Underwater Welding Parameters- A Review

Transcription

1 Underwater Welding Parameters- A Review Ravi Kumar Verma, M.I.Khan Research Scholar, Department of Mechanical Engineering, Integral University, Lucknow, India Professor, Department of Mechanical Engineering, Integral University, Lucknow, India Abstract Welding is an important manufacturing process in fabrication industry. Underwater welding finds its special applications in fabrication of subsea structure like oil and gas pipe lines and repair of ships, submarines and defense structures. The present paper deals with the present status and future trends in underwater fabrications. The text discussed includes underwater arc atmosphere, effect of depth on thermal conductivity of shielding gases (argoned helium), shape characteristics of weld bead cross-section (as affected by welding parameters), microstructure in weld metal, heat affected zones. Index Terms underwater welding, wet welding, underwater welding parameters, Arc atmosphere, weld shape factor. I. INTRODUCTION Underwater welding is similar as arc welding in air except that there is (i) cooling action of water, (ii) a high pressure due to the water head above the welding arc,(iii) a gaseous envelope around arc. Combustion of electrode coating materials and dissociation of water makes a bubble around the arc which contains 93% hydrogen. As welding continues bubble travels to the water surface one after another. [1] Underwater arc welding is performed in either the wet environment or in a protective habitat [2].Although both environments experience increased pressure with depth (0.1 MPa, or 1 bar, for each 10 m, increase in depth), the wet environment also significantly alters the cooling rate during welding, which affects the nature of the weld-metal phase transformation. [3, 4, 5] Now a days an efficient and reliable underwater joining process is necessary for engineering applications. Since welding in dry atmosphere is the best solutions but it can be used for regular shapes the setup used is very expensive it is therefore desirable to study the welding in the wet atmosphere for the purpose of underwater repairs and fabrications. Some experiments have been conducted and reported in the literature. Studies have also been reported on the effect of important weld parameters like arc voltage, arc current, water proof coating, type of flux, electrode size and weld polarity on the properties of underwater welds. Offshore development has accelerated in recent years owing to the fact that more than 50% of undeveloped petroleum and minerals deposits are located under the ocean. In the offshore industry and in underwater oil and gas pipelines fabrication underwater welding is already a routine activity [1, 2] II. UNDERWATER WELDING TECHNIQUES The American Welding Society (AWS) D 3.6, Specification for Underwater Welding classifies underwater welding currently in use into five categories Dry Welding at One Atmosphere Welding is carried out in a pressure vessel at approximately one atmosphere pressure and it is independent of depth. Dry Welding in a Habitat Welding is carried out at ambient pressure in a habitat from which water is displaced with gas such that the welder/diver works without diving equipment. Dry Chamber Welding Welding is performed at ambient pressure in a dry habitat with an open bottom that allows only the head and shoulders of the welder/diver in full diving equipment Dry Spot Welding Welding in a small transparent enclosure that is filled with gas at ambient pressure and the welder/diver is outside in the water Wet Welding Welding at ambient pressure with the diver/welder in the water and the arc is exposed in wet environment. Labanowski et al [6] presented a detailed review of underwater welding processes and gave a schematic classification of the different categories as shown in Figure1. Figure 1: Schematic classification of underwater welding [6] 17 P a g e

