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G as shielded flux cored arc welding(FCAW-G) is a popular and ver-satile welding process Fig. 1. Itis used with mild steel, low-alloy steel, andother alloy materials in a variety of appli-cations and industries, such as heavy fab-rication, structural, shipbuilding, and off-shore. The two most common shieldinggases used with the FCAW-G process are100% carbon dioxide, commonly referredto as CO 2, and the mixture of 7580%argon (Ar) with 2025% CO 2, referred to

as an Ar/CO 2 blend.

So which shielding gas, CO 2 or Ar/CO 2blend, should operators choose for flux cored arc welding applications? Each typeoffers some advantages and disadvan-tages. The factors of cost, quality, and pro-ductivity should be considered when man-ufacturing decisions are made. The choiceof shielding gas affects each of these fac-tors, sometimes in a conflicting way. Thisarticle focuses on the merits of the twobasic gas options for FCAW-G on steelapplications.

Before getting into the particular ad-

vantages of the gas options, it is appropri-ate to review some basics. It should alsobe noted that this article only focuses ona few types of gases. As a more compre-hensive reference, ANSI/AWS

A5.32/A5.32M, Specification for Welding Shielding Gases , prescribes the require-ments for shielding gases, defining re-quirements for testing, packaging, identi-fication, and certification. Additionally, itcontains helpful information on ventila-tion during welding, as well as general

safety considerations.

Choosing aShielding Gasfor FCAW

Fig. 1 Gas shielded flux cored arc welding.

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How Shielding Gas Works

The primary function of all shieldinggases is to protect the molten weld pooland electrode from the oxygen, nitrogen,and moisture in air. Shielding gases flowthrough the welding gun and exit the noz-zle surrounding the electrode, displacingthe air and forming a temporary protec-tive pocket of gas over the weld pool andaround the arc. Both CO 2 and Ar/CO 2

blend shielding gases accomplish thispurpose.

Some shielding gases make it easier tocreate the arc plasma, providing a currentpath for the welding arc. The choice of shielding gas also affects the transfer of thermal energy in the arc and forces onthe pool. For these issues, CO 2 and

Ar/CO 2 blends behave differently.

Properties of ShieldingGases

When exposed to the heat of the weld-ing arc, CO 2 and Ar react in different

ways. Analysis of these differences is use-ful in understanding how the propertiesof each gas affect the welding process and

weld deposit.Ionization potential is a measure of the

energy required to ionize the gas (i.e.,transform to a plasma state in which it ispositively charged), enabling the gas toconduct current. The lower the number,the easier it is to initiate the arc and main-tain arc stability. The ionization potentialfor CO 2 is 14.4 eV vs. 15.7 eV for Ar. Thus,it is easier to initiate an arc in CO 2 thanin Ar.

Thermal conductivity of a gas is its abil-ity to transfer thermal energy. This affectsthe mode of transfer (spray vs. globular,for example), shape of the arc, weld pen-etration, and temperature distribution

within the arc. Carbon dioxide has ahigher thermal conductivity level thanboth Ar or an Ar/CO 2 blend.

Reactivity of a gas refers to whether ornot it will chemically react with the molten

weld pool. Gases can be divided into twogroups, inert and active. Inert gases, or

noble gases, are those that are unreactive with other elements in the weld pool . Argon is an inert gas. Active gases, or re-active gases, are those that combine orreact with other elements in the weld poolto form compounds. At room tempera-ture, CO 2 is inert. However, in the arcplasma, CO 2 will disassociate, formingcarbon monoxide (CO), oxygen (O 2), andsome monotonic oxygen atoms (O).Therefore, CO 2 becomes an active gas inthe welding arc, allowing the oxygen toreact with metals (i.e., oxidize) in the arc.

An Ar/CO 2 blend is also an active gas, butless reactive than CO 2.

