Alexandre Queiroz Bracarense

Development of a robot for orbital welding

Purpose – This work demonstrates the development of a robot, which was designed for the orbital welding of pipes.Design/methodology/approach – The robot consists of a small car pressed against the pipe by means of chains, which are used by the robot to move around it. To provide all necessary torch movements, the robot must have four degrees of freedom: torch travel speed, stick‐out, torch angle and lateral motion. Thus, using a look‐up table‐which was specially designed to this application‐it is possible to follow the optimal parameters (voltage, current, welding speed, torch angle and stick‐out) for each welding position (flat, vertical and overhead).Findings – The robotization of the orbital welding process brings enhancement in the final product quality, considerable increase of repeatability, reduction of rework and reduction of the weld execution time. At the very least, the robot is capable to reproduce the weld bead of the best human welder, through the use of the same paramenters contained in a t...

An investigation on graphene and nanoclay effects on hybrid nanocomposites post fire dynamic behavior

This paper deals with the post fire behavior of hybrid nanocomposites under dynamic loadings. A series of tests were performed to investigate how nanoparticles (i.e. nanoclay and graphene nanosheets) affect the post-fire overall composite behavior. Carbon fiber/epoxy-nanoclay and carbon fiber/epoxy-graphene nanosheets were manufactured. The nanoparticles employed were Cloisite 30B nanoclay, and surface modified graphene nanosheets. The epoxy system used was RemLam M/HY956. The nanocomposites were made using ultrasonic mixer for nanoparticle dispersion in acetone followed by a shear mixing of acetone/nanoparticle/hardener. The following steps involved degassing, the addition of resin to the mixture and, the hand lay-up with vacuum assisted cure. Thermo gravimetric analysis (TGA) indicates an average decrease on peak mass loss around 41% with the addition of small amount of nanoparticles. The sample plates were exposed to a heat flux of 800 kW.m-2 for a period up to 120 seconds. The post-fire low velocity impact tests indicated the impact resistance degraded as a function of heat exposure. However, the addition of nanoclay leads to an increase on impact peak force of 11.69%. The carbon oxidation could be the main cause of the increase on impact peak load is lower than expected, only 6.72%. The model predictions are overestimated by approximately 8%. Even though, it can be a good tool for composites design.

Avaliação da soldagem multipasse de chapas espessas de aços inoxidáveis lean duplex UNS S32304 soldadas pelos processos SMAW, GMAW e FCAW: parte 1: propriedades mecânicas

The duplex stainless steels (DSS ́s) have been placed as an excellent alternative for applications where high corrosion resistance and high mechanical strength are required. However, DSS ́s, including the lean duplex, present lower weldability than the austenitic stainless steels. Thus, this study aims to evaluate the multipass welding of 22 mm plates of lean duplex stainless steel alloy UNS S32304, using the process SMAW, GMAW and FCAW with consumables with two types of chemical composition, 22%Cr9%Ni3%Mo and 23%Cr7%Ni giving a total of six experiments. V-grooves with 60° and ceramic backing for welding the root pass were used and the heat input was maintained constant at 1.6 kJ.mm-1. It was determined the time and sequence of welding passes, aiming an analysis of productivity. The welded joints were subjected to x-ray END and specimens for tensile, bending, Charpy at -30°C and microhardness were extracted. The productivity of the semi-automatic processes proved to be at least 63% greater than the SMAW process while the process FCAW showed 6-18% faster than the GMAW. It was found discontinuities (porosity) acceptable according to ASME B31.3 in some experiments that did not affect negatively the mechanical results, which stayed above the parent material requirement and meet fabrication standards. Key-words: Duplex, Lean Duplex, Weldability, Multipass Welding, Mechanical Properties.The duplex stainless steels (DSSs) have been placed as an excellent alternative for applications where high corrosion resistance and high mechanical strength are required. However, DSSs, including the lean duplex, present lower weldability than the austenitic stainless steels. Thus, this study aims to evaluate the multipass welding of 22 mm plates of lean duplex stainless steel alloy UNS S32304, using the process SMAW, GMAW and FCAW with consumables with two types of chemical composition, 22%Cr9%Ni3%Mo and 23%Cr7%Ni giving a total of six experiments. V-grooves with 60o and ceramic backing for welding the root pass were used and the heat input was maintained constant at 1.6 kJ.mm-1. It was determined the time and sequence of welding passes, aiming an analysis of productivity. The welded joints were subjected to x-ray END and specimens for tensile, bending, Charpy at -30oC and microhardness were extracted. The productivity of the semi-automatic processes proved to be at least 63% greater than the SMAW process while the process FCAW showed 6-18% faster than the GMAW. It was found discontinuities (porosity) acceptable according to ASME B31.3 in some experiments that did not affect negatively the mechanical results, which stayed above the parent material requirement and meet fabrication standards.

