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Welding Technology JOININGSolderingProduces coalescence of materials by heating to soldering temperature (below solidus of base metal) in presence of filler metal with liquidus < 450C

BrazingSame as soldering but coalescence occurs at > 450C

WeldingProcess of achieving complete coalescence of two or more materials through melting & re-solidification of the base metals and filler metalSoldering & Brazing AdvantagesLow temperature heat source requiredChoice of permanent or temporary jointDissimilar materials can be joinedLess chance of damaging partsSlow rate of heating & coolingParts of varying thickness can be joinedEasy realignmentStrength and performance of structural joints need careful evaluation WeldingAdvantagesMost efficient way to join metalsLowest-cost joining methodAffords lighter weight through better utilization of materialsJoins all commercial metalsProvides design flexibilityWeldability Weldability is the ease of a material or a combination of materials to be welded under fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended serviceCommon Arc Welding ProcessesShielded Metal Arc Welding (SMAW) Gas Tungsten Arc Welding (GTAW) or, TIGGas Metal Arc Welding (GMAW) or MIG/MAGFlux Cored Arc Welding (FCAW)Submerged Arc Welding (SAW)WELDABILITY OF STEELSCracking & Embrittlement in Steel WeldsCrackingHot CrackingHydrogen Assisted CrackingLamellar TearingReheat CrackingEmbrittlementTemper EmbrittlementStrain Age Embrittlement6Hot CrackingSolidification CrackingDuring last stages of solidificationLiquation CrackingDuctility Dip CrackingDuctility 0Caused by segregation of alloying elements like S, P etc.Mn improves resistance to hot crackingFormation of (Fe, Mn)S instead of FeS

Crack7Prediction of Hot CrackingHot Cracking Sensitivity HCS =(S + P + Si/25 + Ni/100) x 103 3Mn + Cr + Mo + VHCS < 4,Not sensitiveUnit of Crack Susceptibility[for Submerged Arc Welding (SAW)]UCS = 230C + 90S + 75P + 45Nb 12.3Si 4,5Mn 1UCS 10,Low riskUCS > 30,High risk8Hydrogen Assisted Cracking (HAC)Cold / Delayed CrackingSerious problem in steelsIn carbon steelsHAZ is more susceptibleIn alloy steels Both HAZ and weld metal are susceptibleRequirements for HACSufficient amount of hydrogen (HD)Susceptible microstructure (hardness)Martensitic > Bainitic > FerriticPresence of sufficient restraintProblem needs careful evaluationTechnological solutions possible 9Methods of Preventionof HACBy reducing hydrogen levelsUse of low hydrogen electrodesProper baking of electrodesUse of welding processes without fluxPreheatingBy modifying microstructurePreheatingVarying welding parametersThumb rule (based on experience / experimental results): No preheat if:CE < 0.4 & thickness < 35 mmNot susceptible to HAC ifHAZ hardness < 350 VHN10Graville DiagramZone IC < ~0.1%Zone IIC > ~0.1%CE < ~0.5Zone IIIC > ~0.1%CE > ~0.5

11Determination of Preheat Temperature (#1/2)Hardness Control ApproachDeveloped at The Welding Institute (TWI) UKConsidersCombined ThicknessHD ContentCarbon Equivalent (CE)Heat InputValid for steels of limited range of compositionIn ZoneII of Graville diagram12Hydrogen Control Approach For steels in Zones I & III of Graville diagramCracking ParameterPW = Pcm + (HD/60) + (K/40) x 104, where

Weld restraint, K = Ko x h, withh = combined thicknessKo 69T (C) = 1440 PW 392Determination of Preheat Temperature (#2/2)

13HAC in Weld MetalIf HD levels are highIn Microalloyed SteelsWhere carbon content in base metal is lowDue to lower base metal strengthIn High Alloy Steels (like Cr-Mo steels)Where matching consumables are usedCracking can take place even at hardness as low as 200 VHN14Lamellar TearingOccurs in rolled or forged (thick) productsWhen fusion line is parallel to the surfaceCaused by elongated sulphide inclusions (FeS) in the rolling directionSusceptibility determined by Short Transverse TestIf Reduction in Area>15%, Not susceptible< 5%, Highly susceptible

