Special Considerations When Using Self-Shielded Flux-Cored Electrodes
Tom Myers – Sr. Application Engineer, The Lincoln Electric Company
One of the main arc welding processes used for a variety of steel fabrication applications around the world is the Self-Shielded Flux-Cored Arc Welding (FCAW-S) process. Because it does not require an external shielding gas, it is particularly popular for field or outdoor welding, where wind interference with gas shielded welding processes is a major concern. Because it is also a continuous electrode (i.e., a wire), it often provides a considerable productivity gain over Shielded Metal Arc Welding (SMAW) or stick welding. In order for the FCAW-S process to utilize a self-shielded, flux cored continuous electrode, its design and metallurgy must be unique compared to the other main arc welding processes (SMAW, GTAW, GMAW, FCAW-G and SAW). This often results in the FCAW-S process requiring different welding considerations. Therefore, this article will discuss why the FCAW-S process is unique, and then how this creates some special considerations you should know when using it.
One of the basic principles of all arc welding processes is that you must protect the arc and molten metal or weld pool from the atmosphere. Otherwise, excessive levels of nitrogen and oxygen will be absorbed into the weld pool, creating a weld that is brittle with poor mechanical properties. Therefore, the various arc welding processes use either a slag system and/or an external shielding gas system to protect against the atmosphere. The FCAW-S process uses a slag system. However, it is unique compared to the other processes in that it actually relies on the arc being exposed to the atmosphere and the resulting reactions with the core elements which then cleanse the weld pool and help create the protective slag.
To do this, the FCAW-S process predominantly uses an aluminum-magnesium deoxidizing and de-nitriding cleansing system (vs. a primarily silicon-manganese system used by the other main arc welding processes). The weld metal is typically composed of an average of 1% aluminum, much more than is present in the weld metal from the other welding processes. However, it should be noted that is not in a pure state, but rather in the form of beneficial compounds. The aluminum and magnesium atoms enter the weld pool where they attract oxygen and nitrogen atoms and form compounds of aluminum oxide, aluminum nitride and magnesium oxide. These compounds, particularly magnesium oxide, have high melting temperatures. That means that as the molten weld pool starts to cool, they solidify more rapidly (i.e., are “fast freezing”) than other elements in the pool. These lightweight, fast freezing compounds float to the weld surface quickly and protect the process from further atmospheric contamination. Thus the slag system in effect transforms oxygen and nitrogen, two potential contaminants, into chemical compounds that protect the weld.
Because of self shielded flux cored electrodes’ unique metallurgy and other factors, there are some special considerations when using them. The rest of this article will discuss these issues.
NO SHIELDING GAS
Do NOT use an external shielding gas with FCAW-S electrodes (Figure 1). Granted, arc stability may improve by using a shielding gas. However, the arc would then be shielded from the atmosphere. This would not allow aluminum and magnesium from the electrode’s core to react and combine with nitrogen and oxygen from the atmosphere and form compounds. The weld deposit would then end up much higher than expected levels of primarily aluminum, resulting in very brittle, crack sensitive weld metal. Therefore, using a shielding gas with self shielded flux cored electrodes could result in potential weld cracking issues.
The specific core elements or arc stabilizers used in a particular self shielded flux cored electrode determines the welding polarity in which the arc is the most stable. Most FCAW-S electrodes operate best on direct current electrode negative (DC-) or "straight" polarity (Figure 2). Note that this is opposite of all gas shielded flux cored, solid (i.e., MIG) and metal cored electrodes, which operate best on direct current electrode positive (DC+) or "reverse" polarity. Note also that a few of the self shielded flux cored electrodes also operate best on DC+ polarity. These include electrodes with an American Welding Society (AWS) classification usability designator of "3", "4" and "6" (Example: AWS classification of E70T-4, where "4" is the usability designator).
The FCAW-S process tends to be more sensitive to changes in arc voltage than other processes. Voltage (V) affects the length of the arc (Figure 3). As voltage decreases, arc length gets shorter and the resulting arc cone narrower and smaller. An excessively convex or ropey bead indicates that voltage is too low. As voltage increases, arc length gets longer and resulting arc cone gets wider and larger. As arc voltage becomes excessive, the surface area of the arc cone, and the arc’s exposure to air, gets exponentially larger. There are only so many core elements (i.e. aluminum and magnesium) inside the tubular electrode in which to react with the atmosphere and protect the weld. If the exposure level becomes too much for the core elements to handle, then additional nitrogen is absorbed into the weld metal. Reduced Charpy V-Notch toughness properties and internal and/or external porosity can result. Therefore, too much voltage with self shielded flux cored electrodes can cause reduced CVN values and/or weld porosity.
On multiple pass welds, absorption of excessive nitrogen into the weld metal because voltage is too high is cumulative. The first few weld passes may appear to be sound. But then suddenly porosity is encountered in the last few or cap passes. This could be due to an accumulating build-up of nitrogen in the weld which finally reaches a point of producing visible porosity.
