WELDING PROCESSES IN MARINE APPLICATIONS: This Research article is aimed at highlighting the importance of Welding Processes in marine applications. ... article represents a review

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Int. J. Mech. Eng. & Rob. Res. 2013 A Anand and A Khajuria, 2013

WELDING PROCESSES IN MARINEAPPLICATIONS: A REVIEW

A Anand1* and A Khajuria1

*Corresponding Author: A Anand,[email protected]

This Research article is aimed at highlighting the importance of Welding Processes in marineapplications. The recent developments in the manufacturing world have led to a revolutionarychange in the design and development of various systems. Development in weldingtechnology is one of such changes. Welding processes have been used extensively as ajoining technique. This research article represents a review of underwater welding (UWW)technique which has found a significant role in the joining industry. The paper deals withreview of various underwater welding (UWW) techniques from marine applications point ofview. The paper was structured by taking into consideration the aspects, classification and,importance of UWW process. The paper also describes the characteristics, applicationsand the risk mitigation factors of underwater welding (UWW) techniques from marineapplications point of view.

Keywords: Underwater welding, Marine applications, Processes

INTRODUCTIONThe recent developments in the manufacturingworld have led to a revolutionary change in thedesign and development of various systems.Developments in welding technology is one ofsuch changes. Welding processes have beenused extensively as a joining technique, usedin design and fabrication of various structureslike naval ships, airplanes, automobiles,bridges, pressure vessels, etc. Welding hasemerged as a better option in contrast to other

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Int. J. Mech. Eng. & Rob. Res. 2013

1 School of Mechnaical Engineering, Shri Mata Vaishno Devi University (SMVDU), Katra, J&K, India.

joining techniques in terms of joint efficiency,mechanical properties with a greaterapplication impact.

In some intricate situations, the Robotbased welding processes have replacedhuman welders. During the last few years, theautomation of welding process for pipestructures have gained significant momentumwith an objective to improve the productivityand accuracy in the areas involving marineapplications, etc. Various research studies in

Research Paper

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the welding environment have shown thatproductivity improvement is a major thrustarea of welding industry. The welders intoday’s world are under tremendous pressureto meet two major challenges. These are: thehigher weld quality and; reducedmanufacturing cost.

Another emerging area in marineapplication welding systems is the underwaterwelding technique. Underwater Welding (UW)has been significantly used for over fivedecades. However, its use has not reached toa significant level in a welding environment dueto number of factors. The underwater weldingprocess came into existence with thedevelopment of water-proof electrodes in1940’s (Keats, 2005). It is the process ofwelding at elevated pressures, normallyunderwater. It may be carried out in the wateritself or dry inside a specially constructedpositive pressure envelope, thereby providinga special environment. Under water is alsoknown as “hyperbaric welding” when used ina dry environment, and “underwater welding”when in a wet environment. The applicationareas of this welding technique include a widevariety of structures, such as repair ships,offshore oil platforms, and pipelines. Steel isthe most common material welded (Cary andHelzer, 2005). Various researchers havedefined the concept of underwater welding indifferent ways (Haddad and Farmer, 1985;Oates, 1996; Schmidt, 1996; Khanna, 2004;and Cary and Helzer, 2005).

General Aspects of UnderwaterWelding

There are various ways in which underwaterwelding technique can be performed. Inunderwater welding technique, the main

challenge is to create an environment forwelding. These are discussed in the below inthis article.

One of the main advantages of this weldingtechnique is the savings in terms of time. Forrepair and maintenance jobs in marineapplications, the equipment/vessel need notbe taken out to carry out the welding operation.The main difficulties in underwater welding arethe presence of a higher pressure due to thewater head under which welding takes place,chilling action of the water on the weld metal(which might change the metallurgicalstructures and properties), the possibility ofproducing the arc mixtures of hydrogen andoxygen in pockets, which might set up anexplosion, and the common danger sustainedby divers, of having nitrogen diffused in theblood in dangerous proportions. Furthermore,complete insulation of the welding circuit is anessential requirement of underwater welding.

