Slag Inclusion on a Multi-Pass Fillet
A trainee is making a 12 mm horizontal-vertical (PB) fillet weld on mild steel using a 4 mm covered electrode at 160 A. The weld is built up in three passes. After chipping and brushing, the instructor finds a line of slag trapped between the first and second pass on the radiograph of a test coupon cut from the same procedure. The trainee says the slag was chipped after every pass. The electrode angle looks steep, about 70 degrees from horizontal, and the travel speed is slow. The machine is a DC set on electrode negative.
1. Identify the likely causes of the inter-pass slag inclusion and explain how each contributes.
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Several factors combine:
- Steep electrode angle: at about 70 degrees from horizontal the arc is directed down into the corner, and molten slag can run ahead of the pool and be buried by the next pass. A more typical angle for a PB fillet is around 45 degrees, bisecting the corner.
- Slow travel speed: this puts more metal down per unit length and lets slag float to the surface but also allows it to flow into the root of the joint ahead of the arc, where the next pass covers it.
- Electrode negative polarity: on DCEN, more heat goes into the workpiece and less into the electrode. This gives deeper penetration but a faster-melting, more fluid pool, which can trap slag more easily than DCEP for some electrodes.
- Slag chipping may have been done, but not thoroughly enough at the toes and in the corner, where slag clings.
The inter-pass line is the signature: the slag was left on the first pass surface and covered by the second.
2. Recommend changes to technique and settings that would reduce the risk, and explain why each change helps.
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- Set the electrode angle closer to 45 degrees so the arc bisects the corner and drives slag out rather than into the joint.
- Increase travel speed to a steady rate so the pool does not overfill and slag does not run ahead. Watch the pool shape rather than rushing.
- Reduce current slightly, or change polarity to DCEP if the electrode data sheet allows, to give a less fluid pool and better slag control. Follow the manufacturer's recommended range for the 4 mm electrode.
- Chip and brush each pass thoroughly, paying attention to the toes and the corner, and inspect before the next pass. A wire brush alone is not enough; use a chipping hammer and then brush.
- Keep the arc length short and consistent so the slag is pushed to the surface.
3. The trainee wants to prove the fix works without radiography. Describe a practical shop test and the acceptance check.
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Weld a test coupon using the same material, joint, and settings as the production weld, then cut it across the weld and prepare the cut face by grinding and etching, or bend it in a guided bend jig if one is available.
For a macro-examination, polish the cut face and look for a continuous line of slag between passes. A sound weld shows fused metal across the pass boundaries with no dark line. For a bend test, bend the coupon so the weld is on the tension face; slag inclusions show up as cracks or open defects at the bend.
Acceptance: the section should show complete fusion between passes with no visible slag line. If the defect remains, adjust one variable at a time and retest. Keep the coupons labelled with the settings used so the instructor can compare them.