Crack15Reheat CrackingOccurs during PWHTCoarse-Grain HAZ most susceptibleAlloying elements Cr, Mo, V & Nb promote crackingIn creep resistant steels due to primary creep during PWHT !Variation:Under-clad cracking in pipes and plates clad with stainless steels16Reheat Cracks

Crack

Crack17Reheat Cracking(contd.)Prediction of Reheat CrackingG =Cr + 3.3 Mo + 8.1V + 10C 2Psr =Cr + Cu + 2Mo + 10V + 7Nb + 5Ti 2If G, Psr > 0,Material susceptible to crackingMethods of PreventionChoice of materials with low impurity contentReduce / eliminate CGHAZ by proper welding techniqueButteringTemper-bead techniqueTwo stage PWHT18Temper-bead Techniques

19Temper EmbrittlementCaused by segregation of impurity elements at the grain boundariesTemperature range: 350600 CLow toughnessPredictionJ = (Si + Mn) (P + Sn) x 104If J 180,Not susceptibleFor weld metal PE = C + Mn + Mo + Cr/3 + Si/4 + 3.5(10P + 5Sb + 4Sn + As)PE 3To avoid embrittlement20HAZ Hardness Vs. Heat InputHeat Input is inversely proportional to Cooling Rate

Cr-Mo SteelsCr:112 wt.-%Mo:0.51.0 wt.-%High oxidation & creep resistanceFurther improved by addition of V, Nb, N etc.Application temp. range:400550 CStructureVaries from Bainite to Martensite with increase in alloy contentWeldingSusceptible to Cold cracking & Reheat crackingCr < 3 wt.-%PWHT required:650760 C22Nickel SteelsNi:0.712 wt.-%C:Progressively reduced with increase in Ni For cryogenic applicationsHigh toughness Low DBTTStructureMixture of fine ferrite, carbides & retained austeniteWeldingFor steels with 1% NiHAZ softening & toughness reduction in multipass weldsConsumables: 12.5%Ni Welding (contd.)For steels with 13.5% NiBainite/martensite structureLow HD consumablesMatching / austenitic SSNo PWHT Temper-bead techniqueLow heat inputFor steels with > 3.5% NiMartensite+austenite HAZ Low heat inputPWHT at 650 CAustenitic SS / Ni-base consumable23HSLA SteelsYield strength > 300 MPaHigh strength byGrain refinement throughMicroalloying withNb, Ti, Al, V, BThermo-mechanical processingLow impurity contentLow carbon contentSometimes Cu added to provide precipitation strengtheningWelding problemsDilution from base metalNb, Ti, V etc.Grain growth in CGHAZSoftening in HAZSusceptible to HACCE and methods to predict preheat temperature are of limited validity24STAINLESS STEELSSS defined as Iron-base alloy containing> 10.5% Cr & < 1.5%CBased on microstructure & properties5 major families of SS Austenitic SSFerritic SSMartensitic SSPrecipitation-hardening SSDuplex ferritic-austenitic SSEach family requires Different weldability considerationsDue to varied phase transformation behaviour on cooling from solidificationStainless Steels(contd. 1)All SS types Weldable by virtually all welding processesProcess selection often dictated by available equipmentSimplest & most universal welding processManual SMAW with coated electrodesApplied to material > 1.2 mmOther very commonly used arc welding processes for SSGTAW, GMAW, SAW & FCAWOptimal filler metal (FM)Does not often closely match base metal compositionMost successful procedures for one family Often markedly different for another familyStainless Steels(contd. 