Importance of UnderwaterWelding in Marine Applications

In practice, the use of underwater wet weldingfor offshore repairs has been limited mainlybecause of porosity and low toughness in theresulting welds. With appropriate consumabledesign, however, it is possible to reduceporosity and to enhance weld metal toughnessthrough microstructural refinement. Hence,welding in offshore and marine application isan important area of research and needsconsiderable attention and understandingwhere, many problems are still unsolved. In thepresent review, a brief understanding of theproblems in underwater welding will bediscussed in context to the existing weldingtechniques. Detailed description of a fewadvanced welding techniques has also been

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made. Finally, the scope of further researchwould be recommended.

CLASSIFICATION OFUNDERWATER WELDINGUnderwater welding may be divided into twomain types, wet and dry welding (Oates, 1996).

Wet Welding

This type of welding process is carried out atambient water pressure in which, there existsa relationship between the welder and thediver in the water. This is carried out by meansof a water-proof stick electrode, with nophysical barrier between water and weldingarc (Oates, 1996).

In wet welding technique, the complexstructures may also be welded (Oates, 1996;Shida et al., 1997; Khanna, 2004; andKruusing, 2004). One of the most commonlyused wet welding technique is the ShieldedMetal Arc Welding (SMAW) process and theFlux Cored Arc Welding (FCAW) process. Italso includes the self shielded flux cored arcwelding. From economics point of view, thewet welding technique with coated electrodesis considered. The cooling rate in wet weldsis much higher than in those obtained in drywelding. In the temperature range from 800 to500 °C it can change from 415 to 56 °C/s(Steen, 1991). Underwater wet welds are alsoknown to contain high amounts of porosity(Figure 2). Porosity may be formed bymolecular hydrogen, carbon monoxide or watervapour (Irie et al., 1997; Cavaliere et al., 2006;and Cavaliere et al., 2008). Pores are presentto some extent in all wet welds. The mainfactors affecting this phenomenon are (Shidaet al., 1997; Irie et al., 1997; Cavaliere et al.,

2006; and Cavaliere et al., 2008): water depth,electrode covering and arc stability.

Special precaution should be taken toproduce underwater arc to protect it fromsurrounding water. Wet welding does not needany complicated experiment set up, it’seconomical and can be immediately appliedin case of emergency and accident as it doesnot need water to be evacuated. However,difficulties in welding operation due to lack ofvisibility in water, presence of sea current,ground swells in shallow water and inferior weldqualities (increased porosities, reducedductility, greater hardness in the heat affectedzone, hydrogen pick up from the environment)are the notable disadvantages of wet weldingtechnique.

Dry Welding

Dry welding in underwater may be achievedby several ways (Oates, 1996):

Dry Habitat Welding

Welding at ambient water pressure in a largechamber from which water has beendisplaced, in an atmosphere such that thewelder/diver does not work in diving gear. Thistechnique may be addressed as dry habitatwelding.

Dry Chamber Welding

Welding at ambient water pressure in a simpleopen-bottom dry chamber that accommodatesthe head and shoulders of the welder/diver infull diving gear.

Dry Spot Welding

Welding at ambient water pressure in a smalltransparent, gas filled enclosure with thewelder/diver in the water and no more than thewelder/diver’s arm in the enclosure.

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Dry Welding at One Atmosphere

Welding at a pressure vessel in which thepressure is maintained at approximately oneatmosphere regardless of outside ambientwater pressure.

Cofferdam Welding

Welding inside of a closed bottom, open topenclosure at one atmosphere.

Underwater welding in a dry environment ismade possible by encompassing the area tobe welded with a physical barrier (weldchamber) that excludes water. The weldchamber is designed and custom built toaccommodate braces and other structuralmembers whose centerlines may intersect ator near the area that is to be welded. Thechamber is usually built of steel, but plywood,rubberized canvas, or any other suitablematerial can be used. Size and configurationof the chamber are determined by dimensionsand geometry of the area that must beencompassed and the number of welders thatwill be working in the chamber at the sametime. Dry welding requires a pressurizedenclosure having controlled atmosphere. Weldmetal is not in direct contact with water.Advantages of dry welding are improvementin stability of welding operation, reducedhydrogen problem, lower quench rate of theweld and base metal and restoration of weldstrength and ductility. Dry welding may becarried out under high pressure, whichconsists of preparing an enclosure to be filledwith gas (helium) under high pressure(hyperbaric) to push water back, and have thewelder, fitted with breathing mask and otherprotective equipment (Oates, 1996).Limitations of hyperbaric welding are the

practical difficulties in sealing the chamber andincrease in pressure as weld depth increasesleading to problem which affects both the weldchemistry and microstructures.

