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Comparison of underwater wet welding performed with silicate and polymer agglomerated electrodes
Accepted Manuscript Title: Comparison of Underwater Wet Welding Performed with Silicate and Polymer Agglomerated Electrodes Authors: Pedro Henrique Ribeiro Menezes, Ezequiel Caires Pereira Pessoa, Alexandre Queiroz Bracarense PII: DOI: Reference:
S0924-0136(18)30455-2 https://doi.org/10.1016/j.jmatprotec.2018.10.019 PROTEC 15972
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
26-4-2018 16-10-2018 18-10-2018
Please cite this article as: Ribeiro Menezes PH, Pereira Pessoa EC, Bracarense AQ, Comparison of Underwater Wet Welding Performed with Silicate and Polymer Agglomerated Electrodes, Journal of Materials Processing Tech. (2018), https://doi.org/10.1016/j.jmatprotec.2018.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of Underwater Wet Welding Performed with Silicate and Polymer Agglomerated Electrodes
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Pedro Henrique Ribeiro Menezes1, Ezequiel Caires Pereira Pessoa2, Alexandre Queiroz Bracarense1
Federal University of Minas Gerais - UFMG, Mechanical Engineering Department, Laboratório de
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Robótica, Soldagem e Simulação – LRSS, Belo Horizonte, MG, Brazil. [email protected], [email protected] 2
Federal Institute of Minas Gerais - IFMG-Betim, Industrial Mechanics Department, Betim, Minas
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Gerais, Brazil, [email protected]
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Abstract
Conventional basic electrodes varnished and basic electrodes agglomerated with polymer in
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underwater wet welding in a simulated depth of 10 meters were compared. A hyperbaric tank was used for depth simulation and a gravity welding device for making the bead on plate welds. Bead on plate welds
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were deposited with welding currents of 100, 120, 140, 160 and 180 A with DCEN and DCEP polarities, in order to evaluate the arc stability through a non-dimensional statistical index. The welding current which
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showed better stability level in both polarities was chosen to evaluate weld bead morphology, weld metal porosity, weld metal oxygen and diffusible hydrogen levels. A methodology was used to correlate the heat input and short-circuit frequency with the weld bead morphology. It was observed a direct relation with the
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weld metal oxygen and penetration and an inverse relation of the weld metal oxygen with the diffusible hydrogen. Electrodes agglomerated with polymer showed higher penetration values than conventional electrodes in both polarities and lowest diffusible hydrogen levels in the DCEP polarity. It was observed lower levels of weld metal porosity for the electrodes agglomerated with polymer in comparison to conventional electrodes. This result was related to lower levels of porosity found in the droplets formed at the tip of the electrodes agglomerated with polymer.
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Key-words: Underwater Wet Welding, Polymer Binder, Arc Stability, Weld Metal Porosity, Weld Bead Morphology, Weld Metal Oxygen, Diffusible Hydrogen.
1.
Introduction
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Due to the influences caused by the water surrounding the arc, Santos et al. (2012) affirmed that
there are few reports in the literature regarding underwater wet welds that have reached Class A in
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accordance with the AWS D3.6M: 2010 standard. Consequently, several electrodes have been studied and developed to improve operability and weldability in the aqueous medium. Rutile electrodes are
preferentially used because they present better arc stability and better weld bead appearance, these
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characteristics were reported by West et al. (1990), as well as by Gooch (1983). In contrast, Santos et al.
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(2012) and Pope and Liu (1996) reported that rutile electrodes produce weld metal with high diffusible hydrogen contents (up to 80 ml/100g), making them susceptible to cold cracking. Thus, different
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formulations of oxidizing electrodes have also been widely studied, due to the low diffusible hydrogen
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content produced, which can present results lower than 20 ml/100g according to Santos et al. (2012) and Medeiros and Liu (1998). In contrast, these electrodes have a low arc stability, inferior weld bead
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appearance and poor slag detachability. Santos et al. (2012) developed and studied oxy-rutile electrodes trying to combine the advantages of rutile and oxidizing electrodes. There are also reports in the literature
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related to the use of austenitic stainless steel and nickel-based electrodes, but few literatures are related to
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basic electrodes.
Pessoa et al. (2003) studied the use of electrodes E6013, E7024 and E7018 at depths of 50 and 100
meters, and the welds performed with E7018 electrodes presented high levels of weld metal porosity, bad operability and ugly appearances. It is believed that because of these drawbacks, few literatures are found
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regarding to the values of diffusible hydrogen, oxygen content in the weld bead and in-depth studies of welding parameters seeking better performances of the basic electrodes. Vaz et al. (2012) developed and conducted air-welding with basic electrodes agglomerated with polymer instead of conventional binders which exhibited features that may be desirable in underwater wet welding. These electrodes presented lower levels of diffusible hydrogen in comparison with the conventional E7018 electrodes, not causing the increase of these levels even when exposed to the atmosphere for 30 days. Therefore, it should be
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emphasized that the electrodes agglomerated with polymers have low sensitivity to humidity; they neither need to be placed in ovens for drying or maintenance in portable ovens, allowing their use directly from the packaging, nor the use of varnishes for waterproofing them for underwater wet welding. Vaz and Bracarense (2016) observed that the number of smaller droplets transferred by the polymer agglomerated electrodes is higher than in the conventional electrodes. They also reported that the density of the droplets transferred during the welding was higher for smaller droplets. This phenomenon is related to large voids
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present inside the larger droplets due to the absorption of gases during the formation of the droplet. The same authors also reported a better gas protection provided by the electrode agglomerated with polymer.
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These two last characteristics may be an important factor for decreasing weld metal porosity levels. Pessoa (2007) stated that the weld metal porosity is directly related to the amount of gases absorbed in the droplet being transferred or generated in the welding pool, and this can be minimized through better protection generated by the covered electrode. Vaz and Bracarense (2016) also reported higher amounts of acicular
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ferrite in the weld beads made with electrodes agglomerated with polymers, which is a desirable
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microconstituent in the weld beads because of its higher toughness.
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Fig. 1 shows the electrodes agglomerated with polymer and the varnished conventional electrodes.
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An intriguing feature of the electrode agglomerated with polymer is observed. The covering, besides having resistance to humidity, also has resistance to bending. This is due to the greater flexibility, making it less
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brittle than the conventional electrodes which after bending tend to break the covering and leave the core wire exposed.
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Based on all this knowledge, the present work has the objective of comparing the performance of
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conventional varnished E7018 basic electrodes with E7018 basic electrodes agglomerated with polymer, trough evaluation of operational characteristics, weld bead morphology, weld metal porosity and weld metal oxygen and diffusible hydrogen contents.
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2.
Materials and Methods
2.1. Materials ASTM A-36 steel sheets with 200 mm long, 100 mm wide and 19.05 mm (¾ in) thick were used
as samples. Conventional covered electrodes E7018 and covered electrodes E7018 agglomerated with polymer, both with a core wire of 3.25 mm, were used. Conventional covered electrodes were dried at 350º C for one hour and subsequently protected with vinylic varnish to prevent the covering from coming in
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contact with water and decomposing during welding. The agglomerated electrodes were used directly from the package. The water used was tap water and constantly replaced with new tap water. 2.2. Equipment A hyperbaric tank was used to simulate the wet welding process at a depth of 10 meters. Fig. 2(a) shows the hyperbaric tank. For welding, a gravity welding device was used, as it can be seen in Fig. 2(b).
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This device has main characteristics: welding speed linked to the fusion rate of the electrode and the angles α and β; allows the adjustment of the angles α and β and, consequently, regulation of the welding speed
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with good repeatability.
An IMC welding power source, model HipER-1, specific for underwater wet welding was used. A data acquisition system with an acquisition rate of 1000 points per second was used to monitor the welding current and arc voltage and a software, called Signal, was used for electrical signals treatment. A
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Canon model i3 digital camera was used to capture images and the Image J® software was used to calculate
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weld bead geometry dimensions in macroscopic analysis. A gas chromatograph model ON 900 Oxygen
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Nitrogen Deter minator PC controlled from Eltra, whose principle of measurement is based on the property
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of infrared gas radiation, was used for oxygen content determination. Another gas chromatograph analyser from OERLIKON Yanaco, model HDM ANALIZER G-1006H, was used for diffusible hydrogen content
2.3. Methodology
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measurement.
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The gravity welding device angles values were selected in preliminary tests in order to find the
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best parameters of arc stability. The angles, that presented the best results, were α equal to 60º and β equal to 80º.
Bead on plate welds were deposited at 10 meters simulated water depth with nominal welding
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currents of 100, 120, 140, 160 and 180 A. Two weld beads were made in the DCEN polarity and two other weld beads were made in the DCEP polarity for each welding current. During welding, the arc voltage and welding current data were acquired for each condition. The mean welding current (Im) and standard deviation of the mean welding current (σIm), mean arc voltage (Vm) and standard deviation of the mean arc voltage (σVm) were calculated using the software Sinal in an
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acquisition interval of 10 seconds. The interval of calculations was determined between 10 and 20 seconds after the electric arc was opened to avoid arc voltage and welding current signals transient period. The welding time (t) and the weld bead length (L) were measured for each condition. Welding speed (Ws) was obtained through Equation 1. 𝑊𝑠 =
𝐿
(mm.s-1)
𝑡
(1)
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Afterwards, a welding speed graph (Ws) was generated against mean welding current (Im), to facilitate the comparison in each condition.
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Before morphology, porosities, diffusible hydrogen and oxygen of the weld metal, the stability of
the electric arc was evaluated to determine the mean welding current which the electrodes have better stabilities in each polarity. The Coefficient of Variation (C.V.) was used in the arc voltage data. The C.V.
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is a non-dimensional index that measures the dispersion in relation to the average. Prior to the calculation,
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it is necessary to apply a filter to remove valleys present in the arc voltage due to the short-circuiting metal
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transfer. The C.V. is calculated using Equation 2. The arc voltage is denominated as Vam after the removal
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of short-circuits.
𝜎𝑉𝑎𝑚 𝑉𝑎𝑚
∗ 100
(%)
(2)
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𝐶. 𝑉. =
Fig. 3 schematically shows the sections and the regions of sample withdrawal. As previously stated, two weld beads were made for each condition, thus resulting in 6 samples for each condition. The
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tips of the electrodes were also preserved and cut to evaluate the porosity in the drop being formed.
