SHIELDING GASES SELECTION MANUAL

SHIELDING GASESSELECTION MANUAL

T A B L E O F C O N T E N T SChapter Topic Page▲Introduction 11Properties of Electric Arcs and Cases 3Basic Electrical Concepts 3Physical Properties of Gases 8Basic Gas Properties 102Gas Tungsten Arc Welding (GTAW) 13Process Description 13Gas Flow Rate 14Preflow and Postflow 15Backup Shielding and Trailing Shields 15Shielding Gases for GTAW 153Plasma Arc Processes (PAW and PAC) 19Plasma Arc Welding (PAW) 19– Process Description 19– Application of PAW 21– Shielding Gases for PAW 22Plasma Arc Cutting (PAC) 24– Process Description 24– Gas Flow Rates 27– Shielding and Cutting Gases for PAC 274Gas Metal Arc Welding (GMAW) 29Process Description 29Metal Transfer Modes in GMAW 30Metal-Cored Electrodes 34Shielding Gases for GMAW 35

T A B L E O F C O N T E N T SChapter Topic Page5Flux-Cored Arc Welding (FCAW) 43Process Description 43Flux Effects 44Metal Transfer 44Shielding Gases for FCAW 456The Economics of Gas Selection for GMAW 47Labor and Overhead 47Effect of Shielding Gas on Welding Speed 48Effect of Shielding Gas on Duty Cycle and Cleanup 48Consumable Costs 50Total Cost of a Shielding Gas 51Gas Supply 537Gas Supply Systems 55Cylinder Storage Systems 55High Pressure Cylinders 56Identification of Gases in Cylinders 56Carbon Dioxide Supply 56Gas Mixing 57Pre-Blended Mixtures 57Bulk Storage Systems 57Inert Gas Distribution Systems 588Precautions and Safe Practices 59Welding and Cutting 59Shielding Gases 62Material Safety Data Sheets (MSDS) 63a-zGlossary 64

I N T R O D U C T I O NShielding Gases Selection Manual▲▲During any arc welding process, oxygen andother atmospheric gases can react with themolten metal, causing defects that weaken theweld. The primary function of a shielding gasis to protect the molten weld metal fromatmospheric contamination and the resultingimperfections. In addition to its shieldingfunction, each gas or gas blend has uniquephysical properties that can have a majoreffect on welding speed, penetration, mechanicalproperties, weld appearance and shape,fume generation, and arc stability. A changein the shielding gas composition is usuallyconsidered an “essential variable” in mostqualified welding procedures.The primary gases used for electric weldingand cutting are argon (Ar), helium (He),hydrogen (H 2 ), nitrogen (N 2 ), oxygen (O 2 ),and carbon dioxide (CO 2 ). The compositionand purity of the gas or gas mixture should betailored to meet the process, material, andapplication requirements. Shielding gases areused in either pure form or in blends of varyingcomponents. Therefore, the selection of agas or gas mixture can become quite complexdue to the many combinations available.Selection of the most economical shieldinggas or blend must be based on a knowledgeof the gases available, volume requirements,their applications, and the overall effect theyhave on the welding process. Praxair’sShielding Gas Selection Manual describes thearc plasma characteristics and basic propertiesof shielding gases, as well as their applicationsin the welding and cutting processesmost frequently used by industry today.The processes discussed are Gas TungstenArc Welding (GTAW), Plasma Arc Welding(PAW), Plasma Arc Cutting (PAC), Gas MetalArc Welding (GMAW), and Flux-Cored ArcWelding (FCAW). The shielding gasespresented are Praxair’s Star Gases (puregases) and Star Gas Blends, includingStarGold , Stargon ® , HeliStar Mig Mix Gold and HydroStar blends.Additional sections of this manual explain theeconomics of gas selection, various methodsof gas supply, and the precautions andpractices recommended to ensure a safeworking environment.1

HistoryThe history of shielding gas developmentbegan late in the nineteenth century whenCharles Lewis Coffin replaced the air ina box placed over a welding joint with a nonoxidizingatmosphere. Interest in the use ofinert, non-oxidizing gases for welding wassporadic for the next forty years. Then in1930, two U.S. patents were issued which areconsidered to be the first descriptions of theuse of inert shielding gases for welding.The first patent, issued to H. M. Hobart,described the use of helium with carbon ormetallic arcs. The other, issued to P. K.Devers, described the use of argon and itsmixtures for arc processes.Early in the 1940s, Northrup Aircraft Company,Inc. first used an inert gas shieldedwelding process with a nonconsumabletungsten electrode. The process was developedspecifically for the welding of magnesiumfor aircraft fabrication using helium as theshielding gas. Recognizing its possibilities,Praxair acquired the rights to the invention in1942 and started an extensive research anddevelopment program to expand the use of theprocess. Introduced commercially in 1946 asthe Heliarc process, it is today also known asGas Tungsten Arc Welding (GTAW) orTungsten Inert Gas (TIG).A significant portion of this developmentwork was devoted to the evaluation of argonand helium utilized as shielding gases, and tostudies that identified the importance of gaspurity in producing quality welds. Therefore,it was not until the postwar era, after Praxairpioneered the economical production of highpurityargon on a commercial scale, thatGTAW became a practical reality.In 1950, a patent was issued to Air ReductionCo. Inc. (Airco) covering a process laterknown as Gas Metal Arc Welding (GMAW),Metal Inert Gas (MIG) and Metal ActiveGas (MAG). The initial application was for“spray arc” welding of aluminum in a heliumatmosphere. At that time, the process wascalled SIGMA (Shielded Inert Gas Metal Arc).Extensive development work by Praxair,Airco, and others followed during the 1950s.The effort was dedicated to developing gasmixtures, wire chemistry, and equipmentsystems to improve and expand the applicationrange of the process. This work led tothe rapid growth of GMAW during the nextthirty years and to its wide use today. It alsoprovided the groundwork for the invention ofFlux-Cored Arc Welding (FCAW) by ArthurBernard in the late 1950s. His patent wasassigned to the National Cylinder Gas Co.(NCG) in 1957, where the process was furtherdeveloped and introduced for industrial use.Also during the 1950s, Praxair developed,patented, and commercialized Plasma ArcCutting and Welding Systems. Praxairresearchers discovered that the properties ofthe open arc could be altered by directing thearc through a nozzle located between theelectrode and the workpiece. This proceduregreatly increased arc temperature and voltage,producing a highly constricted jet that wascapable, depending upon its velocity, of either“plasma arc” cutting or welding. The basicpatents covering this discovery were issuedin April 1957.All of these processes, which rely on shielding(or cutting) gases for effectiveness, are now inuse all over the world and play a major role inthe fabrication of products for the automotive,aerospace, railroad, trucking, and constructionindustries, to name a few.2

C H A P T E R 11Properties of Electric Arcs and Gases▲The unique properties of a gas or gas blendcan have a dramatic influence on arc characteristics,heat input, and overall process performance.A basic understanding of the keyelectrical concepts and the fundamentalphysical properties of gases is necessary inorder to select shielding gases wisely.▲BasicElectricalConceptsElectronsIt is convenient to think of electrons as negativecharges that can move about freely in acircuit. They can be thought of simplisticallyas being piled up at the negative end of acircuit, waiting to flow to the other (positive)end. The positive terminal does not haveenough electrons, the negative terminal hastoo many. When the two terminals are connectedto each other by wires, the negativecharges travel to the positive terminal(figure 1).Welding power supplies can be thought ofas a source of more electrons. As long as anelectrical power supply is connected to acircuit, a negative terminal can never use upits surplus of electrons, and a positive terminalcan never receive too many electrons.VoltageVoltage is the unit of pressure or electromotiveforce that pushes current, or electrons,through a circuit. One volt will push one amperethrough a resistance of one ohm. Weldingvoltages typically range from 14 to 35 volts.AmpereAn ampere is the unit of current, or the measureof the number of electrons that flow pasta point in a circuit every second. The quantityof electrons is expressed in coulombs. Onecoulomb equals 6.25 billion electrons. Oneampere is equal to one coulomb per second.Open arc welding amperages typically rangefrom 15 to 400 amps.OhmAn ohm is the unit of resistance to currentflow. One ohm is the quantity of resistancewhich produces a drop of one volt in a circuitcarrying one ampere.▲Figure 1 –Electron flowIn a garden hose, the water pressure is similarto voltage and the quantity of water flowing issimilar to amperage. Any restrictions in thehose, such as a kink, would produce resistanceto the flow of water. The relationship of thesevariables is expressed by the equation:V (volts) = I (amperes) x R (ohms).3

Welding PolarityThe direction of current flow influencesthe melting efficiency of the welding arc.Consequently, the control of the polarity in awelding system is very important. In GMAW,there are two welding polarity connectionsutilized: straight polarity and reverse polarity.When the direct current (DC) straight polarityor the direct current electrode negative(DCEN) connection is used, the electrode isthe nega-tive pole and the workpiece is thepositive pole of the welding arc. See figure 2.When the DC reverse polarity or the directcurrent electrode positive (DCEP) connectionis used, the electrode is the positive pole andthe workpiece is the negative pole (figure 3).WeldingPowerSupplyElectrodeElectronsWhen welding with direct current electrodenegative (DCEN), the direction of electronflow is to the base metal, striking the weldarea at high speed. At the same time, thepositive argon ions flow to the welding electrode,breaking up surface oxides on their way.▲Figure 2 –DC Electrode Negative (DCEN) connectionWorkpieceIn GMAW, by changing polarity to directcurrent electrode positive (DCEP), the heatingeffect of the electron flow is concentrated atthe tip of the electrode. This contributes to theefficient melting of the wire electrode whichprovides molten metal and reinforcement forthe weld. Welding penetration and productivityare increased with DCEP.WeldingPowerSupplyElectrodeElectronsAnother important characteristic of DCEPoperation is the surface cleaning action itproduces. This is especially true when weldingaluminum and magnesium which have aninvisible refractory oxide coating that mustbe removed in some matter to permit soundwelds to be made. It is useful to remove thisoxide prior to welding, as it has a highermelting point than the base material.▲Figure 3 –DC Electrode Positive (DCEP) connectionWorkpieceThe surface cleaning action obtained withDCEP operation is probably the result of ionbombardment, which suggests that the flowof heavy gas ions produces a “sand-blasting”effect on the oxide film. This theory issupported by the fact that argon ions, whichare about ten times heavier than helium ions,yield a greater degree of surface cleaning.4

Applying these basic electrical concepts to awelding arc is complicated by the introductionof a shielding gas. In a normal electricalcircuit, electrons (current) are pushed (byvoltage) through a solid conductor, such asa copper wire, which has some degree of resistance(ohm). In a welding circuit, there isa break or gap between the electrode and theworkpiece where there is no solid wire conductor.The supply of electrons, therefore,must come from the gas.To provide the necessary electrons, theatomic structure of the gas must be physicallychanged to a state that will allow it to conductelectricity. The temperature of the welding arcfacilitates this process.In order to select the most appropriateshielding gas, it is important to understand afew of the fundamental characteristics thatapply to these gases at the elevated temperaturespresent in welding arcs. The characteristicsthat have a major influence on gasselection criteria, such as heat input to theworkpiece, penetration profile, the effect onmetal transfer, arc starting and arc stabilityare of particular interest.LightElectricityHeatArc▲Figure 4 –An (GMAW) arc is a (DCEP)conversion deviceThe Welding ArcConceptually, a welding arc can be thought ofas a conversion device that changes electricalenergy into heat (figure 4).The amount of heat that an arc produces fromelectricity depends upon many factors. One ofthe most important is arc current. When thearc current is increased, the amount of heat thearc produces is increased; the reverse is alsotrue. Another factor which controls arc heatis arc length. Changes in the length of anarc will cause changes in the amount of heatavailable from the arc. Consequently, successfulwelding depends upon control of both thearc current and arc length. Most of the time,when considering manual welding, the arccurrent is controlled by the welding powersupply and the arc length is controlled by thewelder.A welding arc has been defined as “A controlledelectrical discharge between the electrodeand the workpiece that is formed and sustainedby the establishment of a gaseousconductive medium, called an arc plasma.”An arc, or electrical flame, emits bright light,as well as ultraviolet and infrared radiation.Depending on the current level, arc temperaturesmay be very high, relative to the basemetal melting point, producing enough heat tomelt any known material. The characteristicsof the arc depend on the shielding gas presentin the arc environment because it affects theanode, cathode, and plasma regions foundthere. These regions are shown in figure 5and described on the next page.5

▲Figure 5 –The AnodeThe positive end of an arc is called the anode.It can be either the electrode or the workpiece,depending upon polarity of the system. Theanode is the very thin, intensely bright area onthe electrode or workpiece where the arc isattached. The anode voltage does not changewhen the electrode and workpiece are movedcloser together or farther apart. The anodevoltage depends on the composition of thefiller material and the gas that surrounds it andis essentially constant for any set of materials.The CathodeThe negative end of the arc is called thecathode. It also can be either the electrode orthe workpiece, depending upon the polarityselected. The cathode is also a very thin brightarea where the arc is attached. In aluminumwelding, the cathode can appear as rapidlymoving flashes of light at the edge of the arcattachment point to the aluminum surface.Cathode voltage is similar to anode voltage.It depends on materials in the immediatesurroundings of the cathode zone and isessentially constant for any set of materials.Arc PlasmaAn arc plasma is “A gas that has been heatedby an electric arc to at least a partially ionizedcondition, enabling it to conduct an electriccurrent.” It is the visible electrical flame, orarc, in the gap between the anode and cathode.The arc voltage is the sum of the anode andcathode voltages plus the voltage across thearc plasma. With most welding processes,this voltage increases when the electrode andworkpiece move farther apart and decreasesas they move closer together. Arc voltage isalso impacted by the electrical conductivityof the shielding gas selected.Ionization PotentialThe ionization potential is the energy, expressedin electron volts, necessary to remove anelectron from a gas atom, making it an ion, oran electrically charged gas atom. All otherfactors held constant, the ionization potentialdecreases as the molecular weight of the gasincreases. The significance of this is illustratedin figure 6, which shows a simplifiedatomic structure of argon and helium. Simplisticallyspeaking, ionization potential can bedescribed as a measurement of electricalconductivity of the arc shielding gas.The regions of6an arcBase MaterialAnodePlasmaCathodeArgon, with eighteen electrons, is muchheavier than helium, which has only twoelectrons. The force of attraction holdingthe outer electrons in their orbit is inverselyproportional to the square of the distance fromthe nucleus. Simply stated, the energy requiredto remove an electron from the argon atomis significantly less than that required forhelium. Specifically, it takes 15.7 electronvolts to remove the first electron in argoncompared to 24.5 electron volts in helium.At these energy levels, ionization of the gasbegins in the arc gap, which creates the “free”electrons necessary to support current flowacross the gap, forming the plasma.