With all other welding variables beingthe same, different shielding gases pro-duce different welding fume generationrates. Typically, there is a reduction inrates with an Ar/CO 2 blend, as comparedto CO 2, due to the oxidizing potential of CO 2. Specific fume generation levels varyand are dependent on the particular ap-plication and procedures used.

More about Inert Gases

Although inert gases provide weld poolshielding, they are not suitable by them-selves for FCAW-G on ferrous or iron-based metals (mild steel, low-alloy steel,stainless steel, etc.). For example, if only

Ar was used to weld mild steel, the result-ing weld characteristics would be verypoor. Using an inert shielding gas causesexcessive arc length and premature melt-ing of the electrodes outer steel sheath.Wide and uncontrollable arc characteris-tics would lead to excessive weld buildup.Therefore, for the best results for FCAW-G on ferrous metal base materials, a blendof inert gas with active gas is used.

More about CO 2 /Ar Blends

The most common blend for mild steelFCAW-G applications in North Americais 75% Ar/25% CO 2. A less common blendfor mild steel FCAW-G applications is80% Ar/20% CO 2. Some gas shielded flux cored wires are designed for use with asmuch as 90% Ar/10% CO 2. However, lessthan 75% Ar in a shielding gas blend willhave a diminished impact on arc charac-teristics, while you would still incur thecost of having Ar in the shielding gas. Inaddition, nonstandard percentages of

Ar/CO 2 blended cylinders will typically bemore difficult to obtain than standardblended cylinders, like 75% Ar/25% CO 2or 80% Ar/20% CO 2.

Alloy Recovery andMechanical Properties

Due to the reactive nature of CO 2, ahigher level of alloy recovery from a given

Cost, overall operator appeal, and weld quality must all be considered when selecting the shielding gas for

a flux cored arc welding application

BY TOM MYERS

TOM MYERS ( tom_myers@lincolnelectric.com ) is a senior application engineer, The Lincoln Electric Co., Cleveland, Ohio.

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electrode in the weld metal is experienced when using an Ar/CO 2 blend vs. CO 2shielding gas. This is because CO 2 willreact with alloys to form oxides, which,along with oxides from the flux, form theslag. The flux in the core of the electrodemust contain reactive elements, such asmanganese (Mn) and silicon (Si), whichact as deoxidizers, among other purposes.

A portion of these alloys react or oxidize with the free oxygen from the CO 2, end-ing up in the slag instead of being recov-ered in the weld metal. Therefore, higherlevels of Mn and Si result in the weld de-posit (i.e., more alloy recovery) with an

Ar/CO 2 blend than with a CO 2 shieldinggas (see the example in Fig. 2).

The consequences of higher levels of Mn and Si in the weld deposit are an in-crease in weld strength and a decrease inelongation, as well as changes to theCharpy V-notch impact toughness. Bysimply changing from CO 2 to an Ar/CO 2blend, you typically get a 710 ksi increasein tensile and yield strength and 2% de-crease in elongation Fig. 2. This is animportant concept to understand, for asthe percentage of Ar in the shielding gasincreases, the weld strength could becometoo high and ductility too low.

Knowing that shielding gases can affectthe resulting properties in the weld, AWSD1.1/D1.1M:2008, Structural Welding Code

Steel, has a series of requirements to en-sure acceptable properties are achieved.For all welding, the shielding gas must con-form to the requirements of AWS

A5.32/A5.32M. AWS classifications forFCAW-G welding consumables (A5.20/

A5.20M and A5.29/A5.29M) put upperlimits on the strength of the weld deposit .

A shielding gas should be selected to en-sure that these limits are not exceeded, de-pending on the design of the electrode andthe welding procedures used. For prequal-ified welding procedure specifications(WPS), D1.1:2008 requires the specificfiller metal and shielding gas combination

used be supported with test data.Clause 3.7.3 of D1.1:2008 provides twoacceptable forms of support: either theuse of shielding gas used for electrodeclassifications purposes, or data from thefiller metal manufacturer that shows con-formance with the applicable AWS A5 re-quirements, but with the specific shield-ing gas that is to be listed on the WPS. Inthe absence of these two conditions,D1.1:2008 requires that the combinationbe subject to qualification testing.

