAK coefficients did not match those of the classic martensite tempering literature, the softening data from one of the martensitic steels was re-examined. This study revealed that the softening process was a combination of two processes: carbide nucleation and carbide coarsening. The activation energies calculated for each process matched the classic literature values. Carbide coarsening dominated during tempering, which had a non-linear relation with change in hardness. The relationship between carbide coarsening and hardness was responsible for the softening kinetics measured from the rapid tempering experiments.</p> / Doctor of Philosophy (PhD)

In this thesis, the heat affected zone (HAZ) of Gleeble simulated welds, the weldments and the creep specimens for several types of 9%-12% Cr ferritic steels were studied by focusing on the Type IV cracking in the fine grained zone (FZ). The field emission gun transmission electron microscopy (FEGTEM) and scanning electron microscopy (SEM) were used to measure the phosphorus segregation on the grain boundary (GB) and the creep fracture morphologies respectively. Meanwhile the well-developed grain boundary segregation and precipitation (GBSP) model was applied to simulate the experimental results. The experimental results have showed that the HAZ zone was characterised by softening and Type IV cracking. All the high Cr ferritic steel welds gave a microstructure of mainly tempered martensite and M23C6 precipitates after the post weld heat treatment (PWHT). There was no δ-ferrite observed in the HAZ. The Type IV cracking exhibited a mixed cracking mechanism in which the intergranular grain boundary separation is dominant due to the crack initiation by voids and the faster M23C6 growth with the service time. A new model on the mechanism of the Type IV cracking is established. The FEGTEM research has also showed obvious non-equilibrium phosphorus segregation at the grain boundaries, which is affected significantly by the quenching temperature. The phosphorus GB segregation deteriorates the weak grain boundaries. The experimental results were well in agreement with the GBSP modelling.

Neste trabalho foram estudadas as microestruturas, dureza, e resistência ao impacto em aço alta resistência e baixa liga (ARBL) na ZAC. O processo de soldagem foi por arco submerso, utilizando corrente de 600A, tensão de 25V, velocidade de soldagem de 25,5; 24; 20,94; e 19,5 cm/min. Utilizou-se chapa de aço USI-SAC-50. Os quatro aportes de calor produzidos 3,53; 3,75; 4,24; 4,61 kJ/mm, permitiram variações microestruturais na região de granulação grosseira da ZAC, promovendo variações nas medidas de durezas e nos resultados do ensaio de impacto medido à -25ºC e -10ºC. Quando realizado o tratamento térmico de alívio de tensões a 580ºC por 1 hora, as morfologias das microestruturas e medidas de dureza na região grosseira da ZAC, praticamente foram iguais, independente do aporte de calor. Os resultados do ensaio de impacto realizado à -25ºC, com e sem tratamento térmico, não sofreram alterações significativas para os valores das energias absorvidas na ZAC. Já a -10ºC a mudança foi mais significativa. O aspecto da morfologia da fratura na região de granulação grosseira da ZAC, após o ensaio de impacto, foi coerente para os quatro aportes de calor utilizados na soldagem. / It was studied in this report the microstructure, hardness and impact resistance in High Strength Low Alloyed (HSLA) steel in HAZ. The submerged are weld metals were made, using corrent of 600A, are voltage of 25V and travel speed of 25,5; 20,9; 24; and 19,5 cm/min. The USI-SAC-50 steel was used in present report. Four different heat input 3,53; 3,75; 4,24 and 4,61 kJ/mm were used to weld, produced significant changes in the coarse heat affected zone (HAZ) microstructure morphology, and this effect promoted changes on the hardness and the impact results at -25°C and -10°C. Using stress reliev treatment at 580°C for one hour, has changed the microstructure morphology in the coarse heat affected zone and the hardness measurements becames quite similar, therefore independent of the heat input. At -25°C the impact results has shown no variations with the stress reliev heat treatment. However, there were significant changes in the impact results at -10°C. After the impact test, the aspect of morphology facture in the coarse heat affected zone (HAZ), was coherent for the four different heat input, were used to weld.