HeliumAlthough other factors are involved insustaining the plasma, these respectiveenergy levels must be maintained for thisto be accomplished.From this relationship, it becomes readilyapparent that, for equivalent arc lengths andwelding currents (see figure 7), the voltageArgonobtained with a helium enhanced mixture isappreciably higher than it is with argon.Since heat in the arc is roughly measured bythe product of current and voltage (arcpower), the use of helium yields a muchhigher available heat than does argon. This isone reason that helium is commonly referredto as a “hotter” gas (see figure 8).See Table 1, page 11 for specific values ofmolecular weight and ionization potential forthe six pure shielding gases.▲Electrons (–)Figure 6 –Atomic structures of argon and heliumArc starting and arc stability are also largelydependent on the ionization potential of theshielding gas selected. Gases with relativelylow ionization potential, such as argon, giveup electrons more easily, helping to initiateand maintain the arc in a stable operatingmode.30Gas Tungsten Arc Welding, AluminumArgonHelistar A-2525Arc (v)Arc Length2015105HeliumArgonShieldingCupTungstenElectrodeArc Length1/8” (3.18 mm)0.08 Inch 00.16 Inch0 50 100 150 200 250 300 350 400Arc Current (Amperes)▲Figure 7 –Voltage-current relationship(alternative current, AC)▲Amps (DCEN) = 300Arc Voltage = 11Watts = 3300Amps (DCEN) = 300Arc Voltage = 15Watts = 4500Figure 8 –Increase of heat input with pure argonand argon/helium mixtures.7

▲Figure 9 –Pinch ForceCurrent (A).ElectrodePinch EffectForceElectromagnetic Pinch ForceElectromagnetic pinch force has a strongeffect on metal transfer. This pinch force isproduced by current flow in a wire or electrode(see figure 9). Every current-carryingconductor is squeezed by the magnetic fieldthat surrounds it. When the GMAW electrodeis heated close to its melting point and is inits eutectic state, the electromagnetic pinchforce squeezes off drops of metal and assistsin the transfer of the filler metal into theweld pool.▲PhysicalPropertiesof GasesThermal ConductivityThe thermal conductivity of a gas is relatedto its ability to conduct heat. It influences theradial heat loss from the center to the peripheryof the arc column. Pure argon, when usedas a shielding gas has mild thermal conductivity,and produces an arc which has two zones:a narrow hot core and a considerably coolerouter zone (see figure 10). As a result, thepenetration profile of a typical weld fusionarea is characterized by a narrow “finger” atthe root and a wider top. A gas with highthermal conductivity conducts more of theheat outward from the arc core. This resultsin a broader, hotter arc. This type of heatdistribution, which occurs with helium, argonhydrogen,and argon/carbon dioxide mixturesis more uniform and produces a generallywider profile throughout the fusion zone.Dissociation and RecombinationWhen two or more atoms combine they forma molecule. Shielding gases such as carbondioxide, hydrogen, and oxygen are molecules.For example, carbon dioxide is made up ofone atom of carbon and two atoms of oxygen.▲Figure 10 –Comparison of arcs in argonand helium atmospheresNarrowArc CoreWide ArcEnvelopeWorkpieceArgonElectrodePenetration ProfileWorkpieceHeliumWide ArcCoreNarrow ArcEnvelopeWhen heated to the high temperatures presentin the arc plasma (12-15,000 F), these gasesbreak down, or dissociate into their separateatoms. They are then at least partially ionized,producing free electrons and improved currentflow. As the dissociated gas comes in contactwith the relatively cool work surface, theatoms recombine, and it releases energy tothe base material in the form of heat. Thisprocess does not occur with gases such asargon, which consists of a single atom. Therefore,for the same arc temperature, the heatgenerated at the work surface can be greaterwith gases such as carbon dioxide, hydrogen,and oxygen.8

ReactivityReactivity, as it applies to shielding gases, isa comparative measurement of how readily agiven shielding gas (at arc temperatures) willreact with the elements in the puddle.Argon and helium are completely nonreactive,or inert, and therefore have nochemical effect on the weld metal. Nitrogen isan inert gas, but, at the temperatures commonto welding, it may react and have an adverseeffect on weld chemistry.Oxygen and carbon dioxide fall into acategory of reactive gases known as oxidizers.These gases will react to form oxides withthe molten metal in the arc and in the weldpuddle. This property may contribute tocausing welding fume.Hydrogen is a reactive gas, but is reducingin the nature. Hydrogen will (preferentially)react with oxidizing agents, thereby helping toprevent the formation of oxides in the moltenweld metal. However, hydrogen can producedetrimental effects such as underbead cracking,when used on some high strength andlow alloy steels.Surface TensionIn any liquid there is an attractive forceexerted by the molecules below the surfaceupon those at the surface. An inward pull, orinternal pressure is created, which tends torestrain the liquid from flowing. Its strengthvaries with the chemical nature of the liquid.In welding, the surface tension betweenmolten steel and its surrounding atmospherehas a pronounced influence on bead shape.If surface tension is high, a convex, irregularbead will result. Lower values promote flatterbeads with minimum susceptibility forundercutting.Pure argon shielding when used with GMAWis usually associated with high interfacialenergy, producing a sluggish puddle and ahigh-crowned bead when mild steel welding isconsidered. This is partially attributed to thehigh surface tension of liquid iron in an inertatmosphere. For this reason, it is not recommendedfor use in MIG welding of mild steel.Iron oxides, however, have a considerablylower surface tension and thus promote goodwetting to the parent metal. Therefore, theaddition of small percentages of oxygen orcarbon dioxide to argon when performingGMAW result in a more fluid weld puddle.Gas PuritySome base metals, such as carbon steel havea relatively high tolerance for contaminants.Others, such as aluminum, copper andmagnesium, are fairly sensitive to impurities.Still others, such as titanium and zirconium,have extremely low tolerances for anyimpurity in the shielding gas.Depending on the metal being welded and thewelding process used, even minuscule gasimpurities can have a detrimental impact onwelding speed, weld surface appearance, weldbead coalescence, and porosity levels. Theseimpurities can appear in several ways.9

It is always possible that the gas can be contaminatedas delivered; but it is more likelythat it becomes contaminated somewhere betweenthe supply and the end-use points.Praxair is equipped with the analytical equipmentto determine purity levels anywhere inthe gas supply system, and can assist inidentifying the cause of gas purity problemsand their solution.See Table 2, page 12 for standard industryminimum purity levels for welding gradegases.Gas DensityGas density is the weight of the gas per unitvolume. It is usually expressed in pounds percubic feet. Gas density is one of the chieffactors influencing shielding effectiveness.Basically, gases heavier than air require lowerflow rates than gases that are lighter than airto achieve equivalent weld puddle protection.See Table 1, page 11 for specific values.The properties of the gases commonly usedin welding and cutting and how they functionin metal fabrication applications are discussedbelow.▲Basic GasPropertiesArgon (Ar)Slightly less than one percent of the earth’satmosphere is composed of argon, which iscolorless, odorless, tasteless, and nontoxic.As an inert gas, argon does not react withother compounds or elements. Argon is about1.4 times heavier than air and cannot sustainlife. The inert properties of argon make it idealas a shield against atmospheric contamination,thus it is used in many welding processes.Argon promotes good arc starting characteristicsand arc stability due to its low ionizationpotential.Carbon dioxide is commonly mixed withargon to improve productivity and penetrationin GMAW.Helium (He)Helium is the second lightest element, afterhydrogen, and is lighter than air. Like argon,it is chemically inert and will not sustain life.Due to its high thermal conductivity andhigh ionization potential, helium is used as ashielding gas for welding applications whenincreased heat input is desired, and lowtolerance for oxidizing elements exist suchas with aluminum and magnesium welding.Carbon Dioxide (CO 2 )Hydrogen (H 2 )10Carbon dioxide (CO 2 ), a reactive gas, is about1.5 times heavier than air. It is an odorless,colorless gas with a slightly pungent, acidtaste and is slightly toxic. It will not sustainlife or support combustion. Differing fromother reactive gases such as oxygen, CO 2can be used alone for GMAW shielding gasapplications. Its relatively high oxidizingpotential can be countered by the use ofGMAW or FCAW wires higher in alloyingelements, such as silicon and manganese.Hydrogen, the lightest known element, is aflammable gas. Explosive mixtures can beformed when certain concentrations of hydrogenare mixed with oxygen, air, or otheroxidizers. Hydrogen is not life sustaining.Small quantities are useful in gas blends forplasma cutting and some welding applicationsbecause of its high thermal conductivity andreactive nature. It is very useful when GMAWand GTAW – 300 series austenitic stainlesssteels.

Nitrogen (N 2 )Oxygen (O 2 )Nitrogen is a colorless, odorless, and tastelessgas which forms 78 percent of the earth’satmosphere (by volume). It is nonflammable,does not support combustion, and is slightlylighter than air. Nitrogen is inert except at arcwelding temperatures, where it will react withsome metals, such as aluminum, magnesium,and titanium. It is not recommended as aprimary shielding gas with GMAW, but iscommonly applied as an assist gas with lasercutting on stainless steels. It can be used incombination with other gases for some weldingapplications and is also widely used inplasma and laser cutting.Fifty percent of the earth’s crust and approximately21 percent of the earth’s atmosphere(by volume) is oxygen. Oxygen combineswith almost all known elements except rare orinert gases, and it vigorously supports combustion.Because of its highly oxidizing andcombustion-supporting properties, oxygen isan ideal gas for increasing flame temperaturesand improving performance for oxyfuel weldingand cutting. Small amounts of oxygen maybe added to argon for GMAW to increase arcstability and improve the wetting and shape ofthe weld bead when working with mild orstainless steels. It is also used to enhance cuttingspeeds with plasma and laser processes.▲Table 1 –Shieldinggas dataArgon Carbon Helium Hydrogen Nitrogen OxygenDioxideChemical Symbol Ar CO 2 He H 2 N 2 O 2Atomic Number 18 _ 2 1 7 8Molecular Weight 39.95 44.01 4.00 2.016 28.01 32.00Specific Gravity, 1.38 1.53 0.1368 0.0695 0.967 1.105Air = 1Density (lb/cu ft) at 0.1114 0.1235 0.0111 0.0056 0.0782 0.08920 C, 1 atmosphereIonization Potential 15.7 14.4 24.5 13.5 14.5 13.2(ev)Thermal Conductivity 9.69 8.62 85.78 97.22 13.93 14.05(10 -3 x Btu/hr-ft- F) (32 F) (32 F) (32 F) (32 F) (32 F) (32 F)Cubic ft/lb 9.67 8.73 96.71 192 13.8 12.08Cubic ft/gal 113.2 74.0 100.6 103.7 93.2 115.011

▲Table 2 –Gas purity andmoisture content(welding grade)Product Minimum Maximum Approximate Dewpoint atState Purity Moisture* Maximum Moisture Content(percent) (ppm) F CAir Liquid 99.98 120 - 40 - 40Argon Gas 99.995 10 - 77 - 60Liquid 99.997 6 - 83 - 64Carbon Dioxide Gas 99.5 34 - 60 - 51Liquid 99.8 13 - 73 - 58Helium Gas 99.95 32 - 61 - 51Liquid 99.995** 3 - 92 - 69Hydrogen Gas 99.95 8 - 80 - 63Liquid 99.995*** 5 - 86 - 65Nitrogen Gas 99.7 32 - 61 - 51Liquid 99.997 5 - 86 - 65Oxygen Industrial 99.5 50 - 54 - 48Liquid 99.5 6 - 83 - 64* Moisture specifications are measured at full cylinder pressure.** Including neon*** Including heliumBased solely on oxygen content. Minute traces of other inert gases (such as argon, neon, helium, etc.) which remainafter oxygen removal are considered as nitrogen. (This is standard practice in the compressed gas industry.)12

C H A P T E R 22Gas Tungsten Arc Welding (GTAW)▲ProcessDescriptionWelds madewith or withoutfiller metalACHF, DCENor DCEPPower SupplyGasCupTungstenElectrodeGasEnvelopeGas Tungsten Arc Welding (GTAW) is definedas “an open arc welding process that producescoalescence of metals by heating them withan electric arc between a tungsten electrode(nonconsumable) and the workpiece. Themolten weld pool is protected by an externallysupplied shielding gas. Pressure may or maynot be used, and filler metal may or may notbe used.” GTAW is also commonly referred toas TIG (Tungsten Inert Gas) or Heliarc weldingalthough the American Welding Societyrefers to it as GTAW. Figure 11 illustrates theessentials of GTAW.▲Figure 11 –Essentials of GTAWElectricArcWorkpieceGroundConnectionThe use of a nonconsumable tungsten electrodeand inert shielding gases produces thehighest quality welds of any open arc weldingprocess. Welds are bright and shiny, with noslag or spatter, and require little or no postweldcleaning. GTAW is easily used in allwelding positions and provides excellent weldpuddle control, especially on thin and intricateparts. It has found extensive use in theaircraft, aerospace, power generation, chemical,and petroleum industries.Although usually thought of as a manualHigh SpeedWire Feederprocess, GTAW is often automated with orTIG Torchwithout filler wire for high-productionContact Tubeapplications. In 1969, Praxair introduced aAC Hotvariation to the process called “Hot Wire.”DC TIGWireTravelPowerWith this process, the filler wire is independentlypre-heated to a molten state as it entersPower Heated WireWeldArcWorkpiecethe weld puddle. This feature allows arc heatto be fully concentrated on melting the workpiece,▲Figure 12 –Diagram of Gas Tungsten Archot wire systemnot the wire (see figure 12). The HotWire process expands the versatility of automatedGTAW by increasing deposition ratesand travel speeds.13

▲Gas FlowRateGas flow rate, which can range from a fewcubic feet per hour (cfh) to more than 60 cfh,depends on the current developed, the torchsize, the shielding gas composition and thesurrounding environment (drafts, etc.).In general, a higher current will require alarger torch and higher flow rates. In addition,gas density, or the weight of the gasrelative to air, has a major influence on theminimum flow rate required to effectivelyshield the weld.ArgonHeliumArgon is approximately 1.4 times as heavy asair and ten times as heavy as helium. The significanceof these differences in gas densityrelative to air is shown in figure 13.▲Figure 13 –Shielding effectiveness of gas density (GTAW)Nozzle-To-WorkDistance3/16 Inch9/16 Inch▲Air Velocity (Feet Per Second)Figure 14 –Relationship of flow requirementsto cross-draft velocity765432102 to 2 1/2 timesas much gas flowTungsten Arc 5/8 Inch Diameter CupAr75% He25% ArAr0 8 16 24 32 4075% He25% ArMinimum Shielding Gas Flow(Cubic Feet Per Hour)4856Argon shielding gas is delivered to the arczone by the torch nozzle. Its function is toprovide a contaminant free blanket of shieldinggas over the weld area. Because helium ismuch lighter than argon, to produce equivalentshielding effectiveness when welding inthe flat position, the flow of helium must betwo to two and one half times that of argon.The same general relationship is true formixtures of argon and helium, particularlythose high in helium content, although asargon content is increased, shielding gas flowis typically decreased. It should be noted thatfor overhead welding flow rates with heliummixtures can be reduced as the specificgravity of the gas is less than that of air.Figure 14 shows the relationship of flowrequirements to the cross-draft velocity forpure argon and for a 25% argon/75% heliummixture using two different nozzle-toworkpiecedistances.Gas flow rate must be selected with care.It is not productive or economical to use moregas than necessary to achieve good shielding.High gas flows can pull air into the weldingarc, often causing porosity in the weld.To avoid wasting gas and contaminating theweld, use of an inexpensive critical flowdevice that restricts gas flow to an optimalrange is often recommended.14

▲PreflowandPostflowWhen welding materials that are sensitiveto oxidation (such as copper, aluminum andstainless steel), gas preflow and postflow willminimize contamination of the weld zone andelectrode. A pre-flow of shielding gas removesmoisture which may have entered the systemand blankets the weld zone for optimum startingconditions. Changes in room temperaturecan cause air to move in and out of the end ofa torch while not in use; moisture in the aircondenses on the inside of the torch. A preflowof shielding gas for a period of timebefore the arc is initiated will remove themoisture.Postflow works to minimize contamination ofthe weld pool in a different way. When the arcis turned off, the weld metal begins to cool.For a few moments, the weld metal remainshot enough to be contaminated by the surroundingair. To prevent this, the shielding gasis allowed to flow for several seconds after thearc is extinguished. The length of time varieson the size and temperature of the weld but arule of thumb is one second for every tenamps of current. This will provide shieldingto allow the weld to cool. The postflow ofgas also protects the hot electrode fromcontamination.▲BackupShieldingand TrailingShieldsIt is sometimes necessary to use shielding gason the underside of a weld to prevent oxidationof the hot weld bottom. As an example,backup shielding gas is used to purge the airfrom the interior of piping prior to and duringwelding. This procedure prevents contaminationof the backside of the weld while the pipeis being welded from the outside.The same gas may be used for backup andwelding, but it is possible to use a gas blendfor welding and another gas, such as pureargon, nitrogen, or an argon/hydrogen,nitrogen/hydrogen blend for the backup gas,depending on the workpiece material.In some instances, the welding travel speedmay be too great for the shielding gas toprotect the weld until it has cooled. As the arcmoves on, the solidified weld metal remainshot and oxidizes. A trailing gas shield can beused to prevent oxidation on the surface of theweld bead from occurring.▲ShieldingGases forGTAWArgonArgon, an inert rare gas that makes upapproximately 1% of the earth’s atmosphere,is the most commonly used shielding gas forGTAW. Its low thermal conductivity producesa narrow, constricted arc column and excellentelectrical conductivity which allow greatervariations in arc length with minimal influenceon arc power and weld bead shape. Thischaracteristic makes it the preferred choice formanual welding. In addition, argon providesgood arc starting due to its low ionizationpotential.For AC welding applications, high purityargon offers superior cleaning action, arcstability, and weld appearance.While pure argon may be used for mechanizedapplications, argon/helium or argon/hydrogenblends are frequently selected to promotehigher welding travel speeds. The hotter arccharacteristics of these blends also make themmore suitable for welding metals with highthermal conductivity, such as copper orstainless steel. Argon/hydrogen blends shouldonly be used for welding austenitic stainlesssteels.15