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

Study correlating the bubble phenomenon and electrical signals in underwater wet welding with covered electrodes

The bubble phenomenon in underwater wet welding is very important to the final quality of the weld. A good understanding of the formation and collapse of the bubbles and knowledge of the behaviour of welding parameters during the process can improve the practices used in underwater welding and, consequently, improve the final quality of the weld. This study was developed to understand and try to correlate the phenomenon of growth, detachment and frequency of bubbles with electrical signals during the welding process. Beads were made on plate metal at a depth of 0.50 m, with three types of commercial coated electrodes: E6013, E7024 and E7018. High-speed filming to visualize the process of formation, detachment and counting of the bubbles was performed, along with acquisition of their electrical signals. Thus, it was possible to try to correlate the bubble frequency with the electrical signals observed during the process. It was observed in the current data that the mean number of variations under 5% of the mean current is similar to the mean detachment rate of the bubbles. Therefore, it was concluded that there is a possible relationship between bubble detachment and a determined variation in the current.

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

Velocidade de Propagação de Trinca por Fadiga de Soldas Subaquáticas Molhadas: Avaliação Fora da Água

Fatigue cracks propagation characteristics of welds produced in conventional conditions (welds made out of water) and underwater condition (Underwater wet welding) were evaluated out of the water. The growth rates of fatigue crack da/dN showed a significant dependency on the density and distribution of pores, factors which may vary significantly with the welding process and environment. Variations in the propagation speed of the fatigue crack were correlated with the analysis of the fracture surface on the stable propagation crack zone. The results of this study showed that the underwater wet welding procedure produces weld metal resistant to fatigue that is suitable to be used at low stresses applied to offshore structures.

Avaliação da soldagem multipasse de chapas espessas de aços inoxidáveis lean duplex UNS S32304 soldadas pelos processos SMAW, GMAW e FCAW -: resistência à corrosão

Duplex stainless steels (DSSs) have been placed as an excellent alternative for applications where high corrosion resistance and high mechanical strength are required. However, the DSSs, including the lean duplex stainless steels, present lower weldability than the austenitic stainless steels. This study aims to evaluate the corrosion resistance of welded joints using multipass welding of lean duplex stainless alloy UNS S32304 plates 22 mm thick, using the SMAW, GMAW and FCAW consumables with two types of chemical composition, 22%Cr9%Ni3%Mo and 23%Cr7%Ni, giving a total of six experiments. The welded joints were evaluated using the anodic potentiodynamic polarization technique to determine the pitting potential of the material. The electrolyte studied was 3.5% w/v sodium chloride aqueous solution. The lean duplex steel welded by GMAW process with consumable with 22%Cr9%Ni3%Mo showed the best behavior against corrosion in saline solution. The root of the weld region had the highest pitting potential compared to the top of the welded joints. The lean duplex steel joints welded using FCAW process showed a behavior characteristic of general corrosion in the medium of saline solution, unlike the other samples.