Choosing a Shielding Gas for Flux-Cored Welding

Gas-shielded, flux-cored arc welding (FCAW-G) is a very popular and versatile welding process. It is used with mild steel, low-alloy steel and other alloy materials in a variety of applications, such as heavy fabrication, structural, shipbuilding and offshore. The two most common (but not exclusive) shielding gases used with the FCAW-G process are carbon dioxide (CO2) and a binary blend of 75% argon (Ar) / 25% CO2. Other blends, such as 80% Ar / 20% CO2, can also be used.
 
So which shielding gas, 100% CO2 vs. an Ar/CO2 blend, should you choose for your flux-cored welding? Each type offers some advantages and disadvantages. The factors of cost, quality and productivity should be considered when manufacturing decisions are made. The choice of shielding gas affects each of these factors, sometimes in a conflicting way. The merits of the two basic gas options for FCAW on steel applications will be the focus of this article.

Before getting into the particular advantages of the gas options, it is appropriate to review some basics. It should also be noted that this article only focuses on a few types of gases. As a more comprehensive reference, ANSI/AWS A5.32/A5.32M “Specification for Welding Shielding Gases,” prescribes the requirements for shielding gases, defining requirements for testing, packaging, identification and certification. Additionally, it contains helpful information on ventilation during welding as well as general safety considerations.

How Shielding Gas Works
The primary function of all shielding gases is to protect the molten weld puddle and electrode from the oxygen, nitrogen and moisture in air. Shielding gases flow through the welding gun and exit the nozzle surrounding the electrode, displacing the air and forming a temporary protective pocket of gas over the weld puddle and around the arc. Both CO2 and Ar/CO2 blends shielding gases accomplish this purpose.

Some shielding gases make it easier to create the arc plasma, providing a current path for the welding arc. The choice of shielding gas also affects the transfer of thermal energy in the arc and forces on the puddle. For these issues, CO2 and Ar/CO2 blends will behave differently.

Properties of Shielding Gases
Carbon dioxide and argon respond in different ways under the heat of the arc. Three basic criteria are useful in understanding the properties of each shielding gas. 

1. Ionization potential is a measure of the energy required to ionize the gas (i.e.  transform to a plasma state in which it is positively charged), enabling the gas to conduct current.  The lower the number, the easier it is to initiate the arc and maintain arc stability. The ionization potential for CO2 is 14.4 eV, versus 15.7 eV for argon. Thus, it is easier to initiate an arc in pure CO2 than in pure argon.
       
2. Thermal conductivity of a gas is its ability to transfer thermal energy. This affects the mode of transfer (spray versus globular, for example), shape of the arc, weld penetration and temperature distribution within the arc. CO2 has a higher thermal conductivity level than argon and an Ar / CO2 blend.
       
3. Reactivity of a gas is a classification as to whether or not it will chemically react with the molten weld puddle. Gases can be divided into two groups, inert and active. Inert gases, or noble gases, are those that are unreactive with other elements in the weld puddle. Argon is an inert gas. Active gases, or reactive gases, are those that combine or react with other elements in the weld puddle to form compounds. At room temperature, CO2 is inert.  However, in the arc plasma, CO2 will disassociate, forming CO, O2 and some monotonic O. Therefore, CO2 becomes an active gas in the welding arc, allowing the oxygen to react with metals (i.e. oxidize) in the arc. An Ar / CO2 blend is also an active gas, but less reactive than 100% CO2.

With all other welding variables being the same, different shielding gases produce different welding fume generation rates. Typically, there is reduction in rates with an Ar/CO2 blend, as compared to CO2, due to the oxidizing potential of CO2. Specific fume generation levels vary and are dependent on the particular application and welding procedures used.

More about Inert Gases
Although inert gases provide weld puddle shielding, they are not suitable by themselves for FCAW-G welding on ferrous or iron-based metals (carbon steel, low-alloy steel, stainless steel, etc.). If, for example, 100% Ar were used for welding on carbon steel, the resulting weld characteristics would be very poor. The outer steel sheath of the electrode prematurely melts. The arc length is excessive, the arc is wide and uncontrollable, and there is excessive weld build up. Therefore, for ferrous metal FCAW-G welding applications, inert gases are always used in a binary blend with an active gas.

More about CO2 / Argon Blends
The most common blend for carbon steel FCAW-G applications in North America is 75% Ar / 25% CO2. A less common blend for carbon steel FCAW-G applications is 80% Ar / 20% CO2. Some gas-shielded, flux-cored wires are designed for use with as much as 90% Ar / balance CO2. Rarely is a blend used with less than 75% argon. As the argon content is decreased below 75%, the effects of argon on the arc characteristics begin to disappear, yet the costs of having argon in the shielding gas are still incurred. In addition, non-standard percentages of Ar / CO2 blended cylinders will typically be more difficult to obtain than standard blended cylinders, like 75% Ar / 25% CO2 or 80% Ar / 20% CO2. 