2)SS base metal & welding FM chosen based onAdequate corrosion resistance for intended useWelding FM must match/over-match BM content w.r.tAlloying elements, e.g. Cr, Ni & MoAvoidance of crackingUnifying theme in FM selection & procedure developmentHot crackingAt temperatures < bulk solidus temperature of alloy(s)Cold crackingAt rather low temperatures, typically < 150 CStainless Steels(contd. 3)Hot crackingAs large Weld Metal (WM) cracks Usually along weld centrelineAs small, short cracks (microfissures) in WM/HAZAt fusion line & usually perpendicular to itMain concern in Austenitic WMsCommon remedyUse mostly austenitic FM with small amount of ferriteNot suitable when requirement is forLow magnetic permeabilityHigh toughness at cryogenic temperaturesResistance to media that selectively attack ferrite (e.g. urea)PWHT that can embrittle ferriteStainless Steels(contd. 4)Cold crackingDue to interaction of High welding stressesHigh-strength metalDiffusible hydrogenCommonly occurs in Martensitic WMs/HAZs Can occur in Ferritic SS weldments embrittled byGrain coarsening and/or second-phase particlesRemedyUse of mostly austenitic FM (with appropriate corrosion resistance)Martensitic Stainless SteelsFull hardness on air-cooling from ~ 1000 CSoftened by tempering at 500750 CMaximum tempering temperature reducedIf Ni content is significantOn high-temperature tempering at 650750 CHardness generally drops to < ~ RC 30Useful for softening martensitic SS before welding forSufficient bulk material ductilityAccommodating shrinkage stresses due to weldingCoarse Cr-carbides producedDamages corrosion resistance of metalTo restore corrosion resistance after welding necessary toAustenitise + air cool to RT + temper at < 450CMartensitic Stainless SteelsFor use in As-Welded ConditionNot used in as-welded conditionDue to very brittle weld areaExcept forVery small weldmentsVery low carbon BMsRepair situationsBest to avoidAutogenous weldsWelds with matching FMExceptSmall parts welded by GTAW asResidual stresses are very lowAlmost no diffusible hydrogen generated Martensitic Stainless SteelsFor use after PWHTUsually welded with martensitic SS FMsDue to under-matching of WM strength / hardness when welded with austenitic FMsFollowed by PWHTTo improve properties of weld areaPWHT usually of two forms(1)Tempering at < As(2)Heating at > Af (to austenitise)+Cooling to ~ RT(to fully harden)+Heating to < As (to temper metal to desired properties)Ferritic Stainless SteelsGenerally requires rapid cooling from hot-working temperaturesTo avoid grain growth & embrittlement from phaseHence, most ferritic SS used in relatively thin gagesEspecially in alloys with high CrSuper ferritics (e.g. type 444) limited to thin plate, sheet & tube formsTo avoid embrittlement in weldingGeneral rule is weld cold i.e., weld with No / low preheatingLow interpass temperatureLow level of welding heat inputJust enough for fusion & to avoid cold laps/other defectsFerritic Stainless SteelsFor use in As-Welded ConditionUsually used in as-welded condition Weldments in ferritic SSStabilised grades (e.g. types 409 & 405)Super-ferriticsIn contrast to martensitic SSIf weld cold rule is followedEmbrittlement due to grain coarsening in HAZ avoidedIf WM is fully ferriticNot easy to avoid coarse grains in fusion zone Hence to join ferritic SS, considerable amount of austenitic filler metals (usually containing considerable amount of ferrite) are used