[pt] Os aços inoxidáveis Duplex são materiais metálicos que tendem a aliar uma excelente resistência à corrosão em meios agressivos com elevada resistência mecânica devido à sua microestrutura bifásica. Essa microestrutura é composta por uma matriz ferrítica e ilhas de austenita, a temperatura ambiente, com frações volumétricas aproximadamente iguais dessas duas fases. Com isso, estas ligas costumam ser utilizadas em tubulações e equipamentos industriais onde se requer elevada relação resistência/peso, especialmente em empreendimentos de construção e montagem off-shore. Este trabalho apresenta uma metodologia desenvolvida com a utilização de um modelo numérico parametrizado baseado no método de elementos finitos para analisar o efeito dos ciclos térmicos de soldagem nos aços duplex e leanduplex. O modelo desenvolvido faz uso da análise térmica transiente da soldagem, em que as propriedades térmicas dos materiais envolvidos são termo dependentes, e grandes gradientes térmicos são gerados entre a região da solda e as áreas afastadas dela. Procedimentos experimentais para medição dos ciclos térmicos empregados sob determinados regimes de soldagem são utilizados para calibrar o modelo numérico. A metodologia é aplicada ao estudo do comportamento de uma solda na forma de cordão sob chapa contendo um único passe, realizada com o processo GMAW, em chapas planas de 10mm de espessura dos aços duplex UNS32205 e leanduplex UNS32304, através de um modelo de elementos finitos. Os resultados mostram uma boa correspondência entre o modelo numérico e o resultado experimental. / [en] Duplex stainless steels are metallic materials that tend to combine excellent corrosion resistance in aggressive environment with high mechanical strength due to their biphasic microstructure. This microstructure is composed of a ferritic matrix and austenite islands, at room temperature, with approximately equal volume fractions of these two phases. As a result, these alloys are often used in pipes and industrial equipment where high resistance / weight ratio is required, especially in offshore assembly and construction field. This work presents a methodology developed with the use of a parameterized numerical model based on the finite element method to analyze the effect of thermal welding cycles on duplex and leanduplex stainless steels. The developed model uses the transient thermal analysis of welding, in which the thermal properties of the materials involved are thermo-dependent, and large thermal gradients are generated between the region of the weld and the areas away from it. Experimental procedures for measuring the thermal cycles employed under certain welding conditions are used to calibrate the numerical model. The methodology is applied to the study of the behavior of a weld in the form of a strand in sheet form containing a single pass, realized with the GMAW process, in flat plates of 10mm thickness of the duplex steels UNS32205 and leanduplex UNS32304, through a finite element model. The results show a good correspondence between the numerical model and the experimental result.

Neste trabalho foram estudadas as microestruturas, dureza, e resistência ao impacto em aço alta resistência e baixa liga (ARBL) na ZAC. O processo de soldagem foi por arco submerso, utilizando corrente de 600A, tensão de 25V, velocidade de soldagem de 25,5; 24; 20,94; e 19,5 cm/min. Utilizou-se chapa de aço USI-SAC-50. Os quatro aportes de calor produzidos 3,53; 3,75; 4,24; 4,61 kJ/mm, permitiram variações microestruturais na região de granulação grosseira da ZAC, promovendo variações nas medidas de durezas e nos resultados do ensaio de impacto medido à -25ºC e -10ºC. Quando realizado o tratamento térmico de alívio de tensões a 580ºC por 1 hora, as morfologias das microestruturas e medidas de dureza na região grosseira da ZAC, praticamente foram iguais, independente do aporte de calor. Os resultados do ensaio de impacto realizado à -25ºC, com e sem tratamento térmico, não sofreram alterações significativas para os valores das energias absorvidas na ZAC. Já a -10ºC a mudança foi mais significativa. O aspecto da morfologia da fratura na região de granulação grosseira da ZAC, após o ensaio de impacto, foi coerente para os quatro aportes de calor utilizados na soldagem. / It was studied in this report the microstructure, hardness and impact resistance in High Strength Low Alloyed (HSLA) steel in HAZ. The submerged are weld metals were made, using corrent of 600A, are voltage of 25V and travel speed of 25,5; 20,9; 24; and 19,5 cm/min. The USI-SAC-50 steel was used in present report. Four different heat input 3,53; 3,75; 4,24 and 4,61 kJ/mm were used to weld, produced significant changes in the coarse heat affected zone (HAZ) microstructure morphology, and this effect promoted changes on the hardness and the impact results at -25°C and -10°C. Using stress reliev treatment at 580°C for one hour, has changed the microstructure morphology in the coarse heat affected zone and the hardness measurements becames quite similar, therefore independent of the heat input. At -25°C the impact results has shown no variations with the stress reliev heat treatment. However, there were significant changes in the impact results at -10°C. After the impact test, the aspect of morphology facture in the coarse heat affected zone (HAZ), was coherent for the four different heat input, were used to weld.