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The samples of the weld beads and tips of the electrodes were mounted, sanded (sands 80, 180, 320, 400, 600, 1200), polished with 1μm Al2O3 and etched with Nital 2%. Before etching, the samples were photographed to obtain greater contrasts of the formed pores, as it can be seen in Fig. 4(a), this procedure
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facilitating the binarization of the image, as it can be observed in Fig. 4(b). Subsequently, the total area of the pores (Ap) were measured. Binarization and measurement of the total area were performed using Image J® software. Fig. 5 schematically shows the weld bead cross-section and the respective measurements performed.
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To calculate the dilution (𝛿) and weld metal porosity (P), Equations 3 and 4, respectively, were used.
𝛿=
𝑃=
𝐴𝑏𝑚 𝐴𝑏𝑚+𝐴𝑟
𝐴𝑝 𝐴𝑏𝑚+𝐴𝑟
∗ 100
(%)
(3)
∗ 100
(%)
(4)
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The heat input (H) was calculated using Equation 5 and the amount of energy spent to form a cubic millimetre of the weld bead was calculated using Equation 6, which relates the heat input with total area
(At), called Hf. The short-circuit frequency (F) was also calculated to evaluate the influence of short-circuits
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on energy and morphology. For this purpose, the amount of short-circuits (Ncc) were used for a period of
𝑉𝑚∗𝐼𝑚 𝑉𝑠
𝑁𝑐𝑐 10
(5)
(6)
(Hz)
(7)
(J.mm-3)
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𝐹=
𝐻 𝐴𝑡
(J.mm-1)
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𝐻𝑓 =
A
𝐻=
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10 seconds, as described in Equation 7.
To determine the oxygen contents, present in the weld metal, samples weighing approximately
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500 milligrams of reinforcement were removed from the welds, as it can be seen in Fig. 6. The gas chromatography method was applied, using ASTM E260-96 as reference, for determining
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the diffusible hydrogen contents. The welding was performed in an aquarium at 0.5 meters and the specimen had to be inside the capsules in an atmosphere with argon for less than two minutes after the welding for
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the test to be valid. This step would be very difficult to perform in the hyperbaric tank. The welds for the diffusible hydrogen tests were performed on samples that had half the length of the standardized specimen due to the high diffusible hydrogen value produced by underwater welding. Pope and Liu (1996) and
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Medeiros and Liu (1998) also reduced the size of the specimens for the same reasons. Eight tests were performed with the electrode agglomerated with polymer and eight with the commercial electrodes, using nominal welding current in which the electrodes presented better stabilities, four test bodies in the electrode negative polarity (DCEN) and four test bodies in the electrode positive polarity (DCEP). 3.
Results and Discussion
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The data was processed with the Sinal software after the arc voltage and welding current signalled acquisition during the welding. For a better comparison of the data obtained, a mean welding current per mean arc voltage chart was generated, with the respective standard deviations, as observed in Fig. 7. For the nominal welding currents of 100 and 120 A for both electrodes in the DCEN polarity it was not possible to obtain a suitable welding procedure. This can be associated with the welding currents that are insufficient for a stable fusion of the electrode in this polarity or to start the electrical arc. For the
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electrode agglomerated with polymer in the DCEN polarity, it was also not possible to carry out the procedure with the nominal welding current of 180 A, due to a constant increase of the size of the cone at
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the tip of the electrode during the welding and, consequently, a constant increase of the length of the electric
arc until its extinction. Because of this, it was not possible to obtain a stable electric arc for more than 7 seconds.
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It can be observed that the electrodes agglomerated with polymers showed a tendency of mean arc
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voltage higher than the conventional electrodes, in the DCEN polarity, as well as in the DCEP. At the
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beginning of the droplets formation at the electrode tip, no significant differences in cone length between
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the electrodes for the same polarity and same welding current were found. Such occurrence may be related to the presence of fluorine in the electric arc because of the polymer. This hypothesis can be strengthened
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by Vasil'chenko (1987), who found increases in the arc voltage with the application of fluorides in copper TIG welding. Lancaster (1986) also reports that elements that require relatively high energies for ionization to occur, such as fluorine, result in an increase in the electrical resistivity of the plasma column and,
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consequently, tends to cause higher operating arc voltages for a given length due to greater difficulties in
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electric current passage.
It was also observed that for the same electrode, the DCEP polarity presented higher mean arc
voltage than the DCEN polarity. Tsai and Masubuchi (1977) explained that the length of the cone formed
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by the covering of the electrode during welding, and consequently the arc length and arc voltage, are related to the polarity of welding current. According to the model, the DCEP polarity has a longer arc length, resulting in higher arc voltages. Weld bead length and weld time data were used for calculating welding speed and standard deviation, as it can be seen in Fig. 8.
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It is noted that the increase in welding current caused an increase in the welding speed, already expected due to the increase of the power of the arc. The higher welding speeds for the electrodes agglomerated with polymer are a consequence of the higher mean arc voltage presented in relation to the conventional electrodes. Thus, resulting in higher energy values per unit time and consequently higher melting rate and welding speed. The coefficient of variation (C.V.) was used to determine a stability analysis of the arc.
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Undesirable phenomena during the welding process tend to cause large disturbances in the electric arc,
causing variations in the arc voltage and therefore, making it more unstable. Through Equation 2, it can be
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stated that smaller mean arc voltage deviations, consequently lower values of C.V., theoretically result in a more stable process.
Fig. 9 shows the C.V. results for each welding condition. The condition with mean welding current
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approximately of 150 A presented lower C.V. trend in the conventional electrodes at both polarities and for
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the electrode agglomerated with polymer in the polarity DCEP. The electrode agglomerated with polymer
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in the DCEN polarity behaved in a different way from the others, presenting a high C.V. and also a high
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standard deviation at this welding current. The weld beads performed with mean welding currents close to 150 A were chosen for evaluation of the morphology, microstructure, diffusible hydrogen and oxygen
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contents for all cases.
For a given polarity it is observed that the conventional electrode has tendencies of lower C.V.
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This fact may be related to the fluorine content present in the electrode agglomerated with polymer. This tends to decrease the stability of the arc, since the difficulty of ionization tends to cause a more agitated,
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violent arc and with higher indices of disturbances. Bang et al. (2010) in a comparative study on the effect of different fluorides on the flux of tubular wires, reported that elements with high ionization potential tend to cause greater arc instabilities and, in tests performed by Hazlett (1957), it is observed that addition of
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fluoride can cause a violent arc with high levels of spatter. Some macrographs of the weld beads in the mean welding current approximately 150 A are shown
in Fig. 10 and Fig. 11. These examples show the mean values of penetration, reinforcement and width of the welds with respective standard deviations. Pessoa (2003) also found higher penetration values for the conventional electrodes in the DCEN polarity, but higher values of width were also found in this polarity. This contradiction can be related to the
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fact that Pessoa (2003) has carried out the welds in a simulated depth of 50 and 100 meters, being able to cause this change with the penetration. It is observed that the electrode agglomerated with polymer in the polarity DCEP presented greater penetrations, followed by the agglomerated with polymer in the polarity DCEN, conventional DCEN and conventional DCEP. The conventional electrodes in the DCEN polarity presented larger reinforcements, followed by the conventional electrodes in the DCEP polarity, agglomerated with polymer DCEN and
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agglomerated with polymer DCEP. To try to explain these changes, in Table 1 the values of heat input, weld bead area, penetration area for each electrode and dilution are represented in each polarity with a mean
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welding current of approximately 150 A. It was observed higher weld energy values for the electrode agglomerated with polymers, but it is also observed that higher welding energies do not mean larger areas of weld beads. As an example, both electrodes in the DCEP polarity presented higher welding energies
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compared to them in the DCEN polarity, but presented larger areas in the DCEN polarity than in the DCEP
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polarity. This phenomenon can be related to losses of heat of the electric arc and / or losses of material and heat during the welding process. Electrodes agglomerated with polymer showed higher dilution values,
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with DCEP polarity having the highest index, due to the higher penetration areas and smaller areas of
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reinforcement.
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Hf was defined as the energy supplied during the welding process to form a cubic millimetre of the weld bead and the lower the value, the more the process tends to be. Thus, it is worth emphasizing that very high values can mean a low efficiency of the electric arc and, also, high losses of material and heat
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during the welding. After the calculations of Hf and the frequency of short circuits (F), Table 2 was created
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with the values of the reinforcement areas. It is observed that the electrode agglomerated with polymer in the polarity DCEP presented higher
values of Hf and therefore, being theoretically less efficient, with lower frequencies of short-circuits and
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smaller areas of the bead reinforcement. The conventional electrodes in the DCEN polarity have lower Hf, and thus being the most efficient with higher frequencies of short-circuits and larger areas of bead reinforcement. For this reason, a direct short-circuit frequency relation with reinforcement area is observed, that is, larger amounts of short-circuits caused larger reinforcements area or vice versa, and an inverse relation of the short-circuit frequency with the Hf. In this way the electrodes that presented higher
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frequencies of short circuits are more efficient and this fact can be due to smaller losses of heat and material through the spatters. This idea can be understood with the assumption: considering a case where the spatter is nonexistent, thus, the areas of weld bead reinforcements should be the same for all beads because welding is performed through the gravity welding device. For the same angles of the welding device, if the welding speed increases, the melting speed of electrode increases proportionally, causing the same material
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deposition per unit length. Therefore, it is concluded that smaller area generated higher levels of spatters and loss of heat through them. The DCEN polarity is the most efficient for both electrodes, despite having
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a higher C.V., as mentioned previously. Vaz and Bracarense (2016) also reported shorter frequencies of
short circuits for the electrode agglomerated with polymers in the welding to the air, being predominant to globular transfers.
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There is only a small difference with the electrodes agglomerated with polymers in the DCEN
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polarity in relation to the conventional electrodes in the polarity DCEP, where it is observed higher trends
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of frequencies of short circuits in the first, but smaller areas of reinforcement, although the values are very
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close. This may be related to the high standard deviations of these indices for the electrode agglomerated with polymer due to the high C.V. (high instability) and also high standard deviation thereof as noted
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previously in Fig. 9.