Ferritic Stainless SteelsFor use in PWHT ConditionGenerally used in PWHT conditionOnly unstabilised grades of ferritic SSEspecially type 430When welded with matching / no FMBoth WM & HAZ contain fresh martensite in as-welded conditionAlso C gets in solution in ferrite at elevated temperaturesRapid cooling after welding results in ferrite in both WM & HAZ being supersaturated with CHence, joint would be quite brittleDuctility significantly improved by PWHT at 760 C for 1 hr. & followed by rapid cooling to avoid the 475 C embrittlementAustenitic Stainless SteelsFor use in As-Welded ConditionMost weldments of austenitic SS BMs Used in service in as-welded conditionMatching/near-matching FMs available for many BMsFM selection & welding procedure depend onWhether ferrite is possible & acceptable in WMIf ferrite in WM possible & acceptable Then broad choice for suitable FM & proceduresIf WM solidifies as primary ferriteThen broad range of acceptable welding procedures If ferrite in WM not possible & acceptableThen FM & procedure choices restrictedDue to hot-cracking considerationsAustenitic Stainless Steels (As-Welded) (contd. 1)If ferrite possible & acceptableComposite FMs tailored to meet specific needsFor SMAW, FCAW, GMAW & SAW processesE.g. type 308/308L FMs for joining 304/304L BMsDesigned within AWS specification for 0 20 FNFor GMAW, GTAW, SAW processesDesign optimised for 38 FN (as per WRC-1988)Availability limited for ferrite > 10 FNComposition & FN adjusted via alloying inElectrode coating of SMAW electrodes Core of flux-cored & metal-cored wiresAustenitic Stainless SteelsFor use in PWHT ConditionAustenitic SS weldments given PWHTWhen non-low-C grades are welded & Sensitisation by Cr-carbide precipitation cannot be toleratedAnnealing at 10501150 C + water quenchTo dissolve carbides/intermetallic compounds (-phase)Causes much of ferrite to transform to austeniteFor Autogenous welds in high-Mo SS E.g. longitudinal seams in pipeAnnealing to diffuse Mo to erase micro-segregationTo match pitting / crevice corrosion resistance of WM & BMNo ferrite is lost as no ferrite in as-welded conditionAustenitic SS (after PWHT)(contd. 1)Austenitic SS to carbon / low-alloy steel jointsCarbon from mild steel / low-alloy steel adjacent to fusion line migrates to higher-Cr WM producingLayer of carbides along fusion line in WM & Carbon-depleted layer in HAZ of BMCarbon-depleted layer is weak at elevated temperaturesCreep failure can occur (at elevated service temp.)Coefficient of Thermal Expansion (CTE) mismatch between austenitic SS WM & carbon / low-alloy steel BM causesThermal cycling & strain accumulations along interfaceLeads to premature failure in creep In dissimilar joints for elevated-temperature serviceE.g. Austenitic SS to Cr-Mo low-alloy steel jointsNi-base alloy filler metals used Austenitic SS (after PWHT)(contd. 2)PWHT used forStress relief in austenitic SS weldmentsYS of austenitic SS falls slowly with rising temp.Than YS of carbon / low-alloy steelCarbide pptn. & phase formation at 600700 CRelieving residual stresses without damaging corrosion resistance onFull anneal at 10501150 C + rapid coolingAvoids carbide precipitation in unstabilised gradesCauses Nb/Ti carbide pptn. (stabilisation) in stabilized gradesRapid cooling Reintroduces residual stressesAt annealing temp. Significant surface oxidation in airOxide tenacious on SSRemoved by pickling + water rinse + passivationPrecipitation-Hardening SSFor use in As-Welded ConditionMost applications forAerospace & other high-technology industriesPH SS achieve high strength by heat treatmentHence, not reasonable to expect WM to match properties of BM in as-welded conditionDesign of weldment for use in as-welded condition assumes WM will under-match the BM strengthIf acceptableAustenitic FM (types 308 & 309) suitable for martensitic & semi-austenitic PH SSSome