HeliumArgon/Helium BlendsHelium, also an inert gas, has high thermalconductivity and high ionization potential,which require higher arc voltages than argonfor a given current setting and arc length.(See Chapter 1, figure 7, “Voltage-currentrelationship.”) This produces a hotter andbroader arc which improves the depth ofpenetration and weld bead width.Praxair’s HeliStar BlendsEach of these gases (argon and helium),as explained above, has specific advantages.Praxair’s HeliStar blends (argon/heliumblends) are used to increase the heat input tothe base metal while maintaining the favorablecharacteristics of argon, such as arcstability and superior arc starting.The use of helium is generally favored overargon at the higher current levels which areused for welding of thicker materials, especiallythose having high thermal conductivityor relatively high melting temperatures. It isalso often used in high-speed mechanizedapplications, although an addition of argonwill improve arc initiation and cleaningaction.Although argon is widely used for ACwelding of aluminum, helium has beensuccessfully used for DCEN mechanized andhigh current AC welding of this material.It produces greater penetration and highertravel speeds. However, surface oxides mustbe cleaned from the weld joint to obtainacceptable results.The physical properties of helium definitelyoffer advantages in some applications. However,due to it high ionization potential, it alsoproduces a less stable arc and a less desirablearc starting characteristic than argon. Its highercost and higher flow rates are also factorsto be considered. In some cases, an argonmixture is used for igniting the arc and purehelium is used for welding. This techniqueis used for mechanized DCEN-GTAWwelding of heavy aluminum.Praxair’s HeliStar A-75This blend is sometimes used for DC weldingwhen it is desirable to obtain higher heatinput while maintaining the good arc startingbehavior of argon. It is a favorite choice whenMIG welding thick aluminum (> 1/2").Praxair’s HeliStar A-50This blend is used primarily for high-speedmechanized welding of nonferrous materialunder 3/4 inch thick.Praxair’s HeliStar A-25The speed and quality of AC and DC weldingof aluminum, copper and stainless steels canbe improved with this blend. It is sometimesused for manual welding of aluminum pipeand mechanized welding of butt joints inaluminum sheet and plate. Praxair’s HeliStarA-25 blend is also used for many of theGTAW hot wire applications to increasethe energy input and accommodate the highfiller metal deposition rates of the process.16

Argon/Hydrogen BlendsPraxair’s HydroStar BlendsHydrogen is often added to argon to enhanceits thermal properties. Hydrogen’s reducingcharacteristics also improve weld puddlewetting and produce cleaner weld surfacesdue to reduced surface oxidation. Hydrogenenhanced blends are commonly selected toweld 300 series stainless steels.available austenetic stainless steels. It maybe substituted for pure argon in manyapplications.Praxair’s HydroStar H-2 and H-5These blends are used for manual GTAWapplications on 300 series stainless steels.HydroStar H-5 blend is preferred on materialthicknesses above 1/16 inch. These blends arealso used for back purging on stainless pipe.The higher arc voltage associated withhydrogen increases the difficulty of startingthe arc. For this reason, the smallest additionof hydrogen consistent with the desired resultis recommended. Additions up to 5% formanual welding and up to 10% for mechanizedwelding are typical. Ratios beyond thislevel typically cause porosity in GTAW.Argon/hydrogen blends are primarily usedon austenitic stainless steel, nickel, and nickelalloys. Hydrogen is not used to weld carbonor low-alloy steel, copper, aluminum, ortitanium alloys since cracking or porositywill result from the absorption of hydrogen.WarningSpecial safety precautions are requiredwhen mixing argon and hydrogen. DO NOTattempt to mix argon and hydrogen fromseparate cylinders. See the Safety section inthis handbook for more information.Higher ratios of hydrogen may be mixed withargon (up to 35%) depending on the processselected. For example Praxair’s HydroStarH-35 (65%Ar/35% H 2 ) is commonly used inplasma gouging.Praxair’s HydroStar H-10This blend is preferred for high-speedmechanized applications. It is used with 300series stainless steels.Praxair’s HydroStar H-15This blend, which contains 15% hydrogen,is used most often for welding butt jointsin stainless steel (300 series) at speeds comparableto helium, and is typically 50 percentfaster when compared with argon. HydroStarH-15 blend is frequently used to increase thewelding speeds in stainless steel tube mills.It can be used on all thicknesses of stainlesssteel, although concentrations greater than15% may cause weld metal porosity.Praxair’s HydroStar H-35This blend, which contains 35% hydrogen, isused most often for conventional plasma arccutting and gouging of stainless steel.Oxygen and Carbon DioxideThese gases are chemically reactive andshould not be used with GTAW. Their highoxidation potential can cause severe erosionof the tungsten electrode at arc temperatures.Praxair’s HydroStar blends are hydrogenenhancedargon-based blends which areideally suited for general purpose manual andmechanized GTAW of most commerciallySee Table 3, page 18 for the GTAW ShieldingGases Selection Guide.17

▲Table 3 –Shielding GasesSelection Guidefor GTAWMaterial Weld Type Recommended DescriptionShielding GasMild Steel Spot Argon Long electrode life; better weld nuggetcontour; easiest arc startingManual Argon Best puddle control, especially forout-of-position weldingMechanized Argon/Helium High speeds; lower gas flows than withpure heliumHelium Higher speeds than obtained with argon;improved penetrationAluminum Manual Argon Best arc starting, good cleaning action andandweld quality; lower gas consumptionMagnesium Argon/Helium Higher welding speeds, greater weldpenetration than argonMechanized Argon/Helium Good weld quality, lower gas flow than requiredwith straight helium, improved penetrationHelium (dcsp) Deepest weld penetration and greatest weldspeeds; can provide cleaning action foraluminum and magnesium weldingStainless Spot Argon Excellent control of penetration on lightSteelgauge materialsArgon/Helium Higher heat input for heavier gauge materials;faster travel speeds, improved weld puddlefluidityManual Argon Excellent puddle control,controlled penetrationMechanized Argon Excellent control of penetration onlight gauge materialsArgon/Helium Higher heat input, higher weldingspeeds possibleArgon/Minimizes undercutting; produces desirableHydrogen weld contour at low current levels, requireslower gas flows, ideal as a back purge gas on300 series stainless steelNitrogen/ Suitable for back purging on 300 seriesHydrogen stainless alloysCopper, Argon Excellent puddle control, penetration and beadNickel andcontour on thin gauge metalCu-Ni Alloys Argon/Helium Higher heat input to offset high heatconductivity of heavier gauges,faster travelspeedsHelium Highest heat input for sufficient weldingspeed on heavy metal sectionsNote:Argon/helium blendsusually require a watercooled torch and largertungsten diameter due toincreases in arc voltage.Titanium Argon High gas density provides better shieldingArgon/Helium Better penetration for manual welding of thicksections (inert gas backing required to shieldback of weld against contamination)Silicon Bronze Argon Reduces cracking of this hot short metal18Aluminum Bronze Argon Controlled penetration of base metal

C H A P T E R 33Plasma Arc Processes (PAW and PAC)▲Plasma ArcProcess DescriptionWelding (PAW)ShieldingGasShieldingGasPlasmaGasPlasma Arc Welding (PAW) is an evolutionarystep in the overall development of GTAW.Basically, the process uses an open, unrestrictedgas tungsten arc that is “squeezed” througha copper nozzle. The result is a “constricted”TungstenElectrodearc that is longer, thinner, and more focusedthan a GTAW arc. Figure 15 illustrates theessential difference between the GTAW andPAW processes.The constriction process greatly increasesarc voltage and the amount of ionization that▲GTAW – Open ArcPAW – Constricted Arctakes place. In addition to raising arc temperature,the hottest area of the plasma isextended farther down toward the workFigure 15 –surface (figure 16). The overall result is aComparison ofmore concentrated heat source at a higherGTAW with PAWarc temperatures that greatly increases heattransfer efficiency; this promotes fastercutting and welding speeds.PAW is defined as “an arc welding processTemperatures, K10,000 - 14,00014,000 - 18,00018,000 - 24,00024,000 and upthat uses a constricted arc between a nonconsumableelectrode and the weld pool (transferredarc) or between the electrode and theconstricting nozzle (nontransferred arc),see figure 17. Shielding is obtained from theionized gas supplied to the torch, which may▲GTAWPAWbe supplemented by an auxiliary source ofshielding gas. The process is used without theapplication of pressure.”Figure 16 –Arc temperature profile19

The plasma process can produce two types ofShieldingGasPlasmaGasShieldingGasPlasmaGasarcs. If the constricted plasma arc is formedbetween the electrode and the workpiece,it is said to have a “transferred arc.” If thearc is produced between the electrode andthe constricting nozzle, it is called “nontransferredarc.” See figure 17.Plasma arcs have an extremely wide range ofoperation. The nontransferred arc is used inspecial welding applications where it is notdesirable to make the workpiece part of theelectric circuit. It is also used for fusing non-▲TransferredNon-Transferredmetallic materials, such as ceramics andcertain types of glass. Operating currentsrange from 2 to 300 amps.Figure 17 –The two plasma arcsWith the transferred arc, two basic weldingmethods are used: the Melt-in mode, (whichcan be used with or without filler metal), andKeyhole mode. See figure 18 for illustrationsof these methods.▲Melt-in Weldingwithout filler metalFigure 18 –Plasma arc welding modesMelt-in Weldingusing filler metalKeyhole WeldingAlthough similar to GTAW, the Melt-inmethod has some advantages due to its longer,more constricted arc shape. These includeimproved arc stability (particularly at lowcurrent levels), less distortion of the workpiece,higher potential welding speed, andgreater tolerance to changes in torch-to-workdistance. As shown in figure 19, the change inarc plasma area with a change in stand-offdistance, is much greater with GTAW thanwith PAW. This has a major effect on heatingof the work and, subsequently, on penetrationand weld shape.20

GTAWPAWIn Keyhole welding, the workpiece is fusedthrough its entire thickness. The plasma jetpierces through the molten metal giving 100%penetration and forms a welding eyelet (seefigure 20) which moves together with the arcin the direction of welding. Behind the plasmajet, the molten metal flows together again (asa result of surface tension), solidifies, andforms the completed weld.▲ArcLengthArcPlasmaAreaFigure 19 –Variation of heating effectwith stand off distancesArcPlasmaStand-offDistance (L)Application of PAWHigh-quality welds can be made with nickel,nickel-copper, nickel-iron-chromium, copper,heat resisting titanium, refractory alloy steels,and in nickel-chromium alloys up to approximately0.3 inches thick. The process showsits greatest advantage when the keyholeapproach is used in the thickness range of0.062 to 0.0312 inches. The high-quality weldproduced by the single pass keyhole techniqueis illustrated in figure 21.Keyhole▲Figure 20 –PAW Keyhole weldingTorch Travel▲Figure 21 –Cross-section of Keyhole weld21

Shielding Gases for PAWThe physical configuration of PAW requiresthe use of two gases, a “plasma” or orifice gasand a shielding gas. The primary role of theplasma gas, which exits the torch through thecenter orifice, is to control arc characteristicsand shield the electrode. It also effects theheat transfer properties to the base metal. Theshielding gas, introduced around the peripheryof the arc, shields or protects the weld. Inmany applications, the shielding gas is alsopartially ionized to enhance the plasma gasperformance.Low current (< 100 amps)Argon is the preferred orifice gas becauseits low ionization potential ensures easy andreliable starting. Argon/helium and argon/hydrogen mixtures are also used for applicationsrequiring higher heat input.The choice of shielding gas is dependent onthe type and thickness of the base material.When welding aluminum, carbon steel, andcopper, the gases commonly used are argon,helium, and argon/helium mixtures. It isgenerally recommended that the percentageof helium be increased as the base-platethickness increases. When welding low alloysteels, stainless steels, and nickel alloys, theaforementioned gases in addition to argonhydrogenmixtures are used. See Table 4,below for low-current gas selection.High Current (> 100 amps)The choice of gas used when performing highcurrent plasma arc welding also depends onthe composition of the material to be welded.In all but a few cases, the shielding gas is thesame as the orifice gas.▲Table 4 –Low-CurrentPlasma ArcWelding GasSelection GuideMaterial Thickness Keyhole Melt-inAluminum Under 1/16" Keyhole tech. not recommended Argon or HeliumOver 1/16" Helium HeliumCarbon Steel Under 1/16" Keyhole tech. not recommended Argon, Helium or HeliStar A-75(Al. killed) Over 1/16" Argon or HeliStar A-25 Argon or HeliStar A-25Low Alloy Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5Steel Over 1/16" Argon or HeliStar A-25 Argon or HeliumStainless Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5Steel Over 1/16" Argon, HeliStar A-25 Argon, Helium, HydroStar H-5HydroStar H-2 or H-5Copper Under 1/16" Keyhole tech. not recommended Helium or HeliStar A-75Over 1/16" Helium or HeliStar A-25 HeliumNickel Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5Alloys Over 1/16" Argon, HeliStar A-25 Argon, Helium, HydroStar H-522Reactive Under 1/16" Keyhole tech. not recommended ArgonMaterials Over 1/16" Argon, Helium or HeliStar A-25 HeliStar A-75

ArgonArgon/Helium BlendsArgon is suitable as the orifice and shieldinggas for welding all metals, but it does notnecessarily produce optimum welding results.In the Melt-in mode, additions of hydrogento argon produce a hotter arc and offer moreefficient heat transfer to the work. Limits onthe percentage of hydrogen are related to itspotential to cause cracking and porosity.However, when using the Keyhole technique,a given material thickness can be welded withhigher percentages of hydrogen. This may beassociated with the Keyhole effect and thedifferent solidification pattern it produces.Argon is used for welding carbon steel, highstrength steel, and reactive metals such astitanium and zirconium alloys. Even minutequantities of hydrogen in the gas used to weldthese materials may result in porosity, cracking,or reduced mechanical properties.Praxair’s HeliStar BlendsHelium additions to argon produce a hotterarc for a given arc current. Argon/heliummixtures containing between 50% and 75%helium are generally used to make keyholewelds in heavier titanium sections and for filland capping passes on all materials when theadditional heat and wider heat pattern ofthese mixtures prove desirable.Argon-Hydrogen BlendsPraxair’s HydroStar BlendsArgon/hydrogen mixtures are used as theorifice and shielding gases for makingkeyhole welds in stainless steel, Inconel,nickel, and copper-nickel alloys. Permissiblehydrogen percentages vary from 5% to 15%.See Table 5, below for high-current gasselection.▲Table 5 —High-CurrentPlasma ArcWelding GasSelection GuideMaterial Thickness Keyhole Melt-inAluminum Under 1/4" Argon Argon or HeliStar A-25Over 1/4" Helium Helium or HeliStar A-25Carbon Steel Under 1/8" Argon Argon(Al. killed) Over 1/8" Argon HeliStar A-25Low Alloy Under 1/8" Argon ArgonSteel Over 1/8" Argon HeliStar A-25Stainless Under 1/8" Argon or HydroStar H-5 ArgonSteel Over 1/8" Argon or HydroStar H-5 HeliStar A-25Copper Under 3/32" Argon Helium or HeliStar A-25Over 3/32" Keyhole tech. not recommended* HeliumNote:Gas selections shownare for both the orificeand shielding gas.Nickel Under 1/8" HydroStar H-5 ArgonAlloys Over 1/8" HydroStar H-5 HeliStar A-25Reactive Below 1/16" Argon ArgonMaterials Above 1/16" Argon or Argon/Helium HeliStar A-25* The underbead will not form correctly.However, on Cu-Zn alloys a keyhole technique can be used.23