The heat input may have had a fundamental contribution to the penetration, since the electrodes
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agglomerated with polymers presented higher welding energies and higher penetrations, even with lower efficiencies as mentioned above. This would not explain the fact that the highest penetration is not in DCEN
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polarity for the electrodes agglomerated with polymer, contrary to the model of Tsai and Masubuchi (1977). Thus, it is believed that other mechanisms may have had a fundamental contribution in the morphology of the weld bead. The hot oxygen extraction test was carried out to try to confirm this theory, Fig. 12 shows
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the concentration in ppm for welds made with each electrode in each polarity. Comparing with the penetration (Fig. 11), a similarity of behaviour with oxygen is observed. The
condition with higher oxygen levels show higher penetrations and as the oxygen content falls the penetration does as well and therefore, being able to observe a linear relation as evidenced in the graph displayed in Fig. 13.
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Mills and Keene (1990), in a literature review on factors affecting weld penetration, have reported that one of the phenomena that may affect penetration is the direction of the fluid flow of the welding pool resulting from differences in the direction and magnitude of the thermocapillary forces controlled by active elements like oxygen and sulphur, called Marangoni effect. When these elements exceed a critical value the temperature coefficient of the surface tension changes from a negative value to a positive one. High thermal gradients in the welding pool (higher temperatures in the center), together with oxygen contents
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above the critical value, will cause in the center of the pool a higher surface tension than the edges and, thus, a flow of fluid from the region of low surface tension to the region of high surface tension, causing
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an increase of the penetration. Surface tension gradients may become more intense as oxygen and sulphur concentrations increase, making fluid flow more intense and resulting in greater penetrations.
Vaz and Bracarense (2016) also reported a significant increase in penetration for the electrode
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agglomerated with polymer in air-welding, but since they found no significant differences in heat input,
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therefore, a similar theory to what happens with cellulosic covered electrodes is approached. Vaz and Bracarense (2015) suggest that the polymer may cause an increase in the Lorentz electromagnetic forces,
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causing an increase in the plasma jet which would result in higher penetrations. Rokhlin and Guu (1993) in
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a study of arc force, pool depression and weld penetration during gas tungsten arc, state that there is a relation between the plasma jet and the Lorentz electromagnetic forces. Electrical current flow generates
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magnetic fields that interact with current carriers and produce body Lorentz Forces, these forces induce higher pressure in the arc, resulting in plasma jet.
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The appearance of finger penetration in the cross-section of the weld bead deposited by the
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electrode agglomerated with polymer in the DCEP polarity, as it can be seen in Fig. 10 (b), indicates influences of electromagnetic forces on the arc. Reviewing activated TIG (A-TIG) welding, Modenesi (2013) discusses the theories that explain the increased penetration of the bead when fluxes, such as fluorites, are used. The presence of negative ions changes the shape of the bead and the electromagnetic
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pressure in the arc. Mills and Keene (1990) schematically represented the cause of finger penetration is due to high values of Lorentz electromagnetic forces and Ko et al. (2001) studied the effect of arc pressure on TIG welding, reported that high pressures contribute to increased penetration and, in some cases, the appearance of finger penetration.
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According to Santos et al. (2012), the lowest oxygen content found on average is 920 ppm for rutile electrodes. For oxyrutile and oxidizing electrodes the values can reach 2400 ppm. It should be noted that oxygen may be a detrimental element to the toughness of the weld due to the increase in the amount and size of non-metallic inclusions, and high values favour the formation of coarse ferritic microstructures. Alhblom (1984) reported that contents between 250 and 450 ppm are ideal for the formation of larger amounts of acicular ferrite, a more desirable microstructure because, it presents better toughness. The
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values obtained in this work, which maximum content is 576.5 ppm, as shown in Fig. 12, were far lower than the values obtained by Santos et al. (2012).
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The mean and standard deviation of the diffusible hydrogen contents obtained for each electrode at each polarity are shown in Fig. 14.
It is observed that the lowest diffusible hydrogen value was found for the electrode agglomerated
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with polymer in the DCEP polarity (30.5ml/100g of deposited metal), which presented the highest amount
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of oxygen. The highest value was found for the conventional electrode in the DCEP polarity (45 ml/100g
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of deposited metal), which was the one with the lowest oxygen values. The electrode agglomerated with
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polymer in the DCEN polarity and the conventional electrode in the DCEN polarity obtained intermediate and very close values. Pope and Liu (1996) observed a decrease in the values of diffusible hydrogen with
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the increase of the oxygen contents in the weld metal for the rutile electrodes and oxidizing electrodes. In spite of the difference in depth of the test of diffusible hydrogen (0.5 meters) and oxygen (10 meters) in this work, it is possible to note the inverse relationship between hydrogen and oxygen for extreme cases,
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similar to that observed by the last authors, but cannot report this relationship for intermediate cases.
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Perhaps this fact may be more evident if the tests were performed at the same depth. Medeiros and Liu (1998) studied the effect of polarity on the diffusible hydrogen on oxidizing electrodes and reported that welds performed in the DCEN polarity present smaller amounts of diffusible hydrogen, similar to the results found for the conventional electrode but contrary with the results found for the electrodes agglomerated
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with polymers.
Matsushita and Liu (2000) argue that a method to reduce diffusible hydrogen would be the addition
of fluoride like ingredients in the covering to reduce the partial pressure of hydrogen in the arc and form products that are insoluble in liquid iron, such as, for example, hydrogen fluoride (HF). The fact that the polymer has fluorine may have played an important role in reducing the diffusible hydrogen content of the
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polymer-bonded electrode in the DCEP polarity. In addition to Matsushita and Liu (2000), other authors such as Kil et al. (2017) and Bang et al. (2010) also observed the decrease of diffusible hydrogen contents with the addition of fluorides in the tubular wire flux. Santos et al. (2012) carried out tests of diffusible hydrogen for different electrodes, oxidizing, rutile and oxyrutile at a depth of 0.5 meters. Rutile electrodes extrapolated average values of 85 ml/100 g and for oxidizing and oxy-rutile electrodes the highest average value was around 35 ml / 100 g, obtaining
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minimum values of up to 13 ml / 100 g. It is observed that the basic electrodes have diffusible hydrogen
contents far below than the rutile electrodes and close to some oxy-rutile electrodes and just above the
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oxidizing electrodes.
In Fig. 15, the porosity percentage values can be observed in relation to the total transversal area of the weld bead. It is observed that for the electrode agglomerated with polymer there were no differences
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in the mean values of porosity for both polarities, but the same ones in the DCEN polarity presented higher
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values of deviation, caused by the greater variations of porosity along the weld. Conventional electrodes in
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the DCEP polarity showed levels of porosity a little above the electrodes agglomerated with polymer, while
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the same ones in the DCEN polarity presented average levels twice as great as the electrodes agglomerated with polymers. The high standard deviation of the conventional electrodes in the DCEN polarity is due to
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the low presence of porosity at the beginning of the weld bead and an increase in the middle and end of the bead.
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Pessoa (2003) reported that in welds of conventional E7018 electrodes, also performed in A36 steel, higher levels of porosity were found in the DCEP polarity in the depth of 50 meters and higher levels
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in the DCEN polarity in depth of 100 meters, so it is not possible to conclude with a model in which it states that such polarity will present higher or lower levels of porosity for the conventional E7018 electrodes for underwater wet welding.
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Vaz and Bracarense (2016) reported that electrodes agglomerated with polymer in air welding had
better gaseous protections and the transferred droplets were smaller and with higher densities due to the tendency of smaller droplets to absorb smaller amounts of gases and, consequently, lower levels of spaces internally. This may be one of the factors that contributed for the lower levels of porosity in the weld metal of the electrode agglomerated with polymer, as mentioned previously. According to Pessoa (2007), the porosity of the weld metal has a direct relation with the quantities of gases absorbed in the droplets being
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transferred and that the absorption of gases and the porosity can be decreased with better gaseous protections generated by the electrodes. Thus, the tips of the electrodes were collected and evaluated in relation to the porosity levels of the droplets formed in them. Fig. 16 shows droplets formed at the tips of the electrodes for the conventional electrodes and for electrodes agglomerated with polymer in both polarities. The conventional electrode for both polarities had higher porosities in the droplets than the electrode agglomerated with polymer. This is in agreement with Vaz and Bracarense (2016) who also found
be related to the smaller porosities in the weld metal according to Pessoa (2007). Conclusions The results obtained in this work allow to conclude that:
The electrode agglomerated with polymer presented higher arc voltage values in comparison to the
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4.
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lower porosity indices in the electrode agglomerated with polymer droplets in air welding which may also
conventional electrode with the same polarity. The conventional electrode in the DCEP polarity
The conventional electrode presented higher penetration values in the DCEN polarity. For the electrode
A
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presented lower values of C.V., theoretically, being the most stable.
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agglomerated with polymer, the highest penetration value was found in the DCEP polarity. There was a tendency for larger areas of reinforcement and lower energies generated to form one cubic millimeter
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of the weld bead for higher frequencies of short circuits. It was observed an approximately direct relationship between penetration and oxygen content, allowing to assume that the Marangoni effect
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was the phenomenon responsible for the increases in penetration. Another mechanism, that increase plasma jet strength, may have contributed to the higher penetrations of the electrodes agglomerated
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with polymer.
The electrode agglomerated with polymer in the DCEP polarity presented the lowest values of diffusible hydrogen. Although diffusible hydrogen and oxygen tests were performed at different
A
depths, an inverse relationship could be observed for the extremes.
The electrode agglomerated with polymer presented lower levels of weld metal porosity in comparison with the conventional electrode. Higher gas uptakes are observed in the forming droplets at the tips of conventional electrodes, which may be indicative of a poor gas protection compared to the electrode agglomerated with polymer.
14
Acknowledgments The authors would like to acknowledge the Laboratório de Robótica Soldagem e Simulação (LRSS) for the availability of resources for the accomplishment of this work and Prof. Dr. Cláudio Turani Vaz and
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Prof. Dr. Paulo José Modenesi for the academic and discursive help.
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References Alhblom, B., 1984. Oxygen and Its Role in Determining Weld Metal Microstructure and Toughness: A State-of-the-art Review. International Inst. of Welding, pp. 19.
wire, Met. Mater. Int., 16 (3), 489-494. https://doi.org/10.1007/s12540-010-0622-6
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Bang, K. S., Jung, H. C., Han, I. W., 2010. Comparison of the effects of fluorides in rutile-type flux cored
Gooch, T.G., 1983. Properties of underwater welds. Part 1: Procedural trials. Metal Construction. 3, 164-
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167.
Hazlett, T. H., 1957. Coating ingredients’ influence on surface tension, arc stability and bead shape.
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Welding Research Supplement, 36 (1), 18-23.
