|Flux-cored welding offers many advantages when welding on construction applications, including high disposition rates, and good chemical and mechanical properties.|
Self-shielded Flux Cored Arc Welding (FCAW) has been a viable welding process for structural steel erection, heavy equipment repair, bridge construction and other similar applications for many years. That’s not surprising, as it offers high deposition rates, excellent chemical and mechanical properties, and the weldability required for these jobs. Still, it doesn’t mean that the process is without its challenges. Fortunately, with some know-how and a bit of practice, you can prevent some of the common problems associated with the process and gain the weld quality you need.
Tip One: Avoid Wire Feeding Problems
Wire feed stoppages and malfunctions are common problems on many job sites and they can cause a considerable amount of downtime. The two most prevalent type of wire feeding problems—burnback and birdnesting—tend to extinguish the arc prematurely, which in turn can lead to weld defects.
Burnback occurs when the wire melts into a ball at the end of the contact tip and is most often the result of too slow of a wire feed speed and/or holding the welding gun too close to the workpiece. To prevent the problem, be sure to use the correct feed speed for your application and maintain a distance from contact tip to the work of no further than 1 1/4-inch.
To prevent birdnesting—a tangle of wire that halts the wire from being fed—during FCAW welding, always use knurled V- or U-groove drive rolls in your wire feeder. Compared to a GMAW solid welding wire (which uses a smooth V-groove drive roll), FCAW wire is much softer (due to its tubular design) and if you use the incorrect drive roll, it can easily compress the wire.
|Prevent burnback, as shown here, by having the appropriate wire feed speed and MIG gun to work piece distance.|
Additionally, setting the correct drive roll tension can prevent the wire from flattening and becoming tangled. To set the proper tension, begin by releasing the tension on the drive rolls. Increase the tension while feeding the wire into the palm of your welding glove and continue to increase the tension one half turn past wire slippage.
|Using the correct drive rolls and tension settings can prevent birdnesting.|
Other causes of birdnesting include blockages in the liner, improperly trimmed liners or using the wrong liner. Promptly replace your liner if you find a blockage during your routine inspection of your welding gun and cables, and always trim the liner (using the correct tools) according to the manufacturer’s recommendation. Be certain that the liner does not have any burrs or sharp edges and always use the correct size liner for your diameter of welding wire.
Tip Two: Stop Porosity and Worm Tracking
Porosity and wormtracking are both common weld discontinuities that can weaken the integrity of your welds. Porosity results when gas becomes trapped in the weld metal and can appear at any specific point on the weld or along its full length. To prevent this problem, remove any rust, grease, paint, coatings, oil, moisture and dirt from the base metal prior to welding. Using filler metals with added deoxidizers also helps weld through such contaminants, but these products should never replace proper pre-cleaning. Next, maintain an appropriate electrode extension or stick-out. As a general rule, the wire should extend no more than 1 1/4-in. beyond the contact tip.
To prevent worm tracking—marks on the surface of the weld bead caused by gas that the flux in the core of the wire creates—avoid excessive voltage for your given wire feed setting and amperage. It is best to follow the parameters recommended by the filler metal manufacturer for the specific diameter of welding wire. If worm tracking does occur, reduce your voltage by increments of one half volt until you eliminate the problem.
Tip Three: Eliminate Slag Inclusions
Slag inclusions occur when the slag generated by the molten flux in the wire’s core becomes trapped inside of the weld. There are four major causes of slag inclusions, all of which can be prevented with proper welding techniques.
First, avoid incorrect weld bead placement, especially when making multiple passes on thick sections of metal, such as needed for the root passes of welds or wide v-groove openings. Be certain to provide sufficient space in the weld joint for additional passes, particularly on joints requiring multiple passes.
|To prevent worm tracking, use the manufacturer's recommended parameters for your given wire diameter and lower your voltage setting if necessary.|
Second, maintain the correct travel angle and travel speed. In the flat, horizontal, and overhead positions your drag angle should be between 15 and 45 degrees. In the vertical up position, your drag angle should be between 5 and 15 degrees. If you experience slag inclusions at these angles, you should increase your drag angle slightly. Maintain a steady travel speed; if you travel too slowly, the weld puddle will get ahead of the arc and create slag inclusions.
Next, maintain proper weld heat input, as too low of welding heat input can also cause slag inclusions. Always use the manufacturer’s recommended parameters for a given wire diameter. If slag inclusions still occur, increase the voltage until the inclusions cease.
Finally, be certain to clean thoroughly between weld passes, removing any slag with a chipping hammer, wire brush or grinding before beginning your next weld pass.
Tip Four: Prevent Undercutting and Lack of Fusion
Like other weld defects, undercutting and lack of fusion can both affect the quality of your welds and preventing them can go far in reducing downtime and costs for rework.
Undercutting occurs when a groove melts in the base metal next to the toe of the weld, but is not filled by the weld metal. It causes a weaker area at the toe of the weld and often leads to cracking. Using the proper welding current and voltage are key to preventing undercutting (remember to follow your welding parameters), as is adjusting to the right gun angle. Maintain a travel speed that allows the weld metal to fill the melted-out areas of the base metal completely, or if you are using a weaving technique, pause at each side of the weld bead.
To prevent lack of fusion, the failure of the weld metal to fuse completely with the base metal (or the preceding weld bead in multi-pass applications), maintain the correct work angle and heat input. Obtain the correct angle by placing the stringer bead in its proper location at the joint, adjusting the work angle or widening the groove to access the bottom during welding as needed. Keep the arc on the trailing edge of the welding puddle and maintain a gun angle drag of 15 to 45 degrees. If using a weaving technique, momentarily hold the arc on the groove sidewalls when welding. Increase your voltage range and/or adjust the wire feed speed as necessary to obtain complete fusion. Also, if you feel that the wire is getting ahead of the work puddle, simple adjustments, such as increasing travel speed or using a higher welding current, can prevent problems.
Finally, be certain to clean the surface of the base metal prior to welding to remove contaminants to prevent lack of fusion.
Tip Five: Avoid Excessive Penetration or Lack of Penetration
Maintaining the appropriate heat input during welding is key to avoiding problems like excessive penetration. Excessive penetration occurs when the weld metal melts through the base metal and hangs underneath the weld. It most often results from too much heat. If the problem occurs, select a lower voltage range, reduce wire feed speed and increase travel speed.
Conversely, selecting a higher wire feed speed, a higher voltage range and/or reducing travel speed can prevent problems like lack of penetration—the shallow fusion between the weld metal and the base metal. In addition, prepare the joint so as to permit access to the bottom of the groove, while also maintaining proper welding wire extension and arc characteristics.
Self-shielded FCAW is a reliable process for many construction applications, but obtaining high quality welds with it isn’t a matter of luck. It’s the result of good welding technique, the proper choice of parameters and your ability to prevent problems—or identify and rectify them quickly. Remember, arming yourself with some basic information will allow you to prevent most common problems associated with self-shielded FCAW welding without sacrificing time or quality.
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