▲Plasma ArcCutting (PAC)Process DescriptionPlasma Arc Cutting is defined as “an arccutting process that severs metal by melting alocalized area with a constricted arc whichremoves the molten material with a highvelocity jet of hot, ionized gas issuing fromthe constricting orifice.”The major difference between PAC and PAWis the velocity of the orifice gas. In somecases, a shielding gas as well as a cutting, ororifice gas may be used (the shielding gasprevents oxidation of the cut surface.) Thehigher velocity gas used in PAC removes orblows away the molten material. The PACprocess can be used to cut any electricallyconductive metal if its thickness and shapepermit full penetration by the plasma jet.Because the PAC process can be used to cutnonferrous materials, and is faster than oxyfuelcutting in the less-than-two-inch thicknessrange for ferrous materials, it is ideal formany industrial applications.Since PAC was introduced by Praxair in 1954,many process refinements, gas developments,and equipment improvements have occurred.The following sections describe the processvariations that are in use today.Conventional Plasma Arc CuttingIn conventional Plasma Arc Cutting, the arcis constricted by a nozzle only; no shieldinggas is added. Generally the cutting gas(usually nitrogen or air) is tangentially injectedaround the electrode (see figure 22).The swirling action of the gas causes thecooler (heavier) portions of the gas to moveradially outward, forming a protective boundarylayer on the inside of the nozzle bore. Thishelps prevent nozzle damage and extends itslife. Electrode life is also improved since thearc attachment point (cathode spot) is forcedto move about and distribute its heat loadmore uniformly. Until the introduction ofWater Injection Plasma Arc Cutting in 1970(see page 26) conventional Plasma Arc Cuttingwas the most popular technique. It is stillthe best method for cutting thicker stainlessand aluminum plate.▲Figure 22 –Conventional Plasma Arc CuttingPlasmaJetElectrodeWorkpieceNozzleAir Plasma Arc CuttingAir Plasma Arc Cutting was introduced in theearly 1960s for cutting mild steel. Oxygen inthe air provides additional energy by creatingan exothermic reaction with molten steel,boosting cutting speeds about 25 percent.Although this process can also be usedto cut stainless steel and aluminum, the cutsurface will be heavily oxidized and is oftenunacceptable for many applications. Electrodeand tip life are also reduced when comparedto the use of nitrogen as the plasma gas.24

NozzleElectrodePlasma Gas InletShieldingGas InletShieldOxygen Plasma Arc CuttingIn Oxygen Plasma Arc Cutting, oxygen isused as the plasma (orifice) gas in place ofnitrogen or air. The oxygen in the plasmastream has a similar effect on steel as withoxyfuel cutting; it produces an exothermicreaction which increases cutting speed. It ispossible to achieve cutting speeds similar tonitrogen at much lower currents. Oxygenplasma cutting is used primarily on mild steel.▲Figure 23 –Oxygen PAC nozzleWorkpieceLimitations of Oxygen Plasma Arc CuttingThe conventional PAC process (with nitrogen)uses tungsten electrodes which cannot be usedin an oxygen environment. Halfnium is substitutedas the electrode material for oxygencutting. The halfnium must be kept cool andthe current capacity of the torch limited toensure longer life (see figure 23).NozzleElectrodeShield Gas orWater ShieldShield CupDual-Flow Plasma Arc CuttingDual-Flow Plasma Arc Cutting is a slightmodification of conventional Plasma ArcCutting (see figure 24). It incorporatesmost of the features of conventional PlasmaArc Cutting, but adds a secondary shieldinggas around the nozzle.▲Figure 24 –Dual-flow Plasma Arc CuttingPlasma JetWorkpieceUsually the cutting gas is nitrogen and theshielding gas is selected according to themetal to be cut. Cutting speeds are slightlybetter than “conventional” plasma arc cuttingon mild steel; however, cut quality is notacceptable for many applications. Cuttingspeed and quality on stainless steel andaluminum are essentially the same as inconventional Plasma Arc Cutting.25

▲Figure 25 –WaterInjectionPlasma ArcCuttingWater Injection Plasma Arc CuttingIn Water Injection Plasma Arc Cutting, wateris introduced inside the nozzle to provideadditional arc constriction (see figure 25)and nozzle cooling.Two modes of water injection have beendeveloped: Radial Injection (the waterimpinges the arc with no swirl component),and Swirl Injection (the water is introduced asa vortex swirling in the same direction as thecutting gas).The increased arc constriction provided by thewater improves cut squareness and increasescutting speed. The water also protects thenozzle since it provides cooling at the pointof arc constriction. The water completelyprotects the bottom half of the nozzle fromintense radiation, allowing complete insulationof the nozzle; hence, resistance to damageis greatly improved. This approach ensurescomponent durability, superior cut qualityand high cutting speeds.Underwater Plasma Arc CuttingUnderwater Plasma Arc Cutting is ideallysuited to numerically-controlled shape cuttingWater Injection(Radial or Swirl)ElectrodeNozzleCeramicPlasma Jetand produces a comfortable noise level of85 dBA or less under normal operatingconditions. (Conventional Plasma Arc Cuttingtypically produces noise levels in the rangeof 105 to 115 dBA.) Underwater cuttingvirtually eliminates the ultraviolet radiationand fumes associated with conventionalPlasma Arc Cutting.In underwater PAC, the steel plate being cutis supported on a cutting table with the topsurface of the plate two to three inches beneaththe surface of the water. A device thatlocates the submerged top surface of the metalis critical to this fully-automated process.Accurate height control is maintained by asensor that monitors arc voltage. Cuttingspeed and quality are comparable to thoseattained with plasma arc cutting by waterinjection.WarningIt is hazardous to cut aluminum underwater.Hydrogen generated by the processcan be trapped under the plate creating thepotential for explosion.Precision Plasma Arc CuttingPrecision Plasma Arc Cutting utilizes an improvednozzle design to increase arc constrictionand dramatically increase energy density.Because of the higher energy density, the edgequality and squareness of the cut is improved,particularly on thinner material (3/8").Recent developments in plasma torch designallow the operator to drag the nozzle on thematerial surface without the arcing problemsnormally associated with other PAC processvariations (see figure 26).WorkpieceThe Precision Plasma Arc Cutting process isemployed in cutting sheet in the range of 20gauge to 3/8". Conventional plasma can cut upto 2" thicknesses.26

Gas Flow RatesThe orifice gas will often have a lower flowrate than the shielding gas, but both will varyas changes in cutting current are made toaccommodate different base metals and thicknesses.Most PAC equipment use only anorifice gas with no shielding gas.Gas flow with most PAC equipment iscontrolled by a gas pressure regulator and aflowmeter. The range of gas flow can varybetween 1.0 and 100 standard cubic feet perhour (scfh) for the orifice gas and 8.0 and 200scfh for the shielding gas, as determined bythe cutting requirements. Because PAC equipmentdesign can vary significantly betweenmodels, specific flow rates are not listed here.Shielding and Cutting Gases for PACInert gases, such as argon, helium, andnitrogen (except at elevated temperatures) areused with tungsten electrodes. Air may beused as the cutting gas when special electrodes,made from water-cooled copper withhigh temperature resistant inserts of metalslike hafnium, are used. Recently, PAC unitsshielded by compressed air have beendeveloped to cut thin gauge materials.Virtually all plasma cutting of mild steel isdone with one of four gas types: (1) Air,(2) Nitrogen with carbon dioxide shielding orwater injection (mechanized), (3) Nitrogen/oxygen or air, and (4) Argon/hydrogen andnitrogen/hydrogen mixtures. The first twohave become the standard for high-speedmechanized applications. Argon/hydrogen andnitrogen/hydrogen (20% to 35% hydrogen)are occasionally used for manual cutting, butdross formation is a problem with the argonblend. Dross is a tenacious deposit of resolidifiedmetal attached at the bottom of thecut. A possible explanation for the heavier,more tenacious dross formed in argon is thegreater surface tension of the molten metal.The surface tension of liquid steel is 30 percenthigher in an argon atmosphere than innitrogen. Air cutting gives a dross similar tothat formed in a nitrogen atmosphere.▲Figure 26 –High FlowVortex NozzleWorkpieceElectrodePlasma Gas InletPlasmaGas VentShieldingGas InletDuring cutting, the plasma jet tends to removemore metal from the upper part of the workpiecethan from the lower part. This results incuts with non-parallel cut surfaces which aregenerally wider at the top than at the bottom.The use of argon/hydrogen, because of itsuniform heat pattern or the injection of waterinto the torch nozzle (mechanized only), canproduce cuts that are square on one side andbeveled on the other side. For base metalover three inches thick, argon/hydrogen isfrequently used without water injection. Airis used as a low cost plasma gas, but specificprecautions must be taken to ensure that it ismoisture and oil free. Table 6, page 28 liststhe combinations of orifice and shieldinggases that may be used with PAC.Precision PAC nozzle27

▲Table 6 –Thickness RangeGas SelectionGuide forPlasma ArcAirOrifice1/4" 1/2" 1" 2" 3" 4" 5" 6"CuttingAuxiliaryNitrogen*OrificeKeyAuxiliaryCarbon SteelStainless Steeland Nickel AlloysOxygenOrificeAuxiliaryAluminumCarbon DioxideOrificeAuxiliaryHydroStarH-35OrificeAuxiliaryArgon/NitrogenOrificeAuxiliary* For Water Injection Plasma Cutting, nitrogen is the preferred plasma gas.Notes – Depending upon equipment type the following applies:(1) An orifice gas is often used with no auxiliary gas.(2) When multiple auxiliary gases are shown for a single orifice gas, only one auxiliary gas applies for a given application.(3) Cutting speed and quality can vary with gas selection.(4) This table is a composite based on gas requirements for currently available PAC equipment.Use manufacturer’s recommendations for selecting gases.28

C H A P T E R 44Gas Metal Arc Welding (GMAW)▲ProcessDescriptionWeldingTorchElectrodeWire FeedSolidifiedWeld MetalGas Metal Arc Welding is defined as “anelectric arc welding process that producescoalescence of metals by heating them withan arc between a continuous filler metalelectrode and the workpiece. Shielding isobtained entirely from an externally suppliedgas.” Figure 27 shows the essential elementsof a basic GMAW welding process.▲ConsumableElectrodeFigure 27 –Basic GMAW weldingSpool ofWireElectricArcMolten WeldGasEnvelopeWorkpieceGMAW is used to weld all commerciallyimportant metals, including steel, aluminum,copper, and stainless steel. The process canbe used to weld in any position, including flat,vertical, horizontal, and overhead. It is usuallyconnected to use direct current electrodepositive (DCEP). It is an arc welding processwhich incorporates the automatic feeding ofa continuous, consumable electrode that isshielded by an externally supplied gas (seefigure 28). Since the equipment provides forautomatic control of the arc, the only manualcontrols required by the welder for semiautomaticoperation are gun positioning, guidanceand travel speed.WirefeederTorch orGunPowerSupplyArcWorkpiece▲Figure 28 –Basic GMAW system29

▲MetalTransferModes inGMAWThe GMAW process has five distinctive metaltransfer modes:• Short circuiting• Globular• Spray• Pulsed spray• High-current density (rotational andnonrotational) spray.The metal transfer mode is determined bymany factors, including operating current,wire diameter, arc length or voltage, powersupply characteristics, and shielding gas.Short-Circuit Gas Metal Arc Welding(GMAW-S)GMAW-S is defined as “a gas metal arcwelding process variation in which theconsumable electrode is deposited duringrepeated short circuits.”In the short-circuiting mode, metal transferoccurs when the electrode is in direct contactwith the weld pool. In this mode of metaltransfer, the relationship between the electrodemelt rate and its feed rate into the weld zonedetermines the intermittent establishment ofan arc and the short circuiting of the electrodeto the workpiece.30▲DropFigure 29 –Short-circuitingtransferMeltRateFeedRateArc No Arc New ArcBefore Transfer During Short Circuit After TransferSpecifically, the electrode is fed at a constantspeed at a rate that exceeds the melt rate.When it contacts the molten pool a shortcircuit occurs, at which time there is no arc.The current then begins to rise and heats thewire to a plastic state. At the same time, thewire begins to deform or neck down due to anelectromagnetic pinch force. Eventually, thecurrent value and resulting pinch force causesa drop of metal to transfer into the weldpuddle. At this point, an arc is established.This sequence repeats itself approximately50 to 250 times per second (see figure 29).Since there is less “arc on time” establishedduring the short circuit, the overall heat inputis low, and the depth of fusion is relativelyshallow; thus, care must be exercised inselecting the procedure and weld technique toassure complete fusion when welding thickermaterials. Due to its low heat input characteristics,the process produces a small, fastfreezingweld puddle which make it ideal forwelding in all positions. Short-circuitingtransfer is also particularly adaptable to weldingsheet metal with minimum distortion andfor filling gapped or poorly fitted parts withless tendency for burn-through of the partbeing welded.

Globular TransferGlobular Transfer is characterized by thetransfer of molten metal in large drops acrossthe arc. This transfer mode takes place whenthe current and arc voltage are between theshort-circuiting and spray transfer current andvoltage levels; it occurs with all types ofshielding gas. Carbon dioxide yields this typeof transfer at all usable welding currentsabove the short circuiting range. Globulartransfer is characterized by a drop sizeapproximately two to four times greater thanthe diameter of the electrode (see figure 30).With carbon dioxide, the droplet is notpropelled across the arc, due to the repellingforces acting upward toward the wire tip.These forces tend to hold the droplet on theend of the wire. During this time the dropgrows in size and eventually either transfersby gravity due to its weight, or short circuitsacross the arc gap.Spray Transfer▲Figure 30 –Globular transferIn Spray Transfer, the molten metal ispropelled axially across the arc in smalldroplets. In a gas blend of at least 80% argon(see table 7, page 39), when combined withthe proper operating conditions, the electrodemetal transfer changes from globular to aspray or spray-like mode. The minimumcurrent and voltage levels required vary forany given electrode diameter. The changetakes place at a value called the globular-spraytransition current. Spray transfer in argon ischaracterized by a constricted arc column andpointed electrode tip (see figure 31).▲Figure 31 –Spray transferMolten metal transfers across the arc as smalldroplets equal to or less than the electrodediameter. The metal transfer is axially directedto the workpiece. Since the metal droplets aresmall, the transfer rate can be as high asseveral hundred droplets per second. Due topuddle fluidity, spray transfer is limited to theflat or horizontal welding position.31

Pulsed Gas Metal Arc Welding(GMAW-P)No WireNeckingDropletTransfersWithPulseIn this variation, the power source providestwo output levels: a steady background level,too low in magnitude to produce any transfer,but able to maintain an arc; and a pulsed,high-output level which causes melting ofdroplets from the electrode which are thentransferred across the arc. This pulsed highoutput (peak) occurs at regular controlledintervals. The current can be cycled between ahigh and low value at up to several hundredcycles per second. The net result is to producea spray arc with average current levels muchbelow the transition current required for aparticular diameter and type of electrode.▲Figure 32 –Pulsed spray transferIn pulsed spray welding the shielding gasmust be able to support spray transfer. Metalis transferred to the workpiece only during thehigh current pulse. Ideally, one droplet istransferred during each pulse (see figure 32).The pulsing rate can be varied depending onthe base material, thickness, wire diameterand weld position. The control of backgroundcurrent maintains the arc and heat input. Theresulting lower average current level allowsthe joining of base metals less than 1/8 inchthick with a spray type metal transfer. Pulsedspray welding may be used in all positions.Welding fume levels are the lowest obtainablewith solid wire GMAW.32

High Current Density Metal TransferHigh current density metal transfer is a namegiven to a GMAW process having specificcharacteristics created by a unique combinationof wire feed speed, wire extension,and shielding gas. Weld metal deposition ratescan range between 10 and 30 pounds per hour,whereas most GMAW Spray Arc is in the 8 to12 pound/hr range. The arc characteristics ofhigh density metal transfer are further dividedinto rotational spray transfer and nonrotationalspray transfer.When using a solid carbon steel wire, a highwire feed speed is combined with a longelectrode extension and an argon/carbondioxide/oxygen shielding gas to create an arcphenomenon known as rotational spray arctransfer. The long electrode extension createshigh resistance heating of the wire electrodecausing the electrode end to become molten.The electro-mechanical forces generatedby the current flow in the wire cause themolten wire end to rotate in a helical path(see figure 33).The shielding gas affects the rotationaltransition current by changing the surfacetension at the molten electrode end. Praxair’sStargon and StarGold O-5 blends (see pages36-38) produce rotational spray transfer atdeposition rates of 10 to 30 pounds per hourwith 0.035 and 0.045 diameter wires usingcontact-tip to workpiece distances (ESO) of7/8 inch to 1 1/2 inch.▲Figure 33 –Rotational spray transferNonrotational spray high current densitytransfer is produced when the molten wire enddoes not rotate. This also develops a depositionrate range of 10 to 30 pounds per hour.Rotation is suppressed when the thermalconductivity of the shielding gas increases andthe surface tension of the molten electrode endincreases. The droplet rate decreases resultingin larger droplets across the arc. Shieldinggases with carbon dioxide or helium additions,such as Praxair’s Stargon and HeliStar CSshielding gases (see page 38), will raise therotational spray transition current and suppressthe tendency for the arc to rotate. Thearc appears elongated and diffuse but lookssimilar to conventional spray transfer. Theplasma stream is axial and narrower thanrotational spray transfer. Because the heatsource is more concentrated, the depth offusion is greater than rotational spray transferat the same welding current.33

