WELDINGWelding is a process of joining two or more pieces of thesame or dissimilar materials to achieve completecoalescence.DEFINITION AS PER AWS / ASMEA localized coalescence of metals or non-metals producedeither by heating the materials to the welding temperaturewith or without the application of pressure alone, and withor without the application of filler material.WELDING PROCESSES1. Shielded Metal Arc Welding (SMAW)2. Submerged Arc Welding (SAW)3. Gas Metal Arc Welding (GMAW )1. - MIG , MAG4. Gas Tungsten Arc Welding (GTAW/ TIG)SELECTION OF WELDING PROCESS DEPENDS ON Thickness & Size of part to be welded Location and position of weld joints Joint design Welding equipment availability Process and operation requirement Production costSHIELDED METAL ARC WELDING SMAWprocess defined as an arc weldingprocess which produces coalescence of metalsby heating them with an arc between a coveredmetal electrode and the work-piece. Shielding is obtained from decomposition of theelectrode covering. Pressure is not used and filler metal is obtainedfrom the electrode.SHIELDED METAL ARC WELDINGMODE OF OPERATIONArc melts parent plate and electrode to form a weldpool which is protected by flux cover.Operator adjusts electrode feed rate, I.e. handmovement to keep arc length constant. Slag must beremoved after depositing each bead. Normally asmall degree of penetration, requiring plate edgepreparation. Butt weldsinthickplateorlargefilletsare deposited in a number of passes.Theprocesscanalsobeusedtodeposit metal toform a surface with alternative properties.CONSUMABLES Metal rods 1.5 - 8mmdiameter with fluxcovering (1-5mm radial thickness).Widely used: 2.5x 250, 3.15 x350/450,4 mm x 450 mm The characteristics of manual metal arcelectrodes, i.e., arc stability, depth ofpenetration, rate of deposition, position ofwelding, depends on the chemical compositionof the electrode coating classified by AWSspecifications.SMAW - OPERATING PARAMETERS Current range : 75-300 A Open Circuit Voltage : 65 - 80 V Range of Thickness: 2mm upwards Deposition rate : 2-11 lb/Hr Types of joint : All Welding position: all (depending on flux coating) Access : Good Portability : GoodMATERIALSMostlyconfinedtoferrousalloys, but canbeused for some others.TYPICAL APPLICATIONSStructural steel work.Ship BuildingRepair and Hard facing of constructionplant.OVERALL ADVANTAGESLow Equipment CostUsage possible with restricted access.OVERALL LIMITATIONSSkilled operator is required for good quality welds.Slow, mainly because flux must be chipped away.SAFETYArc emits visible and ultraviolet radiation.High open circuit voltage present while electrode is fitted to holder.SUBMERGED ARC WELDING SAWisdefinedasanarcweldingprocess inwhich an arc is maintained between an end of abare wire electrode and work piece. The arc is fully submerged in a layer of drygranular flux. Pressure is not used and filler metalis obtainedfrom the electrode.SUBMERGED ARC WELDINGOVERALL ADVANTAGES High weld metal Quality Smooth and uniform weld bead with no spatters. Extremely high deposition rate and welding speed. Current Range: 450 - 3000 A High Arc time can be achieved through automation. Minimum operator fatigue.OVERALL LIMITATIONS High initial cost Limited welding positions. Requirement of special jigs and fixtures. Difficulty in welding low thickness metals. Full penetration weld joints not possible from single side.GAS METAL ARC WELDINGGMAW is defined as an arc welding process whichproducescoalescenceof metalsbyheatingthemwith an arc between a continuous filler metal(consumable) electrode and the work piece.Shieldingis obtainedentirely fromanexternallysupplied gas or gas mixture.GAS METAL ARC WELDINGTYPE OF OPERATION : Semi-automatic / Fullyautomatic.EQUIPMENT : Power SourceWire feed UnitWelding gun or torchGas supply system travelling unit, if fully