ferrite in WM required to avoid hot crackingPrecipitation-Hardening SS For use in PWHT ConditionPWHT to obtain comparable WM & BM strengthWM must also be a PH SSAs per AWS classificationOnly martensitic type 630 (17-4 PH) available as FMAs per Aerospace Material Specifications (AMS)Some FM (bare wires only) match BM compositionsUsed for GTAW & GMAW Make FM by shearing BM into narrow strips for GTAWMany PH SS weldments light-gage materialsReadily welded by autogenous GTAWWM matches BM & responds similarly to heat treatmentDuplex Ferritic-Austenitic Stainless SteelsOptimum phase balanceApproximately equal amounts of ferrite & austeniteBM composition adjusted as equilibrium structure at ~1040CAfter hot working and/or annealingCarbon undesirable for reasons of corrosion resistanceAll other elements (except N) diffuse slowlyContribute to determine equilibrium phase balanceN most impt. (for near-equilibrium phase balance)Earlier duplex SS (e.g. types 329 & CD-4MCu)N not a deliberate alloying elementUnder normal weld cooling conditionsWeld HAZ & matching WMs reach RT with very little Poor mechanical properties & corrosion resistanceFor useful propertieswelds to be annealed + quenching To avoid embrittlement of ferrite by / other phasesDuplex SS(contd. 1)Over-alloying of weld metal with Ni causesTransformation to begin at higher temp. (diffusion very rapid)Better phase balance obtained in as-welded WMNothing done for HAZAlloying with N (in newer duplex SS)Usually solves the HAZ problemWith normal welding heat input & ~0.15%NiReasonable phase balance achieved in HAZN diffuses to austeniteImparts improved pitting resistanceIf cooling rate is too rapidN trapped in ferriteThen Cr-nitride precipitatesDamages corrosion resistanceAvoid low welding heat inputs with duplex SSDuplex SSFor use in As-Welded ConditionMatching composition WMHas inferior ductility & toughnessDue to high ferrite contentProblem less critical with GTAW, GMAW (but significant)Compared to SMAW, SAW, FCAWSafest procedure for as-welded conditionUse FM that matches BM With higher Ni contentAvoid autogenous weldsWith GTAW process (esp. root pass)Welding procedure to limit dilution of WM by BMUse wider root opening & more filler metal in the root Compared to that for an austenitic SS jointDuplex SS (As-Welded)(contd. 1)SAW processBest results with high-basicity fluxesWM toughnessStrongly sensitive to O2 contentBasic fluxes provide lowest O2 content in WMGTAW processAr-H2 gas mixtures used earlierFor better wetting & bead shapeBut causes significant hydrogen embrittlementAvoid for weldments used in as-welded conditionSMAW process (covered electrodes)To be treated as low-hydrogen electrodes for low alloy steelsDuplex SSFor use in PWHT ConditionAnnealing after weldingOften used for longitudinal seams in pipe lengths, welds in forgings & repair welds in castingsHeating to > 1040 CAvoid slow heating Pptn. of / other phases occurs in few minutes at 800 CPipes produced by very rapid induction heatingBrief hold near 1040 C necessary for phase balance controlFollowed by rapid cooling (water quench)To avoid phase formationAnnealing permits use of exactly matched / no FM As annealing adjusts phase balance to near equilibriumDuplex SS (after PWHT)(contd. 1)Furnace annealingProduce slow heating phase expected to form during heatingLonger hold (> 1 hour) necessary at annealing temp. To dissolve all phaseProperly run continuous furnaces Provide high heating ratesUsed for light wall tubes & other thin sectionsIf phase pptn. can be avoided during heatingLong anneals not necessaryDistortion during annealing can be due toExtremely low creep strength of duplex SS at annealing temp. Rapid cooling to avoid phaseMajor Problem with welding ofAl, Ti & Zr alloysProblemDue to great affinity for oxygenCombines with oxygen in air to form a high melting point oxide on metal