2 III. DEVELOPMENTS IN UNDERWATER WELDING A. Arc Atmosphere Zhu Jialei (2) in 2014 studied the characteristic of underwater welding droplet transfer and arc stability of wet welding process. To achieve the droplet transfer in special environment with relatively low current, a set of experimental systems of laser enhanced droplet transfer under atmospheric pressure has been established which can meet the need of the experimental research. From high-speed images of the droplet transfer and the processing results of the droplet edge detection, it can be seen that laser with a certain power density can obviously improve the statue of the droplet transfer in GMAW welding, and the size of droplet can also be controlled by laser. B. Effect of Increased Pressure on Underwater Arc When pressure increases it increases the density of gas in the chamber due to this cooling of molten metal becomes faster. Thermal conductivity of gases increases with increase in density. Amin et al [12] have shown in Figure 2 and Figure 3 the dependency of the thermal conductivity of argon and helium, commonly used as chamber gases in hyperbaric welding on pressure and temperature. Amin reported that the thermal conductivity of argon increases with both ambient temperature and pressure. He also reported that helium is less pressure sensitive than argon. [12] Figure 3: Thermal conductivity of helium as a function of pressure and temperature C. Cooling Rate The cooling rate in underwater wet welding was much higher compared to air welding. For instance, the cooling rate in the temperature range from 800 to 500 C could rise sharply to 100 C/s, which was more than the critical cooling rate for the formation of martensite. [8] Jingqing Zhang [9] in 2014 performed friction stir lap welding of 6061 T-6 aluminum alloy and concluded that external water could decrease the peak temperature and shorten the hot working time. Joshua E. Omajene [10] in (2014) compared air welds with underwater welds and observed that weld bead size is similar but HAZ in underwater welding is reduced by 30-50%. Figure 2: Thermal conductivity of argon as a function of pressure and temperature D. Weld Shape Characteristic Experimental evidence indicates that weld bead geometry is influenced by the change of the contact tube-to-work distance (CTWD) (Fig. 2) during welding. The CTWD influences the formation of the weld pool and the resulting weld shape by changing the arc length and welding current. the arc length which is related to the welding voltage and CTWD are closely related to the weld bead geometry. The convection in the weld pool affects the weld pool geometry. An increase in arc length due to an increase in CTWD increases the bead width because of the widened arc area at the surface of the weld. Increase in arc length reduces the reinforcement height because the volume of filler metal is used remains the same.[10] 18 P a g e

3 affected zone (HAZ). The detrimental effects of hydrogen on steel are well established and are one of the major issues affecting the structural integrity of offshore installations. [13] E. Heat Transfer Fig. 4. Contact tube-to-work distance [10]. Heat transfer equations have been developed for modelling underwater welding. Finite element method has been used to develop the equations. [11] F. Weld Microstructure Three types of ferrite are associated with low-carbon steel weld metal: grain-boundary ferrite (GBF), Widmanstätten, or sideplate, ferrite (SPF), and acicular ferrite (AF). Prior austenite grain boundaries can promote the formation of GBF and represent the sites for the nucleation of SPF, which protrudes from the grain boundaries into the prior austenite grains. Acicular ferrite is intra granular and has a much finer basket-weave structure. Other micro constituents, such as pearlite, cementite, and martensite, can also form. At faster cooling rates, the formation of bainite or ferrite with aligned carbide (AC) and martensite is possible. It is recognized that acicular ferrite is the microstructural constituent that provides a high resistance to cleavage fracture. [14] Figure 6: Hydrogen assisted crack in a sleeve repair fillet weld H. Effect of induction heating Induction heating during underwater welding was investigated by H.T.Zhang in In this procedure they provided induction heating to the water surrounded by the welds due to which cooling rate of the welded joints in water atmosphere were reduced, thus improved the microstructural and mechanical properties of the joint.[8] IV. UNDERWATER WELDING PARAMETERS In order to improve the quality of weld it is necessary to study in detail the various underwater welding parameters. Welding parameters which affect the quality of the weld are as follows: Figure 6: welding parameters of underwater welding Figure 5: Percentage of weld-metal microstructural constituents for wet underwater welds as a function of water depth G. Hydrogen and its Effects on Underwater Welds Hydrogen assisted cracking shown in Figure 5 typically occurs at stress concentration points such as the root or toe of the weld and can propagate through the weld metal to the heat A. Characteristic of the Arc A welding arc can be defined as electric discharge between two electrodes and a heated and ionized gas, called plasma [15]. Therefore, the arc stability could be affected as a result of the magnetic field of the induction heating and eddy current. Welding arc struck inside the bubble create due to the combustion of electrode and the water when pressure increases 19 P a g e

