Numerical simulation was used to predict the thermal behavior and the resulting microstructure at the heat-affected zone (HAZ) of a 170 mm diameter and 3.5 mm thickness super duplex stainless steel (SDSS) UNS S32750 tube. In order to evaluate the thermal response from the model, a usual welding situation involving interpass temperature (IT) and its influence on the HAZ microstructure was exploited. Thus, two superimposed autogenous welding passes were simulated, the first clockwise with the tube in the room temperature and the second, counterclockwise, with the tube at the temperature of 250oC. Even subjected to successive thermal cycles and high interpass temperature, the proportion and morphology of the phases at the HAZ and Fusion Zone (FZ) did not present significant differences when comparing the two welding passes. Meanwhile, nitrogen losses should be avoided during welding in order to obtain a balanced microstructure in DSS welds, contributing to guarantee satisfactory toughness in addition to resistance to pitting corrosion. The predictions from the simulation were validated by using experimental results obtained from the autogenous TIG (Tungsten Inert Gas) process.

The planning and execution of the adequate welding procedure for the superduplex stainless steel (SAF 2507 SDSS) in multiple passes is a complex technological task since thermal cycling may cause the precipitation of harmful phases, such as the sigma (σ) and chi (χ) phases. For this reason, technical standards limit the maximum interpass temperature (T.I.) in an extremely conservative manner at 100 °C. In order to investigate a possibly wider temperature range for this procedure, SAF 2507-SDSS tubes were subjected to autogenous Tungsten Inert Gas (TIG) welding with interpass temperatures ranging from 150°C to 400°C. The different welding zones were characterized by microscopy, Vickers microhardness, and polarization corrosion tests. The results indicated that in the Fusion Zone (FZ), columnar grains of ferrite were formed surrounded by allotriomorphic austenite (AA), besides intragranular austenite (AI) and Widmanstätten austenite (AW). The heat-affected zone (HAZ) had a lower ferrite content and was composed of equiaxed grains of ferrite, also surrounded by AA, besides AI and AW. Hardnesses varied between HV272 and HV293, regardless of the region or the interpass temperature used. Furthermore, the corrosion resistance has not been significantly affected by the interpass temperatures between 150 °C and 400°C.

Resumo: Um desafio bem conhecido é prever as transformações que ocorrem durante a soldagem das ligas metálicas com o objetivo de controlar as propriedades da solda. Desta forma, este estudo apresenta um modelo Termo-Mecânico-Metalúrgico para predizer numericamente a história térmica, as transformações de fase no estado sólido, a microestrutura de solidificação e a distribuição de dureza durante e após a soldagem de aços de alta resistência e baixa liga. O modelo foi implementado numericamente em um código computacional próprio baseado no Método dos Volumes Finitos, o que permitiu rastrear e calcular dinamicamente as frações volumétricas de ferrita, perlita, bainita e martensita na zona afetada pelo calor, além da formação e determinação do espaçamento do braço dendrítico na zona de fusão, enquanto que a distribuição da dureza na zona afetada pelo calor foi calculada aplicando-se a regra da mistura de fases. Para tanto, soldas autógenas usando o processo Gas Tungsten Arc Welding de passe único foram simuladas numericamente e experimentalmente realizadas em amostras do aço de alta resistência e baixa liga AISI 4130, incluindo seu pré-aquecimento com o objetivo de avaliar a eficácia do modelo proposto para simular a soldagem de peças em diferentes condições térmicas iniciais, tendo sido obtida uma estreita concordância entre os resultados calculados e experimentais.

Abstract: A well-known challenge is to predict the transformations occurring during the metal alloys welding aiming to control the weldment properties. Thus, this study presents a Thermo-Mechanical-Metallurgical model to numerically predict the thermal history, the solid-state phase transformations, the solidification microstructure and the hardness distribution during and after the welding of high strength low-alloy steels. The model was numerically implemented in an in-house computational code based on the Finite Volume Method, which allowed to dynamically track and calculate the volume fractions of ferrite, pearlite, bainite and martensite at the heat-affected zone, besides the formation and determination of dendrite arm spacing at the fusion zone, whereas the hardness distribution at the heat-affected zone was calculated by applying the phase mixture rule. For this, single-pass autogenous Gas Tungsten Arc Welding welds were numerically simulated and experimentally carried out on high strength low-alloy AISI 4130 steel samples, including their preheating to evaluate the effectiveness of the proposed model to simulate the workpieces welding in different initial thermal conditions and a close agreement between the calculated and experimental results were obtained.

Stainless steel and nickel alloy have high corrosion resistance in high-temperature environments due to the high Cr content present in their chemical composition, being widely used in components of nuclear reactors, petrochemical industries, etc. Through proper processes and procedures, it becomes possible to join these alloys. However, this union can generate detrimental factors in its performance, among them, the residual stresses. In this work, the residual stresses generated by the autogenous GTAW process, due to different interpass temperatures on the weld bead geometry, were analyzed by the Hole-Drilling technique in dissimilar welding joints of stainless steel AISI 316L and Inconel 718 alloy. In addition, the Vickers microhardness measurements were carried out to evaluate the hardness profile in the cross section of the weld bead covering base metal (BM), heat affect zone (HAZ) and weld metal (WM). We found that in the interface region between BM and HAZ of each dissimilar joint metal, residual stresses increased above 300 MPa, while hardness increased above 160 HV.