▲Metal-CoredElectrodesMetal-cored wire welding is considered avariation of GMAW. A metal-cored wireoperates like a solid wire, has generally lowfume levels, no slag and a high depositionefficiency (95 percent or better) despite itscored-type construction.A metal-cored wire is a composite filler metalelectrode consisting of a metal tube filledwith alloying materials. These metal powdersprovide arc stabilization and fluxing of oxides.Metal-cored wires provide high depositionrates with excellent deposition efficiency, andcan be used to weld in all positions. They areused successfully in applications where fit-upis poor.Metal-cored wires are designed to give qualitywelds over some rust and mill scale usingargon-based shielding gases. They generallyhave a higher level of deoxidizers whichprovides a good bead profile with excellentpuddle control. This type of wire combines thehigh deposition rate of flux-cored wires withthe approximate deposition efficiency andfume levels of a solid wire. The weld metalmechanical properties are comparable tocarbon steel solid wires and, as with solidwires, little slag is formed on the weld beadsurface.The advantages of metal-cored wires are:• High deposition rate• High deposition efficiency(95 percent or better)• Quality welds over light rust and mill scale• Low spatter levels• Little slag clean-up• Easy to use• All position welding capability• Low fume levels• Greater resistance to undercut• Improved performance with poorbase metal fit up• Large variety of alloys availableMetal-cored carbon steel wires operate bestin an argon/carbon dioxide blend (8 to 20%carbon dioxide) or an argon/carbon dioxide/oxygen blend, while stainless wires operatewell with a argon/oxygen (1 to 2% oxygen)or an argon/CO 2 blends (2 to 10%).34

▲Shielding Gasesfor GMAWArgonArgon is used on nonferrous base metals suchas aluminum, nickel, copper, magnesiumalloys, and reactive metals, such as zirconiumand titanium. Argon provides excellent arcwelding stability, penetration, and bead profileon these base metals. When welding ferrousbasedmetals, argon is usually mixed withother gases, such as oxygen, helium, carbondioxide, or hydrogen.The low ionization potential (good electricalconductivity) of argon helps create an excellentcurrent path and superior arc stability.Argon produces a constricted arc column withhigh current density which causes the arcenergy to be concentrated over a small surfacearea. The result is a penetration profile, havinga distinct “finger like” shape, as shown infigure 34.Carbon Dioxide▲Figure 34 –GMAW weld bead profileswith several shielding gases% Spatter (by weight)Argon/O 2 Argon Helium/Argon Helium1210864Spatter and Deposition Efficiency.045 (1.1 mm)ØCarbon and Low AlloySteel WiresCO 2STARGON8890929496Deposition Efficiency %Carbon dioxide, a reactive gas, dissociatesinto carbon monoxide and free oxygen in theheat of the arc. Oxygen then combines withelements transferring across the arc to formoxide in the form of slag and scale, and alsohelps to generate a great deal of fumes.Although carbon dioxide is an active gas andproduces oxidation of the weld material,sound welds can be consistently achievedwith careful filler metal selection.Carbon dioxide is often used for weldingcarbon steel because it is readily availableand produces good welds at low cost. However,the low cost per unit of gas does notalways translate to the lowest cost per foot ofdeposited weld. Other factors, such as lowerdeposition efficiency due to spatter loss,high levels of welding fume, poor weldbead profile and reduced tensile strength caninfluence the final weld cost and should becarefully considered.▲20 100 200 300Welding Current (Amps DGEP)To approximate spatter for 0.062 inches (1.6 mm)Ø wire add 50A to welding current.To approximate spatter for 0.035 inches (0.89 mm)Ø wire subtract 50A to welding current.Figure 35–Typical spatter levels withtwo common shielding gases98Carbon dioxide will not support spray transfer.Metal transfer is restricted to the short circuitingand globular modes. A major disadvantageof carbon dioxide is harsh globular metal transferwith its characteristic spatter (see figure 35).The weld surface resulting from use of carbondioxide shielding is usually heavily oxidized.An electrode with higher amounts of deoxidizingelements is needed to compensate forthe loss of alloying elements across the arc. 35

Welded parts may require a cleaning operationprior to painting which can more than offsetthe lower cost of CO 2 shielding gas. Theadvantages of carbon dioxide are good depthand width of fusion and the achievement ofacceptable mechanical properties.HeliumHelium is a chemically inert gas that is usedfor welding applications requiring higher heatinputs. Helium may improve wetting action,depth of fusion, and travel speed. It does notproduce the stable arc provided by argon.Helium has greater thermal conductivity thanargon and produces a wider arc column.The higher voltage gradient needed for stableoperation generates a higher heat input thanargon, promoting greater weld pool fluidityand better wetting action. This is an advantagewhen welding aluminum, magnesium, andcopper alloys.properties and corrosion resistance of weldsmade with 1% and 2% oxygen additions aresimilar. However, bead appearance will bedarker and more oxidized for the 2% blendswith stainless steels.Praxair’s StarGold O-5This blend provides a more fluid but controllableweld pool. It is the most commonlyused argon/oxygen mixture for general carbonsteel welding. The additional oxygen permitshigher travel speeds.Argon/Carbon Dioxide BlendsPraxair’s StarGold andMig Mix Gold BlendsArgon/carbon dioxide blends are used withcarbon, low-alloy and some stainless steels.Greater amounts of carbon dioxide whenadded to argon and used at higher currentlevels, increase spatter.Argon/Oxygen BlendsPraxair’s StarGold BlendsThe addition of small amounts of oxygen toargon greatly stabilizes the welding arc, increasesthe metal transfer droplet rate, lowersthe spray transition current, and enhancesbead shape. The weld pool is more fluid andstays molten longer, allowing the metal toflow out towards the weld toes. Welding fumemay be reduced with these mixtures.Praxair’s StarGold O-1Primarily used for spray transfer on stainlesssteels, one percent oxygen is usually sufficientto stabilize the arc and improve the dropletrate and bead appearance.In conventional GMAW, slightly highercurrent levels must be exceeded when usingargon/carbon dioxide in order to establish andmaintain stable spray transfer. Above approximately20% carbon dioxide, spray transferbecomes unstable and periodic short-circuitingand globular transfer occurs.Praxair’s StarGold C-5Used for pulsed spray transfer and conventionalspray transfer with a variety of materialthicknesses. A 5% mixture may be used forGMAW-P of low alloy steels for out-ofpositionwelding. This blend provides goodarc stability when welding over mill scale anda more controllable puddle than a argon/oxygen blend.36Praxair’s StarGold O-2This blend is used for spray arc welding ofcarbon steels, low-alloy steels and stainlesssteels. It provides better wetting action thanthe 1% oxygen mixture. Weld mechanicalPraxair’s Mig Mix GoldThis blend performs similarly to C-5, but itsincreased heat input provides a wider, morefluid weld puddle in either short-circuit, sprayor pulsed spray transfer.

Praxair’s StarGold C-10This blend performs similarly to Praxair’sMig Mix Gold but its additional CO 2 contentprovides a wider, more fluid weld puddle.This blend is frequently recommended foruse with metal-cored wires.Praxair’s StarGold C-15Used for a variety of applications on carbonand low-alloy steel. In the short-circuitingmode, maximum productivity on thin gaugemetals can be achieved with this blend. Thisis done by minimizing the excessive meltthroughtendency of higher carbon dioxidemixes, while increasing deposition rates andtravel speeds. As the carbon dioxide percentagesare lowered from the 20% range (maximumspray arc levels), improvements indeposition efficiency occur due to decreasingspatter loss. This blend will support the sprayarc mode of transfer.Praxair’s StarGold C-20May be used for short circuiting or spraytransfer welding of carbon steel.Praxair’s StarGold C-25Commonly used for GMAW with shortcircuitingtransfer on carbon steel. It wasformulated to provide optimum dropletfrequency on short-circuiting transfer usingArgonHelium▲Figure 36 –Effect of argon and helium shielding gases onweld profile when welding aluminum (DCEP).035 and .045 diameter wire. Praxair’sStarGold C-25 blend operates well in highcurrent applications on heavy base metal. Itpromotes good arc stability, weld pool control,and weld bead appearance. This blend will notsupport spray type metal transfer. StarGoldC-25 can also be used with flux-cored wires(see manufacturer’s recommendations).Praxair’s StarGold C-40This mixture is recommended for use withsome flux cored wires where improved arcstability, reduced spatter levels and improvedperformance over light surface contaminationare desirable.Praxair’s StarGold C-50This mixture is used for short arc welding ofpipe, particularly when contaminants arepresent on the surfaces to be welded.Argon/Helium BlendsPraxair’s HeliStar BlendsHelium is often mixed with argon to obtainthe advantages of both gases. Argon providesgood arc stability and cleaning action, whilehelium promotes wetting with a greater widthof fusion.Argon/helium blends are used primarily fornonferrous base metals, such as aluminum,copper, nickel alloys, magnesium alloys, andreactive metals. Helium additions to an argonbasedgas increase the effective heat input.Generally, the thicker the base metal, the higherthe percentage of helium. Small percentagesof helium, as low as 20%, will affect thearc. As the helium percentage increases, therequired arc voltage, spatter, and weld widthto depth ratio increase, while porosity isminimized (see Figure 36). The argon percentagemust be at least 20% when mixedwith helium to produce and maintain a stablespray transfer.37

38Praxair’s HeliStar A-25This blend is used for welding nonferrousbase metals when an increase in heat input isneeded and weld bead appearance is ofprimary importance. It is ideal for bothGMAW and GTAW of aluminum alloys.Praxair’s HeliStar A-50Helistar A-50 blend is used primarily forhigh-speed mechanized welding of nonferrousmaterials under 3/4 inch thick.Praxair’s HeliStar A-75This blend is used for mechanized weldingof aluminum greater than 3/4" in the flatposition. It increases heat input and reducesporosity of welds made in copper and copperalloys.Argon/Oxygen/Carbon Dioxide BlendsPraxair’s Stargon BlendsThese three component mixtures provideversatility due to their ability to operate inshort-circuiting, globular, spray, pulsed orhigh-density transfer modes. Several ternarycompositions are available and their usedepends on the desired metal transfer modeand welding position.The advantage of this blend is its ability toshield carbon steel and low-alloy steel of allthicknesses using any metal transfer modeapplicable. Praxair’s Stargon blend producesstable welding characteristics and mechanicalproperties on carbon and low-alloy steels andsome stainless steels. On thin gauge basemetals, the oxygen constituent promotes arcstability at very low current levels (30 to 60amps) permitting the arc to be kept short andcontrollable. This helps minimize excessivemelt-through and distortion by lowering thetotal heat input to the base material. Stargon isgenerally used for spray arc welding, providinghigh deposition rates and often highertravel speeds than carbon dioxide.Argon/Helium/Carbon Dioxide BlendsPraxair’s HeliStar BlendsHelium and carbon dioxide additions to argonincrease the heat input to the weld, whichimproves wetting, fluidity, and weld beadprofile.Praxair’s HeliStar CSThis blend has been developed for spray andpulsed spray welding of both carbon and lowalloysteels. It can be used on all materialthicknesses in any position. This blend canproduce high quality welds over rust, oil andmill scale when compared with conventionaltwo-part mixtures. It produces good mechanicalproperties and weld puddle control.Praxair’s HeliStar SSHeliStar SS is used for short arc, spray, andpulsed spray arc welding of stainless steel.It provides a higher welding speed, a broadweld with a flat crown and good color match,reduced porosity, and excellent alloy retentionwith good corrosion resistance.Praxair’s HeliStar A-1025Helistar A-1025 blend is widely used forshort-circuiting transfer welding of stainlesssteel in all welding positions. The carbondioxide content is kept low to minimizecarbon pick-up and assure good corrosionresistance, especially in multipass welds. Theargon and carbon dioxide additions providegood arc stability and increased depth of weldfusion. The high helium content providessignificant heat input to overcome the sluggishnature of the stainless steel weld pool.See Table 8, pages 40-42 for a Gas SelectionGuide for GMAW.

▲Table 7 –Globular-to-spraytransitioncurrentsElectrode Type Wire Dia. Shielding Gas Minimum Spray(inch)Arc Current(amp)Low Carbon Steel 0.023 98% Argon/2% O 2 135(ER70S-3 or 0.030 98% Argon/2% O 2 150ER70S-6) 0.035 98% Argon/2% O 2 1650.045 98% Argon/2% O 2 2200.062 98% Argon/2% O 2 2750.035 95% Argon/5% O 2 1550.045 95% Argon/5% O 2 2000.062 95% Argon/5% O 2 2650.035 92% Argon/8% CO 2 1750.045 92% Argon/8% CO 2 2250.062 92% Argon/8% CO 2 2900.035 85% Argon/15% CO 2 1800.045 85% Argon/15% CO 2 2400.062 85% Argon/15% CO 2 2950.035 80% Argon/20% CO 2 1950.045 80% Argon/20% CO 2 2550.062 80% Argon/20% CO 2 345Stainless Steel 0.035 99% Argon/1% O 2 1500.045 99% Argon/1% O 2 1950.062 99% Argon/1% O 2 2650.035 Argon/Helium/CO 2 1600.045 Argon/Helium/CO 2 2050.062 Argon/Helium/CO 2 2800.035 Argon/Hydrogen/CO 2 1450.045 Argon/Hydrogen/CO 2 1850.062 Argon/Hydrogen/CO 2 255Aluminum 0.030 Argon 950.047 Argon 1350.062 Argon 180Deoxidized Copper 0.035 Argon 1800.045 Argon 2100.062 Argon 310Silicon Bronze 0.035 Argon 1650.045 Argon 2050.062 Argon 27039

▲Table 8 –Shielding Gas Selection Guide for GMAWMaterial Thickness Transfer Recommended DescriptionModeShielding GasCarbon Steel Up to 14 gauge Short Circuiting StarGold C-8, Good penetration and distortion control to reduceC-15, C-25potential burnthrough; good gap-bridging abilityStargon14 gauge – 1/8" Short Circuiting StarGold C-8, Higher deposition rates without burnthrough;C-15, C-25minimum distortion and spatter; good puddleStargoncontrol for out-of-position weldingOver 1/8" Short Circuiting StarGold C-15, High welding speeds; good penetration andC-25 puddle control; applicable for out-of-positionStargonweldsCO 2Globular StarGold C-8, Suitable for high current and high speed weldingC-25CO 2Short Circuiting StarGold C-50 Deep penetration; low spatter, high travel speeds;good out-of-position weldingShort Circuiting CO 2 Deep penetration and fast travel speeds but withand Globularhigher burnthrough potential; high current(Buried Arc)mechanized weldingSpray Arc StarGold O-1, Good arc stability; produces a more fluid puddleO-2, O-5as O 2 increases; good coalescence and beadStargoncontour; good weld appearance and puddle controlShort Circuiting StarGold C-5, Applicable to both short circuiting and sprayand Spray C-8 transfer modes; has wide welding current rangeTransferand good arc performance; weld puddle has goodcontrol which results in improved weld contourHigh Density Stargon Used for high deposition rate welding whereRotational StarGold C-8 15 to 30 lbs/hr is typical; special weldingTransfer HeliStar CS equipment and techniques are sometimes requiredto achieve these deposition levelsGauge Pulsed Spray StarGold C-5 Used for both light gauge and out-of-positionStargonweldments; achieves good pulsed spray stabilityHeliStar CSover a wide range of arc characteristics anddeposition rangesShort Circuiting Stargon Good coalescence and bead contour withStarGold C-5 excellent mechanical propertiesHeliStar CS40