Alloy Recovery in Weld and Resulting Mechanical Properties
Due to the reactive nature of CO2, a higher level of alloy recovery from a given electrode in the weld metal is experienced when using an Ar/CO2 blend vs. CO2 shielding gas. This is because CO2 will react with alloys to form oxides, which, along with oxides from the flux, form the slag. The flux in the core of the electrode must contain reactive elements, such as manganese (Mn) and silicon (Si), which act as deoxidizers, among other purposes. A portion of these alloys react or oxidize with the free oxygen from the CO2, ending up in the slag instead of being recovered in the weld metal. Therefore, higher levels of Mn and Si result in the weld deposit (i.e., more alloy recovery) with an Ar/CO2 blend than with a CO2 shielding gas (see example in Table 1).

The consequences of higher levels of Mn and Si in the weld deposit are an increase in weld strength and a decrease in elongation, as well as changes to the impact properties (i.e., Charpy V-Notch values).  By simply changing from CO2 to an Ar/CO2 blend, you typically get a 7 – 10 ksi increase in tensile and yield strength and 2% decrease in elongation (see example in Table 1). This is an important concept to understand, for as the percentage of argon in the shielding gas increases, the weld strength could become too high and ductility too low.

Knowing that shielding gases can affect the resulting properties in the weld, AWS D1.1/D1.1M:2008 "Structural Welding Code"has a series of requirements to ensure acceptable properties are achieved. or all welding, the shielding gas must conform to the requirements of A5.32/A5.32M. or prequalified WPSs, D1.1 requires that the specific filler metal and shielding gas combination that is used be supported with test data. 

Clause 3.7.3 of D1.1:2008 provides two acceptable forms of support: either a) the shielding gas that is used for electrode classifications purposes, or b) data from the filler metal manufacturer that shows conformance with the applicable AWS A5 requirements, but with the specific shielding gas that is to be listed on the WPS.  In the absence of these two conditions, D1.1:2008 requires that the combination be subject to qualification testing.  

Filler Metal Classification by Gas Type
Beginning in 2005, American Welding Society (AWS) Flux-Cored Filler Metal Specifications made the type of shielding gas used for classification part of the classification designation. A carbon steel electrode’s AWS classification is "EXXT-XX", where the last X is the “Shielding Gas Designator.” It will either be "C" for 100% CO2 or “M” for mixed gas of 75 – 80% argon/ balance CO2 (for example, E71T-1C or E71T-1M). For a low alloy steel electrode, the shielding gas designator follows the deposit composition designator (for example, E81T1-Ni1C). In contrast, self-shielded, flux-cored electrodes, which require no shielding gas, would have no shielding gas designator in its classification (for example, E71T-8).

Some electrodes are designed to be used solely with 100% CO2. Other electrodes are designed to be used solely with an Argon / CO2 blend. Still others are designed to be used with either 100% CO2 or an Argon / CO2 blend. In this latter case, the electrode must meet the requirements of both classifications. 

Comparing the Types of Shielding Gases for FCAW-G Welding
In choosing either CO2 or an Ar/CO2 blend shielding gas for flux-cored welding, consider the following three comparison points:

1. Shielding Gas Cost
   Total welding costs are a significant factor for many companies and controlling these welding costs crucial to maintaining profitability. In general, 80% of total welding costs can be attributed to labor and overhead expenses and 20% to material costs; with shielding gases accounting for as much as a quarter of material costs, or 5% of total welding costs. If cost of shielding gas is the only deciding factor, then significant cost savings can be achieved by using CO2 over an Ar/CO2 blend. However, often times other factors influence total welding costs as well and those are discussed in later sections.
   
   CO2 costs less than Ar/CO2 blends because it is a less costly gas to collect and the sources are plentiful and widely available all over the world. CO2 is generally collected as a by-product of some other process. For the welding industry, a common source is from the processing or cracking of natural gas. Argon, on the other hand, can only be collected from air. With argon constituting just less than 1% of the atmosphere, a tremendous amount of air must be processed to get argon in large quantities. Special air separation plants are required to process air. Air separation plants consume large quantities of electricity and are only located in specific areas of the world.
   