Kil, W., Shin, M. J., Ban, K.S., 2017. Effects of Fluoride in the Flux on Hydrogen Content in Weld Metal
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and Operating Behavior in FCAW-S. Journal of Welding and Joining, 35 (5), 65-70.
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https://doi.org/10.5781/JWJ.2017.35.5.9
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Ko, S. H., Choi, S. K., Yoo, C. D., 2001 . Effects of Surface Depression on Pool Convection and
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Geometry in Stationary GTAW. Welding Journal, 80, 39s-45s. Lancaster, J. F., (1986). The Physics of Welding. Pergamon Press & International Institute of Welding,
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pp. 340.
Matsushita, M., Liu, S., 2000. Hydrogen Control in Steel Weld Metal by Means of Fluoride Additions in
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Welding Flux. Welding Journal, 79 (10), 295-303. Medeiros, R. C., Liu, S., 1998. A Predictive Electrochemical Model for Weld Metal Hydrogen Pickup in Underwater Wet Welds. OMAE 1998: Proceedings of the 17th International Conference on Offshores
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Mechanics and Arctic Engineering. Lisbon, Portugal. https://doi.org/10.1115/1.2829547 Mills, K. C., Keene, B. J., 1990. Factors affecting variable weld penetration. International Materials Reviwes, 35 (4), 185-216. https://doi.org/10.1179/095066090790323966 Modenesi, P. J., 2013. A química da formação do cordão na soldagem TIG. Soldagem & Inspeção, 18 (3), 287-300. http://dx.doi.org/10.1590/S0104-92242013000300011
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Pessoa, E. C. P., 2003. Estudo Comparativo do Desempenho de Eletrodos Revestidos Comerciais E6013, E7018 e E7024 em Soldagem Subaquática Molhada. MSc dissertation. Federal University of Minas Gerais. Brazil (in Portuguese). Pessoa, E. C. P., 2007. Estudo da Variação da Porosidade ao Longo do Cordão em Soldas Subaquáticas Molhadas. PhD dissertation. Federal University of Minas Gerais. Brazil (in Portuguese).
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Pessoa, E. C.P., Bracarense, A. Q., Liu, S., Perez, F., 2003. Estudo Comparativo do Desempenho de Eletrodos Revestidos E6013, E7024 e E7018 em Soldagem Subaquática em Água Doce do Aço A36 à profundidade de 50 e 100 Metros. 2º COBEF – Congresso Brasileiro de Engenharia de Fabricação.
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Uberlândia – MG. Brazil.
Pope, A. M., Liu, S., 1996. Hydrogen content of underwater wet welds deposited by rutile and oxidizing
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electrodes. Proc. Conf. OMAE 96, American Society of Mechanical Engineers. 3, 85-92.
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Rokhlin, S. I., Guu, A. C., 1993. A Study of Arc Force, Pool Depression, and Weld Penetration During
A
Gas Tungsten Arc Welding. Welding Journal. 72 (8), 381s-390s.
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Santos, V.R., Monteiro, M.J., Rizzo, F.C., Bracarense, A.Q., Pessoa, E.C.P., Marinho, R.R., Vieira, L.A., 2012. Development of an Oxyrutile Electrode For Wet Welding. Welding Journal. 91, 319-328.
Bulletin, 224, 1-37.
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Tsai, C. L., Masubuchi, K., 1977. Interpretive Report on Underwater Welding. Welding Research Council
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Vasil'chenko, V V., 1987. Effects of taper angle of tungsten electrodes and additions of different fluorides on arc stability in the argon TIG welding of copper. Welding International. 39 (10), 28-31.
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https://doi.org/10.1080/09507118709453009 Vaz, C. T., Bracarense, A. Q., 2015. The Effect of the Use of PTFE as a Covered-Electrode Binder on
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Metal Transfer. Soldagem & Inspeção. 20 (2), 160-170. http://dx.doi.org/10.1590/0104-9224/SI2002.04 Vaz, C. T., Bracarense, A. Q., 2016. The influence of PTFE used as basic covered electrode binder on weld metal acicular ferrite formation. Welding International. 30 (5), 359-371. https://doi.org/10.1080/09507116.2015.1096507
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Vaz, C. T., Bracarense, A. Q., Felizardo, I., Pessoa, E. C. P., 2012. Impermeable Low Hydrogen Covered Electrodes: Weld Metal, Slag, and Fumes Evaluation. Journal of Materials Research and Technology. 1 (2), 64-70. https://doi.org/10.1016/S2238-7854(12)70012-1 West, T.C., Mitchell, G., Lindberg, E., 1990. Wet Welding electrode evaluation for ship repair. Welding
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A
N
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Journal. 69, 47-56.
18
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CC E
PT
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M
A
Fig. 1. Electrodes agglomerated with polymer and conventional electrodes.
19
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PT
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A
N
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Fig. 2. Equipment for simulation of underwater wet welding. (a) Hyperbaric tank and (b) gravity welding device.
20
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A
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PT
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M
A
N
Fig. 3. Schematic representation of cut planes and sample withdrawal regions.
21
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Fig. 4. (a) Macrograph of the weld bead without chemical attack. (b) Binarization of the macrograph
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CC E
PT
ED
M
A
N
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making the pore region dark areas.
22
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A
CC E
PT
ED
M
A
N
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Fig. 5. Schematic weld bead cross-section showing the measurements made to evaluate the morphology.
23
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Fig. 6. (a) Schematic drawing representing the cut plane for sample extraction. (b) Samples removed from
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PT
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A
N
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the reinforcement of the weld beads for oxygen testing.
24
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
45
35 30 25 20 15 10 100
110
120
130
140
150
Mean Welding Current (A)
160
170
180
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90
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Mean Arc Voltage (V)
40
Fig. 7. Mean arc voltage per mean welding current chart for conventional electrode and electrode
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PT
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A
N
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agglomerated with polymer in DCEP and DCEN polarity.
25
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
4.5 4 3.5 3 2.5 100
110
120
130
140
150
Mean Welding Current (A)
160
170
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90
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Welding speed (mm/s)
5
180
Fig. 8. Welding speed per mean welding current chart for conventional electrode and electrode
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PT
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M
A
N
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agglomerated with polymer in DCEP and DCEN polarity.
26
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
22 20
16 14 12 10 8 6 100
110
120
130
140
150
Mean Welding Current (A)
160
170
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90
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C.V. (%)
18
180
Fig. 9. Coefficient of variation (C.V.) per mean welding current chart for conventional electrode and
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PT
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M
A
N
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electrode agglomerated with polymer in DCEP and DCEN polarity.
27
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N
Fig. 10. Macrographs of weld bead produced with electrodes: (a) agglomerate with polymer DCEN
A
polarity; (b) agglomerated with polymer DCEP polarity; (c) conventional DCEN polarity; (d)
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PT
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conventional DCEP polarity
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Penetration 11
reinforcement
10.1
9.9
9.9
10
width
8.9
9 7 6 5 4
2.8
3
3.3 2.5
2.2
2.4
2.7 1.9
1.4
2
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1 0
Pol. DCEN
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Millimeters
8
Pol. DCEP
Conv. DCEN
Conv. DCEP
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Fig. 11. Mean values of penetration, reinforcement and width for each electrode (Pol. = With polymer
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PT
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A
N
and conv. = Conventional), at each polarity, for the mean welding current approximately 150 A.
29
700 576.5 502.3 432.8
500
368.0
400 300 200 100 0
Pol. DCEP
Conv. DCEN
Conv. DCEP
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Pol. DCEN
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Oxygen concentration (ppm)
600
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PT
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A
N
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Fig. 12. Oxygen concentration in welds for both electrodes in both polarities.
30
550
Pol. DCEP
Pol. DCEN 500 450 400
Conv. DCEP
Conv. DCEN
350 300 2
2.5
3
Penetration (mm)
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1.5
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Oxygen concentration (ppm)
600
3.5
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A
N
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Fig. 13. Linear relationship between penetration and concentration of oxygen in the weld metal.
31
60
45.0 36.5
40
35.0 30.5
30 20 10 0
Pol. DCEP
Conv. DCEN
Conv. DCEP
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Pol. DCEN
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Hdf (ml/100g)
50
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PT
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A
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0.5 meters deep.
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Fig. 14. Diffusible hydrogen contents in the weld metal for both electrodes, in both polarities, performed
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6
5
Porosity (%)
4
3.4
3 1.9
1.7
1.7
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2
1
Pol. DCEN
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0
Pol. DCEP
Conv. DCEN
Conv. DCEP
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PT
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M
A
N
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Fig. 15. Percentage of porosity in relation to the total cross-section weld bead area for each condition.
33
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Fig. 16. Droplets at the tips of the electrodes, being: (a) conventional electrode DCEP polarity; (b) conventional electrode DCEN polarity; (c) electrode agglomerated with polymer DCEP polarity; (d)
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PT
electrode agglomerated with polymer DCEN polarity.