RISKS ASSOCIATED WITHUNDERWATER WELDINGThere is a risk to the welder/diver of electricshock. Precautions include achievingadequate electrical insulation of the weldingequipment, shutting off the electricity supplyimmediately the arc is extinguished, andlimiting the open-circuit voltage of MMA (SMA)welding sets. Secondly, hydrogen and oxygenare produced by the arc in wet welding.Precautions must be taken to avoid the build-up of pockets of gas, which are potentiallyexplosive. The other main area of risk is to thelife or health of the welder/diver from nitrogenintroduced into the blood steam duringexposure to air at increased pressure.Precautions include the provision of anemergency air or gas supply, stand-by divers,and decompression chambers to avoidnitrogen narcosis following rapid surfacingafter saturation diving. For the structures beingwelded by wet underwater welding, inspectionfollowing welding may be more difficult thanfor welds deposited in air. Assuring the integrityof such underwater welds may be moredifficult, and there is a risk that defects mayremain undetected.

CHARACTERISTICS OF AGOOD UNDERWATERWELDING• Requirement of inexpensive welding

equipment, low welding cost, easy tooperate and flexibility of operation in allpositions.

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• Minimum electrical hazards, a minimum of20 cm/min welding speed at least.

• Permit good visibility.

• Produce good quality and reliable welds.

• Operator should be capable in supportinghimself.

• Easily automated.

APPLICATIONS OFUNDERWATER WELDING• Offshore construction for tapping sea

resources.

• Temporary repair work caused by ship’scollisions or unexpected accidents.

• Salvaging vessels sunk in the sea.

• Repair and maintenance of ships.

• Construction of large ships beyond thecapacity of existing docks.

UNDERWATER WELDINGTECHNIQUESThe fusion welding processes of greatestpractical significance in underwater weldingare manual shielded metal arc welding,tungsten inert gas welding, metal inert gaswelding are used (Khanna, 2004). Theprinciples of the above mentioned weldingtechniques are summarized below:

Shielded Metal Arc Welding

Shielded Metal Arc Welding (SMAW) is amongthe most widely used welding processes.During the process, the flux covering theelectrode melts during welding. This forms thegas and slag to shield the arc and molten weldpool. Figure 1 shows the schematic ofshielded metal arc welding process. The slag

must be chipped off the weld bead afterwelding. The flux also provides a method ofadding scavengers, deoxidizers, and alloyingelements to the weld metal. For underwaterwet welding with Shielded Metal Arc Welding(SMAW) technique, direct current is used andusually polarity is straight (Khanna, 2004).Electrodes are usually water proofed.Furthermore, it is flux coated which causesgeneration of bubble during welding anddisplaces water from the welding arc and weldpool area. Hence, the flux composition anddepth of flux coating should be optimized toensure adequate protection. Electrodes forshielded metal arc welding are classified byAWS as E6013 and E7014 (Khanna, 2004).Versatility, simple experiment set-up, economyin operation and finished product quality arenotable advantages of the technique. However,during welding, all electrical leads, lightinggear, electrode holder, gloves, etc., must befully insulated and in good condition. Ferriteelectrodes with a coating based on iron oxideshould be used as they resist hydrogencracking. Flux cored arc welding is anothertechnique which could not yet competed with

Figure 1: Schematic of Shielded Metal ArcWelding Process

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SMAW because of reported excessiveporosities and problems with underwater wirefeeding system (Oates, 1996).

Flux Cored Arc Welding

Flux Cored Arc Welding (FCAW) is acommonly used high deposition rate weldingprocess that adds the benefits of flux to thewelding simplicity of MIG welding (Khanna,2004). As in MIG welding wire is continuouslyfed from a spool. Figure 2 shows theschematic of flux cored arc welding process.Flux cored welding is therefore referred to asa semiautomatic welding process. Selfshielding flux cored arc welding wires areavailable or gas shielded welding wires maybe used. Less pre-cleaning may be necessarythan MIG welding. However, the condition ofthe base metal can affect weld quality.Excessive contamination must be eliminated.Flux cored welding produces a flux that mustbe removed. Flux cored welding has goodweld appearance (smooth, uniform weldshaving good contour). Flexibility in operation,higher deposition rate, low operator skill and

good quality of the weld deposits are thenotable advantages of flux cored arc welding.However, presence of porosities and burnbackare the problems associated with the process.Recent development of nickel based flux coredfiller materials have provided improved wetweldability and halogen free flux formulationspecif ically designed for wet weldingapplication (Oates, 1996).

