3What is Fabrication / Welding? “Metal Fabrication is the forming of metal, usually steel plate, into various forms either by welding or other forms of metal joining processesWelding“A fabrication process that joins materials, usually metals by causing coalescence”Coalescence means “Fusion”
10WELDED JOINT CATEGORY ASME Code defines welded joints by category. The term “Category” defines the location of a joint in a vessel.The joints included in each category are designated as joints of Categories A, B, C, and D.The “Categories” established by UW-3 are for use in specifying special requirements (based on Service, Material, and Thickness) regarding joint type and degree of inspection for certain welded pressure joints.Weld joint efficiency “E”It is a measure of weld quality and accounts for stress concentrations.E is needed in component thickness calculations
11Figure illustrates typical joint locations included in each category.
12Category A:Longitudinal welded joints within the main shell or nozzles; any welded joint within a formed or flat head; circumferential welded joints connecting hemispherical heads to main shells, to transitions in diameters.Category B:Circumferential welded joints within the main shell, nozzles, or transitions in diameter; circumferential welded joints connecting formed heads other than hemispherical to main shells, to transitions in diameter.Category C:Welded joints connecting flanges, tubesheets, or flat heads to main shell, to formed heads, to transitions in diameter, to nozzles.Category D:Welded joints connecting nozzles to main shells, to spheres, to transitions in diameter, to heads.
13Weld joint efficiency vs Weld joint efficiency vs. Joint Type, Category & Radiographic Examination
14Welding PositionsThere are various positions that a weld can be made in:1414
15Fill Pass Cover Pass Root Pass Hot Pass Weld Passes Hot pass - one or two beads. But keep it open. If we did end up with a tight area, it would be best to open up the area with a grinder and repair it before moving to the next pass.
17Welding Process Arc Welding “Arc welding process use electric power supply to create & maintain an electric arc b/w an electrode and the base material to melt metals at the welding point”.
18How an arc is formed?The arc is like a flame of intense heat that is generated as the electrical current passes through a highly resistant air gap.
19The Arc Welding Circuit The electricity flows from the power source, through the electrode and across the arc, through the base material to the work lead and back to the power source1919
20Basic Steps of Arc Welding Choose the right welding processPreparation of WeldingPrepare the base materials: remove scale, rust, oil, grease and any foreign materialQualified WeldersWelding ProcedureTack WeldsChoose the right filler materialPre-heatingConnectionsWelding & Removal of Temporary AttachmentAssess and comply with safety requirementsUse proper welding techniques and be sure to protect the molten puddle from contaminants in the airInspect the weld2020
21Factors Affecting the Welding Welding ProcedureThicknessElectrode extensionAngle of bevelTravel speedCleanlinessType of jointPolarity, Current & VoltageRoot Opening distanceWelding Position
23Gas Tungsten Arc Welding (GTAW) Also called Tungsten inert gas (TIG)This uses a similar inert gas shield to MIG, but the tungsten electrode is not consumed.Filler metal is provided from a separate rod fed automatically into the molten pool
25Type of Current & polarity Direct CurrentDirect current straight polarity (DCSP)Direct current reverse polarity (DCRP)Alternating CurrentType of Power SourceGeneratorTransformer
26Tungsten Electrode Non consumable Melting point = 6170°F Tungsten Alloys :Thoria, Zirconia, Ceria, Lanthana or a combination of oxides.Length = 3 to 24 inchesDiameter = .01 to .25inchesExtension & Shape
27GTAW Shielding Gases Argon, Helium or mixture of Gases Inert gases don't form compound with other elements.These gases protect the tungsten electrode and weld metal from contamination.
28Properties Of Inert Gases Properties of ArgonProperties of HeliumArgon is a heavy gas that is obtained from the atmosphere by the liquification of airHelium is a light gas that is obtained by separation from natural gasMay be used as a compressed gasQuieter & smoother arc actionDeeper penetrationLower costExpensiveSuitable gas for GTAWGives a smaller heat affected zoneBetter for thin metalsBetter for thicker metalsGood cleaning actionLower flow rates are requiredHigher flow rates are requiredBetter for welding dissimilar metalsMore availabilityLess availableBetter for welding at higher speedsArgon - helium mixture are used when better control of argon and the deeper penetration of helium are needed. (75% helium, 25% Argon)
29Filler MetalsSelection of Filler metal depends upon chemical composition of base metal.To increase the tensile strength, ductility, and corrosion resistance.Filler metal is kept in “HOT OVENS” for preheating for smooth operation.Classification of Filler metals in AWSStainless steel A5.9Carbon steels A5.18Low alloy steel A5.28Deposition RateER7018A1Allow Constituent of FillerCurrent ConditionPosition of Welding ( )Tensile Strength in KsiType of Covering (usually for GTAW only)Electrode
30Cost of GTAW Labor (20% to 40%) Overhead cost (Major Cost) Filler metal costShielded gas costElectric power cost (Minor cost)Tungsten Electrode cost (4%of Shielding gas cost)Welding Torch
31Advantages High Quality Weld No Flux or Slag Used for both Ferrous & Non-Ferrous metalsNo Smokes or FumesWelding can be done in all positionFiller metal is not always requiredThe arc & weld pool is clearly visible to the welderFor many application, it is the best methodExcellent for welding thin metals and pipeline welding
32Limitations Welding speed is slow Electrode is easily contaminated Not efficient for welding thick sectionsThickness should not exceed ¾”.Lower filler metal deposition rateNot EconomicalHand eye co-ordination skill is requiredHighly skilled labor needed for this process
33Shielded Metal Arc Welding (SMAW) This is the most commonly used technique. There is a wide choice of electrodes, metal and fluxes, allowing application to different welding conditions.The gas shield is evolved from the flux, preventing oxidation of the molten metal pool.Also referred to as “Stick Welding”Used for everything from pipeline welding, farm repair and complex fabrication.Uses a “stick” shaped electrode.Can weld: steel, cast iron, stainless steel, etc.