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Table Captions Table 1 Values of heat input, area of penetration, total area and dilution for each electrode, in each polarity, realized with the mean welding current approximately 150 A. Pol. DCEN Mean
Deviation
Mean
Conv. DCEN
Deviation
Mean
Deviation
Conv. DCEP Mean
Deviation
1088.4
360.0
1237.1
295.7
977.2
304.6
15.4
0.8
18.3
1.9
12.7
0.9
28.7
2.7
26.7
2.2
26.7
1.2
25.4
1.2
54.1
5.3
68.4
1.2
47.5
2.1
46.0
0.6
(J/mm) Penetration
Total area
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PT
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M
A
N
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(mm²) Dilution (%)
35
11.7
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area (mm²)
1023.0
236.2
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Heat input
Pol. DCEP
0.8
Table 2 Hf values, area of reinforcement and short-circuit frequency for each electrode, at each polarity, carried out with a mean welding current approximately 150 A. (arrows pointing up indicate higher values for calculated indices and arrows pointing down indicate the lowest values). Pol. DCEP
Mean
Deviation
Mean
Deviation
Hf (J/mm³)
38.0
16.1
46.4
14.8
Reinforcement
13.3
2.6
8.4
3.8
2.1
0.6
Conv. DCEN Mean
Deviation
Mean
Deviation
36.6
13.1
40.3
11.3
1.0
14.0
1.1
0.1
6.4
0.1
13.7
1.1
3.2
0.1
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area (mm²) Short-circuit
Conv. DCEP
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Pol. DCEN
A
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PT
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A
N
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frequency (Hz)
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S0924-0136(18)30455-2 https://doi.org/10.1016/j.jmatprotec.2018.10.019 PROTEC 15972
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
26-4-2018 16-10-2018 18-10-2018
Please cite this article as: Ribeiro Menezes PH, Pereira Pessoa EC, Bracarense AQ, Comparison of Underwater Wet Welding Performed with Silicate and Polymer Agglomerated Electrodes, Journal of Materials Processing Tech. (2018), https://doi.org/10.1016/j.jmatprotec.2018.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of Underwater Wet Welding Performed with Silicate and Polymer Agglomerated Electrodes
1
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Pedro Henrique Ribeiro Menezes1, Ezequiel Caires Pereira Pessoa2, Alexandre Queiroz Bracarense1
Federal University of Minas Gerais - UFMG, Mechanical Engineering Department, Laboratório de
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Robótica, Soldagem e Simulação – LRSS, Belo Horizonte, MG, Brazil. [email protected], [email protected] 2
Federal Institute of Minas Gerais - IFMG-Betim, Industrial Mechanics Department, Betim, Minas
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Gerais, Brazil, [email protected]
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Abstract
Conventional basic electrodes varnished and basic electrodes agglomerated with polymer in
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underwater wet welding in a simulated depth of 10 meters were compared. A hyperbaric tank was used for depth simulation and a gravity welding device for making the bead on plate welds. Bead on plate welds
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were deposited with welding currents of 100, 120, 140, 160 and 180 A with DCEN and DCEP polarities, in order to evaluate the arc stability through a non-dimensional statistical index. The welding current which
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showed better stability level in both polarities was chosen to evaluate weld bead morphology, weld metal porosity, weld metal oxygen and diffusible hydrogen levels. A methodology was used to correlate the heat input and short-circuit frequency with the weld bead morphology. It was observed a direct relation with the
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weld metal oxygen and penetration and an inverse relation of the weld metal oxygen with the diffusible hydrogen. Electrodes agglomerated with polymer showed higher penetration values than conventional electrodes in both polarities and lowest diffusible hydrogen levels in the DCEP polarity. It was observed lower levels of weld metal porosity for the electrodes agglomerated with polymer in comparison to conventional electrodes. This result was related to lower levels of porosity found in the droplets formed at the tip of the electrodes agglomerated with polymer.
1
Key-words: Underwater Wet Welding, Polymer Binder, Arc Stability, Weld Metal Porosity, Weld Bead Morphology, Weld Metal Oxygen, Diffusible Hydrogen.
1.
Introduction
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Due to the influences caused by the water surrounding the arc, Santos et al. (2012) affirmed that
there are few reports in the literature regarding underwater wet welds that have reached Class A in
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accordance with the AWS D3.6M: 2010 standard. Consequently, several electrodes have been studied and developed to improve operability and weldability in the aqueous medium. Rutile electrodes are
preferentially used because they present better arc stability and better weld bead appearance, these
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characteristics were reported by West et al. (1990), as well as by Gooch (1983). In contrast, Santos et al.
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(2012) and Pope and Liu (1996) reported that rutile electrodes produce weld metal with high diffusible hydrogen contents (up to 80 ml/100g), making them susceptible to cold cracking. Thus, different
A
formulations of oxidizing electrodes have also been widely studied, due to the low diffusible hydrogen
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content produced, which can present results lower than 20 ml/100g according to Santos et al. (2012) and Medeiros and Liu (1998). In contrast, these electrodes have a low arc stability, inferior weld bead
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appearance and poor slag detachability. Santos et al. (2012) developed and studied oxy-rutile electrodes trying to combine the advantages of rutile and oxidizing electrodes. There are also reports in the literature
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related to the use of austenitic stainless steel and nickel-based electrodes, but few literatures are related to
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basic electrodes.
Pessoa et al. (2003) studied the use of electrodes E6013, E7024 and E7018 at depths of 50 and 100
meters, and the welds performed with E7018 electrodes presented high levels of weld metal porosity, bad operability and ugly appearances. It is believed that because of these drawbacks, few literatures are found
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regarding to the values of diffusible hydrogen, oxygen content in the weld bead and in-depth studies of welding parameters seeking better performances of the basic electrodes. Vaz et al. (2012) developed and conducted air-welding with basic electrodes agglomerated with polymer instead of conventional binders which exhibited features that may be desirable in underwater wet welding. These electrodes presented lower levels of diffusible hydrogen in comparison with the conventional E7018 electrodes, not causing the increase of these levels even when exposed to the atmosphere for 30 days. Therefore, it should be
2
emphasized that the electrodes agglomerated with polymers have low sensitivity to humidity; they neither need to be placed in ovens for drying or maintenance in portable ovens, allowing their use directly from the packaging, nor the use of varnishes for waterproofing them for underwater wet welding. Vaz and Bracarense (2016) observed that the number of smaller droplets transferred by the polymer agglomerated electrodes is higher than in the conventional electrodes. They also reported that the density of the droplets transferred during the welding was higher for smaller droplets. This phenomenon is related to large voids
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present inside the larger droplets due to the absorption of gases during the formation of the droplet. The same authors also reported a better gas protection provided by the electrode agglomerated with polymer.
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These two last characteristics may be an important factor for decreasing weld metal porosity levels. Pessoa (2007) stated that the weld metal porosity is directly related to the amount of gases absorbed in the droplet being transferred or generated in the welding pool, and this can be minimized through better protection generated by the covered electrode. Vaz and Bracarense (2016) also reported higher amounts of acicular
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ferrite in the weld beads made with electrodes agglomerated with polymers, which is a desirable
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microconstituent in the weld beads because of its higher toughness.
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Fig. 1 shows the electrodes agglomerated with polymer and the varnished conventional electrodes.
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An intriguing feature of the electrode agglomerated with polymer is observed. The covering, besides having resistance to humidity, also has resistance to bending. This is due to the greater flexibility, making it less
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brittle than the conventional electrodes which after bending tend to break the covering and leave the core wire exposed.
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Based on all this knowledge, the present work has the objective of comparing the performance of
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conventional varnished E7018 basic electrodes with E7018 basic electrodes agglomerated with polymer, trough evaluation of operational characteristics, weld bead morphology, weld metal porosity and weld metal oxygen and diffusible hydrogen contents.
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2.
Materials and Methods
2.1. Materials ASTM A-36 steel sheets with 200 mm long, 100 mm wide and 19.05 mm (¾ in) thick were used
as samples. Conventional covered electrodes E7018 and covered electrodes E7018 agglomerated with polymer, both with a core wire of 3.25 mm, were used. Conventional covered electrodes were dried at 350º C for one hour and subsequently protected with vinylic varnish to prevent the covering from coming in
3
contact with water and decomposing during welding. The agglomerated electrodes were used directly from the package. The water used was tap water and constantly replaced with new tap water. 2.2. Equipment A hyperbaric tank was used to simulate the wet welding process at a depth of 10 meters. Fig. 2(a) shows the hyperbaric tank. For welding, a gravity welding device was used, as it can be seen in Fig. 2(b).
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This device has main characteristics: welding speed linked to the fusion rate of the electrode and the angles α and β; allows the adjustment of the angles α and β and, consequently, regulation of the welding speed
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with good repeatability.
An IMC welding power source, model HipER-1, specific for underwater wet welding was used. A data acquisition system with an acquisition rate of 1000 points per second was used to monitor the welding current and arc voltage and a software, called Signal, was used for electrical signals treatment. A
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Canon model i3 digital camera was used to capture images and the Image J® software was used to calculate
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weld bead geometry dimensions in macroscopic analysis. A gas chromatograph model ON 900 Oxygen
A
Nitrogen Deter minator PC controlled from Eltra, whose principle of measurement is based on the property
M
of infrared gas radiation, was used for oxygen content determination. Another gas chromatograph analyser from OERLIKON Yanaco, model HDM ANALIZER G-1006H, was used for diffusible hydrogen content
2.3. Methodology
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measurement.
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The gravity welding device angles values were selected in preliminary tests in order to find the
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best parameters of arc stability. The angles, that presented the best results, were α equal to 60º and β equal to 80º.
Bead on plate welds were deposited at 10 meters simulated water depth with nominal welding
A
currents of 100, 120, 140, 160 and 180 A. Two weld beads were made in the DCEN polarity and two other weld beads were made in the DCEP polarity for each welding current. During welding, the arc voltage and welding current data were acquired for each condition. The mean welding current (Im) and standard deviation of the mean welding current (σIm), mean arc voltage (Vm) and standard deviation of the mean arc voltage (σVm) were calculated using the software Sinal in an
4
acquisition interval of 10 seconds. The interval of calculations was determined between 10 and 20 seconds after the electric arc was opened to avoid arc voltage and welding current signals transient period. The welding time (t) and the weld bead length (L) were measured for each condition. Welding speed (Ws) was obtained through Equation 1. 𝑊𝑠 =
𝐿
(mm.s-1)
𝑡
(1)
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Afterwards, a welding speed graph (Ws) was generated against mean welding current (Im), to facilitate the comparison in each condition.
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Before morphology, porosities, diffusible hydrogen and oxygen of the weld metal, the stability of
the electric arc was evaluated to determine the mean welding current which the electrodes have better stabilities in each polarity. The Coefficient of Variation (C.V.) was used in the arc voltage data. The C.V.
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is a non-dimensional index that measures the dispersion in relation to the average. Prior to the calculation,
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it is necessary to apply a filter to remove valleys present in the arc voltage due to the short-circuiting metal
A
transfer. The C.V. is calculated using Equation 2. The arc voltage is denominated as Vam after the removal
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of short-circuits.
𝜎𝑉𝑎𝑚 𝑉𝑎𝑚
∗ 100
(%)
(2)
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𝐶. 𝑉. =
Fig. 3 schematically shows the sections and the regions of sample withdrawal. As previously stated, two weld beads were made for each condition, thus resulting in 6 samples for each condition. The
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tips of the electrodes were also preserved and cut to evaluate the porosity in the drop being formed.
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The samples of the weld beads and tips of the electrodes were mounted, sanded (sands 80, 180, 320, 400, 600, 1200), polished with 1μm Al2O3 and etched with Nital 2%. Before etching, the samples were photographed to obtain greater contrasts of the formed pores, as it can be seen in Fig. 4(a), this procedure
A
facilitating the binarization of the image, as it can be observed in Fig. 4(b). Subsequently, the total area of the pores (Ap) were measured. Binarization and measurement of the total area were performed using Image J® software. Fig. 5 schematically shows the weld bead cross-section and the respective measurements performed.