▲Table 8 –Shielding Gas Selection Guide for GMAW (continued)Material Thickness Transfer Recommended DescriptionModeShielding GasAlloy Steel Up to 3/32" Short Circuiting StarGold C-8, High welding speeds; good penetration andC-15 puddle control; applicable for out-of-positionStargonwelds; suitable for high current and high speedweldingSpray Arc StarGold O-5 Reduces undercutting; higher deposition rates(High Current Stargon and improved bead wetting; deep penetrationDensity and HeliStar CS and good mechanical propertiesRotational)Over 3/32" Pulsed Spray StarGold O-2, Used for both light gauge and heavy out-of-C-5, C-8position weldments; achieves good pulsed sprayStargonstability over a wide range of arc characteristicsand deposition rangesStainless Steel* Up to 14 gauge Short Circuiting StarGold O-2 Good control of burnthrough and distortion; usedStargonalso for spray arc welding; puddle fluiditysometimes sluggish, depending on the base alloyOver 14 gauge Short Circuiting HeliStar SS, Low CO 2 percentages in He mix minimize carbonTransfer A-1025 pick-up, which can cause intergranular corrosionStarGold O-2 with some alloys; helium improves wetting action;StargonCO 2 percentages over 5% should be used withcaution on some alloys; applicable for allposition weldingSpray Arc HeliStar SS Good arc stability; produces a fluid butStarGold O-1, controllable weld puddle; good coalescence andC-5, C-10bead contour; minimizes undercutting on heavierMig Mix Gold thicknessesSpray Arc StarGold O-2 Can be used on more sluggish alloys to improvepuddle fluidity, coalescence and bead contourPulsed Spray HeliStar SS Used for both light gauge and heavy out-of-StarGold O-1, position weldments; achieves good pulsed sprayO-2 stability over a wide range of arc characteristicsStargonand deposition ranges* Stargon and StarGold C-5 and C-10 shielding gases may be used for certain stainless steel welding.Call your Praxair Shielding Gases representative for specific information.41

▲Table 8 –Shielding Gas Selection Guide for GMAW (continued)Material Thickness Transfer Recommended DescriptionModeShielding GasCopper, Nickel Up to 1/8" Short Circuiting HeliStar SS Good arc stability, weld puddle controland Copper- StarGold O-1, and wettingNickel Alloys O-2Over 1/8" Short Circuiting HeliStar SS, Higher heat input of helium mixtures offsetsA-1025 high heat conductivity of heavier gauges; goodwetting and bead contour; using 100% helium onheavier material thicknesses improves wetting andpenetration; can be used for out-of-positionweldingPulsed Spray HeliStar SS, Used for both light gauge and heavy out-of-A-50, A-75position weldments; achieves good pulsed sprayArgonstability over a wide range of arc characteristicsStarGold O-1, and deposition rangesO-2Aluminum Up to 1/2" Spray Arc Argon Best metal transfer, arc stability, and platecleaning; little or no spatter; removes oxides whenused with DCEP (Reverse Polarity)Spray Arc HeliStar A-25, High heat input; produces fluid puddle, flat beadA-50 contour, and deep penetration; minimizes porosityOver 1/2" Spray Arc Helium High heat input; good for mechanized weldingHeliStar A-25 and overhead; applicable to heavy section weldingPulsed Spray Argon Good wetting; good puddle controlMagnesium All thicknesses Spray Arc Argon Excellent cleaning section; provides more stableTitanium andarc than helium-rich mixturesother reactiveSpray Arc HeliStar A-25, Higher heat input and less chance of porosity;materialsA-50, A-75more fluid weld puddle and improved wetting42

C H A P T E R 55Flux-Cored Arc Welding (FCAW)▲ProcessDescriptionFlux-Cored Arc Welding is defined as “an arcwelding process that uses an arc between acontinuous filler metal electrode and the weldpool. The process is used with shielding gasfrom a flux contained within the tubularelectrode, with or without additional shieldingfrom an externally supplied gas, and withoutthe application of pressure.” FCAW is alsoknown as Cored-Rod Welding or Cored-WireWelding.1When the process is used with an externallysupplied shielding gas, the process equipmentis virtually identical to that of GMAW (Seefigure 37). FCAW is principally intendedfor use with carbon steel, stainless steel, andlow-alloy steels. The process can use DCEPor DCEN polarity, depending on the fluxcomposition.APower SupplyBWorkpieceCDWelding TorchWire Feed Motor2BEFWire ReelWelding ControlACGShielding GasE3D4▲FFigure 37 –Flux-Cored Arcwelding equipmentG6578910111Power Cable (negative)7Shielding Gas from Cylinder or Bulk Supply2To Primary Power 230/460/575 V8Cooling Water In3115 VAC In – Welding Contactor Control9Cooling Water to Torch (optional)4Wire Conduit10Cooling Water from Torch (optional)5Torch Switch11Cooling Water Out (optional)6Power Cable (positive)43

▲Flux EffectsThe constituents in the flux influence thecharacteristics of the arc and its weldingperformance. They can increase penetration,clean foreign matter from the workpiecesurface, influence the speed of welding andaffect the mechanical properties of the welddeposit.The flux contains deoxidizers, slag formers,arc stabilizers, and alloying materials as required.The FCAW process typically generatesmore fumes and spatter than solid wires usingthe same shielding gases. Recent developmentshave resulted in the production of a“new generation” of cored wires with significantlyreduced levels of welding fume.During welding, the flux is heated by the arcas the electrode is consumed. The chemicalreactions occurring in the flux create gaseswhich help shield the arc and the weld zone.Carbon dioxide is generally used for additionalshielding with flux-cored wires greaterthan 1/16" diameter. Argon/carbon dioxide,and other argon-based mixtures are used withsmaller diameter wires which are designedfor enhanced all-position performance andincreased productivity.▲MetalTransferThe same metal transfer variations found inGMAW are also found in FCAW. However,in FCAW, the transfer typically is not spatterfree. There are more short circuits andexploding gas bubbles which generateweld spatter.Small-diameter cored wires using an argonbasedshielding gas can develop any of thethree modes of metal transfer: short-circuit,globular, or spray-like, and most can weld inall positions.The large-diameter cored wires that useargon-based gases are capable of a globularor spray-like transfer, while large-diameterwires using carbon dioxide transfer metalby a globular mode only.44

▲ShieldingGases forFCAWCarbon DioxideThe majority of large-diameter flux-coredwires that require a secondary shielding gasuse carbon dioxide. They are generally usedfor welding over rust and mill scale on heavyplate. Flux-cored wires designed for weldingstainless steel frequently use CO 2 ; Argon/CO 2blends can be used to enhance productivityand reduce base metal distortion.Argon/Carbon Dioxide BlendsMany small-diameter cored wires use argonbasedshielding gases. These gas blendsprovide greater weld puddle control and arepreferred by welders because of ease of use.CO 2 content varies from 5-25% dependingupon the wire formulation. Follow manufacturer’srecommendations for optimumperformance. Utilize a push welding techniquewith small diameter wires and argon-basedblends.Argon/Oxygen BlendsSmall amounts of oxygen may be addedto argon for welding carbon steels;however, shielding gas blends, such asargon/5% oxygen, are used primarily onclean base materials.See Table 9, below for a Gas Selection Guidefor FCAW.▲Table 9 –Shielding GasSelection Guidefor FCAW*Material Recommended DescriptionShielding GasCarbon Steel StarGold C-5, For wires so formulated:C-10, C-15Small-diameter cored wire; spray transfer;excellent alloy retention, quiet arc, goodbead profile, low smoke, fume and spatterStarGold C-25Carbon DioxideSmall- and large-diameter cored-wire,short-circuiting and globular transfer;excellent alloy retention, quiet arc, good beadprofile, low smoke, fume, and spatterSmall- and large-diameter cored-wire,short-circuiting and globular transfer; widepenetration profile, higher smoke and spatterlevels, greater loss of alloying elementsStainless Steel StarGold O-2 Small-diameter cored wire, spray transfer,Carbon Dioxide good bead profile, little smoke and spatter,lower fume, clean welds* Please refer to the wire manufacturer’s recommendations whenselecting shielding gases with flux-cored consumables.45

C H A P T E R 66The Economics of Gas Selection for GMAW▲Figure 38 –Welding costs▲When considering the total cost of welding, itis important to understand how the choice ofconsumables can influence overall costs. It isnot the “cost per pound” or “cost per cubicfoot” for the consumables that is reallyimportant, it is their impact on productivityand possible labor savings that really counts.Power 2%Equipment 4%Wire andGas 9%Figure 38 illustrates how total consumablescosts relate to overall welding cost for industrializednations (North America, Europe,Japan, etc.). From this chart it can be seenthat if the labor cost could be reduced by 10%through productivity improvements (such asincreased travel speed, less overweld, minimumpost-weld cleanup, etc.), significantsavings would be generated overall. Thesehidden savings would far exceed a 10%reduction in the price of wire or shielding gas.Many times the focus on cutting the cost ofconsumables misses the larger objective ofsignificant cost reductions that accrue throughlabor/overhead savings.* For industrialized nationsLabor andOverhead*85%▲Labor andOverheadThe dominant cost factors in most weldingoperations are labor and overhead. The properselection of shielding gas and wire electrodecan reduce these costs by increasing weldingspeed and duty cycle, while also decreasingpost-weld cleanup time. The effect of propershielding gas selection on the weldingoperation can result in a significant reductionin overall cost.47

▲Effect ofShielding Gason WeldingSpeedIf welding speed can be increased, laborand overhead charges per unit of weld willdecrease accordingly; this decreases bottomlinewelding costs. Maximum welding speedis largely dependent upon the shielding gasselected and its inherent heat transfer properties,oxidizing potential, and metal transfercharacteristics. Gases with high thermalconductivity produce the hottest and mostfluid weld puddle. Gases with high oxidizingpotential are more effective at reducingsurface tension of the weld puddle to providebetter wetting of the weld bead to the basematerial. Inert gas blends that promote spraytransfer provide the highest level of wireelectrode deposition efficiency and generallyhigher travel speeds.In addition to the shielding gas and weld beadsize, other factors that affect welding speedinclude current and voltage, weld position(flat, vertical up, etc.), joint fit-up, and themode of travel (manual vs. mechanized).Maximum usable welding speed for anygrouping of consumables is higher in mechanizedapplications with clean plate and verygood fit-up. In manual welding, operator skillis often the limiting factor, as all shieldinggases are capable of producing welds atspeeds higher than the capabilities of theoperator.If welding is mechanized, carbon dioxide(CO 2 ) is capable of producing a higher speedweld on thin gauge material when burnthroughis not a problem. However, when fitupis poor or burn-through problems areevident, argon blends with lower carbondioxide percentage (by volume) will moreconsistently achieve the desired results.Plate surface conditions can also have a majoreffect on welding speed and bead appearance.Heavy rust, mill scale, or oil can reduce thetravel speed and will adversely affect weldbead appearance. The use of clean, sandblastedplate or plate with only a light millscale minimizes these problems.Welding speeds increases for Gas TungstenArc Welding (GTAW) and Plasma ArcWelding (PAW) can be easily measureddirectly with the substitution of the gas orthe gas blend. On the other hand, whenconsidering Gas Metal Arc Welding (GMAW),additional factors must be taken in account(i.e. wire feed speed and arc voltage) in orderto achieve the full potential improvement.▲Effect ofShielding Gason Duty Cycleand CleanupOperator duty cycle is the amount of time anoperator is actually welding versus time spenton related activities such as setup, cleanup, orother non-welding functions. When carbondioxide shielding is used, spatter will accumulatein the gas nozzle of the welding gun andinterfere with proper shielding of the weldpuddle. Spatter can also accumulate on thecontact tip and interfere with wire feeding.Removal of spatter from nozzles, tips, andthe finished workpiece decreases the dutycycle of the welder. Since the use of argonbasedshielding gases can greatly decreasethe amount and size of the spatter generated,there is significantly less cleanup requiredthus maintaining the duty cycle at a higherlevel than when CO 2 is used. The lowerspatter levels associated with argon blendsalso means more welding between requiredcleanup periods, greater operator comfort,and less chance of operator injury resultingfrom contact with hot spatter.48

▲mg/g =miligrams offume per gramof depositedweld metalmg/gFigure 39 –Fume levels(CO 2 vs. argon/CO 2 blends)151050124CO 2 Argon/CO 2Weld spatter is weld metal that does notbecome part of the weld itself. This is “lost”material and increases the overall materialcosts for welding. This spatter loss is reflectedin the “deposition efficiency” of the weldingprocess. As spatter losses decrease, depositionefficiency increases. As can be seen in thetable below, the wide range of efficiencies forthe gas-shielded arc processes are largely aresult of the different gas blends that can beused. The use of argon blends will result inthe highest levels of deposition efficiency.Typical Deposition Efficiencies▲mg/g =miligrams offume per gramof depositedweld metalmg/g25201510Figure 40 –Fume levels(CO 2 flux-cored vs. argon metal-cored)5022CO 2Flux-Cored8ArgonMetal-CoredGTAW (TIG): 95-100%GMAW (MIG): 88-98%GMAW-P: 97-99%FCAW (gas shield): 85-90%FCAW (self shield): 75-85%PAW (Plasma arc): 95-100%SMAW (stick): 50-65%Use of the proper shielding gas will ensurethat the deposition efficiency is as high aspossible for the process used. For example,the deposition efficiency of a MIG operationusing 100% CO 2 shielding is typically 88-92%. By replacing CO 2 with an argon/10%CO 2 shielding gas blend, the efficiency wasincreased to 95-97%. More of the wirepurchased ended up as deposited weld metal.Fumes and gases are created by the effect ofthe intense heat of the arc on the consumableused and the interaction between the ultravioletradiation of the arc and its surroundingatmosphere. Significantly less fume is generatedby argon-shielded welding processes asis shown in the figures 39 and 40. This meansthat it will be easier to maintain an acceptableworking environment for the welder, and therewill be less cost associated with filtering andrecirculating shop air near welding operations.49

▲ConsumableCostsWhen determining consumable costs, the wireand shielding gas price should be consideredtogether because of the effect shielding gashas on deposition efficiency. The lowest pricewire and gas may also produce the worstdeposition efficiency. Although the initialcost may be higher, the recommended wireand argon based shielding gas combinationmay increase deposition efficiency and resultin lower overall cost (see table 10).Highly oxidizing shielding gases such as purecarbon dioxide, require welding wires withadditional deoxidizers. This is due to the lossof micro alloys from the reactivity of the CO 2in the welding arc. If 100% CO 2 shielding ischanged to argon with 8% CO 2 added, a lessexpensive welding wire may be substitutedbecause of the good alloy retention feature ofargon-based shielding gases. This change mayresult in as much as a 25% savings in wirecost while maintaining mechanical properties,ease of operation, and increasing weldingtravel speed.▲Table 10 –The effect ofshielding gason wire cost(CO 2 vs. argon/CO 2 blends)To determine the extent to which differences in bead shape and overweldaffect wire cost, examine the following 3/16" and 1/4" fillet weld examples:Deposition Efficiency (%)Lbs. of Weld Metal Required per Foot (lb/ft)Lbs. of Wire Required per Foot (lb/ft)Wire Cost per Foot @ $0.65/lb** ($/ft)Extra Wire Cost per Foot of Using CO 2 ($/ft)3/16" Fillet 1/4" FilletCO 2 Ar/CO 2 * CO 2 Ar/CO 2 *89 97 89 970.100 0.080 0.151 0.1280.109 0.082 0.170 0.1320.076 0.057 0.116 0.0860.019 0.030* Contains 92% argon/8% CO 2 .** The wire cost will vary depending onAWS type, diameter, geographical locationand quantity purchased.50