In this thesis, new methods for shear strength are proposed and backed up through extensive experimentation, ABAQUS models and data analysis of Titanium welds of three different alloys. The results are compared with those obtained by using the procedure outlined by AWS B4 for calculating Shear Strength in the transverse and longitudinal directions; this equation is widely used by the American Welding Society (AWS) and all those in search of more efficient designs involving welding. It is a well-documented issue that the equation provided by AWS yields a large discrepancy between the values for shear strength of longitudinal and transverse welds.

A tecnologia dos aços microligados para transporte de gás natural e petróleo tem sido pressionada pelo descobrimento das novas jazidas e o aumento da demanda no mundo. As solicitações ambientais e de resistência mecânica são os parâmetros para o desenvolvimento de aços de alta resistência baixa liga para o transporte de gás e petróleo a menor custo e de forma segura. Neste contexto esta pesquisa investiga, em um aço microligado para tubos API 5L X80, o efeito das transformações de fase obtidas por resfriamentos controlados na fratura induzida por hidrogênio Hydrogen Induced Cracking-(HIC) e nas propriedades mecânicas. Os testes de HIC foram realizados no material como recebido, na espessura da chapa submetida a resfriamentos contínuos e em amostras do material tratadas de forma a simular as regiões de grão grosso da zona afetada pelo calor (GGZAC). Segundo o ciclo de resfriamento, os aços microligados têm microestruturas complexas, como é caso do aço microligado em estudo, onde sua microestrutura, estudada em trabalhos anteriores, é formada principalmente por ferrita, bainita, perlita e microconstituinte austenita/martensita (AM). A morfologia, tamanho, quantidade e distribuição dos produtos de transformação na chapa mudam as propriedades do aço. Esses fenômenos são de grande interesse tecnológico em aços microligados para a fabricação de tubos soldados para o transporte de gás e petróleo, tanto quando a solda é realizada em campo como também durante o encurvamento por indução; aqui as propriedades mecânicas do tubo decorrentes do processo de fabricação termomecânico podem ser degradadas pela ação do aquecimento e dos resfriamentos experimentados na zona afetada pelo calor (ZAC), principalmente na região de GGZAC. A simulação dos ciclos térmicos para o estudo da HIC na espessura da chapa foram realizados em CP austenitizados a 900ºC e submetidos a resfriamentos contínuos no dilatômetro de têmpera. Para simular os ciclos térmicos com resfriamentos controlados focados na GGZAC e a seguir obter CP de tamanho adequado para testes de tração e Charpy, foi necessário fazer os tratamentos térmicos a 1300ºC e resfriamentos contínuos em um simulador termomecânico e dilatômetro Gleeble. O maior tamanho da amostra tratada termicamente neste último equipamento permitiu extrair amostras para avaliar as propriedades mecânicas e a HIC do material, pois as diferentes regiões da ZAC em uma solda real são restritas e não permitem este tipo de ensaios em uma região específica da ZAC. Os resultados permitiram identificar a suscetibilidade de cada microestrutura produto da transformação da austenita na espessura da chapa, sendo a região central da chapa a mais sensível ao hidrogênio no aço como recebido e quando tratado a baixas taxas de resfriamento de 0,5°C/s após austenitizado a 900°C. As bandas grosseiras formadas por estruturas de maior dureza que a matriz na região central diminuíram a resistência à HIC. Da mesma forma nos corpos de prova que simulam a região GGZAC, a fratura induzida pelo hidrogênio foi localizada na região central da espessura embora apresente bainita e ferrita acicular. A falha possivelmente se deve aos elementos remanescentes segregados nesta região central e partição de carbono para os sub contornos de grão da bainita e ferrita