5
To calculate the dilution (𝛿) and weld metal porosity (P), Equations 3 and 4, respectively, were used.
𝛿=
𝑃=
𝐴𝑏𝑚 𝐴𝑏𝑚+𝐴𝑟
𝐴𝑝 𝐴𝑏𝑚+𝐴𝑟
∗ 100
(%)
(3)
∗ 100
(%)
(4)
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The heat input (H) was calculated using Equation 5 and the amount of energy spent to form a cubic millimetre of the weld bead was calculated using Equation 6, which relates the heat input with total area
(At), called Hf. The short-circuit frequency (F) was also calculated to evaluate the influence of short-circuits
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on energy and morphology. For this purpose, the amount of short-circuits (Ncc) were used for a period of
𝑉𝑚∗𝐼𝑚 𝑉𝑠
𝑁𝑐𝑐 10
(5)
(6)
(Hz)
(7)
(J.mm-3)
M
𝐹=
𝐻 𝐴𝑡
(J.mm-1)
N
𝐻𝑓 =
A
𝐻=
U
10 seconds, as described in Equation 7.
To determine the oxygen contents, present in the weld metal, samples weighing approximately
ED
500 milligrams of reinforcement were removed from the welds, as it can be seen in Fig. 6. The gas chromatography method was applied, using ASTM E260-96 as reference, for determining
PT
the diffusible hydrogen contents. The welding was performed in an aquarium at 0.5 meters and the specimen had to be inside the capsules in an atmosphere with argon for less than two minutes after the welding for
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the test to be valid. This step would be very difficult to perform in the hyperbaric tank. The welds for the diffusible hydrogen tests were performed on samples that had half the length of the standardized specimen due to the high diffusible hydrogen value produced by underwater welding. Pope and Liu (1996) and
A
Medeiros and Liu (1998) also reduced the size of the specimens for the same reasons. Eight tests were performed with the electrode agglomerated with polymer and eight with the commercial electrodes, using nominal welding current in which the electrodes presented better stabilities, four test bodies in the electrode negative polarity (DCEN) and four test bodies in the electrode positive polarity (DCEP). 3.
Results and Discussion
6
The data was processed with the Sinal software after the arc voltage and welding current signalled acquisition during the welding. For a better comparison of the data obtained, a mean welding current per mean arc voltage chart was generated, with the respective standard deviations, as observed in Fig. 7. For the nominal welding currents of 100 and 120 A for both electrodes in the DCEN polarity it was not possible to obtain a suitable welding procedure. This can be associated with the welding currents that are insufficient for a stable fusion of the electrode in this polarity or to start the electrical arc. For the
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electrode agglomerated with polymer in the DCEN polarity, it was also not possible to carry out the procedure with the nominal welding current of 180 A, due to a constant increase of the size of the cone at
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the tip of the electrode during the welding and, consequently, a constant increase of the length of the electric
arc until its extinction. Because of this, it was not possible to obtain a stable electric arc for more than 7 seconds.
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It can be observed that the electrodes agglomerated with polymers showed a tendency of mean arc
N
voltage higher than the conventional electrodes, in the DCEN polarity, as well as in the DCEP. At the
A
beginning of the droplets formation at the electrode tip, no significant differences in cone length between
M
the electrodes for the same polarity and same welding current were found. Such occurrence may be related to the presence of fluorine in the electric arc because of the polymer. This hypothesis can be strengthened
ED
by Vasil'chenko (1987), who found increases in the arc voltage with the application of fluorides in copper TIG welding. Lancaster (1986) also reports that elements that require relatively high energies for ionization to occur, such as fluorine, result in an increase in the electrical resistivity of the plasma column and,
PT
consequently, tends to cause higher operating arc voltages for a given length due to greater difficulties in
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electric current passage.
It was also observed that for the same electrode, the DCEP polarity presented higher mean arc
voltage than the DCEN polarity. Tsai and Masubuchi (1977) explained that the length of the cone formed
A
by the covering of the electrode during welding, and consequently the arc length and arc voltage, are related to the polarity of welding current. According to the model, the DCEP polarity has a longer arc length, resulting in higher arc voltages. Weld bead length and weld time data were used for calculating welding speed and standard deviation, as it can be seen in Fig. 8.
7
It is noted that the increase in welding current caused an increase in the welding speed, already expected due to the increase of the power of the arc. The higher welding speeds for the electrodes agglomerated with polymer are a consequence of the higher mean arc voltage presented in relation to the conventional electrodes. Thus, resulting in higher energy values per unit time and consequently higher melting rate and welding speed. The coefficient of variation (C.V.) was used to determine a stability analysis of the arc.
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Undesirable phenomena during the welding process tend to cause large disturbances in the electric arc,
causing variations in the arc voltage and therefore, making it more unstable. Through Equation 2, it can be
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stated that smaller mean arc voltage deviations, consequently lower values of C.V., theoretically result in a more stable process.
Fig. 9 shows the C.V. results for each welding condition. The condition with mean welding current
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approximately of 150 A presented lower C.V. trend in the conventional electrodes at both polarities and for
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the electrode agglomerated with polymer in the polarity DCEP. The electrode agglomerated with polymer
A
in the DCEN polarity behaved in a different way from the others, presenting a high C.V. and also a high
M
standard deviation at this welding current. The weld beads performed with mean welding currents close to 150 A were chosen for evaluation of the morphology, microstructure, diffusible hydrogen and oxygen
ED
contents for all cases.
For a given polarity it is observed that the conventional electrode has tendencies of lower C.V.
PT
This fact may be related to the fluorine content present in the electrode agglomerated with polymer. This tends to decrease the stability of the arc, since the difficulty of ionization tends to cause a more agitated,
CC E
violent arc and with higher indices of disturbances. Bang et al. (2010) in a comparative study on the effect of different fluorides on the flux of tubular wires, reported that elements with high ionization potential tend to cause greater arc instabilities and, in tests performed by Hazlett (1957), it is observed that addition of
A
fluoride can cause a violent arc with high levels of spatter. Some macrographs of the weld beads in the mean welding current approximately 150 A are shown
in Fig. 10 and Fig. 11. These examples show the mean values of penetration, reinforcement and width of the welds with respective standard deviations. Pessoa (2003) also found higher penetration values for the conventional electrodes in the DCEN polarity, but higher values of width were also found in this polarity. This contradiction can be related to the
8
fact that Pessoa (2003) has carried out the welds in a simulated depth of 50 and 100 meters, being able to cause this change with the penetration. It is observed that the electrode agglomerated with polymer in the polarity DCEP presented greater penetrations, followed by the agglomerated with polymer in the polarity DCEN, conventional DCEN and conventional DCEP. The conventional electrodes in the DCEN polarity presented larger reinforcements, followed by the conventional electrodes in the DCEP polarity, agglomerated with polymer DCEN and
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agglomerated with polymer DCEP. To try to explain these changes, in Table 1 the values of heat input, weld bead area, penetration area for each electrode and dilution are represented in each polarity with a mean
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welding current of approximately 150 A. It was observed higher weld energy values for the electrode agglomerated with polymers, but it is also observed that higher welding energies do not mean larger areas of weld beads. As an example, both electrodes in the DCEP polarity presented higher welding energies
U
compared to them in the DCEN polarity, but presented larger areas in the DCEN polarity than in the DCEP
N
polarity. This phenomenon can be related to losses of heat of the electric arc and / or losses of material and heat during the welding process. Electrodes agglomerated with polymer showed higher dilution values,
A
with DCEP polarity having the highest index, due to the higher penetration areas and smaller areas of
M
reinforcement.
ED
Hf was defined as the energy supplied during the welding process to form a cubic millimetre of the weld bead and the lower the value, the more the process tends to be. Thus, it is worth emphasizing that very high values can mean a low efficiency of the electric arc and, also, high losses of material and heat
PT
during the welding. After the calculations of Hf and the frequency of short circuits (F), Table 2 was created
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with the values of the reinforcement areas. It is observed that the electrode agglomerated with polymer in the polarity DCEP presented higher
values of Hf and therefore, being theoretically less efficient, with lower frequencies of short-circuits and
A
smaller areas of the bead reinforcement. The conventional electrodes in the DCEN polarity have lower Hf, and thus being the most efficient with higher frequencies of short-circuits and larger areas of bead reinforcement. For this reason, a direct short-circuit frequency relation with reinforcement area is observed, that is, larger amounts of short-circuits caused larger reinforcements area or vice versa, and an inverse relation of the short-circuit frequency with the Hf. In this way the electrodes that presented higher
9
frequencies of short circuits are more efficient and this fact can be due to smaller losses of heat and material through the spatters. This idea can be understood with the assumption: considering a case where the spatter is nonexistent, thus, the areas of weld bead reinforcements should be the same for all beads because welding is performed through the gravity welding device. For the same angles of the welding device, if the welding speed increases, the melting speed of electrode increases proportionally, causing the same material
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deposition per unit length. Therefore, it is concluded that smaller area generated higher levels of spatters and loss of heat through them. The DCEN polarity is the most efficient for both electrodes, despite having
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a higher C.V., as mentioned previously. Vaz and Bracarense (2016) also reported shorter frequencies of
short circuits for the electrode agglomerated with polymers in the welding to the air, being predominant to globular transfers.
U
There is only a small difference with the electrodes agglomerated with polymers in the DCEN
N
polarity in relation to the conventional electrodes in the polarity DCEP, where it is observed higher trends
A
of frequencies of short circuits in the first, but smaller areas of reinforcement, although the values are very
M
close. This may be related to the high standard deviations of these indices for the electrode agglomerated with polymer due to the high C.V. (high instability) and also high standard deviation thereof as noted
ED
previously in Fig. 9.
The heat input may have had a fundamental contribution to the penetration, since the electrodes
PT
agglomerated with polymers presented higher welding energies and higher penetrations, even with lower efficiencies as mentioned above. This would not explain the fact that the highest penetration is not in DCEN
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polarity for the electrodes agglomerated with polymer, contrary to the model of Tsai and Masubuchi (1977). Thus, it is believed that other mechanisms may have had a fundamental contribution in the morphology of the weld bead. The hot oxygen extraction test was carried out to try to confirm this theory, Fig. 12 shows
A
the concentration in ppm for welds made with each electrode in each polarity. Comparing with the penetration (Fig. 11), a similarity of behaviour with oxygen is observed. The
condition with higher oxygen levels show higher penetrations and as the oxygen content falls the penetration does as well and therefore, being able to observe a linear relation as evidenced in the graph displayed in Fig. 13.
