March 7, 2011
A reliable process for heavy-duty applications, self-shielded flux-cored welding (FCAW-SS) has its challenges. Here are some FAQs about these challenges and suggestions for overcoming them.
For structural steel applications, bridge construction, and heavy equipment repair, self-shielded flux-cored welding (FCAW-SS) has become a standard and reliable process because of its ability to provide high deposition rates and good weld quality. The process also provides the chemical and mechanical properties necessary to withstand low temperatures and is equally well-suited for ship and barge construction applications.
Like every welding process, however, FCAW-SS has its challenges. From selecting the right power source and welding gun to determining the best filler metal for the application, it’s important to take care with each to obtain the best welding performance. It’s also important to know the causes of and solutions to problems when they arise. Doing so can save you time, money, and frustration.
Here are some frequently asked questions about FCAW-SS problems, along with some advice on how to remedy them.
Slag inclusions are common problems that can arise during the FCAW-SS process. They occur often in out-of-position welding and are caused by molten flux from inside the welding wire becoming trapped inside the weld. The potential for slag inclusions also is prevalent during multipass welding applications.
Slag inclusions can be prevented in several ways. First, be certain to maintain the correct travel speed and angle. When welding in the vertical-up position, keep your gun’s drag angle between 5 and 15 degrees and increase it as necessary if the problem still occurs. If you are welding in the flat or horizontal positions, maintain a drag angle between 15 and 45 degrees to prevent the problem.
Second, always use the filler metal manufacturer’s recommended voltage for your welding wire to ensure that you are maintaining the proper heat input. Heat input that is too low can cause slag inclusions.
Other ways to prevent slag inclusions include cleaning thoroughly between weld passes to remove any slag (use a chipping hammer, grinder, or wire brush) and placing beads correctly. Allow enough space in the weld joint, especially on root passes and wide groove openings, for the weld metal to fill it.
Burnbacks (Figure 1), which occur when the welding wire melts into a ball and fuses to the end of the contact tip, are a common cause of downtime in the FCAW-SS process. The two main reasons for burnback are holding your gun too close to the joint or workpiece and using a wire feed speed that is too low.
You can prevent the problem by maintaining a work-to-contact-tip distance of no more than 1-1/4 inch. Also, use the correct wire feed speed for your application. When in doubt as to what that wire feed speed should be, contact your local welding distributor and/or consult the manual provided with your wire feeder.
Lack of fusion often is caused by using an improper gun angle. If you do not place the stringer bead in the proper location at the joint, the weld metal fails to fuse completely with the base metal. This problem can also occur in multipass welding applications if the weld metal does not fuse with the preceding weld bead. A dirty work surface can also be the culprit.
To prevent lack of fusion, be sure to clean the material thoroughly before the initial weld pass and in-between passes if you have a multipass application. Adjust your work angle or widen the joint groove so that you can fully access the bottom of the joint. Maintain a gun angle drag of 15 to 45 degrees, or hold the arc on the groove’s sidewall if you are using a weaving technique.
Consider increasing your voltage range if you experience lack of fusion, and adjust your wire feed speed and your travel speed until you see complete weld fusion.
Several factors can cause birdnesting (Figure 2), a tangle of wire that prevents it from feeding properly through the wire feeder and gun. Two causes are using the wrong drive rolls in your wire feeder and setting the wrong drive roll tension.
To prevent birdnesting, use knurled V- or U-groove drive rolls that keep the soft FCAW wire from compressing. Also, set the proper drive roll tension by first releasing the tension on the drive rolls, then increasing it slightly, while you carefully feed the wire into a gloved palm. Continue increasing the tension until you reach a half-turn past wire slippage. As with using the correct drive rolls, the proper drive roll tension keeps the FCAW wire from being crushed.
Issues with the gun liner also can cause birdnesting. These include using an improperly trimmed liner or using the wrong liner size for the FCAW wire diameter. Trim the liner according to the manufacturer’s recommendation, and make sure that there are no sharp edges on it afterward. Inspect your liner on a regular basis to look for blockages, and replace the liner if any are found.
Good joint penetration is essential to achieving high-quality, sound welds, and the goal is to prevent too much or too little weld metal from flowing into the joint. To prevent excessive penetration (weld metal melting through the base metal and hanging under the weld), maintain the proper heat input for your application. Lower your voltage range if the problem occurs, while also decreasing your wire feed speed and increasing your travel speed.
Conversely, if you find that you are not gaining enough penetration into the weld joint (called lack of penetration — the shallow fusion between the weld and base metal), increase your voltage range and wire feed speed, but reduce your travel speed. Remember, too, to prepare your joint properly. You need to be able to access the bottom portion of the groove to maintain the proper wire extension and obtain the arc characteristics necessary for a good-quality weld. When possible, prepare the joint so that the material you are welding isn’t too thick.
6. What Can I Do to Prevent Porosity?
The best prevention against porosity — a weld defect that occurs when gas becomes trapped in a weld — is to clean your base material thoroughly before welding. Be certain to remove all dirt, rust, grease or oil, paint, and other potential contaminants along the full length of the joint you plan to weld. When you begin welding, maintain a wire extension of no more than 1-1/4 inch beyond the end of the contact tip.
For some applications, using a filler metal that contains additional deoxidizers can help prevent porosity, as these products often can weld through light contaminants. Remember, however, no filler metal should be a replacement for proper cleaning procedures.
Like any welding process, you may encounter a number of challenges with FCAW-SS, and those discussed here are by no means an exhaustive list. A trusted welding distributor and a welding equipment manufacturer are good resources to help you address problems related to wire feeding, weld quality, and more. And remember, the more that you know about the causes of welding problems, the faster you can resolve them and get back to work.