Ezequiel C. P. Pessoa

Estudo comparativo de eletrodos comerciais para soldagem subaquática molhada

With the objective to supply subsidies to select ferritic electrodes for underwater wet welding, some weld metal characteristics of commercial electrodes were determined. The welding trials were carried out at 0.5m fresh water depth in an aquarium, using a gravity device for welding. The study aimed also to obtain data to help to elect the consumables of best performance to be tested in higher depths. The following characterizations were performed: microstructural analysis, quantification of inclusions, weld metal and inclusions chemical composition, mechanical properties, diffusible hydrogen and weldability evaluation. The obtained results helped to classify the tested consumables in two groups with quite different characteristics. The first group is composed of one oxidizing type electrode and the second one is composed of four rutile type electrodes. Regarding to hydrogen in the weld metal, the obtained results show that the oxidizing electrode is able to produce welds with considerable low diffusible hydrogen content. As a consequence, smaller potential risk of cold cracking is expected when using this electrode. Meantime, regarding to the arc stability and other operational indicators, the oxidizing electrode presented inferior performance. Considering mechanical properties, the rutile electrodes presented the best results. The properties differences among the electrodes tested are shown and discussed in the present work.

Arc Stability Indexes Evaluation on Underwater Wet Welding

Underwater wet welding (UWW) with shielded metal arc welding (SMAW) is employed basically in repairs of offshore structures, including platforms, ships and others. The main problems of this type of welds are related, of course, with water presence in the electric arc that causes higher cooling rates, Oxygen and Hydrogen availability in the arc atmosphere and arc instability. Many of research and test welding programs in laboratory are undertaken in shallow water performed by automatic devices using hyperbaric chambers to simulate depths. Also, welding arc signals are acquired using data acquisition systems and the arc stability is estimated through indexes calculated from values acquired and analyzed. It is very well known the reduced stability of the wet welding process at shallow depths — less than approximately five meters. So this effect would be considerable significant since it can be used to make correlations between the arc stability indexes and the welds quality results. The main objective of this work was to evaluate the efficiency of the most used arc stability indexes reported in the literature in detect the arc instability effect of shallow water wet welding. Bead-on-plate welds had been made using a gravity feeding system device inside a hyperbaric chamber, applying straight polarity (DCEN) in ASTM A36 steel plates, using the same weld parameters in two different depths, 0.5 and 20.0 meters. Rutile, basic and oxidizing commercial electrodes types prepared for UWW with 3.25mm rod diameter were used. Visual analysis, bead morphology and arc stability were the criteria used to evaluate the weld quality. The voltage and current arc signals were acquired at 10 KHz rate. The arc stability indexes measured were average voltage and current and its standard deviation, S (Imax/Imin) parameter, voltage and current square mean, arc “re-ignition” voltage and current, metal transfer time and its deviation, metal transfer frequency and its deviation, short circuit time and its deviation and the voltage versus current graph area. The results shown that none of the stability indexes tested has been shown to indicate, alone, a good relationship to the surface appearance obtained for the three electrodes studied. The rutile type electrode was the only one that clearly produced better weld appearance at 20 meters than in shallow water depth. The rutile and oxidizing electrodes showed better surface appearance with the increased number of short circuits. For the rutile electrode, the globular transfer mode with high voltage were directly related with poor weld bead surface appearance.Copyright

Study of Porosity Location in Fresh Water Wet Welds

The objective of this work was to determine for particular water depth, the optimum current ranges in terms of best porosity location. Pores located at the top part of each weld bead were generally eliminated through re-melting by the next weld bead. This finding was consistent in fresh water wet welds on ASTM A36 steel using three commercial electrodes: E6013, E7018 and E7024 grades. Two depth levels were studied, 50 and 100 meters. A gravity feed welding system was used in a hyperbaric chamber of 200 m pressure capability. Through visual analysis and with help of a computer software, the optimal weld parameters for each electrode and depth regarding pore location were determined. Additionally, it was observed that porosity increases with increasing current and depth. Similar to welding in seawater, it was also verified that the E6013 electrodes presented greater arc stability and produced the best beads, while the E7018 electrodes did not produce good results. The E7024 electrode exhibited easy arc opening and reopening but did not present good stability. Finally, it was observed that the optimal current range reduced as depth increases.© 2003 ASME