10
Mills and Keene (1990), in a literature review on factors affecting weld penetration, have reported that one of the phenomena that may affect penetration is the direction of the fluid flow of the welding pool resulting from differences in the direction and magnitude of the thermocapillary forces controlled by active elements like oxygen and sulphur, called Marangoni effect. When these elements exceed a critical value the temperature coefficient of the surface tension changes from a negative value to a positive one. High thermal gradients in the welding pool (higher temperatures in the center), together with oxygen contents
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above the critical value, will cause in the center of the pool a higher surface tension than the edges and, thus, a flow of fluid from the region of low surface tension to the region of high surface tension, causing
SC R
an increase of the penetration. Surface tension gradients may become more intense as oxygen and sulphur concentrations increase, making fluid flow more intense and resulting in greater penetrations.
Vaz and Bracarense (2016) also reported a significant increase in penetration for the electrode
U
agglomerated with polymer in air-welding, but since they found no significant differences in heat input,
N
therefore, a similar theory to what happens with cellulosic covered electrodes is approached. Vaz and Bracarense (2015) suggest that the polymer may cause an increase in the Lorentz electromagnetic forces,
A
causing an increase in the plasma jet which would result in higher penetrations. Rokhlin and Guu (1993) in
M
a study of arc force, pool depression and weld penetration during gas tungsten arc, state that there is a relation between the plasma jet and the Lorentz electromagnetic forces. Electrical current flow generates
ED
magnetic fields that interact with current carriers and produce body Lorentz Forces, these forces induce higher pressure in the arc, resulting in plasma jet.
PT
The appearance of finger penetration in the cross-section of the weld bead deposited by the
CC E
electrode agglomerated with polymer in the DCEP polarity, as it can be seen in Fig. 10 (b), indicates influences of electromagnetic forces on the arc. Reviewing activated TIG (A-TIG) welding, Modenesi (2013) discusses the theories that explain the increased penetration of the bead when fluxes, such as fluorites, are used. The presence of negative ions changes the shape of the bead and the electromagnetic
A
pressure in the arc. Mills and Keene (1990) schematically represented the cause of finger penetration is due to high values of Lorentz electromagnetic forces and Ko et al. (2001) studied the effect of arc pressure on TIG welding, reported that high pressures contribute to increased penetration and, in some cases, the appearance of finger penetration.
11
According to Santos et al. (2012), the lowest oxygen content found on average is 920 ppm for rutile electrodes. For oxyrutile and oxidizing electrodes the values can reach 2400 ppm. It should be noted that oxygen may be a detrimental element to the toughness of the weld due to the increase in the amount and size of non-metallic inclusions, and high values favour the formation of coarse ferritic microstructures. Alhblom (1984) reported that contents between 250 and 450 ppm are ideal for the formation of larger amounts of acicular ferrite, a more desirable microstructure because, it presents better toughness. The
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values obtained in this work, which maximum content is 576.5 ppm, as shown in Fig. 12, were far lower than the values obtained by Santos et al. (2012).
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The mean and standard deviation of the diffusible hydrogen contents obtained for each electrode at each polarity are shown in Fig. 14.
It is observed that the lowest diffusible hydrogen value was found for the electrode agglomerated
U
with polymer in the DCEP polarity (30.5ml/100g of deposited metal), which presented the highest amount
N
of oxygen. The highest value was found for the conventional electrode in the DCEP polarity (45 ml/100g
A
of deposited metal), which was the one with the lowest oxygen values. The electrode agglomerated with
M
polymer in the DCEN polarity and the conventional electrode in the DCEN polarity obtained intermediate and very close values. Pope and Liu (1996) observed a decrease in the values of diffusible hydrogen with
ED
the increase of the oxygen contents in the weld metal for the rutile electrodes and oxidizing electrodes. In spite of the difference in depth of the test of diffusible hydrogen (0.5 meters) and oxygen (10 meters) in this work, it is possible to note the inverse relationship between hydrogen and oxygen for extreme cases,
PT
similar to that observed by the last authors, but cannot report this relationship for intermediate cases.
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Perhaps this fact may be more evident if the tests were performed at the same depth. Medeiros and Liu (1998) studied the effect of polarity on the diffusible hydrogen on oxidizing electrodes and reported that welds performed in the DCEN polarity present smaller amounts of diffusible hydrogen, similar to the results found for the conventional electrode but contrary with the results found for the electrodes agglomerated
A
with polymers.
Matsushita and Liu (2000) argue that a method to reduce diffusible hydrogen would be the addition
of fluoride like ingredients in the covering to reduce the partial pressure of hydrogen in the arc and form products that are insoluble in liquid iron, such as, for example, hydrogen fluoride (HF). The fact that the polymer has fluorine may have played an important role in reducing the diffusible hydrogen content of the
12
polymer-bonded electrode in the DCEP polarity. In addition to Matsushita and Liu (2000), other authors such as Kil et al. (2017) and Bang et al. (2010) also observed the decrease of diffusible hydrogen contents with the addition of fluorides in the tubular wire flux. Santos et al. (2012) carried out tests of diffusible hydrogen for different electrodes, oxidizing, rutile and oxyrutile at a depth of 0.5 meters. Rutile electrodes extrapolated average values of 85 ml/100 g and for oxidizing and oxy-rutile electrodes the highest average value was around 35 ml / 100 g, obtaining
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minimum values of up to 13 ml / 100 g. It is observed that the basic electrodes have diffusible hydrogen
contents far below than the rutile electrodes and close to some oxy-rutile electrodes and just above the
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oxidizing electrodes.
In Fig. 15, the porosity percentage values can be observed in relation to the total transversal area of the weld bead. It is observed that for the electrode agglomerated with polymer there were no differences
U
in the mean values of porosity for both polarities, but the same ones in the DCEN polarity presented higher
N
values of deviation, caused by the greater variations of porosity along the weld. Conventional electrodes in
A
the DCEP polarity showed levels of porosity a little above the electrodes agglomerated with polymer, while
M
the same ones in the DCEN polarity presented average levels twice as great as the electrodes agglomerated with polymers. The high standard deviation of the conventional electrodes in the DCEN polarity is due to
ED
the low presence of porosity at the beginning of the weld bead and an increase in the middle and end of the bead.
PT
Pessoa (2003) reported that in welds of conventional E7018 electrodes, also performed in A36 steel, higher levels of porosity were found in the DCEP polarity in the depth of 50 meters and higher levels
CC E
in the DCEN polarity in depth of 100 meters, so it is not possible to conclude with a model in which it states that such polarity will present higher or lower levels of porosity for the conventional E7018 electrodes for underwater wet welding.
A
Vaz and Bracarense (2016) reported that electrodes agglomerated with polymer in air welding had
better gaseous protections and the transferred droplets were smaller and with higher densities due to the tendency of smaller droplets to absorb smaller amounts of gases and, consequently, lower levels of spaces internally. This may be one of the factors that contributed for the lower levels of porosity in the weld metal of the electrode agglomerated with polymer, as mentioned previously. According to Pessoa (2007), the porosity of the weld metal has a direct relation with the quantities of gases absorbed in the droplets being
13
transferred and that the absorption of gases and the porosity can be decreased with better gaseous protections generated by the electrodes. Thus, the tips of the electrodes were collected and evaluated in relation to the porosity levels of the droplets formed in them. Fig. 16 shows droplets formed at the tips of the electrodes for the conventional electrodes and for electrodes agglomerated with polymer in both polarities. The conventional electrode for both polarities had higher porosities in the droplets than the electrode agglomerated with polymer. This is in agreement with Vaz and Bracarense (2016) who also found
be related to the smaller porosities in the weld metal according to Pessoa (2007). Conclusions The results obtained in this work allow to conclude that:
The electrode agglomerated with polymer presented higher arc voltage values in comparison to the
U
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4.
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lower porosity indices in the electrode agglomerated with polymer droplets in air welding which may also
conventional electrode with the same polarity. The conventional electrode in the DCEP polarity
The conventional electrode presented higher penetration values in the DCEN polarity. For the electrode
A
N
presented lower values of C.V., theoretically, being the most stable.
M
agglomerated with polymer, the highest penetration value was found in the DCEP polarity. There was a tendency for larger areas of reinforcement and lower energies generated to form one cubic millimeter
ED
of the weld bead for higher frequencies of short circuits. It was observed an approximately direct relationship between penetration and oxygen content, allowing to assume that the Marangoni effect
PT
was the phenomenon responsible for the increases in penetration. Another mechanism, that increase plasma jet strength, may have contributed to the higher penetrations of the electrodes agglomerated
CC E
with polymer.
The electrode agglomerated with polymer in the DCEP polarity presented the lowest values of diffusible hydrogen. Although diffusible hydrogen and oxygen tests were performed at different
A
depths, an inverse relationship could be observed for the extremes.
The electrode agglomerated with polymer presented lower levels of weld metal porosity in comparison with the conventional electrode. Higher gas uptakes are observed in the forming droplets at the tips of conventional electrodes, which may be indicative of a poor gas protection compared to the electrode agglomerated with polymer.
14
Acknowledgments The authors would like to acknowledge the Laboratório de Robótica Soldagem e Simulação (LRSS) for the availability of resources for the accomplishment of this work and Prof. Dr. Cláudio Turani Vaz and
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A
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Prof. Dr. Paulo José Modenesi for the academic and discursive help.
15
References Alhblom, B., 1984. Oxygen and Its Role in Determining Weld Metal Microstructure and Toughness: A State-of-the-art Review. International Inst. of Welding, pp. 19.
wire, Met. Mater. Int., 16 (3), 489-494. https://doi.org/10.1007/s12540-010-0622-6
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Bang, K. S., Jung, H. C., Han, I. W., 2010. Comparison of the effects of fluorides in rutile-type flux cored
Gooch, T.G., 1983. Properties of underwater welds. Part 1: Procedural trials. Metal Construction. 3, 164-
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167.
Hazlett, T. H., 1957. Coating ingredients’ influence on surface tension, arc stability and bead shape.
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Welding Research Supplement, 36 (1), 18-23.
Kil, W., Shin, M. J., Ban, K.S., 2017. Effects of Fluoride in the Flux on Hydrogen Content in Weld Metal
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and Operating Behavior in FCAW-S. Journal of Welding and Joining, 35 (5), 65-70.