mechanised.GAS METAL ARC WELDINGAn arc is maintained between the end of the barewire electrode and the work. The wire is fed at aconstant speed, selected to give the requiredcurrent, and the arc length is controlled by thepower source. The operator is not thereforeconcerned with controlling the arc length and canconcentrate on depositing the weld metal in thecorrect manner. Hence the name semi-automaticfor manual operation, in which wire, gas andpower are fed to a hand held gun via a flexibleconduit.GAS METAL ARC WELDINGContd.The process can be operated at high currents(250-500A) when metal transfer is in the form ofspray but except for aluminium, this technique isconfined to welding in the flat and horizontalpositions. For welding in the vertical andoverhead positions. Special low-currenttechniques must be used, I.e. dip transfer orpulsed arc shielded by a stream of Gas.GAS METAL ARC WELDINGGMAW - OPERATING PARAMETERSCurrent range : 65-500 AMetal Transfer modes:Globular - in CO2 welding ( Non-axial metal transfer) for StructuresSpray Transfer - Thk > =6mm; in MIG ( Axial Transfer) for piping ( with Argo-CO2 shield) high thickness range.Dip Transfer ( Short Circuit ) - suitable for thin sheet metal works ( thk @ 3 mm or less): Dip transfer, pulsed arc Above 0.5mm Spray transfer 6mm upwardsDeposition rate :12-10 Kg/Hr (2-20 Lb/Hr)Types of joint : All including spot weldsConsumablesElectrodes, bare wire (m.s. is normally copper coated)0.6-1.6mm dia. Layer wound on spools 0.5-12 Kg (1-25lb) weight. Composition of wire selected to suit parentmaterial; wire specifications covered by BS 2901.Electrodes, flux cored are available for high depositionrate welding.Shielding gas in cylinders containing compressed gaswhich will expand to 7Cu.M (250 Cu.Ft) at atmosphericpressure. Composition to suit parent material usuallycarbon dioxide (CO2) for mild steel, argon for non-ferrous materials and argon with 1.5% oxygen or 5-20% CO2 for high strength and corrosion resistingferrous alloys.Other mixtures are used in specialcircumstances.TYPICAL APPLICATIONSCO2 Welding ( MAG )- For structuresMIGWeldingwithArgon/ Helium/ MixedGases - for Low Alloy, Mild Steel, Aluminium alloy,Copper and Copper allloys Spray Transfer method - for heavywall thickness pipes, structuresDip Transfer ( Short Circuit) - for Thinsheet metal working. Vehiclemanufacturing.OVERALL ADVANTAGESContinuous processReduced finishing operationsThin sheets can be welded in all positions by dip transfer mode.OVERALL LIMITATIONSNo independent control of filler addition.Range of action limited by wire feed.SAFETYArc emits visible and ultraviolet radiation.GAS TUNGSTEN ARC WELDINGGTAW is defined as an arc welding process whichproducescoalescenceof metalsbyheatingthemwith an arc between a tungsten (non- consumable)electrode and the work piece.GasTungstenArcWelding(GTAW) usesanonconsumable tungsten electrode which must beshielded with an inert gas.The arc is initiated between the tip of the electrodeand work to melt the metalbeing welded, as wellas the filler metal, when used.Agas shieldprotects theelectrode andmoltenweld pool, and provides the required arccharacteristics.GAS TUNGSTEN ARC WELDINGOVERALL ADVANTAGES Superior quality welds Welds can be made with or without filler metal Precise control of welding variables (heat) Free of spatter Slag free Low distortionOVERALL LIMITATIONS Requires greater welder dexterity than MIG or stick welding Lower deposition rates More costly for welding thick sectionsSTRAIGHT POLARITYREVERSE POLARITYSTANDARD REBAKING PROCEDURE FOR LOW HYDROGEN ELECTRODES1. Rebake the electrodes at 250-300 Deg. C for one hour.2. Cool them in the same oven to 100 Deg. C3. Transfer them to a holding oven maintained at 60-70 Deg. C4. Draw from this oven for use.5. Do not keep the rutile type electrodes in the same oven.WELD SYMBOLS