Exothermic Additions in a Tubular Covered Electrode and Oxidizing Reactions Influence on Underwater Wet Welding

During Underwater Wet Welding (UWW), the water that surrounds the arc decomposes liberating large amount of hydrogen and oxygen. As a consequence of the presence of these gases in the arc atmosphere and weld pool, porosity in the weld metal occurs. In the past years, many research programs had been carried out with the objective to reduce or eliminate porosity in wet welds. A simple way to accomplish this goal is using chemical elements or ingredients to promote or avoid certain chemical reactions in the weld pool. In conventional stick (shielded metal arc - SMA) electrodes, it is possible to add alloying elements or other ingredients through the external covering. A tubular covered electrode (TCE) (a hybrid process between SMA and flux cored arc - FCA welding) allows the addition of reactive elements in the hollow rod, separate from the other ingredients used in the flux covering. This way, it is possible to use exothermic elements, placed inside the tube, to control the oxidation reactions, but limiting these reactions to the arc plasma and in the weld pool. Exothermic additions in welding consumables can promote desirable oxidation reactions, change the metal transfer mode, reduce the cooling rate, and decrease the electrical dependence of the welding process. Theoretically, the application of flux cored shielded metal arc (FC-SMA) welding with exothermic additions will permit better control the weld metal composition and reduce the porosity in wet welds. This paper describes underwater wet welding with tubular covered electrodes that contain exothermic additions such as (CaC2 ) and aluminum (A1), and the influence of these ingredients on weld metal composition and porosity.© 2007 ASME

Influência do molibdênio em propriedades do metal de solda na soldagem molhada com eletrodos óxi-rutílicos

A tecnica de soldagem subaquatica molhada com eletrodos revestidos apresenta um crescente potencial de aplicacao para reparos submarinos em elementos estruturais de unidades flutuantes de producao de petroleo (profundidade ate 20 m). Porem, ela apresenta problemas tais como o maior risco de fissuracao a frio e de formacao acentuada de porosidade. O presente trabalho tem como objetivo melhorar a resistencia mecânica do metal de solda de um eletrodo experimental do tipo oxi-rutilico. Foram estudadas as influencias de adicoes de Mo (ate 0,4% no metal de solda) na microestrutura e em propriedades mecânicas. As soldas foram realizadas em simulador de soldagem subaquatica em profundidade equivalente de 10m utilizando um sistema de soldagem por gravidade. As analises das micrografias mostrou que o aumento do teor de Mo no metal de solda diminui significantemente o tamanho medio de grao da regiao reaquecida de graos finos. O aumento do teor de Mo no metal de solda resultou, ainda, em aumento do limite de resistencia a tracao sem perdas de tenacidade e ductilidade ate aproximadamente 0,25%Mo.

Thermodynamic considerations of pore formation and hydrostatic pressure during underwater wet welding

The pore and crack formations in the weld bead during underwater wet welding are the main cause of failure to reach the required mechanical properties of the weld metal. These defects are closely associated with the decomposition of the water molecule under electric arc conditions. In this paper, the thermodynamic calculations of the complex process of the water decomposition under the conditions of high electric arc temperatures at a pressure of 1 atm of steam are discussed. The values of the partial pressures of the five main products of the vaporizations and decomposition of the water (H2O(g), H2, O2, H and O) are calculated for temperatures between 1870 and 4000 K. Due to the fact that atomic hydrogen is mainly responsible for pore formation in the weld metal, its partial pressure as a function of partial pressures of atomic oxygen and steam (water vapour) is expressed. Hydrogen solubility values in the liquid metal under the conditions of underwater wet welding at depths of 50 and 100 m, and a comparison between the thermodynamic calculation and porosity measurement results at depths of 50 and 100 m, are presented.