▲Total Cost of aShielding GasShielding gas costs are typically between 1%to 5% of total welding costs, depending on thedeposition rate of the application being considered.A typical cost comparison betweenan argon-based gas mixture and carbon dioxideis shown in the accompanying analysis(table 11). It can be seen that even though theargon blend costs substantially more than thecarbon dioxide ($ 0.100 vs. $ 0.025 per cubicfoot), it is still a small percentage of theoverall welding cost.When the increase in duty cycle (from 40to 45% due to less postweld cleanup andequipment maintenance) and depositionefficiency (from 89% to 97% due to considerablyreduced spatter and fume) resulting fromthe use of argon are considered, the total costper foot of weld is less with an argon mixturethan with CO 2 ($ 0.796 vs. $0.881 per foot).▲Table 11 –Cost comparisonper foot of wire(CO 2 vs. argon/CO 2 blends)CO 2 Argon/CO 2a. Shielding Gas Cost ($/ft 3 ) 0.025 0.100b. Gas Flow Rate (ft 3 /h) 35 35c. Welding Speed (in/min) 22 221. Shielding Gas Total Cost (1) ($/ft) 0.008 0.032d. Labor and Overhead Rate ($/h) 35 35e. Operator Duty Cycle (4) 0.40 (40%) 0.45 (45%)2. Labor and Overhead Total Costs (2) ($/ft) 0.800 0.710f. Wire Cost ($/lb) 0.65 0.65g. Deposition Efficiency (5) 0.89 (89%) 0.97 (97%)h. Lbs. of Weld Metal Required for 3/16" fillet (lb/ft) 0.100 0.0803. Wire Total Cost (3) ($/ft) 0.073 0.0544. Total (1 + 2 + 3) ($/ft) 0.881 0.7965. Savings Using Argon/CO 2 Blend ($/ft) 0.085Note 1: Pricing reflects cylinder gases supply. The shielding gas and wire costs will vary dependingon the geographical location, quantity purchased and supply method.Note 2: The foregoing computations were based on average figures obtained from different sectionsof the country. To determine how your operation might fare by substituting an argon mixture for CO 2 ,simply insert the correct numbers for your area and your company in the appropriate columns.(1)Shielding Gas Total Cost = a x bc x 60 (min/h)12 (in/ft)(2)Labor and Overhead Total Costs = d(3)Wire Total Cost = f x hc x e x 60 (min/h)12 (in/ft)g(4)Operator duty cycle is the amount of time an operator is actually welding versus time spent onrelated activities such as setup, cleanup, or other non-welding functions.(5)Deposition Efficiency in arc welding is the ratio of the weight of deposited weld metal to thenet weight of the filler metal used.Deposition Efficiency (%) = Deposited Weld MetalFiller Metal Consumed51

Another cost analysis is shown in table 12 andfigure 41, where the company currently uses100% CO 2 shielding with solid wire. Theypurchase approximately 70,000 of wire/yearand employ a total of 54 welders on all shifts/day. The increase in welding speed of 38%(13 in/min to 18 in/min), the 3% increase inwelder duty cycle, and the decrease in over-welding (shown by the required weld metalper foot of fillet weld), generated savings ofover one quarter of a million dollars per year.In addition, fume generation rates weresubstantially reduced; this, too, will lead toadditional cost savings.▲Table 12 –Cost comparisonper foot of weld(CO 2 vs. argon/CO 2 blends)CO 2 Argon/CO 2a. Shielding Gas Cost ($/ft 3 ) 0.010 0.025b. Gas Flow Rate (ft 3 /h) 40 40c. Welding Speed (in/min) 13 181. Shielding Gas Total Cost (1) ($/ft) 0.006 0.011d. Labor and Overhead Rate ($/h) 35 35e. Operator Duty Cycle (4) 0.30 (30%) 0.33 (33%)2. Labor and Overhead Total Costs (2) ($/ft) 1.795 1.179f. Wire Cost ($/lb) 0.65 0.75g. Deposition Efficiency (5) 0.88 (88%) 0.96 (96%)h. Lbs. of Weld Metal Required for 1/4" fillet (lb/ft) 0.155 0.135i. Fume Generation Level (mg/g) 12 43. Wire Total Cost (3) ($/ft) 0.115 0.1064. Total (1 + 2 + 3) ($/ft) 1.916 1.2965. Savings Using Argon/CO 2 Blend ($/ft) 0.6206. Yearly Savings $ 285,000Note 1: Pricing reflects bulk gases supply. The shielding gas and wire costs will vary dependingon the geographical location, quantity purchased and supply method.Note 2: The foregoing computations were based on average figures obtained from different sectionsof the country. To determine how your operation might fare by substituting an argon mixture for CO 2 ,simply insert the correct numbers for your area and your company in the appropriate columns.(1)Shielding Gas Total Cost = a x bc x 60 (min/h)12 (in/ft)(2)Labor and Overhead Total Costs = d(3)Wire Total Cost = f x hc x e x 60 (min/h)12 (in/ft)52g(4)Operator duty cycle is the amount of time an operator is actually welding versus time spent onrelated activities such as setup, cleanup, or other non-welding functions.(5)Deposition Efficiency in arc welding is the ratio of the weight of deposited weld metal to thenet weight of the filler metal used.Deposition Efficiency (%) = Deposited Weld MetalFiller Metal Consumed

▲Figure 41 –Cost comparisonper foot of weld(CO 2 vs. argon/CO 2 blends)$/ft2.521.5CO 21Ar/CO 20.570,000 lb ofwire/year54 totalwelders allshifts0Gas($/ft)Labor andOverhead($/ft)Wire($/ft)Total($/ft)Savings($/ft)Total yearlysavings –$ 285,000CO 2Ar/CO 20.0060.0111.7951.1790.1150.1061.9161.2960.620▲Gas SupplyShielding gases are available in vapor formwith high pressure cylinders and in liquidform with transportable or stationary storagetanks (the product is vaporized prior to beingused). The cost of the product, as well as thedistribution system, depends upon the gasblend used, consumption patterns, locationof equipment, and the presence of a gasdistribution piping system (see chapter 7for more details).A second portion of gas cost is managing theamount of gas used in the welding operation.Pipelines should be frequently inspected forleaks and repaired as appropriate. Cylindergas blends should be purchased from areliable supplier in order to be certain thatproduct composition and quality does not varyfrom the top to the bottom of the cylinder.This minimizes the return of unused productas a result of poor shielding gas performance.The use of metered orifices in place of flowmeterscan significantly reduce gas consumptionon a per station basis.Praxair works with customers on a individualbasis to select the distribution system thatwill control gas flow and mixture consistencymost efficiently. Praxair will make recommendationsabout sizing a system correctly, theproper use of regulation and mixing equipment,and flowmeters to help ensure economicalgas consumption.53

C H A P T E R 77Gas Supply Systems▲After a shielding gas or gas blend has beenselected, it is necessary to consider which ofthe various gas supply systems is best for theapplication. This chapter describes the mostfrequently used cylinder and bulk supplysystems.▲CylinderStorageSystemsWhen gas volume requirements are small,individual cylinders of argon, helium, carbondioxide, oxygen, or blends of these gasescan be used directly in the welding operation.If the consumption rate increases, two or morecylinders can be manifolded together in banksto provide a greater source of supply and toreduce the amount of cylinder handling (seefigure 42).A manifold usually has two independent setsof controls to permit alternate or simultaneousoperation of the two cylinder banks. Emptycylinders can be replaced with full ones aftershutting down one or both cylinder banks.Liquid gases supplied in cylinders may besubstituted for gaseous cylinders to increasevolumes.▲Figure 42 –Manifold system55

▲High-PressureCylinders▲Figure 43 –High-pressurecylindersSince gases have a relatively low density, thevolume of gas at atmospheric pressure can beconsiderably reduced by compressing it intoa cylinder under greater pressure. Thesecylinders must be constructed to withstand thehigh pressures and meet the requirements ofthe Department of Transportation (DOT)which govern transportation of these cylinders(see figure 43).▲Identificationof Gases inCylindersThe gas in a compressed gas cylinder isidentified by the chemical or trade namemarked on the cylinder. The cylinder mustbe labeled in accordance with the requirementsof the Occupational Safety andHealth Administrations (OSHA) HazardCommunication Standard.While cylinders are painted in various colorsand combinations of colors, these colors donot provide identification of gas contents andshould not be used for that purpose. Suppliersdo not intend that users rely on cylinder colorto identify gas content.▲CarbonDioxideSupplyGasCarbon dioxide welding grade gas is availablein either cylinders or bulk containers. Themost widely used container is the highpressuresteel cylinder, approved by theDepartment of Transportation (DOT).▲At 70 F, a full carbon dioxide gas cylinderFigure 44 –contains both liquid and gas. Liquid carbonStandarddioxide takes up approximately two-thirds ofcarbon dioxidegas cylinderLiquidNote:At 70 F there are8.76 cubic feet ofcarbon dioxideper pound.the space in the cylinder; the rest is filled withcarbon dioxide in gaseous form.56

▲Gas MixingShielding gases are often used in a blend tooptimize the arc welding process. Gas mixers,first developed by Praxair in the early 1950s,are used to accurately mix two or more gasesinto a required blend. They are available forsingle weld stations or entire plants.Mixers can provide blends that meet very tighttolerances, assuring the user that they are consistentlyreceiving the correct blend. Often,mixers are equipped with analyzers and alarmdevices or that signal when mixture tolerancesare not met. Pure gases can be purchasedand blended to meet changing productionrequirements by changing the mixer settings.WarningDo not mix hydrogen with other gasesfrom separate cylinders. Always purchasehydrogen blends ready-mixed.▲Pre-BlendedMixtures▲Figure 45 –Cylinder withPraxair’s Star Blendmixing systemMany gas cylinders contain Praxair’s Starblends mixing system (patented). Eachcylinder is evacuated and purged priorto filling to ensure that the cylinder containsonly the prescribed gases. Praxair’s mixingsystem solves the problem of gas stratificationand assures gas consistency throughout theuse of a cylinder (see figure 45).▲Bulk StorageSystems▲Figure 46 –Tube trailerFor users with consumption in the range of15,000 to 50,000 ft 3 /mo., bulk storage systemsshould be considered. They can resultin savings in space and cost. Bulk systemsconsist of permanent receivers or tube trailers(see figure 46) into which the gas is pumpedfrom trailers, or exchanged as required.57

15’ 6”▲5’Figure 47 –Liquid storagetanks10’ 2”36’ 2”Another storage system for gas consumptionof approximately 25,000 ft 3 /mo. or greater is aliquid storage tank (see figure 47) which isfilled from trailer trucks or railway tank cars.The liquid is converted into gas through avaporizer and piped to each destination withinthe plant. Liquid storage systems are notgenerally applicable for liquid helium.The liquid storage tanks in figure 47 aretypical of the many thousands of cryogenictanks in existence today. Standard capacitiesrange from 250 to 11,000 gallons; customdesigns to hold up to 60,000 gallons areavailable. Praxair engineers, designs, installs,and services entire bulk liquefied gas deliverysystems including tanks, vaporizers, instrumentation,computer controls for mixtureregulation, and total safety controls.▲Inert GasDistributionSystemsWhen individual cylinders are used to supplya gas, a pressure regulator is attached to thecylinder to reduce the exiting gas pressure to asafe working level. A flowmeter, connected tothe welding equipment by tubing with appropriatefittings, is used to maintain preset gasflow rates.A typical secondary line has a station valvethat is connected to the welding machinethough a pressure regulator and flowmeter(often a combined unit). This is connected tothe welding machines by an inert gas hose ortubing with appropriate fittings and a solenoid-operatedshut-off valve.When large volumes are required or wherethere are multiple weld stations, it may bepreferable to install a distribution pipingsystem.A system of this type contains main andsecondary distribution lines. The gas supply,mixing device (supplied if necessary), maindistribution line, and shut-off valves form thebasis for the system. Secondary, or branchlines to station valves near the weldingmachines at various locations throughout theplant are also included. These lines areequipped with valves that allow the lines tobe purged.It is recommended that the design, materials,fabrication, inspection, and tests meet therequirements of ANSI B 31.3, “PetroleumRefinery Piping.” The designer is cautionedthat the code is not a design handbook. Thecode does not do away with the need forcompetent engineering judgment.The Compressed Gas Association pamphlet,Industrial Practices for Gaseous Transmissionand Distribution Piping Systems, containsadditional information about pipelines.58

C H A P T E R 88Precautions and Safe Practices▲Precautionsand SafePractices forWelding andCuttingAlways read the safety information and theMaterial Safety Data Sheet (MSDS) suppliedby the manufacturers of gases, materials andequipment used in welding operations. Thisinformation will recommend safe practicesthat will protect you from health hazards, suchas fumes, gases, and arc burns. Recommendationsconcerning ventilation and protectivedevices should be carefully followed.To weld and cut safely, you must have athorough knowledge of the welding processand equipment you will be using, and all ofthe hazards involved. The information foundhere is excerpted from Praxair’s Precautionsand Safe Practices for Electric Welding andCutting (P52-529), Precautions and SafePractices for Gas Welding, Cutting andHeating (P-2035). Also see Praxair’s SafetyPrecautions (P-3499 A), Condensed SafetyInformation – Compressed Gases andCryogenic Liquids (P-12-237), Guidelines forHandling Compressed Gas Cylinders &Cryogenic Liquid Containers (P-14-153).Fumes and Gases Can Harm Your HealthKeep your head out of the fumes. Do notbreathe fumes and gases caused by the arc.Use enough ventilation. The type and theamount of fumes and gases depend on theequipment and supplies used. Air samples canbe used to find out what respiratory protectionis needed.Provide enough ventilation wherever weldingand cutting are performed. Proper ventilationcan protect the operator from the evolvingfumes and gases. The degree and type ofventilation needed will depend on the specificwelding and cutting operation. It varies withthe size of the work area, on the number ofoperators, and on the types of materials to bewelded or cut. Potentially hazardous materialsmay exist in certain fluxes, coatings, and fillermetals. They can be released into the atmosphereduring welding and cutting. In somecases, general natural-draft ventilation maybe adequate. Other operations may requireforced-draft ventilation, local exhaust hoodsor booths, or personal filter respirators or airsuppliedmasks. Welding inside tanks, boilers,or other confined spaces requires specialprocedures, such as the use of an air-suppliedhood or hose mask.Sample the welding atmosphere and check theventilation system if workers develop unusualsymptoms or complaints. Measurements maybe needed to determine whether adequateventilation is being provided. A qualifiedperson, such as an industrial hygienist, shouldsurvey the welding operations and surroundingenvironment.Do not weld on plate contaminated withunknown material. The fumes and gaseswhich are formed could be hazardous to yourhealth. Remove all paint and other coatingsbefore welding.More complete information on health protectionand ventilation recommendations forgeneral welding and cutting can be found inthe American National Standard Z49.1, Safetyin Welding and Cutting. This document isavailable from the American Welding Society,550 N.W. LeJeune Road, Miami, FL 33126.59

▲Work TableFigure 48 –Illustration of grounding electricalequipment and the workpieceElectric Shock Can Kill YouDo not touch live electrical parts.To avoid electric shock follow the recommendedpractices listed below. Faulty installation,improper grounding, and incorrectoperation and maintenance of electricalequipment can be sources of danger.1. Ground all electrical equipment and theworkpiece. Prevent accidental electricalshocks. Connect power supply, control cabinets,and workpiece to an approved electricalground. The work lead is not a ground lead. Itis used to complete the welding circuit. Aseparate connection is required to ground thework, or the work lead terminal on the powersupply may be connected to ground. Do notmistake the work lead for a ground connection.See figure 48.2. Use the correct cable size. Sustained overloadingwill cause cable failure and result inpossible electrical shock or fire hazard. Workcable should be the same rating as the torchcable.To ApprovedEarth GroundGroundConnectionTo ApprovedEarth GroundPowerSupply3. Make sure all electrical connections aretight, clean, and dry. Poor electrical connectionscan become heated and even melt.They can also cause poor welds and producedangerous arcs and sparks. Do not allowwater, grease, or dirt to accumulate onplugs, sockets, or electrical units.4. Moisture and water can conduct electricity.To prevent shock, it is advisable to keep workareas, equipment, and clothing dry at alltimes. Fix water leaks immediately. Make surethat you are well insulated. Wear dry gloves,rubber-soled shoes, or stand on a dry board orplatform.5. Keep cables and connectors in goodcondition. Improper or worn electricalconnections can cause short circuits and canincrease the chance of an electrical shock.Do not use worn, damaged, or bare cables.6. Avoid open-circuit voltage. Open-circuitvoltage can cause electric shock. Whenseveral welders are working with arcs ofdifferent polarities, or when using multiplealternating-current machines, the open-circuitvoltages can be additive. The added voltagesincrease the severity of the shock hazard.7. Wear insulated gloves when adjustingequipment. Power should be shut off andinsulated gloves should be worn when makingany equipment adjustment to assure shockprotection.8. Follow recognized safety standards. Followthe recommendations in American NationalStandard Z,49.1, Safety in Welding andCutting available from the American WeldingSociety, 550 N.W. LeJeune Road, Miami, FL33126, and also the National Electrical Code,NFPA No. 70, which is available from theNational Fire Protection Association,Batterymarch Park, Quincy, MA 02269.60