The properties of the super duplex stainless steels (SDSS) are strongly affected by the thermal history imposed by welding procedures. The controlled dual phase microstructure (ferrite and austenite) guarantee excellent mechanical properties such as mechanical strength and corrosion resistance, in addition to small thermal expansion coefficient and high thermal conductivity. In this paper, we newly proposed a model able to predict the thermal history of the welding pieces coupled with local mechanical properties developed during welding procedure that combine the effects of temperature and phase changes during welding. We applied inverse method to fit the thermophysical parameters based on measured data. The model was verified by comparing measured and predicted temperature profiles using thermocouples located within the heat affected zone. Thus, an inverse method was implemented to obtain the parameters fitting for the grain growth evolution compatible with the final microstructure and grain size measured using SEM images and stereological techniques. We demonstrated that very small amount of non-equilibrium deleterious phases and nanosized precipitated are expected during the welding procedures depending upon the local conditions of temperature, compositions and alloying dilution evolutions.

A phenomenological model to predict the multiphase diffusional decomposition of the austenite in low-alloy hypoeutectoid steels was adapted for welding conditions. The kinetics of phase transformations coupled with the heat transfer phenomena was numerically implemented using the Finite Volume Method (FVM) in a computational code. The model was applied to simulate the welding of a commercial type of low-alloy hypoeutectoid steel, making it possible to track the phase formations and to predict the volume fractions of ferrite, pearlite and bainite at the heat-affected zone (HAZ). The volume fraction of martensite was calculated using a novel kinetic model based on the optimization of the well-known Koistinen-Marburger model. Results were confronted with the predictions provided by the continuous cooling transformation (CCT) diagram for the investigated steel, allowing the use of the proposed methodology for the microstructure and hardness predictions at the HAZ of low-alloy hypoeutectoid steels.

Resumo Este trabalho constitui um estudo dos vergalhões CA-50 da classe dos aços SAE 1026. Para um melhor controle das propriedades mecânicas da junta soldada foi realizado este estudo, o conhecimento de certas variáveis é de grande importância para encontrar os parâmetros ideais de soldagem, e os respectivos resultados microestruturais e consequentemente sobre as propriedades mecânicas da junta soldada. Para realização deste estudo foi realizado um estudo do material como recebido para a fixação de uma base sólida de comparação. Foi realizada a soldagem do material utilizando como gás de proteção argônio com 20% de dióxido de carbono, o arame utilizado foi o cobreado ER70S-6, as juntas soldadas foram do tipo transpasse com monitoramento de temperatura através de termopares para dois aportes térmicos distintos. Um código numérico computacional foi desenvolvido para simular os fenômenos que ocorrem no processo (gradiente de temperatura, transformações de fases, transferência de calor). As juntas soldadas não apresentaram martensita como fase frágil, o metal de solda apresentou estrutura dendrítica, a ZTA apresentou duas regiões distintas com tamanhos de grão diferentes tudo isso devido ao gradiente de temperatura, que também originou características distintas do cordão de solda, ZTA, fases formadas e tamanhos de grão distintos.

Abstract This work is a study of rebar CA-50 class of steels SAE 1026. To better control the mechanical properties of the weld was carried out this study, knowledge of certain variables is very important to find the optimal parameters of welding, and the results microstructure and therefore on the mechanical properties of the welded joint. For this study was conducted a study of the material as received for establishing a solid basis for comparison. Welding the material using argon as protective gas of 20% carbon dioxide was performed, the wire used was ER70S-6 coppered, welded joints were of the type with crossover temperature monitoring thermocouple through two distinct thermal contributions. A computational numerical code was developed to simulate the phenomena occurring in the process (temperature gradient, phase transformations, heat transfer). Welded joints did not show how fragile martensite phase, the weld metal showed dendritic structure, the ZTA presented two distinct regions with different grain sizes all this due to the temperature gradient, which also originated distinct characteristics of the weld bead, ZTA, stages formed and different grain sizes.

Key microstructural changes that occur when Duplex Stainless Steels (DSS) are welded could be evaluated when bead-on-plate welding was carried out on a 2205 DSS by the GMAW process. By using numerical simulations, it was possible to calculate locally the heating and cooling rates taking place during the 2205 DSS welding and discuss its correlation to the microstructural changes experimented by the parent metal. Results showed that increasing heat input has promoted the ferritic grain growth with a slight reduction in the austenite content present at the high temperature heat affected zone (HTHAZ), whereas the cooling rates remained above from those reported as critical for sigma phase precipitation in 2205 DSS. Furthermore, nitrogen has proved to be an effective austenite former at the fusion zone (FZ), which can contributes to get a balanced microstructure in DSS welds in contrast to the effects from the elevated cooling rates.

A model based on transport equations was numerically implemented by the finite volume method (FVM) in a computational code in order to simulate the influence of the heat input, base metal thickness and preheating temperature on the thermal evolution and the cooling rate during the welding of the low alloy ferritic steels T/P23 and T/P24. As a consequence, it was possible to evaluate qualitatively the microstructure at the heat affected zone (HAZ) of these steels when a single weld bead was deposited on their surface and calculate the maximum hardness reached at this region. Goldakfs double-ellipsoid heat source model for power density distribution was utilized in order to obtain a good estimate of the cooling rate and dimensions of the fusion zone (FZ). The results are discussed in light of previous work and good agreement between experimental and simulated results was verified.