Using the proper arc voltage with FCAW-S electrodes, per the manufacturer’s recommendations, is imperative. Set the arc voltage via the power source’s or wire feeder’s voltmeter, or with a hand held voltmeter placed between the gun connecter block and work piece. Note that poor welding cable connections, undersized or damaged cables and poor cable clamps can cause a significant drop between the set voltage at the power source and the actual voltage at the arc.
Because a stable arc length is critical with self shielded flux cored electrodes, use only constant voltage (CV) output (Figure 4). Power sources producing CV output and used with constant speed wire feeders produce a very consistent arc length. This produces a stable, well protected arc. With constant current (CC) (i.e., variable voltage) output, the arc length varies too much. This results in an erratic arc, particularly at procedures of 22 volts or less (where most FCAW-S electrodes operate). In addition, variable arc lengths can cause too long of arc length. This in turn can increase the arc’s exposure to the atmosphere and potentially cause reduced Charpy v-notch toughness properties and weld porosity (as explained above in the "arc voltage" section). Therefore, do not use a welding machine intended for SMAW or GTAW welding, with its CC output only, for welding with self shielded flux cored electrodes. Instead, only use CV machines or multi-process machines with CC/CV output.
CONTACT TIP TO WORK DISTANCE
Contact tip to work distance (CTWD) is the distance from the end of the welding gun’s contact tip to the work piece or plate (Figure 5). It is very important to hold a consistent CTWD with the gun while welding for good arc stability. Maintain this length within ±1/8 in. (3.2 mm) for CTWD ≤1 in. (25 mm) or within ±1/4 in. (6.4 mm) for CTWD >1 in. (25 mm) during welding. Refer to a particular self shielded flux cored electrode’s recommended procedure datasheet for its proper CTWD.
Note that the normal recommended CTWD for flux cored electrodes is long (1 in. (25 mm) average) compared to short circuit MIG welding (3/8 in. (10 mm) average). The electrode becomes electrically hot as soon as it touches the inside of the contact tip. This longer CTWD for cored electrodes allows for a split second more time of resistance heating in the wire, which allows the core elements to fully react or activate and provide proper protection of the arc. If the CTWD is too long (with no change to wire feed speed), it can cause an unstable arc, increased spatter, and decreased penetration. Conversely, if CTWD is too short, incomplete activation of the core elements may occur, potentially resulting in gas marks on the weld surface or internal weld porosity. In addition, with a constant wire feed speed, if the gun is pushed too far into the weld puddle (i.e., shortening CTWD), then voltage is momentarily increased in order to stabilize the arc. This in turn could introduce excessive nitrogen into the weld metal.
Some self shielded flux cored electrodes utilize extended stickout distances of 1-½ in. to 3-¾ in. (38 – 95 mm) with faster wire feed speeds for higher productivity. (Note that an extended stickout of these distances are not possible with gas shielded electrodes, as a lack of shielding and resulting weld metal contamination will most likely occur). The longer CTWDs and resulting increase in resistance heating increases the melt off rate of the electrode. Therefore, much faster wire feed speeds must be used, which greatly increases deposition rates. To consistently maintain these long CTWDs at ± ¼ in. (6.4 mm), nozzle “insulated guides” of various lengths are used (Figure 6). While still using extended CTWDs, the visible portion of electrode extended beyond the end of the insulated guide, called the visible stickout (VSO), is much shorter and easier to maintain at a consistent distance. These insulated guides screw onto the end of the gun tube.
To obtain the proper CTWD when using an insulated guide, first remove the insulated guide from the end of the gun tube. Inch the electrode out beyond the end of the contact tip until you obtain the desired CTWD specified for each size and type electrode. Then replace the insulated guide. Note the VSO distance that must then be maintained.
SINGLE PASS LIMITATIONS
Certain FCAW-S electrodes are limited to single pass welding only. They rely on admixture or dilution with the base metal to produce the weld deposit. If used for multiple pass welding, then after the first few passes you would begin to have an all filler metal or all weld metal deposit. The resulting alloy content of the weld bead could be undesirable and weld cracking could potentially result. Self shielded flux cored electrodes which are intended for single pass welding only include those with an AWS classification usability designator of "3", "10", "13", "14" and "GS" (where "G" means general and "S" means single pass only).
Certain FCAW-S electrodes are limited to a maximum steel plate thickness in which they can be used. If used with plate thicknesses beyond these recommended limits, then the cooling rate of the weld metal could be faster than desired, due to the thermal conductivity or weld quenching effect of the thicker plate. This in turn could potentially create an undesirable microstructure in the weld metal, in which weld cracking issues could potentially result. Table 1 lists examples of self shielded flux cored electrodes with maximum plate thickness restrictions. Note that these material thickness limitations apply to self shielded flux cored electrodes with a usability designator of "3", "11" and "14", as well as potentially to "G" and "GS" electrodes.