Similarly, improved underwater wet weldingcapabilities and halogen-free flux formulationshave been developed with stainless steel flux-cored wires.

Tungsten Inert Gas WeldingTIG-welding (Tungsten Inert Gas) or GTAW-welding (Gas Tungsten Arc Welding) uses apermanent non-melting electrode made oftungsten (Khanna, 2004). Filler metal is addedseparately, which makes the process veryflexible. It is also possible to weld without fillermaterial. TIG welding has got the advantagethat it gives a stable arc and less porous weld.Figure 3 shows the schematic of tungsten inertgas welding.

The most used power source for TIG-welding generates Alternating Current (AC).

Figure 2: Schematic of Flux Cored ArcWelding

Figure 3: Schematic of a Gas TungstenArc Welding Technique

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Direct current can be used. AC TIG-weldingusually uses argon as a shielding gas. Theprocess is a multi purpose process, whichoffers the user great flexibility. By changing thediameter of the tungsten electrode, weldingmay be performed with a wide range of heatinput at different thicknesses. AC TIG-weldingis possible with thicknesses down to about 0.5mm. For larger thicknesses, > 5 mm, AC TIG-welding is less economical compared to MIG-welding due to lower welding speed. DC TIG-welding with electrode negative is used forwelding thicknesses above 4 mm. Thenegative electrode gives a poor oxide cleaningcompared to AC-TIG and MIG, and specialcleaning of joint surfaces is necessary. Theprocess usually uses helium shielding gas.This gives a better penetration in thickersections. In deep see construction, freeburning arc is used for fusion welding. The arcis then operated in a localized dry regioncreated around the weldment at elevatedpressures. Similar ambient conditions can befound in high pressure discharge lamps andin some plasma heaters and torches. Thetungsten inert gas welding process atatmospheric pressures has been investigatedextensively from the experimental andtheoretical side (Haddad and Farmer, 1985;and Lancester, 1987). The properties of thefree-burning arc column are studied forambient pressures of 0.1 MPa (i.e.,atmospheric) to 10 MPa for applications inunderwater welding (Schmidt, 1996).

CURRENT UNDERWATERWELDING TECHNIQUES

Friction Welding (FRW)

Friction welding is a solid state weldingprocess which produces coalescence of

materials by the heat obtained frommechanically-induced sliding motion betweenrubbing surfaces (Khanna, 2004). The workparts are held together under pressure. Thisprocess usually involves rotating of one partagainst another to generate frictional heat atthe junction. When a suitable high temperaturehas been reached, rotational motion ceasesand additional pressure is applied andcoalescence occurs. Figure 4 shows theschematic of friction welding process.

Figure 4: Schematic of Friction WeldingProcess

The start of the new millennium will see theintroduction of friction welding for underwaterrepair of cracks to marine structures andpipelines. There are two variations of thefriction welding process. In the original processone part is held stationary and the other partis rotated by a motor which maintains anessentially constant rotational speed. The twoparts are brought in contact under pressurefor a specified period of time with a specificpressure. Rotating power is disengaged fromthe rotating piece and the pressure isincreased. When the rotating piece stops theweld is completed. This process can beaccurately controlled when speed, pressure,

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and time are closely regulated. Cavaliereet al. (2006) investigated the effect of processparameters on fatigue behavior of AA6056joints produced by friction stir welding.Cavaliere et al. (2008) carried out a researchstudy on the effect of friction stir weldingparameters to analyse the fatigue propertiesof AA6082 joints. Pessoa et al. (2006) carriedout a study of Porosity variation for weldingspecimens. In this research study, theresearchers have focussed on the multipassunderwater wet welds and its influence onmechanical properties. There is a loss ofductility in the welded specimen and the HeatAffected Zone (HAZ). There is large amountof porosity in the underwater wet welds (Roweet al., 2002). Labanowski and Fydrych carriedout a research study on some aspects ofunderwater welding processes. In thisresearch study, the authors have observed thatamount of hydrogen in weld metal is in therange from 5 to 21 ml/100 g Fe and dependson welding parameters, especially flow rate ofshielding gas (Labanowski and Fydrych,2008).