4 inside the bubble it forces to leave the arc and meet the surrounded water suddenly another bubble form and takes its place. When electrode is too far then weld metal is unable to transfer the metal in proper way and when electrode is too close bubble will collapse around the weld. B. Arc Current Welding current controls the deposition rate because it controls the melting rate of electrode. Low welding current results in less heat generation which increases the chances of poor penetration. Also too high welding current may lead to undercut in the weld joint. Increasing the welding current in general increases the depth of penetration. Changing the polarity from direct current electrode positive (DCEP) to direct current electrode negative (DCEN) affects the amount of heat generated at the electrode and work piece which affects the metal deposition rate, weld bead geometry and mechanical properties of the weld metal. The positive electrode generates two third of the total heat, while the negative electrode generates one third of the total heat. The DCEN polarity produces higher deposition rate and reinforcement than the DCEP polarity in submerged arc welding. The level of hydrogen absorption in underwater wet welding which results in porosity can be minimized by using low current DCEP or a high current DCEN [16, 17]. C. Arc Voltage Low arc voltage results produces unstable arc which in turn results in poor weld bead geometry. Also high arc voltage causes increased arc gap and wide bead and shallow penetration. An increase in the arc length increases the voltage. The voltage determines the shape of the weld bead cross section and external appearance. Increasing the voltage at a constant current will result in a flatter, wider, and reduced penetration, which also leads to reduced porosity caused by rust on steels. Increase in arc voltage results to an increase in the size of droplets and thereby reduce the number of droplets. Increase in voltage enhances flux consumption, but a further increase in voltage will increase the possibility of breaking the arc and hinder normal welding process. Increase in arc voltage beyond the optimum value leads to an increase in loss of alloying elements which affects the metallurgical and mechanical properties of the weld metal. Arc voltage beyond the optimum value produces a wide bead shape that is susceptible to cracking, increase undercut and difficulty of slag removal. A decrease in the welding voltage will decrease the diffusible hydrogen content during underwater welding. [18]. less filler metal is applied per unit length of the weld, which will result in less weld reinforcement and a smaller weld bead. The welding speed is the most influencing factor on weld penetration than other parameters except welding current. Excessive welding speed may result in porosity, undercutting, arc blow, uneven bead shape, cracking and increased slag inclusion in the weld metal. Higher welding speed results in less HAZ and finer grains. A relatively slow welding speed gives room for gases to escape from the molten metal, thereby reducing porosity. The bead width at any current is inversely proportional to the welding speed. A fast welding speed with low DCEN or high DCEP can minimize the level of porosity in wet welding [18] E. Water environment & its Effect In underwater conditions, static pressure caused by depth, cooling effect of water, moving the electrode and high hydrogen level, cause a rising voltage/ampere characteristic for underwater arcs. In water atmosphere due to fast cooling there is not enough time for slag to come to the top of the molten weld metal. These entrapped slags cause non uniform solidification and high residual stresses which cause problems. The mobility of the molten bead is restricted by the environment and temperature. The weld bead usually appears to be convex. The decomposed hydrogen near the arc during underwater welding may become entrapped and cause porosity. This decreases the impact toughness and leads to hydrogen induced cracking. F. Water Temperature The presence of water and the type of water surrounding the weld metal affects the welding process and the resulting temperatures. This effect is due to the greater convective heat transfer coefficient of water as compared to air which results in the rapid cooling in underwater welding [19]. At higher water temperature, for higher oxygen content, the diffusible hydrogen contents are likely to be less thereby, lowering the tendency of hydrogen assisted cracking. When welding at lower water temperature, there are few inclusions resulting in higher diffusible hydrogen level in the coarse grain heat affected zone which leads to greater cracking tendency. G. Waterproof Coating When commercially produced waterproofed wet welding electrodes are not available, surface welding electrodes can be used if waterproofed. The electrodes listed in Table below are suitable for on-site waterproofing. D. Weld Speed The linear rate at which the arc is moved along the weld joint influences the heat input per unit length of the weld. An increase in the welding speed will decrease the heat input and 20 P a g e