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https://doi.org/10.5781/JWJ.2017.35.5.9
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Ko, S. H., Choi, S. K., Yoo, C. D., 2001 . Effects of Surface Depression on Pool Convection and
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Geometry in Stationary GTAW. Welding Journal, 80, 39s-45s. Lancaster, J. F., (1986). The Physics of Welding. Pergamon Press & International Institute of Welding,
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pp. 340.
Matsushita, M., Liu, S., 2000. Hydrogen Control in Steel Weld Metal by Means of Fluoride Additions in
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Welding Flux. Welding Journal, 79 (10), 295-303. Medeiros, R. C., Liu, S., 1998. A Predictive Electrochemical Model for Weld Metal Hydrogen Pickup in Underwater Wet Welds. OMAE 1998: Proceedings of the 17th International Conference on Offshores
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Mechanics and Arctic Engineering. Lisbon, Portugal. https://doi.org/10.1115/1.2829547 Mills, K. C., Keene, B. J., 1990. Factors affecting variable weld penetration. International Materials Reviwes, 35 (4), 185-216. https://doi.org/10.1179/095066090790323966 Modenesi, P. J., 2013. A química da formação do cordão na soldagem TIG. Soldagem & Inspeção, 18 (3), 287-300. http://dx.doi.org/10.1590/S0104-92242013000300011
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Pessoa, E. C. P., 2003. Estudo Comparativo do Desempenho de Eletrodos Revestidos Comerciais E6013, E7018 e E7024 em Soldagem Subaquática Molhada. MSc dissertation. Federal University of Minas Gerais. Brazil (in Portuguese). Pessoa, E. C. P., 2007. Estudo da Variação da Porosidade ao Longo do Cordão em Soldas Subaquáticas Molhadas. PhD dissertation. Federal University of Minas Gerais. Brazil (in Portuguese).
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Pessoa, E. C.P., Bracarense, A. Q., Liu, S., Perez, F., 2003. Estudo Comparativo do Desempenho de Eletrodos Revestidos E6013, E7024 e E7018 em Soldagem Subaquática em Água Doce do Aço A36 à profundidade de 50 e 100 Metros. 2º COBEF – Congresso Brasileiro de Engenharia de Fabricação.
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Uberlândia – MG. Brazil.
Pope, A. M., Liu, S., 1996. Hydrogen content of underwater wet welds deposited by rutile and oxidizing
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electrodes. Proc. Conf. OMAE 96, American Society of Mechanical Engineers. 3, 85-92.
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Rokhlin, S. I., Guu, A. C., 1993. A Study of Arc Force, Pool Depression, and Weld Penetration During
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Gas Tungsten Arc Welding. Welding Journal. 72 (8), 381s-390s.
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Santos, V.R., Monteiro, M.J., Rizzo, F.C., Bracarense, A.Q., Pessoa, E.C.P., Marinho, R.R., Vieira, L.A., 2012. Development of an Oxyrutile Electrode For Wet Welding. Welding Journal. 91, 319-328.
Bulletin, 224, 1-37.
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Tsai, C. L., Masubuchi, K., 1977. Interpretive Report on Underwater Welding. Welding Research Council
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Vasil'chenko, V V., 1987. Effects of taper angle of tungsten electrodes and additions of different fluorides on arc stability in the argon TIG welding of copper. Welding International. 39 (10), 28-31.
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https://doi.org/10.1080/09507118709453009 Vaz, C. T., Bracarense, A. Q., 2015. The Effect of the Use of PTFE as a Covered-Electrode Binder on
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Metal Transfer. Soldagem & Inspeção. 20 (2), 160-170. http://dx.doi.org/10.1590/0104-9224/SI2002.04 Vaz, C. T., Bracarense, A. Q., 2016. The influence of PTFE used as basic covered electrode binder on weld metal acicular ferrite formation. Welding International. 30 (5), 359-371. https://doi.org/10.1080/09507116.2015.1096507
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Vaz, C. T., Bracarense, A. Q., Felizardo, I., Pessoa, E. C. P., 2012. Impermeable Low Hydrogen Covered Electrodes: Weld Metal, Slag, and Fumes Evaluation. Journal of Materials Research and Technology. 1 (2), 64-70. https://doi.org/10.1016/S2238-7854(12)70012-1 West, T.C., Mitchell, G., Lindberg, E., 1990. Wet Welding electrode evaluation for ship repair. Welding
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Journal. 69, 47-56.
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A
Fig. 1. Electrodes agglomerated with polymer and conventional electrodes.
19
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A
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PT
ED
M
A
N
U
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Fig. 2. Equipment for simulation of underwater wet welding. (a) Hyperbaric tank and (b) gravity welding device.
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IP T SC R U
A
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PT
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M
A
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Fig. 3. Schematic representation of cut planes and sample withdrawal regions.
21
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Fig. 4. (a) Macrograph of the weld bead without chemical attack. (b) Binarization of the macrograph
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PT
ED
M
A
N
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making the pore region dark areas.
22
IP T
A
CC E
PT
ED
M
A
N
U
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Fig. 5. Schematic weld bead cross-section showing the measurements made to evaluate the morphology.
23
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Fig. 6. (a) Schematic drawing representing the cut plane for sample extraction. (b) Samples removed from
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PT
ED
M
A
N
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the reinforcement of the weld beads for oxygen testing.
24
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
45
35 30 25 20 15 10 100
110
120
130
140
150
Mean Welding Current (A)
160
170
180
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90
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Mean Arc Voltage (V)
40
Fig. 7. Mean arc voltage per mean welding current chart for conventional electrode and electrode
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PT
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M
A
N
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agglomerated with polymer in DCEP and DCEN polarity.
25
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
4.5 4 3.5 3 2.5 100
110
120
130
140
150
Mean Welding Current (A)
160
170
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90
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Welding speed (mm/s)
5
180
Fig. 8. Welding speed per mean welding current chart for conventional electrode and electrode
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PT
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M
A
N
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agglomerated with polymer in DCEP and DCEN polarity.
26
Polymer DCEN
Polymer DCEP
Conventional DCEN
Conventional DCEP
22 20
16 14 12 10 8 6 100
110
120
130
140
150
Mean Welding Current (A)
160
170
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90
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C.V. (%)
18
180
Fig. 9. Coefficient of variation (C.V.) per mean welding current chart for conventional electrode and
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PT
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M
A
N
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electrode agglomerated with polymer in DCEP and DCEN polarity.
27
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Fig. 10. Macrographs of weld bead produced with electrodes: (a) agglomerate with polymer DCEN
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polarity; (b) agglomerated with polymer DCEP polarity; (c) conventional DCEN polarity; (d)
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PT
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conventional DCEP polarity
28
Penetration 11
reinforcement
10.1
9.9
9.9
10
width
8.9
9 7 6 5 4
2.8
3
3.3 2.5
2.2
2.4
2.7 1.9
1.4
2
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1 0
Pol. DCEN
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Millimeters
8
Pol. DCEP
Conv. DCEN
Conv. DCEP
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Fig. 11. Mean values of penetration, reinforcement and width for each electrode (Pol. = With polymer
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PT
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and conv. = Conventional), at each polarity, for the mean welding current approximately 150 A.
29
700 576.5 502.3 432.8
500
368.0
400 300 200 100 0
Pol. DCEP
Conv. DCEN
Conv. DCEP
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Pol. DCEN
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Oxygen concentration (ppm)
600
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N
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Fig. 12. Oxygen concentration in welds for both electrodes in both polarities.
30
550
Pol. DCEP
Pol. DCEN 500 450 400
Conv. DCEP
Conv. DCEN
350 300 2
2.5
3
Penetration (mm)
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1.5
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Oxygen concentration (ppm)
600
3.5
A
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M
A
N
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Fig. 13. Linear relationship between penetration and concentration of oxygen in the weld metal.
31
60
45.0 36.5
40
35.0 30.5
30 20 10 0
Pol. DCEP
Conv. DCEN
Conv. DCEP
SC R
Pol. DCEN
IP T
Hdf (ml/100g)
50
A
CC E
PT
ED
M
A
N
0.5 meters deep.
U
Fig. 14. Diffusible hydrogen contents in the weld metal for both electrodes, in both polarities, performed
32
6
5
Porosity (%)
4
3.4
3 1.9
1.7
1.7
IP T
2
1
Pol. DCEN
SC R
0
Pol. DCEP
Conv. DCEN
Conv. DCEP
A
CC E
PT
ED
M
A
N
U
Fig. 15. Percentage of porosity in relation to the total cross-section weld bead area for each condition.
33
IP T SC R U N A M ED
Fig. 16. Droplets at the tips of the electrodes, being: (a) conventional electrode DCEP polarity; (b) conventional electrode DCEN polarity; (c) electrode agglomerated with polymer DCEP polarity; (d)
A
CC E
PT
electrode agglomerated with polymer DCEN polarity.
34
Table Captions Table 1 Values of heat input, area of penetration, total area and dilution for each electrode, in each polarity, realized with the mean welding current approximately 150 A. Pol. DCEN Mean
Deviation
Mean
Conv. DCEN
Deviation
Mean
Deviation
Conv. DCEP Mean
Deviation
1088.4
360.0
1237.1
295.7
977.2
304.6
15.4
0.8
18.3
1.9
12.7
0.9
28.7
2.7
26.7
2.2
26.7
1.2
25.4
1.2
54.1
5.3
68.4
1.2
47.5
2.1
46.0
0.6
(J/mm) Penetration
Total area
A
CC E
PT
ED
M
A
N
U
(mm²) Dilution (%)
35
11.7
SC R
area (mm²)
1023.0
236.2
IP T
Heat input
Pol. DCEP
0.8
Table 2 Hf values, area of reinforcement and short-circuit frequency for each electrode, at each polarity, carried out with a mean welding current approximately 150 A. (arrows pointing up indicate higher values for calculated indices and arrows pointing down indicate the lowest values). Pol. DCEP
Mean
Deviation
Mean
Deviation
Hf (J/mm³)
38.0
16.1
46.4
14.8
Reinforcement
13.3
2.6
8.4
3.8
2.1
0.6
Conv. DCEN Mean
Deviation
Mean
Deviation
36.6
13.1
40.3
11.3
1.0
14.0
1.1
0.1
6.4
0.1
13.7
1.1
3.2
0.1
SC R
area (mm²) Short-circuit
Conv. DCEP
IP T
Pol. DCEN
A
CC E
PT
ED
M
A
N
U
frequency (Hz)
36