Friction welding requires relativelyexpensive apparatus similar to a machine tool.There are three important factors involved inmaking a friction weld:

1. The rotational speed which is related to thematerial to be welded and the diameter ofthe weld at the interface.

2. The pressure between the two parts to bewelded. Pressure changes during the weldsequence. At the start it is very low, but it isincreased to create the frictional heat. Whenthe rotation is stopped pressure is rapidlyincreased so that forging takes place

immediately before or after rotation isstopped.

3. The welding time. Time is related to theshape and the type of metal and the surfacearea. It is normally a matter of a fewseconds. The actual operation of themachine is automatic and is controlled bya sequence controller which can be setaccording to the weld schedule establishedfor the parts to be joined.

Laser Welding

Laser as a source of coherent andmonochromatic radiation, has a wide scopeof application in materials processing (Steen,1991; and Dutta et al., 2003). Laser assistedwelding, because of the sheer volume/proportion of work and advancement over theyears, constitutes the most importantoperations among the laser joining processesFigure 5 shows the front view of the schematicset up for laser underwater welding with a fillerrod (Kruusing, 2004). The focused laser beamis made to irradiate the work piece or joint atthe given level and speed. A shroud gasprotects the weld pool from undue oxidationand provides with the required oxygen flow.Laser heating fuses the work piece or plateedges and joins once the beam is withdrawn.In case of welding with filler, melting is primarilyconfined to the feeding wire tip while a part ofthe substrate being irradiated melts to insurea smooth joint. In either case, the work piecerather than the beam travels at a rate conducivefor welding and maintaining a minimum HeatAffected Zone (HAZ). There are twofundamental modes of laser weldingdepending on the beam power/configurationand its focus with respect to the work piece:(a) conduction welding, and (b) keyhole or

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in CO2 laser assisted welding has shielding

effect on the laser beam, but the plume inducedin Nd:YAG laser assisted welding has not suchshielding effect on laser energy transferring.No matter CO

2 or Nd:YAG laser assisted

welding, the optical emissions induced in thewelding process indicate the basiccharacteristics of the keyhole and the variationof welding parameters.

ULBW COMPARED TO GTAWThe filler metal used for Underwater LaserBeam Welding (ULBW) is the same as thatfor Gas Tungsten Arc Welding (GTAW), and isselected for suitability for the application andbase material being joined. It is introduced into

penetration welding (Figures 6a and 6b)(Dutta, 2003). Conduction limited weldingoccurs when the beam is out of focus andpower density is low/insufficient to causeboiling at the given welding speed. In deeppenetration or keyhole welding, there issufficient energy/unit length to causeevaporation and hence, a hole forms in themelt pool. The ‘keyhole’ behaves like an opticalblack body in that the radiation enters the holeand is Underwater laser assisted weldingcompared to the other underwater weldingmethods (Irie et al., 1997; Ogawa, 1998; andOgawa et al., 1998;) has the advantages oflow heat input, easy to transfer energy andcontrol adaptability. The low heat input is ofsignificance for reducing of the sensitivity ofstainless steels to Stress Corrosion Cracking(SCC). Underwater LBW has not been usedin application, however, because a series ofproblems have not been solved yet. Theplasma induced by the interaction of the laserbeam and the metal vapor or the shielding gas

Figure 5: Schematic of Laser Welding witha Filler Rod. Argon Shroud Removes Heat

and Prevents Undue Oxidation andDisplaces Water. The Relative Position ofthe Laser Focus Determines the Quality

and Configuration of the Weld

Figure 6: Schematic View of(a) Conduction Melt Pool, and

(b) Deep Penetration Welding Mode.The Surface Boiling and Marangoni Effect

are More in (a)

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the pool created by the laser beam in a mannervery similar to that seen in during welding witha gas metal arc welding machine (Figure 5).however, ULBW is a completely automaticwelding process. In this respect, ULBW differsfrom GTAW performed with a machine, wherethe operator makes adjustments duringwelding (www.westinghousenuclear.com).