Filler Metal Classificationby Gas Type

Beginning in 2005, the AmericanWelding Societys flux cored filler metalspecifications made the type of shieldinggas used for classification part of the clas-sification designation. A mild steelFCAW-G electrodes AWS classificationis EXXT-XX, where the last X is theshielding gas designator. It will either beC for CO 2 or M for mixed gas, or Ar/CO 2blends (for example, E71T-1 C or E71T-1M ). For a low-alloy steel electrode, theshielding gas designator follows the de-posit composition designator at the end

of the mandatory designators (for exam-ple, E81T1-Ni1 C ). In contrast, self shielded flux cored electrodes, which re-quire no shielding gas, would have noshielding gas designator in its classifica-tion (for example, E71T-8).

Some electrodes are designed to beused solely with CO 2. Other electrodesare designed to be used solely with

Ar/CO 2 blends. Still others are designed

to be used with both types of shieldinggases, CO 2 and Ar/CO 2 blends. In this lat-ter case, the electrode must meet the re-quirements of both classifications.

Selecting Shielding Gasesfor FCAW-G

In choosing either CO 2 or an Ar/CO 2blend shielding gas for flux cored weld-ing, consider the following three compar-ison points.

1) Shielding Gas Cost. Total weldingcosts are a significant factor for many com-

panies and controlling these welding costsis crucial to maintaining profitability. Ingeneral, 80% of total welding costs can beattributed to labor and overhead expensesand 20% to material costs, with shieldinggases accounting for as much as a quarterof material costs, or 5% of total weldingcosts. If cost of shielding gas is the onlydeciding factor, then significant cost sav-ings can be achieved by using CO 2 overan Ar/CO 2 blend. However, oftentimesother factors influence total welding costsas well and those are discussed in latersections.

Carbon dioxide costs less than Ar/CO 2blends because it is a less costly gas to col-lect, and the sources are plentiful and

widely available all over the world. Car-bon dioxide is generally collected as a by-product of some other process. For the

welding industry, a common source isfrom the processing or cracking of natu-ral gas. On the other hand, Ar can only becollected from air. With Ar constituting

just less than 1% of the atmosphere, atremendous amount of air must beprocessed to extract Ar in large quanti-ties. Special air-separation plants are re-quired to process air. Air-separationplants consume large quantities of elec-tricity and are located in specific areas of the world.

2) Overall Operator Appeal and Im-pact on Productivity. When comparingshielding gases for use on the same typeand size electrode, smoother, softer arccharacteristics and lower spatter levels areseen with an Ar/CO 2 blend, resulting inan increased overall appeal to operators,

vs. CO 2 shielding gas. A welding arc inCO 2 shielding gas has a more globular arctransfer with larger droplet sizes (typicallylarger than the diameter of wire), result-ing in a harsher, more erratic arc and

Fig. 2 Deposition composition and mechanical properties results of a typical gas shielded flux cored wire designed for use with both CO 2 and an Ar/CO 2 blend.

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higher levels of spatter. A welding arc inan Ar/CO 2 blend has more of a spray arctransfer with smaller droplet sizes (typi-cally smaller than the diameter of wire),resulting in a smoother, softer arc andlower levels of spatter.

Another feature of an Ar/CO 2 blendthat increases its overall operator appeal,

with its lower level of thermal conductiv-ity, is that it tends to keep the weld hot-ter or more fluid, compared to a weld pro-duced using CO 2. This makes it easier to

work the pool and wet the bead at the toesof the weld. Higher operator appeal is par-ticularly valuable when welding out of po-sition (i.e., uphill and overhead positions).Some fabricators find that using an

Ar/CO 2 blend gives less-experienced welders more control over the welding arc, which results in the ability to weld athigher productivity levels.

