Not Your Father’s Gas-Shielded Flux-Cored Electrodes
FCAW-G consumables have steadily evolved into application-specific productivity workhorses
by Tom Myers, The Lincoln Electric Company
|Electrodes for the gas-shielded flux-cored arc welding (FCAW-G) process were first developed in the late 1950s. Over the next 40 years or so, manufacturers refined and improved these products, offering a fairly limited line of carbon steel and low alloy steel electrodes for either all-position welding or flat and horizontal only (i.e., in-position) welding.
During this time, there also were relatively few variations in the formulations of electrodes within a specific American Welding Society (AWS) classification. In many cases, one particular electrode was intended to be used for a variety of applications. Often times it was intended to be used with either 100% carbon dioxide (CO2) shielding gas or a mixed argon (Ar) / CO2 shielding gas.
Back then, the general manufacturing philosophy was to develop an electrode that was in essence a “one-size-fits-all” product. This philosophy was partially successful in meeting the demands of the welding market during this time. However, today’s structural engineers and industrial designers are increasingly specifying higher-strength, lower-weight steels for cost savings and productivity considerations, making these base materials a popular choice in many industries.
These new specifications demand the need for low-alloy FCAW-G electrodes which produce welds with increased tensile and yield strengths (compared to carbon steel electrodes) for welding these higher-strength steels. Other applications require electrodes which produce welds with improved impact properties. Generally, electrodes needed to produce welds with low temperature toughness of at least 20 ft•lbf (27 J) at a test temperature of 0⁰F (-18⁰C) or -20⁰F (-29⁰C). Some applications now require these same absorbed energy values at temperatures of -40⁰F (-40⁰C) or even lower.
Similarly, operator demand for just the "right" application-specific, flux-cored electrode has steadily increased over the past five to ten years, to keep up with a growing desire for increased weld productivity, performance and quality, not to mention aesthetics.
Because of this increased specification of new materials, combined with demands for more customized, more efficient electrodes, manufacturers have been returning to the proverbial R&D drawing boards to develop new gas-shielded flux-cored consumables. No longer does the one-size-fits-all approach to electrode design work for many FCAW-G applications. This consumable category has come a long way. In short, they aren’t your father’s gas shielded flux-cored electrodes anymore.
Fundamentals and Advantages of FCAW Electrodes
Flux-cored electrodes were originally developed as a higher productivity extension of shielded metal arc welding (SMAW) electrodes. They are, in fact, like a SMAW electrode turned inside out. They are a steel tube (i.e. outer steel sheath) with flux inside the tube or at the electrode’s core, hence the name, “flux core.” Because of this design, the electrode can be wound onto a coil or spool, and with the use of a wire feeder and welding gun, fed continuously into the weld joint.
Flux-cored electrodes fall into two fundamentally different categories: self-shielded, flux-cored electrodes (FCAW-S) and gas-shielded, flux-cored electrodes (FCAW-G).
||Gas-shielded, flux-cored electrodes incorporate a double shielding system by using an external shielding gas as well as a slag system. The shielding gas is required to protect the arc and molten metal from the atmosphere. It also results in exceptionally smooth arc characteristics, compared to self-shielded electrodes. They use either a rutile slag system or a basic slag system. The rutile system is the most common and is characterized by a smooth arc with complete slag coverage of the weld. The basic slag system, while producing a globular metal transfer and thinner slag coverage, can be more resistant to weld cold cracking. |
Most FCAW-G electrodes are ideal for all-position welding and all deliver great mechanical properties with high deposition rates. They are used effectively in general shop fabrication, structural steel (including seismic applications), shipbuilding, offshore, pipeline and other applications.
FCAW electrodes can be used similarly to SMAW electrodes with a few notable benefits in the process itself. First, SMAW electrodes must be fed manually into a weld joint, making only short welds and resulting in a lot of stop and restart areas in the weld. Restart areas generally have a higher chance of containing a weld defect than any other part of the weld. With the FCAW process, the weld can be made for as long as the welder can comfortably reach before having to stop the arc and reposition themselves. This results in less restart areas in the weld, and ultimately, less chances for weld defects.
The FCAW process also has a higher operating factor than the SMAW process (where operating factor (%) equals arc time divided by total fabrication time). It’s also easier to use. It operates at higher current levels, which yields higher deposition rates and resulting higher productivity. Finally, FCAW electrodes have higher electrode efficiency than SMAW electrodes. This means that more of the purchased pounds (kg) of electrode end up as deposited weld metal and less is lost through stubs.