   
2. Overall Operator Appeal and Impact on Productivity
   When comparing shielding gases for use on the same type and size electrode, smoother, softer arc characteristics and lower spatter levels are seen with an Ar/CO2 blend, resulting in an increased overall appeal to operators, vs. CO2 shielding gas. A welding arc in CO2 shielding gas has a more globular arc transfer with larger droplet sizes (typically larger than the diameter of wire), resulting in a harsher, more erratic arc and higher levels of spatter affecting the operator. A welding arc in an Ar/CO2 blend has more of a spray arc transfer with smaller droplet sizes (typically smaller than the diameter of wire), resulting in a smoother, softer arc and lower levels of spatter.
   
   

   Figure 3: A comparison of the metal transfer through the arc with CO2 (left) vs. 75%Ar/25%CO2 (right) blend using the same wire feed speed and voltage welding procedures.

   
   Another feature of an Ar/CO2 blend that increases its overall operator appeal, with its lower level of thermal conductivity, is that it tends to keep the weld hotter or more fluid, compared to a weld with CO2. This makes it easier to work the puddle and wet in the bead at the toes of the weld. This improvement in operator appeal is particularly noticeable when welding out of position (i.e., vertical up and overhead positions). Some fabricators find that by using an argon blend, less experienced welders are able to control the arc easier, which results in the ability to weld at higher productivity levels.

   One disadvantage of an Ar/CO2 blend, because of the high argon content, is that it radiates more heat up towards the welder than CO2. This means that it feels hotter when welding. In addition, welding guns will run hotter with an Ar/CO2 blend (guns have a lower duty cycle rating with Ar/CO2 blend than with CO2). This may require the use of larger guns or potentially incur higher annual replacement costs of the same size guns and consumable parts.
   

3. Weld Quality
   As discussed earlier, an Ar/CO2 blend, compared to CO2, tends to keep the weld puddle more fluid, making it easier to work the puddle and wet in the bead at the toes of the weld. Some fabricators find this allows welders to improve the weld profile and resulting quality of the weld. In addition, the welding arc in an Ar/CO2 blend produces less weld spatter. his results in higher weld quality and reduction in weld cleaning time and cost. Lower spatter levels can also improve ultrasonic weld testing costs, as excessive spatter must first be removed to ensure proper weld inspection with the U.T. equipment.
     
   Another quality issue is a shielding gas’ susceptibility to gas marks, which are not considered a weld defect, but rather a cosmetic imperfection. as marks, also referred to as worm tracks or chicken scratch, are small grooves that sometimes appear on the weld surface. hey are caused by dissolved gases in the weld metal that have escaped before the puddle freezes, but then are trapped underneath the slag after it has solidified. There is a higher susceptibility to gas marks with an Ar/CO2 blend than with a CO2 shielding gas. There is more of a spray arc transfer with argon in the shielding gas, which results in smaller metal droplet size and a greater number of droplets. This increases the total surface area of the molten droplets, resulting in a higher level of dissolved gases in the weld metal. There are factors besides shielding gas type that affect susceptibility to gas marks, however they are outside the scope of this article.
   
   

Typical Shielding Gas Used for Some Main Applications and Industries
Over the years, the type of shielding gas used for FCAW-G welding has been standardized for some main applications and industries. For example, for high deposition applications using flat and horizontal only type wires, CO2 is preferred, as little benefit is achieved with an Ar/CO2 blend in the down hand position. Shipyards also generally prefer to use CO2 because its arc characteristics have proven a greater ability to burn off primer on the base material. In the North American offshore fabrication industry, vertical down final passes on T-, Y- and K-connection groove welds require a very smooth weld contour and minimal spatter levels, making an Ar/CO2 blend the preferred shielding gas. For some regions of the world, CO2 is the gas of choice for all applications, as the supply of argon is too inconsistent. 

Conclusion 
When choosing a shielding gas for your FCAW-G applications, you should consider more than just the cost of the gas. Instead, consider all three comparison points discussed in this article. How does each gas type affect your total welding costs? Which gas type reduces your total cost to make one foot or one meter of weld? Some fabricators find that the merits of an Ar/CO2 blend allow them to improve their quality and productivity. For other fabricators, the benefits of an Ar/CO2 blend are not realized or do not outweigh the cost savings of CO2. And for yet other fabricators, CO2 provides the best cost and benefits for their particular welding application. For users of the FCAW-G process, the choice of which shielding gas to use should be based on how it most positively influences the overall driving factors of cost, quality and productivity to their welding operations. Then once the choice of shielding gas is made, the FCAW-G electrode used should be one that is designed for that particular shielding gas.

Tom Myers is a Senior Application Engineer with The Lincoln Electric Company in Cleveland, Ohio.