5 S.No 1 2 Size of Electrode 1/8, 5/32 & 3/16 1/8, 5/32 & 3/16 3 1/8 & 5/32 Series Material Source E6013,E7014, E7016 & E7018 E309,E310 & E312 ENiCrFe-2,ENiCrFe-3 & ENiCrMo-3 Carbon steel Stainless steel High nickel American Welding Society, AWS A5.1 American Welding Society, AWS A5.1 American Welding Society, AWS A5.1 Table 1: List of electrodes can be used in underwater welding H. Bead Width The bead width of a weld is the maximum width of weld metal deposited. This influences the flux consumption rate. The bead width is in direct proportion to the arc current, welding voltage and diameter of the electrode. It is inversely proportional to the welding speed [20, 21]. I. Penetration Penetration is the maximum distance between the top of the base plate and depth of fusion. Penetration is influenced by welding current, welding speed, polarity, electrode stick out, basicity index and physical properties of the flux. Penetration is directly proportional to welding current and inversely proportional to welding speed and diameter of electrode. Increase in thermal conductivity of the weld metal decreases penetration. Deepest penetration is achieved with DCEP polarity than DCEN polarity. A stable arc increases penetration because the arc wanders is minimized which allows more efficient heat transfer [21]. J. Rreinforcement The reinforcement influences the strength of the weld and wire feed rate. Increasing the wire rate increases the reinforcement irrespective of welding current and polarity. Reinforcement is inversely proportional to welding voltage, welding speed and diameter of electrode. A bigger reinforcement is achieved with DCEN polarity than with DCEP polarity [21]. V. CONCLUSION The major problems in underwater welding are the fast cooling rate of the base metal by the surrounding water. This leads to the formation of microstructures that are susceptible to cracking. The effect of water depth and the cooling rate of the weld metal from the water temperature influence the effect of arc bubble generation, welding current, welding speed, and voltage on the weld bead geometry during underwater welding. It is evident that a proper adjustment of welding process parameter to minimize hydrogen pick up and alter the influence of the water environment can yield a sound weld in underwater welding as compared to air welding. So by selecting a proper welding parameters mechanical strength of the parent material and weld bead microstructure can be improved. REFERENCES [1] M.I.Khan,Shankar lal State of art in underwater welding IE(1) Journal ME vol-56,pt ME 5 May [2] C.l. Tsai and k. Masubuchi, interpretive report on underwater welding, weld. Res. Counc. Bull., vol 224, 1977, p 1-37 [3] N. Christensen, the metallurgy of underwater welding, underwater welding, pergamon press, 1983, p [4] A. Hausi and y. Suga, on cooling of underwater welds, trans. Jpn. Weld. Soc.,vol 2 (no. 1), 1980 [5] C.l. Tsai and k. Masubuchi, mechanisms of rapid cooling in underwater welding, appl. Ocean res., vol 1 (no. 2), 1979, p [6] Labanowski, D., Fydrych, D. and Rogalski, G. (2008), "Underwater Welding - Review", Advances in Materials Science, vol. 8, no. 3, pp [7] S. Pak, Y. Kim, K. Y. Park, K. D. Lee, M. S. Cheon, and H. G.Lee, Laser welding and ablation cutting process for hydraulic connections by remote handling in the ITER diagnostic port plug, Fusion Engineering and Design, vol. 85, no. 2, pp ,2010. [8] H. T. ZHANG, X. Y. DAI, J. C. FENG, AND L. L. HU,(2015), Preliminary Investigation on Real Time Induction Heating Assisted Underwater Wet Welding, Welding Journal, Vol.94, pp [9] Jingqing Zhang, Yifu Shen, Xin Yao, Haisheng Xu, Bo Li,(2014), Investigation on dissimilar underwater friction stir lap welding of 6061-T6 aluminum alloy to pure copper, Materials and Design, Vol.64, pp [10] Joshua E. Omajene, Jukka Martikainen, Paul Kah, Markku Pirinen( 2014), Fundamental Difficulties Associated With Underwater Wet Welding, Int. Journal of Engineering Research and Applications, Vol.4, Issue.6,pp [11] Yasar Vehbi ISIKLAR, Ibrahim GIRGIN (2011) Numerical Modelling of Underwater Welding Journal of Naval Science and Engineering Vol.7, No.2, pp [12] Amin, S. A. (2012), Dry Hyperbaric Gas Metal Arc Welding of Subsea Piplines (PhD thesis), Norwegian University of Science and Technology, Trondhein, Norway. [13] Fostervoll, H. and Akselsen, O. M. (2004), Remote PRS: Hydrogen Pick-up at 1 bar MIG Welding, STF24 F04240, SINTEF, Norway. [14] ASM Welding handbook (1993) vol.6, pp [15] Welding Handbook. 8th edition Ed. R. L. O'Brien. Miami, Fla.: American Welding Society [16] B. K. Srivastava, S. P. Tewari & J. Prakash, "A review on effect of arc welding parameters on mechanical behaviour 21 P a g e

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