Reliable Weld Characteristics

The laser beam’s precise heat input anddilution controls result in consistent weld quality.Weld chemistry testing shows high depositpurity as a result of the low heat input.

Various Application Uses

The optical fiber delivery of the laser lightminimizes weld system complexity and allowsweld heads to be developed for tight andremote applications. Being an automaticprocess also makes it ideal for use in locationssuch as the high-radiation areas found innuclear power plants.

• Mechanized underwater welding foractual usage of a very large floatingstructures.

• Investigation of the potential of using a robotmanipulator for underwater ultrasonictesting of welds in joints of complexgeometry.

• Application of advanced welding technique,like friction, laser welding and understandthe behavior of materials after the weldingand process optimization.

• Invention of new welding techniques andexplore the possibility of its application inunderwater welding.

• Generation of research data book on weldability of materials during underwaterwelding.

REFERENCES1. Cary H B and Helzer S C (2005), Modern

Welding Technology, Upper SaddleRiver, Pearson Education, New Jersey.

2. Cavaliere P, Campanile G, Panella F andSquillace A (2006), “Effect of WeldingParameters on Mechanical andMicrostructural Properties of AA6056Joints Produced by Friction Stir Welding”,J. Mater. Process. Technol., Vol. 180,pp. 263-270.

3. Cavaliere P, Squillace A and Panella F(2008), “Effect of Welding Parameters onMechanical and MicrostructuralProperties of AA6082 Joints Produced byFriction Stir Welding”, J. Mater. Process.Technol., Vol. 200, pp. 364-372.

4. Dutta Majumdar J and Manna I (2003),Sadhana, Vol. 28, p. 495.

Figure 7: A Schematic View of UnderwaterLaser Beam Welding

A Schematic view of Underwater laserbeam welding (www.westinghousenuclear.com).

FUTURE SCOPE• Automation of the underwater joining and

inspection of the welded structures.

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5. Elangovan K, Balasubramanian V andBabu S (2009), “Predicting TensileStrength of Friction Stir Welded AA6061Aluminum Alloy Joints by a MathematicalModel”, J. Mater. & Des., Vol. 30,pp. 188-193.

6. Haddad G N and Farmer A J (1985), Weld.J., Vol. 64, No. 12, pp. 339-342.

7. Irie T, Ono Y, Matsushita H et al. (1997),Proceedings of 16th OMAE, pp. 43-50.

8. Keats D J (2005), Underwater WetWelding—A Welder’s Mate, SpecialityWelds Ltd.

9. Khanna O P (2004), A Textbook ofWelding Technology, Dhanpat RaiPublications (P) Ltd., New Delhi, India.

10. Kruusing A (2004), Optics and Lasers inEngineering, Vol. 41, pp. 329-352.

11. Labanowski J and Fydrych D (2008),“Investigations of Underwater WeldingProcesses”, Report, Gdañsk University ofTechnology, Gdañsk.

12. Lancester J F (1987), “The Physics ofFusion Welding – Part I: The Electric Arc

in Welding”, IEE Proc., Vol. 134,pp. 233-254.

13. Oates W A (Ed.) (1996), WeldingHandbook, Vol. 3, American WeldingSociety, Miami, USA.

14. Pessoa E, Bracarense A, Zica E, Liu S andGuerrero F (2006), “Porosity VariationAlong Multipass Underwater Wet Weldsand its Influence on MechanicalProperties”, Journal of MaterialsProcessing Technology, Vol. 179.

15. Rowe M, Liu S and Reynolds T J (2002),“The Effect of Ferro-Alloy Additions andDepth on the Quality of Underwater WetWelds”, Welding Journal, Vol. 08.

16. Schmidt H-P (1996), IEEE Transactions onPlasma Science, Vol. 24, pp. 1229-1238.

17. Shida T, Hirokawa M and Sato S (1997),Welding Research Abroad, Vol. 43,No. 5, p. 36.

18. Steen W M (Ed.) (1991), Laser MaterialProcessing, Springer Verlag, New York.

19. www.westinghousenuclear.com, June2011, NS-IMS-0050-2011.

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WELDING PROCESSES IN MARINE APPLICATIONS: This Research article is aimed at highlighting the importance of Welding Processes in marine applications. ... article represents a review