surfaceRemedyOxide must be cleaned from metal surface before start of weldingSpecial procedures must be employedUse of large gas nozzlesUse of trailing shields to shield face of weld poolWhen using GTAW, thoriated tungsten electrode to be usedWelding must be done with direct current electrode positive with matching filler wireJob is negative (cathode)Cathode spots, formed on weld pool, scavenges the oxide filmALUMINUM ALLOYSImportant PropertiesHigh electrical conductivityHigh strength to weight ratioAbsence of a transition temperatureGood corrosion resistanceTypes of aluminum alloysNon-heat treatableHeat treatable (age-hardenable) Non-Heat TreatableAluminum AlloysGets strength from cold workingImportant alloy typesCommercially pure (>98%) Al Al with 1% MnAl with 1, 2, 3 and 5% MgAl with 2% Mg and 1% MnAl with 4, 5% Mg and 1% MnAl-Mg alloys often used in welded constructionHeat-treatableAluminum AlloysCu, Mg, Zn & Li added to AlConfer age-hardening behavior after suitable heat-treatmentOn solution annealing, quenching & agingImportant alloy typesAl-Cu-MgAl-Mg-SiAl-Zn-MgAl-Cu-Mg-LiAl-Zn-Mg alloys are the most easily weldedWelding of Aluminum AlloysMost widely used welding processInert gas-shielded weldingFor thin sheetGas tungsten-arc welding (GTAW)For thicker sectionsGas metal-arc welding (GMAW)GMAW preferred over GTAW due toHigh efficiency of heat utilizationDeeper penetrationHigh welding speedNarrower HAZFine porosityLess distortionWelding of Aluminum Alloys(contd...1)Other welding processes usedElectron beam welding (EBW)AdvantagesNarrow & deep penetrationHigh depth/width ratio for weld metalLimits extent of metallurgical reactionsReduces residual stresses & distortionLess contamination of weld poolPressure weldingTITANIUM ALLOYSImportant propertiesHigh strength to weight ratioHigh creep strengthHigh fracture toughnessGood ductilityExcellent corrosion resistanceTitanium Alloys(contd...1)Classification of Titanium alloysBased on annealed microstructureAlpha alloysTi-5Al-2.5SnTi-0.2PdNear Alpha alloysTi-8Al-1Mo-1VTi-6Al-4Zr-2Mo-2SnAlpha-Beta alloysTi-6Al-4VTi-8MnTi-6Al-6V-2SnBeta alloysTi-13V-11Cr-3AlWelding of Titanium alloysMost commonly used processes GTAWGMAWPlasma Arc Welding (PAW)Other processes usedDiffusion bondingResistance weldingElectron weldingLaser weldingZIRCONIUM ALLOYSFeatures of Zirconium alloysLow neutron absorption cross-sectionUsed as structural material for nuclear reactorUnequal thermal expansion due to anisotropic propertiesHigh reactivity with O, N & CPresence of a transition temperatureZirconium Alloys(contd.1)Common Zirconium alloysZircaloy-2ContainingSn = 1.21.7%Fe = 0.070.20%Cr = 0.050.15%Ni = 0.030.08%Zircaloy-4ContainingSn = 1.21.7%Fe = 0.180.24%Cr = 0.070.13%Zr-2.5%NbWeldability Demands For Nuclear IndustriesWeld joint requirementsTo match properties of base metalTo perform equal to (or better than) base metalWelding introduces features that degrade mechanical & corrosion properties of weld metalPlanar defectsHot cracks, Cold cracks, Lack of bead penetration (LOP), Lack of side-wall fusion (LOF), etc.Volumetric defectsPorosities, Slag inclusionsType, nature, distribution & locations of defects affect design critical weld joint propertiesCreep, LCF, creep-fatigue interaction, fracture toughness, etc.Welding of Zirconium AlloysMost widely used welding processesElectron Beam Welding (EBW)Resistance WeldingGTAWLaser Beam Welding (LBW)For Zircaloy-2, Zircaloy-4 & Zr-2.5%Nb alloys in PHWRs, PWRs & BWRsBy resistance weldingSpot & Projection weldingEBWGTAWWelding Zirconium Alloysin Nuclear IndustryFor PHWR componentsEnd plug welding by resistance weldingAppendage welding by resistance weldingEnd plate welding by resistance weldingCobalt Absorber Assemblies by EBW & GTAWGuide Tubes, Liquid Poison Tubes etc by circumferential EBWWelding of Zirconium to Stainless steel by Flash welding