FCAW electrodes, with their slag systems, also have inherent advantages over slag-less processes, such as gas metal arc welding (GMAW). The fast-freezing slag system of all-position classified FCAW electrodes allows for better out-of-position welding capability, including vertical and overhead, as the slag helps hold the molten metal against gravity. FCAW electrodes produce higher deposition rates when welding out of position than GMAW consumables do. In addition, many in-position classified FCAW electrodes have good penetration characteristics, making them ideal for thicker sections of steel plate.
Concerns with One-Size-Fits-All Electrodes
|FCAW electrodes also handle surface contaminants on steel plate better than solid GMAW electrodes (aka MIG wires). Not only are deoxidizers present in FCAW electrodes’ outer carbon steel sheath, but deoxidizers, denitrifiers and scavenger elements are also added to the core elements. While solid GMAW electrodes can only rely on the deoxidizers that are present in the raw green rod material, which is drawn down to make them. FCAW electrodes are considered to be “fabricated” electrodes and, thus, provide a good platform for manufacturing new low-alloy electrodes. The outer sheath on FCAW electrodes – even low-alloy types – are fabricated from types of carbon steels that are either strip-based or green rod-based, both of which are commonly available from steel mills. As such, the core ingredients for various FCAW electrodes then can be altered to produce low alloy weld deposits with differing mechanical properties.
On the contrary, low-alloy solid GMAW cannot be fabricated. The final chemistry of the electrode can only be achieved by purchasing it as the raw green rod steel. Low-alloy green rod can be more expense and difficult to source than carbon-steel green rod.
The traditional multipurpose approach towards FCAW-G electrodes has proven to be increasingly ineffective over the years. While the use of a one-size-fits-all electrode for a wide range of applications can deliver adequate arc performance, the reach for a single electrode to perform well in every application is just too broad. As a result, the arc is never optimized.
||Why? One electrode used with both 100% CO2 and mixed gas (i.e., 75% Ar / 25% CO2) has to have a fine balance on chemistry in order to meet the minimum and maximum mechanical property requirements of its AWS classifications with either type of shielding gas. Carbon dioxide is an active gas, meaning that it actively reacts with some of the electrode’s alloys. Less alloy recovery from the electrode occurs in the weld puddle, resulting in a slight decrease in mechanical properties, such as ultimate tensile strength and yield strength. Argon, an inert gas, is non-reactive in the arc. Therefore, the more argon in a mixed gas, the more alloy recovery that occurs in the weld puddle. This results in a slight increase in both tensile and yield strengths.
Hydrogen levels also play a role in why FCAW-G electrodes are becoming more specific, moving away from the one-size-fits-all design structure of the past. Lower levels of diffusible hydrogen in weld deposits means that such welds will have higher resistance to hydrogen-induced cracking.
Welding consumables can be classified with an optional diffusible hydrogen designator. These designators include the letter “H” and a number, which indicate maximum milliliters of diffusible hydrogen per 100 grams of weld metal. Most FCAW-G electrodes today meet a diffusible hydrogen rating of H8, with some meeting a very low rating of H4.
Some industries, such as shipbuilding / barge building, have increasingly pushed the deposition rate capabilities of all position FCAW-G electrodes. Generally when welding in position or with gravity, welders can utilize faster wire feed speed procedures to produce higher deposition rates than they can when welding out of position or against gravity. However, because of remote welding locations and limited access to their welding equipment, welders often cannot easily turn up their procedures when they transition from out of position to in position welding. Therefore they need one set of welding procedures for FCAW-G electrodes which produce maximum deposition rates for out of position welding and still produce high deposition rates for in position welding. Many of the original one-size-fits-all FCAW-G electrodes could only be pushed so far before the slag system would not support the additional molten weld metal. Therefore, new FCAW-G electrodes were needed with a different type of slag system. These high deposition or “HD” electrodes have very fast freezing slag systems which better support higher wire feed speed welding procedures.
||Additionally, many industries have had increased requirements for weld metal with improved impact properties. The original one-size-fits-all FCAW-G electrodes were designed to produce welds with a minimum low temperature toughness of 20 ft-lbf (27 J) @ 0⁰F (-18⁰C) or 20 ft-lbf (27 J) @ -20⁰F (-29⁰C). Some industries, such as offshore and pipeline fabrication, often require FCAW-G electrodes that produce welds with a minimum low temperature toughness of 20 ft-lbf (27 J) @ -40⁰F (-40⁰C). These more stringent requirements have necessitated the need for new FCAW-G electrodes with improved impact properties.
In other cases, FCAW-G electrodes have been increasingly used on weldments that must be stress relieved after welding. In general, after post weld heat treatment (PWHT), the tensile and yield strength of the weld drops to a certain degree. When the weld from a one-size-fits-all FCAW-G electrode is stress relieved, you can run the risk of the tensile and yield strengths dropping below the minimum specified levels. Therefore, PWHT applications have led to the need for more specialized FCAW-G electrodes that have an altered chemical formulation. These electrodes are designed to have a minimal drop in tensile and yield strength after stress relief.