WarningArc rays and spatter can injure eyes andburn skin. Wear correct eye, ear, and bodyprotection.Electric arc radiation can burn eyes and skinthe same way as strong sunlight. Electricarcs emit both ultraviolet and infrared rays.Operators, and particularly those peoplesusceptible to sunburn, may receive eye andskin burns after brief exposure to arc rays.Reddening of the skin by ultraviolet raysbecomes apparent seven or eight hours later.Long exposures may cause a severe skin burn.Eyes may be severely burned by both ultravioletand infrared rays. Hot welding spatter cancause painful skin burns and permanent eyedamage.Be sure you are fully protected from arcradiation and spatter. Cover all skin surfacesand wear safety glasses for protection fromarc burns and burns from sparks or spatter.2. Protect against arc flashes, mechanicalinjury, or other mishaps. Wear spectacles orgoggles with No. 2 shade filter lens and sideshields inside the welding helmet or handshield. Helpers and observers should wearsimilar protection.3. Wear protective clothing such as heat resistantjackets, aprons, and leggings. Exposure toprolonged or intense arc radiation can causeinjury. Thin cotton clothing is inadequateprotection. Cotton deteriorates with this typeof radiation.4. Wear high, snug fitting shoes. Avoidwearing low or loose shoes which wouldallow hot spatter to get inside.5. Wear cuffless pants. By wearing pants withno cuffs, you eliminate a dangerous spark andspatter trap. Pant legs should overlap shoetops to prevent spatter from getting into yourshoes.1. Keep sleeves rolled down. Wear gloves anda helmet. Use correct lens shade to preventeye injury. Choose the correct shade from thetable below. Observers should also use properprotection.Filter Recommendations(Adapted from ANSI Safety Standard Z49.1)ApplicationMIG (Gas Metal andFlux Cored Arc)60 to 160 amps 11160 to 250 amps 12250 to 500 amps 14Lens Shade No.** As a rule of thumb, start with a shade that is too darkto see the arc zone. Then go to a lighter shade whichgives sufficient view of the arc zone without exerting astrain on your eyes.6. Wear clean clothes. Do not wear clothingthat has been stained with oil and grease.It may burn if ignited by the heat of the arc.7. Wear ear protection, not only where there isnoise, but where there is a chance that spatteror sparks could get into your ears.8. Wear a leather cap or other protection toprotect the head from sparks or spatter.9. Protect neighboring workers from exposureto arc radiation. Shield your station with metalor heat resistant shields. If your station cannotbe shielded, everyone within about 75 feetshould wear eye protection when weldingor cutting is in progress. Assure properventilation.10. Do not breathe welding fumes.61

▲Precautionsand SafePractices forShieldingGasesTo handle shielding gases safely, you musthave a thorough knowledge of the characteristicsof the gases you use, including thehazards. This section includes informationabout the major hazards of shielding gases.Read and understand Praxair’s SafetyPrecautions for Gases (P-3499) andCondensed Safety Information, CompressedGases and Cryogenic Liquids (P-12-237).Always use a pressure-reducing regulatorwhen withdrawing gaseous hydrogen from acylinder or other high-pressure source.Take every precaution against hydrogen leaks.Escaping hydrogen cannot be detected bysight, smell, or taste. Because of its lightness,it has a tendency to accumulate beneathroofs and in the upper portions of otherconfined areas.Nitrogen, Argon, Helium andCarbon DioxideWarningNitrogen, argon, helium, and carbondioxide can all cause rapid asphyxiationand death if released in confined, poorlyventilated areas.Nitrogen, argon, and helium as liquids or coldgases and carbon dioxide as cold gas maycause severe frostbite to the skin or eyes.Do not touch frosted pipes or valves withbare skin.Use a pressure-reducing regulator whenwithdrawing gaseous nitrogen, argon, helium,or carbon dioxide from a cylinder or otherhigh-pressure source.HydrogenDangerHydrogen is a flammable gas. A mixture ofhydrogen with oxygen or air in a confinedarea will explode if ignited by a spark,flame, or other source of ignition. Ahydrogen flame is virtually invisible inwell-lighted areas.Do not mix hydrogen with other gases fromseparate cylinders. Always purchasehydrogen blends ready-mixed.OxygenWarningOxygen supports and can greatly acceleratecombustion. Oxygen, as a liquid or cold gas,may cause severe frostbite to the skin oreyes. Do not touch frosted pipes or valves.Always use a pressure-reducing regulatorwhen withdrawing gaseous oxygen from acylinder or other high-pressure source.Read all labels on containers of liquid orgaseous oxygen and observe all safetyprecautions on the label. If a label is missingor illegible, do not use the product. Make noassumptions about what is in the container;return it to the supplier.Keep combustibles away from oxygen andeliminate ignition sources. Many substanceswhich do not normally burn in air andother substances which are combustiblein air may burn violently when a highpercentage of oxygen is present.Hydrogen as a liquid or cold gas may causesevere frostbite to the eyes or skin. Do nottouch frosted pipes or valves.62

▲Material SafetyData Sheets(MSDS)The Occupational Safety and Health Actregulations, published in the Code of FederalRegulations (CFR), 29CFR 1910.1200,requires that MSDS be provided to users ofhazardous materials. The physical andchemical hazards of materials used in welding,including gas mixtures can be evaluatedon the basis of the information provided.The MSDS provides guidance which can beapplied to any specific welding application.Praxair provides an MSDS for each puregas, pure liquid or shielding gas mixture.See the following list for the appropriateMSDS numbers:Pure Gases Product Gas – MSDS# Liquid – MSDS#and LiquidsArgon P-4563 P-4564Carbon Dioxide P-4574 –Helium P-4602 P-4600Hydrogen P-4604 P-4603Nitrogen P-4631 P-4630Oxygen P-4638 P-4637Praxair’s Mixtures Product Description MSDS #Argon/Oxygen StarGold O-1 1% Oxygen/99% Argon Mixture P-4718StarGold O-2 2% Oxygen/98% Argon Mixture P-4719StarGold O-5 5% Oxygen/95% Argon Mixture P-4720Argon/Carbon Dioxide StarGold C-5, C-10, Argon/Carbon Dioxide Mixtures P-4715C-15, C-20, C-25,C-40, C-50MigMix Gold Argon/Carbon Dioxide Mixtures P-4715Argon/Carbon Dioxide/ Stargon Argon/Carbon Dioxide/Oxygen Mixture P-4718OxygenHelium/Argon HeliStar Helium/Argon Mixtures P-4716A-25, A-50, A-75Helium/Argon/ HeliStar Helium/Argon/Carbon Dioxide Mixtures P-4713Carbon Dioxide A 1025, CS, SSHydrogen/Argon HydroStar H-2 Hydrogen/Argon Mixtures - Nonflammable P-4860HydroStar H-5, H-10, Hydrogen/Argon Mixtures - Flammable P-4861H-15, H-35For more information call 1-800-PRAXAIR.63

A P P E N D I X Aa-zGlossary64▲Arc blow — The deviation of an arc from itsnormal path because of magnetic forces.Arc force — The axial force developed byan arc plasma.Arc length — The distance from the tip of theelectrode or wire to the workpiece.Arc welding deposition efficiency —The ratio of the weight of filler metal depositedto the weight of filler metal melted. (%)Arc welding electrode — A part of the weldingsystem through which current is conducted thatends at the arc.As-welded — The condition of the weld metal,after completion of welding, and prior to anysubsequent thermal or mechanical treatment.Automatic — The control of a process withequipment that requires little or no observationof the welding, and no manual adjustment of theequipment controls.Backhand welding — A welding techniquewhere the welding torch or gun is directedopposite to the direction of welding.Brazing (B) — A welding process where thecoalescence of materials is produced by heatingthem in the presence of a filler metal having amelting point above 450 C (840 F) but belowthe melting point of the base metal. The fillermetal is distributed between the closely fittedsurfaces of the joint by capillary action.Buttering — A process that deposits surfacingmetal on one or more surfaces of a joint toprovide metallurgically compatible weld metalfor the subsequent completion of the weld.Cold lap — Incomplete fusion or overlap.Constant current power source — An arcwelding power source with a volt-ampere outputcharacteristic that produces a small weldingcurrent change from a large arc voltage change.Constant voltage power source — An arcwelding power source with a volt-ampere outputcharacteristic that produces a large weldingcurrent change from a small arc voltage change.Contact tube — A system component thattransfers current from the torch gun to acontinuous electrode.Cryogenic — Refers to low temperatures,usually -200 F or below.Density — The ratio of the weight of a substanceper unit volume; e.g. mass of a solid, liquid, orgas per unit volume at a specific temperature.Deposited metal — Filler metal that has beenadded during welding, brazing or soldering.Dew point — The temperature and pressure atwhich the liquefaction of a vapor begins, usuallyapplied to condensation of moisture from thewater vapor in the atmosphere.Direct current electrode negative (DCEN) —The arrangement of direct current arc weldingleads in where the electrode is the negative poleand workpiece is the positive pole of thewelding arc.Direct current electrode positive (DCEP) —The arrangement of direct current arc weldingleads in where the electrode is the positive poleand workpiece is the negative pole of thewelding arc.Duty cycle — The percentage of time during atime period that a power source can be operated atrated output without overheating.Electrode extension — The length of electrodeextending beyond the end of the contact tube.Filler material — The material to be added inmaking a welded, brazed, or soldered joint.Fillet weld — A weld of approximately triangularcross section which joins two surfaces approximatelyat right angles to each other in a lap joint,T-joint, or corner joint.Flammable Range — The range over which a gasat normal temperature (NTP) forms a flammablemixture with air.Flat welding position — A welding positionwhere the weld axis is approximately horizontal,and the weld face lies in an approximatelyhorizontal plane.

Forehead welding — A welding technique wherethe welding torch or gun is directed toward thedirection of welding.Gas Metal Arc Welding (GMAW) — An arcwelding process where the arc is between acontinuous filler metal electrode and the weldpool. Shielding from an externally supplied gassource is required.Gas Tungsten Arc Welding (GTAW) —An arc welding process where the arc is between atungsten electrode (nonconsumable) and the weldpool. The process is used with an externallysupplied shielding gas.Heat-affected zone — That section of the basemetal, generally adjacent to the weld zone, whosemechanical properties or microstructure have beenaltered by the heat of welding.Hot crack — A crack formed at temperatures nearthe completion of weld solidification.Incomplete fusion — A weld discontinuity wherefusion did not occur between weld metal and thejoint or adjoining weld beads.Incomplete joint penetration — A condition in agroove weld where weld metal does not extendthrough the joint thickness.Interpass temperature — In a multipass weld,the temperature of the weld area between passes.Kerf — The width of the cut produced during acutting process.Manual welding — A welding process where thetorch or electrode holder is manipulated by hand.Mechanized welding — Welding with equipmentwhere manual adjustment of controls is requiredin response to variations in the welding process.The torch or electrode holder is held by amechanical device.Metal cored electrode — A composite tubularelectrode consisting of a metal sheath and a coreof various powdered materials, producing no morethan slag islands on the face of the weld bead.External shielding is required.Molecular Weight — The sum of the atomicweights of all the constituent atoms in themolecule of an element or compound.Pilot arc — A low current arc between theelectrode and the constricting nozzle of a plasmatorch which ionizes the gas and facilitates the startof the welding arc.Plasma Arc Cutting (PAC) — An arc cuttingprocess using a constricted arc to remove themolten metal with a high-velocity jet of ionizedgas from the constricting orifice.Plasma Arc Welding (PAW) — An arc weldingprocess that uses a constricted arc between anonconsumable electrode and the weld pool(transferred arc) or between the electrode and theconstricting nozzle (non-transferred arc). Shieldingis obtained from the ionized gas issuing fromthe torch.Plasma Spraying (PSP) — A thermal sprayingprocess in which a nontransferred arc is used tocreate an arc plasma for melting and propellingthe surfacing material to the substrate.Porosity — A hole-like discontinuity formed bygas entrapment during solidification.Preheat temperature, welding — The temperatureof the base metal immediately before weldingis started.Pull gun technique — Same as backhandwelding.Pulsed spray welding — An arc welding processvariation in which the current is pulsed to achievespray metal transfer at average currents equal toor less than the globular to spray transitioncurrent.Push angle — The travel angle where theelectrode is pointing in the direction of travel.Root opening — A separation at the joint rootbetween the workpieces.Self-shielded Flux Cored Arc Welding(FCAW-S) — A flux cored arc welding processvariation in which shielding gas is obtainedexclusively from the flux within the electrode.Shielded Metal Arc Welding (SMAW) —An arc welding process where the arc isbetween a covered electrode and the weld pool.Decomposition of the electrode covering,provides the shielding.Spatter — Metal particles expelled duringwelding that do not form a part of the weld.Standard Temperature and Pressure (STP) —An internationally accepted reference base wherestandard temperature is 0 C and standardpressure is one atmosphere, or 14.6960 psia.Stress-relief heat treatment — Uniform heatingof a welded component to a temperature sufficientto relieve a major portion of the residual stresses.65

Thermal conductivity — The quantity of heatpassing through a plate.Underbead crack — A crack in the heat-affectedzone generally not extending to the surface of thebase metal.Undercut — A groove melted into the base plateadjacent to the weld toe or weld root and leftunfilled by weld metal.Vapor Pressure — The pressure exerted by avapor when a state of equilibrium has beenreached between a liquid, solid or solution andits vapor. When the vapor pressure of a liquidexceeds that of the confining atmosphere, theliquid is commonly said to be boiling.Viscosity — The resistance offered by a fluid(liquid or gas) to flow.Welding leads — The workpiece lead andelectrode lead of an arc welding circuit.Welding wire — A form of welding filler metal,normally packaged as coils or spools, that may ormay not conduct electrical current dependingupon the welding process used.Weld metal — The portion of a fusion weld thathas been completely melted during welding.Weld pass — A single progression of weldingalong a joint. The result of a pass is a weld beador layer.Weld pool — The localized volume of moltenmetal in a weld prior to its solidification as weldmetal.Weld reinforcement — Weld metal in excess ofthe quantity required to fill a joint.Wetting — The phenomenon whereby a liquidfiller metal or flux spreads and adheres in a thincontinuous layer on a solid base metal.Wire feed speed — The rate at which wire isconsumed in welding.66

Operations in over 40 countries worldwideNorth AmericaWorld HeadquartersPraxair, Inc.39 Old Ridgebury RoadDanbury, CT 06810-5113USATel: 1-800-PRAXAIR(1-800-772-9247)(716) 879-4077Fax: 1-800-772-9985(716) 879-2040Praxair Canada, Inc.1 City Centre DriveSuite 1200Mississauga, Ontario,L5B 1M2CanadaTel: (905) 803-1600Fax: (905) 803-1690Praxair México S.A.México CityApartado Postal 10-788Blvd. Manuel AvilaCamacho 32Col. Lomas deChapultepec11000 MéxicoTel: 52 (5) 627-9500Fax: 52 (5) 627-9515Central America/CaribbeanPraxair Puerto Rico, Inc.P.O. Box 307Road #189Intersection 931Barrio NavarroGurabo 00778Puerto RicoTel: (787) 258-7200Fax: (787) 258-7233Belize, Costa RicaSouth AmericaS.A. White MartinsRua Mayrink Veiga, 9Rio de Janeiro, RJ20090-050BrazilTel: 55 (21) 588-6622Fax: 55 (21) 588-6794Argentina, Bolivia, Chile,Colombia, Ecuador,Paraguay, Peru, Uruguay,VenezuelaEuropePraxair N.V.Fountain PlazaBelgicastraat 7B-1930 ZaventemBelgiumTel: 32 (2) 716-0580Fax: 32 (2) 720-9174Austria, Croatia,Czech Republic, France,Germany, Israel, Italy,The Netherlands, Poland,Portugal, Slovenia, Spain,TurkeyAsiaPraxair Asia, Inc.541 Orchard Road#20-00-Liat TowersSingapore 238881Tel: (65) 736-3800Fax: (65) 736-4143Australia, India, Indonesia,Japan, People’s Republicof China, South Korea,ThailandFor more information,or a detailed listing ofPraxair’s internationallocations visit our website at: www.praxair.come-mail: info@praxair.comThe information contained herein isoffered for use by technically qualifiedpersonnel at their discretion and risk,without warranty of any kind.Praxair is a registered trademark andHeliStar, HydroStar, Mig Mix Gold,Star Gases, StarGold and Stargon aretrademarks of Praxair Technology, Inc.© 1998, Praxair Technology, Inc.P-8201 10M 6/98