Consideraciones termodinámicas entre la formación de poros y la presión hidrostática durante la soldadura subacuatica mojada

The pores and cracks formations in weld bead during underwater wet welding are the main cause that prevent to reach the required mechanical properties of the weld metal. These defects are closely associated with the decomposition of the water molecule under conditions of electric arc. In this paper the thermodynamic calculations of the complex process of the water decomposition under the conditions of high temperatures of electric arc to a pressure of one atmosphere of steam is exposed. The values of the partial pressures of the five main products of the vaporizations and decomposition of the water (H2O(g), H2, O2, H and O) are calculated for temperatures among 1870 and 4000 K. Due to the fact that atomic hydrogen is the main responsible for the pore formation in the weld metal its partial pressure as function of partial pressures of atomic oxygen and steam (water vapor) is expressed. Values of hydrogen solubility in the liquid metal in conditions of underwater wet welding at 50 and 100 m of depth and, a comparison between the thermodynamic calculation and porosity measurement results are presented.

Efeito do teor de carbono do metal de base e da alma do eletrodo revestido sobre a porosidade em soldas molhadas

Porosidade e um defeito comum em soldagem subaquatica molhada. Varios estudos tem sido realizados com o objetivo de avaliar os mecanismos que controlam sua formacao e assim encontrar solucoes para minimiza-la ou elimina-la. Este trabalho tem como objetivo avaliar o efeito da variacao dos teores de carbono do metal de base e da alma do eletrodo sobre a porosidade do metal de solda. Dois diferentes metais de base, com variacoes apenas no teor de carbono (C2 - 0,1% e C7 - 0,7%), foram utilizados para deposicao das soldas feitas a 50 metros de profundidade, utilizando eletrodos comerciais E6013 com diferentes teores de carbono na alma (E2 - 0,002% e E6 -0,6%), em polaridade direta (DCEN). O metodo macrografico de analise de imagens foi utilizado para a quantificacao da porosidade. Utilizando os metais de base C2 e C7, observou-se que a porosidade aumentou significativamente com o aumento do teor de carbono da alma do eletrodo. Em contrapartida, observou-se uma reducao da porosidade com o aumento do teor de carbono do metal de base utilizando-se os eletrodos E2 e E6.

Characterization of Pores and Cracks in Underwater Welds by μCT and Digital Optical Microscopy

Underwater shielded metal arc wet welding with coated electrodes is a common procedure for in situ repair of structural parts of offshore oil production units. However, there are serious difficulties in obtaining sound welds similar to the ones achieved under atmospheric conditions. The water environment surrounding the weld pool is usually responsible for very high cooling rates and high hydrogen content in the weld metal. These factors may lead to the formation of porosity and cracks in the weld metal which are not precisely analyzed by the conventional 2D techniques. The 3D visualization and measurement of these defects can contribute significantly to a better characterization and prediction of weld metal properties. In the present work wet welded steel samples were analyzed by µCT, with the aim of analyzing the presence of pores and cracks, their concentration, spatial distribution, and orientation.

Evaluation and Development of Electrodes for Wet Welding of Structural Ship Steels

To better understand the behavior of some commercial wet welding electrodes down to 20 m equivalent water depth as well as to develop a new electrode, a research program was initiated in 2007. This depth is considered the maximum expected in the in situ repair of structural parts of floating production units. The weld metal evaluation was done by microstructural characterization, Vickers hardness, Charpy and tensile tests, chemical analysis, and diffusible hydrogen measurement. The influences of pressure on the mechanical properties of the weld metal are presented and discussed on the basis of chemical composition, microstructure and porosity. The electrode under development showed promising results concerning the possibility to broaden the range of qualified welding procedures. This expectation is based on the good results of diffusible hydrogen, porosity, impact toughness and ductility down to 20 m equivalent water depth. The susceptibility of hydrogen cracking in weld metal and in heat affected zone was estimated comparatively. The presence of hydrogen cracks is discussed as an important limiting factor for the qualification of welding procedures in the class A of the AWS D3.6M:1999 specification.© 2010 ASME