With such changeable factors as shielding gas, diffusible hydrogen levels, deposition rate needs and mechanical property requirements, as well as different grades of steel, coming into play in the FCAW-G arena, the scope and versatility of a one-size-fits all electrode has become increasingly narrower. This challenges manufacturers to meet stringent mechanical properties on a consistent basis with traditional multipurpose FCAW-G electrode design.
|New Approach to Design
To meet various industry-specific requirements and operator demands for mechanical properties, performance and aesthetics, manufacturers now are designing and producing application-targeted, next-generation FCAW-G consumables with specific industries in mind – abandoning the one-size-fits-all approach. Instead, operators can match the electrode to the job.
In addition, many of the FCAW-G electrodes today have been designed for use with only one type of shielding gas in order to produce optimum operator appeal and the targeted mechanical properties. They either will be for use with 100% CO2 or a mixed blend, consisting of 75% - 85% Ar / balance CO2 (with 75% Ar / 25% CO2 the most popular blend).
The required shielding gas now is also incorporated into the electrode’s AWS classification number. For example, the "C" in an E71T-1C classified electrode specifies that it is for use with carbon dioxide shielding gas, while the "M" in an E71T-1M classified electrode specifies that it is for use with mixed shielding gas. Electrodes that are still designed for use with either type of shielding gas are dual classified, such as “E71T-1C / E71T-1M.
Furthermore, operator appeal of low-alloy FCAW-G electrodes also has improved. A welder can weld with a carbon steel FCAW-G electrode or a low-alloy FCAW-G electrode and not really see a difference in arc performance. This results from the fact that manufacturers have succeeded in coming up with a standard slag system for families of electrodes. Individual electrodes can be modified for different applications by tweaking the alloy formulation in the electrode’s core so that welders and fabricators will see similar operating characteristics, no matter the application.
Application Specific FCAW Electrodes
Producing a successful FCAW-G electrode comes down to balance in the design and manufacture of the electrodes. Manufacturers have worked to develop FCAW-G consumables that consistently meet mechanical properties, without compromising quality and aesthetics. They do so without taking it to the extreme. They avoid creating an electrode that produces mechanical properties that are so robust that they tip the scale on operability.
Think of the three key components of electrode design as a triangle. On one side, you have operability. On the second side, you have mechanical properties. On the third side, you have diffusible hydrogen levels.
Targeted product development has allowed manufacturers to design "families" of FCAW-G electrodes, each aimed for different applications in specific industry segments. Each family balances the three sides of the design triangle to avoid compromising any one of those components and, thus, delivers a robustly performing electrode.
Today, manufacturers of FCAW electrodes offer broad product lines, with many electrodes designed for specific applications and industries. Examples of more specialized electrodes include the following:
FCAW-G electrodes designed for use with one specific type of shielding gas. (i.e., UltraCore® 71C, UltraCore® 71A85)
FCAW-G electrodes designed for higher-strength steels (i.e. 80 ksi, 90 ksi and 100 ksi minimum tensile strength). (i.e., UltraCore® 81Ni1A75-H, Outershield® 91K2-H, Outershield® 690-H)
"HD" type FCAW-G electrodes designed for high-deposition, out-of-position capability. (i.e., UltraCore® HD-C, UltraCore® HD-M)
FCAW-G electrodes designed for exceptionally high deposition rates in the flat and horizontal positions. (i.e., UltraCore® 70C, UltraCore® 75C)
FCAW-G electrodes designed for improved low temperature toughness properties. (i.e., UltraCore® 712A80, UltraCore® 81Ni2A75-H)
"SR" type FCAW-G electrodes designed for stress-relieved applications. (i.e., UltraCore® SR-12)
FCAW-G electrodes designed for pipe welding applications. (i.e., Pipeliner® 81M, Pipeliner® 101M, Pipeliner® 111M)
FCAW-G electrodes designed for chromium-molybdenum (Cr-Mo) steels. (i.e., Cormet 1, Cormet 2)
Again, gone are the days of FCAW-G electrodes simply being of the “one-size-fits-all” variety. Today, welders have a broad choice of electrodes that have been designed for a variety of specific applications and industries, expanding the range of use, as well as overall quality and productivity. With improved operating characteristics and performance, these enhanced, highly efficient products are truly not our father’s flux-cored electrodes anymore.
Tom Myers is a Senior Applications Engineer at Lincoln Electric in Cleveland, Ohio.