CHARPY V-NOTCH TOUGHNESS PROPERTIES
Per the appropriate carbon steel filler metal specification, note that several of the self shielded flux cored electrode classifications do not have a requirement for Charpy V-Notch (CVN) impact energy or toughness. Therefore, some of these electrodes will not include any notch toughness data in their product information and will not meet any specified minimum CVN values. To help illustrate this point only, Table 2 includes partially excerpted information from Table 1U, "A5.20 Mechanical Property Requirements" from the AWS A5.20/A5.20M:2005 Specification for Carbon Steel Electrodes for Flux Cored Arc Welding.
Note also that all "low-alloy" classified self shielded flux cored electrodes do have a minimum Charpy V-Notch Impact Energy specification.
It is recommended to use the same self shielded flux cored electrode to tack weld plates together that will also be used to weld the joint. If using stick electrodes to tack, often the self shielded flux cored electrode’s slag adheres strongly where you have welded over the stick electrode tack welds. The higher levels of aluminum in FCAW-S electrodes react with a rutile electrode (i.e., titanium oxide) and form a thin coating on the bottom of the slag that is very hard to remove. The severity of this problem varies with different types of FCAW-S electrodes.
However, tack welding with stick electrodes is often the most practical method. If this is the case, then the following stick electrodes are recommended for tack welding prior to FCAW-S welding. Also thoroughly remove slag from tacks before welding with self shielded flux cored electrodes.
Special cellulosic electrodes. For example, Lincoln Electric’s "Fleetweld® 35LS" (E6011), which is designed specifically as a tacking electrode for use under Innershield® welds (LS = low silicon)
Secondly, other cellulosic stick electrodes (E6010 or E6011) or low hydrogen electrodes (E7016, E7018)
Avoid using electrodes with a rutile coating (E6013, E7014, etc.)
INTERMIXING IN SAME JOINT WITH OTHER PROCESSES
When FCAW-S weld deposits are intermixed in the same joint with weld deposits from other welding processes (e.g., SMAW, GTAW, GMAW, FCAW-G or SAW), a decrease in weld metal Charpy V-Notch toughness properties may occur. In particular, this issue can occur when the FCAW-S weld deposit is in the root pass(es) and the weld pass(es) from another process are on top of them. Again, the aluminum – magnesium based weld pool cleansing and slagging system of the FCAW-S process is different than the silicon-manganese system of the other arc welding processes. In the self shielded flux cored welds, the aluminum and magnesium is present in the form of compounds. However, when you weld over it with another arc welding process, you can break apart these compounds and create an increased presence of aluminum (particularly) into the resulting intermixed weld metal. You also change the level of manganese in the resulting intermixed weld metal. These alterations to the weld metal can in turn affect the weld’s notch toughness. Therefore, for applications requiring good notch toughness, testing of an actual intermixed weld sample, made with the specific procedures and the two specific electrodes to be used, is recommended to ensure that it meets the minimum CVN requirement.
Inductance is a naturally occurring phenomenon in any electrical circuit in which current is flowing, including a welding circuit. In simplest terms, inductance is resistance to a change in current flow (either increasing or decreasing current). A welding arc is dynamic, in which current and voltage are changing constantly. Therefore inductance can be beneficial in that it helps resist these current changes up or down and thus produces a smoother arc characteristic.
However, inductance can have unwanted effects also, particularly when it is unintentional inductance. This can occur when large external inductors are inadvertently created in the welding circuit resulting in high, uncontrolled external inductance. This inductance can result in disturbing the welding arc and can significantly reduce the useable current output of the power source. The effect can be more noticeable with the FCAW-S process, where arc stability is decreased and / or the welding machine’s output does not feel "hot enough".
A common source of unintentional inductance can come from long lengths of welding cable, particularly when current is flowing through them while they are still coiled up (Figure 7). Long lengths of cable should be unrolled and stretched out when using. Also avoid winding the welding cable directly around the power source. Additionally, when the full length of cable is not needed for a particular job, the use of male and female cable quick connect plugs can make it easy to take most of the cable completely out of the welding circuit.
Unintentional inductance can also occur in the particular case of when self shielded flux cored electrodes are being used with pure DC generator type engine driven welders with Dual Continuous Control. When using these types of welders with an optional Wire Feed Module and CV output, the “Course Current Switch” or “Current Range Selector Switch” should be set to the maximum tap (Figure 8). Then arc voltage is controlled with the “Fine Current and OCV Switch” or “Voltage Adjustment Dial”. While the course current switch is not controlling the level of welding output, there is still current flowing through it and therefore it is producing a certain degree of self-inductance. The higher this self-inductance, the more arc interference you can get. This inductance is lowest when the tap is its maximum setting. For a crisper arc characteristic, move the tap one position less than maximum. Note that this issue is only related to pure DC generator type engine drives. Chopper Technology® and Reactor Technology engine drives use different circuitry, where this issue is not present.
When arc gouging welds made with self shielded flux cored electrodes, black smudges or spots may appear on the surface of the gouged out groove. The condition is aggravated when the carbon is allowed to touch the surface. These spots are often mistakenly identified as porosity. This black residue does not indicate the presence of porosity or poor weld quality. However, if desired, it can be easily removed by wire brushing or light grinding.