35Gas Metal Arc Welding (GMAW) Electrode metal is fused directly into the molten pool. The electrode is, therefore, consumed rapidly, being fed from a motorized reel down the centre of the welding torchAlso referred to as “MIG” weldingUses a shield gas and a continuous wire electrodeUsed for all types of fabricatioGreat for thin metals up to ¼”Excellent speed of depositionUsed for metals such as: steel, aluminum and stainless steel.
37MIG Welding Benefits Higher deposition rates than SMAW All position capabilityLess operator skill required Long welds can be made without starts and stops Minimal post weld cleaning is required
38Submerged arc welding (SAW) Instead of using shielding gas, the arc and weld zone are completely submerged under a blanket of granulated flux.A continuous wire electrode is fed into the weld. This is a common process for welding
39Flux-cored arc welding (FCAW) This is similar to the MIG process, but uses a continuous hollow electrode filled with flux, which produces the shielding gas.The advantage of the technique is that it can be used for outdoor welding, as the gas shield is less susceptible to draughts.
40Plasma - Arc Welding (PAW) Plasma welding is similar to the TIG process.A needle-like plasma arc is formed through an orifice and fuses the base metal.Shielding gas is used. Plasma welding is most suited to high-quality and precision welding applications.
41Oxygen/ Fuel WeldingUtilizes oxygen and a fuel gas to heat metal until it is in a molten state and fuse multiple pieces of metal together. Can be used with or without a filler rod.Great for brazing dissimilar metals together.Older technology that can be replaced by GTAW
42Welding SymbolsWelding symbols contain information about the weld to be madeSECTION OVERVIEW:The next slides are intended to help students learn about welding symbolsTEACHER NOTES:Slides 44-46: These slides show some examples of actual welding symbols which can help students more easily grasp the concepts discussed.For more information on welding symbols, see “How to Read Shop Drawings” available for purchase from the James F. Lincoln Foundation (www.jflf.org)To order free posters on the parts of a weld, types of joints, positions and welding symbols go to (Winter 2006)4242
43Welding symbols give the welder specific instructions about the weld including: Placement, Size, Length, ProcessAny other special notesWelding symbols areUniversally usedGoverned by the AWSFound on engineering drawings4343
44Reference LineReference Line (Required element)Always Horizontal
45Arrow LineReference Line (Required element)Arrow
47Arrow Tail Reference Line must always be horizontal, Arrow points to the line or lines on drawing which clearly identify the proposed joint or weld area.Reference Line (Required element)ArrowTailThe tail of the welding symbol is used to indicate the welding or cutting processes, as well as the welding specification, procedures, or the supplementary information to be used in making the weld.
48Basic components of a WELDING SYMBOL Reference Line must always be horizontal,Arrow points to the line or lines on drawing which clearly identify the proposed joint or weld area.Basic components of a WELDING SYMBOLArrow connects reference line to arrow side member of joint or arrow side of jointReference Line (Required element)ArrowTail omitted when reference not usedTailThe tail of the welding symbol is used to indicate the welding or cutting processes, as well as the welding specification, procedures, or the supplementary information to be used in making the weld.
49All the way AroundA circle at the tangent of the arrow and the reference line means welding to be all around.
50Field Weld SymbolA flag at the tangent of the reference line and arrow means Field Weld.
64Back Purging Back Purging Procedure Used to protect the electrode and the molten weld metal from atmospheric contaminationProtect the underside of the weld & its adjacent base metal surfaces from oxidation during WeldingApplication : All e.g.: Pipe FabricationGas Backing : for minimum of the first two passesMinimum purge time before welding, flow rate and venting, etcShielding Gas type (Argon, He or mixture of 2-Inert Gases for shielding).
66Arc Welding Safety Fumes and Gases Electric Shock Arc Rays Protect yourself and others from potential hazards including:Fumes and GasesElectric ShockArc RaysFire and Explosion HazardsNoiseHot objects6666
69REFERENCES Technical Guide for GTAW by Hobart Institute American Welding Society (AWS)ASME Section II- Part C : Specifications for Welding Rods, Electrodes, and Filler MetalsASME Section VIII – Div.1: Rules for Construction of Pressure VesselsASME Section IX - Welding and Brazing Qualifications for personnel and procedures