C Sridhar, Director-Technical, Advance Institute of Welding Technology, Chennai, Tamilnadu
With some tips and practice, you can avoid flux core welding problems and improve your FCAW welds
When it comes to structural steel erection, bridge construction, heavy equipment repair, and other same applications, FCAW (Self-Shielded Flux Cored Arc Welding) has been, for many years, a feasible welding process. Unsurprisingly, because it provides high deposition rates, fantastic mechanical and chemical properties and weldability needed for those jobs. Yet, there exist some challenges related to this process. With some tips and practice, you can avert the problems and create qualified welds you need.
1. How to avoid wire feed welding problems:
Common problems at workshops are wire feed malfunctions and stoppages caused by a significant number of downtime. Bird-nesting and burn-back are 2 most notorious types of wire feed welding problems. Those types tend to prematurely extinguish the arc, inducing weld defects.
Burn-back happens when the wire melts into the ball at the end of contact tip. Most often it’s the consequence of too slow speed of wire feed or/ and the welding gun held too close to the work-piece. For this problem prevention, make sure you use the proper wire feed speed and keep a distance of no more than 25 to 35 mm from the contact tip to the work.
To avoid bird-nesting – a wire tangle halting the wire from being fed – when FCAW welding, use knurled U or V-groove drive rolls in the wire feeder. Comparatively with GMAW solid welding wire that employs smooth V-groove drive roll, the FCAW wire is far softer because of tube-shaped design. It can compress the wire if you use wrong drive roll.
Also, setting the right drive roll tension can stave off the wire flattening and being tangled. In order to set the correct tension, start with releasing tension on drive rolls. Raise the tension whilst feeding the wire into the palm of welding glove and keep raising the tension one half turn to pass wire slippage.
Bird-nesting also results from blockages in the liner, wrong liner used, or inappropriately trimmed liners. Quickly replace the liner if you find out a blockage when routinely inspecting your welding cables and gun. Also, always trim your liner (employing the correct tools) in line with the manufacturer’s recommendations. Make sure that there isn’t any sharp edges or burrs on the liner, and always apply the proper size liner for the diameter of welding wire.
2. How to prevent worm tracking and porosity
As typical weld discontinuities, worm tracking and porosity can weaken the weld integrity.
Porosity occurs when gas is trapped in the weld metal, and can appear along its full length or at any particular point of the weld. For preventing porosity in welding, get rid of any grease, rust, oil, paint, coatings, dirt and moisture from the base metal before you weld. You can also use filler metals with added de-oxidizers to prevent contaminated welds. Still, those products can’t replace appropriate pre-cleaning. After that, maintain a proper electrode stick-out or extension. Generally, the wire shouldn’t extend more than 25 to 35 mm beyond contact tip.
To avert worm tracking (also, marks on the weld bead’s surface as a consequence of gas created by the flux in the wire core), avoid immoderate voltage for the wire feed setting as well as amperage. For the best, follow the recommended parameters by the manufacturer of filler metal for the particular diameter of welding wire. In case worm tracking occurs, decrease your voltage by one half volt’s increments until you remove the problem.
3. How to remove slag inclusions:
Slag inclusions eventuate when the slag produced by the molten flux in the wire core gets trapped inside the weld. Whatever causes of this problem, they can be averted with appropriate welding techniques.
First, prevent wrong placement of weld bead, especially when making many passes on the thick metal sections. Be sure to give adequate space in the weld joint for added passes, particularly, on the joints in need of multiple passes.
Second, keep up with correct travel speed and travel angle. From 15 to 45º should be the drag angle in the overhead, horizontal and flat positions. For vertical up position, the angle should be in range of 10 to15º. In case there are slag inclusions at those angles, raise slightly the drag angle. Keep a steady travel speed. If you make too slow travel speed, slag inclusion due to weld puddle getting ahead of the arc will eventuate.
After that, maintain appropriate welding heat input, because too low heat input can be conductive to slag inclusions. Always apply the parameters recommended by the manufacturer for a certain wire diameter. In case slag inclusions still happen, raise the voltage until this problem stops.Finally, make sure you thoroughly clean between the weld passes, taking away any slag using a wire brush, chipping hammer or grinding before you begin the next weld pass.
4. How to avoid weld undercutting and lack of fusion
Same as other weld defects, lack of fusion and undercutting can lower your welds quality and prevent them from going far in reduction of downtime and rework costs.
Undercutting is the aftermath of a groove melting in the base metal next to the weld toe but not being filled by weld metal. It weakens the toe of weld and often induces cracking. Applying appropriate welding voltage and current helps prevent this problem (don’t forget to conform to your welding parameters). Maintain the travel speed that enables the weld metal to completely fill the base metal’s melt-out areas. If you are applying the weaving technique, pause at the weld bead’s each side.
To avoid lack of fusion that eventuates due to the weld metal failing to completely fuse with the base metal (or the preceding weld bead in multi passes), maintain right work angle and heat input. Get the right angle by putting the stringer bead in its appropriate location at the joint, widening the groove or adjusting the work angle to approach the bottom during welding as necessary. Keep the arc on the welding puddle’s trailing edge, and maintain the gun angle drag of 15 – 45°. If you are applying a weaving technique, hold the arc on groove sidewalls for a moment during your welding. Raise your voltage range or/ and adjust wire-feed speed as needed to get complete fusion. In case you feel the wire is going ahead of the work puddle, such simple adjustments as applying higher welding current or raising travel speed can avoid problems.
Finally, make sure you clean the surface of base metal before welding to get rid of contaminants to avert lack of fusion.
5. How to prevent lack of penetration or excessive penetration
Maintain proper heat input when welding is critical to preventing such problems as excessive penetration. The excessive penetration happens due to the weld metal melting through base metal and hanging underneath the weld. Most often, it is the consequence of too much heat. If you encounter the problem, choose a lower voltage range, decrease the wire-feed speed and raise the travel speed.
By contrast, choosing a higher voltage range, higher wire-feed speed or/ and reducing the travel speed can avert such problems as lack of penetration (i.e. shallow fusion between weld metal and base metal). Also, prepare the joint for permitting access to the groove bottom whilst maintaining appropriate extension of welding wire and arc characteristics.
6. How to get qualified FCAW welds
Self-shielded FCAW is a trusted process for various construction applications though, getting qualified welds with it is not the luck of the draw. It results from good welding techniques, appropriate choice of parameters, and your ability to avert problems or quickly define and rectify them. Remember, preparing yourself with some essential information will enable you to avoid most common problems related to FCAW welding without sacrificing quality or time