Because of the high Ar content, onedisadvantage of an Ar/CO 2 blend is thatit radiates more heat up toward the oper-ator than CO 2. This means that it feelshotter when welding. In addition, weldingguns run hotter with an Ar/CO 2 blend(guns have a lower duty cycle rating with

Ar/CO 2 blend than with CO 2). This mayrequire the use of larger guns or poten-

tially incur higher annual replacementcosts of the same size guns and consum-able parts.

3) Weld Quality. As discussed earlier,an Ar/CO 2 blend when compared to CO 2,tends to keep the weld pool more fluid,making it easier to work the pool and wetthe bead at the toes of the weld. Some fab-ricators find this allows welders to im-prove the weld profile and resulting qual-ity of the weld.

In addition, the welding arc in an Ar/CO 2 blend produces less weld spatter.This results in higher weld quality and re-duction in weld cleaning time and cost.

Lower spatter levels can also improve ul-trasonic weld testing costs, as excessivespatter must first be removed to ensureproper weld inspection with ultrasonictesting equipment.

Another quality issue impacting a welds cosmetic appearance is a shieldinggass susceptibility to gas marks. Gasmarks, also referred to as worm tracks orchicken scratch, are small grooves thatsometimes appear on the weld surface.They are caused by dissolved gases in the

weld metal that have escaped before thepool freezes, but then are trapped under-neath the slag after it has solidified.

There is a higher susceptibility to gasmarks with an Ar/CO 2 blend than with aCO 2 shielding gas. The spray arc transfercharacteristics of Ar/CO 2 shielding gasproduce a large number of small metaldroplets. This increases the total surfacearea of the molten droplets, resulting in ahigher level of dissolved gases in the weldmetal. There are factors besides shieldinggas type that affect susceptibility to gasmarks; however, they are outside thescope of this article.

Typical Shielding Gas Usedfor Some Main Applicationsand Industries

Over the years, the type of shieldinggas used for FCAW-G has been standard-ized for some main applications and in-dustries. For example, with high-deposi-tion applications in the flat and horizon-tal positions only, CO 2 is preferred, as lit-tle benefit is achieved with an Ar/CO 2blend in these positions.

Shipyards also generally prefer to useCO 2 because its arc characteristics have agreater ability to burn off primer on thebase material. In the North American off-

shore fabrication industry, downhill finalpasses on T-, Y-, and K-connection groove

welds require a very smooth weld contourand minimal spatter levels, making an

Ar/CO 2 blend the preferred shielding gas.In some regions of the world, CO 2 is thegas of choice for all applications due to aninconsistent supply of Ar. Fabricators whouse more than one gas shielded weldingprocess in their shop, such as GMAW andFCAW-G, often standardize bothprocesses with on type of shielding gas. Inthat situation, many fabricators choose an

Ar/CO 2 blend to obtain the GMAW ben-efits of spray or pulsed arc transfer.

Conclusion

When choosing a shielding gas for yourFCAW-G applications, you should con-sider more than just the cost of the gas.Instead, consider all three comparisonpoints discussed in this article. How doeseach gas type affect your total weldingcosts? Which gas type reduces your totalcost to make one foot or one meter of

weld? Some fabricators find that the mer-its of an Ar/CO 2 blend allow them to im-prove their quality and productivity. For

other fabricators, the benefits of an Ar/CO 2 blend are not realized or do notoutweigh the cost savings of CO 2. And for

yet other fabricators, CO 2 provides thebest cost and benefits for their particular

welding application. For users of theFCAW-G process, the choice of whichshielding gas to use should be based onhow it most positively influences the over-all driving factors of cost, quality, and pro-ductivity to their welding operations.Then once the choice of shielding gas ismade, the FCAW-G electrode usedshould be one that is designed for that par-

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Fig. 3 A comparison of the metal transfer through the arc with CO 2 (left) vs. 75% Ar/25% CO 2 blend using the same wire feed speed and voltage welding procedures.