que cresceram a partir a austenita primária. As inclusões e precipitados, segundo seu tipo, forma e localização na microestrutura, participam ou não da nucleação e propagação da trinca, sendo a posição mais crítica quando localizadas dentro das estruturas bandeadas. Não foi observada a nucleação de trincas na presença de hidrogênio em precipitados de Nb e Ti. / The technology of microalloyed steels for the transportation of natural gas and oil has been pressed by the discovery of new deposits and the increased demand in the world. Environmental requests for safety and ever increasing mechanical strength are the parameters for the development of high strength low alloy steels for transporting gas and oil at lower cost and safely. In this context, this research investigates, in a microalloyed steel pipe API 5L X80, the effect of phase transformations obtained by controlled cooling on the behavior when loaded with hydrogen - Hydrogen Induced Cracking - (HIC) and in the mechanical properties. HIC tests were performed on as-received material, on samples extracted from the thickness of the plate and subjected to continuous cooling and on samples of the material treated to simulate the coarse-grained regions of heat affected zone (CGHAZ). According to the cooling cycle, the microalloyed steels have complex microstructures: in the steel under evaluation its microstructure, studied in a previous work, consists mainly of ferrite, bainite, pearlite and austenite/martensite constituent (AM). The morphology, size, quantity and distribution of the products of transformation change the properties of plate steel. These phenomena are of great technological interest in microalloyed steels for the fabrication of welded tubes for the transport of gas and oil, when the welding is performed in the field as well as during hot bending; here the mechanical properties of the tube from the process of thermomechanical fabrication can be degraded by the action of heating and cooling experienced in the heat affected zone (HAZ), mainly in the region of CGHAZ. Simulations of thermal cycles for the study of HIC on sheet thickness were performed in coupons subjected to austenitization at 900ºC followed by continuous cooling in the dilatometer. To simulate the thermal cycles with controlled cooling, focused in the CGHAZ, and getting suitable sample sizes for tensile testing and Charpy, it was necessary to austenitize at 1300ºC followed by continuous cooling using the thermal and thermomechanical simulator in a Gleeble dilatometer. Samples heat treated in this equipment were suitable to evaluate the mechanical properties and the HIC of the material for different regions of HAZ, while a real weld would not have enough material to allow this type of testing on a specific region of HAZ. The results showed the susceptibility of each microstructure product of austenite transformation and of the position on the plate thickness. The central region of the plate was more sensitive to hydrogen in the steel as-received and when treated at low cooling rates of 0.5°C/s after austenitization at 900°C. The bands formed by coarse structures of greater hardness than the matrix in the central region decreased the resistance to HIC. Likewise in coupons that simulate the CGHAZ region, the fracture induced by hydrogen was located in the central thickness line, even when the microstructure were bainite and acicular ferrite. Failure there was possibly due to remnants of segregated elements in this central region and carbon partition to the subboundaries of the bainite and ferrite grain that grew from the primary austenite. Inclusions and precipitates, according to their type, shape and location in the microstructure, participating or not in the nucleation and propagation of the crack, were more critical when located within the banded structures; crack nucleation in the presence of hydrogen was not observed at Nb and Ti precipitates.