Root Gap Trouble on a 6 mm Butt Weld
A trainee in a BTEB short-course shop is asked to butt-weld two 6 mm mild steel plates, 200 mm long, in the flat position using a 3.2 mm covered electrode on a 200 A AC transformer set to about 110 A. The drawing calls for a 2 mm root gap with a 60-degree included V-groove. The plates were sheared, so the edges are square and slightly bowed; when the trainee tacks both ends and checks with a 2 mm feeler gauge, the gap at the middle has closed to nearly zero while the ends hold 2.5 mm. The first root pass shows incomplete penetration in the centre and burn-through at one end. The instructor says to stop welding and redo the fit-up before continuing.
1. Explain why the gap closed in the middle, and describe a fit-up sequence that would give a uniform 2 mm root gap along the full 200 mm length.
Read the worked answer
The gap closed because the plates were sheared square rather than bevelled, and the two tack welds at the ends contracted as they cooled, pulling the free middle together like a bowstring. The 2.5 mm gap at the ends is really the tacks holding the plates apart. Also, sheared edges are not straight, so the gap varies along the length even before welding.
A workable sequence:
- Bevel both edges to the 60-degree included angle first, by grinder or by thermal cutting followed by grinding to clean metal. Square sheared edges cannot give a controlled root gap.
- Straighten or clamp the plates flat on a jig or strong-back so they cannot bow.
- Set the 2 mm gap using a 2 mm spacer or feeler gauge at three points: each end and the middle.
- Tack at both ends and the middle, then re-check the gap at the quarter points before welding. If a tack pulls the gap closed, grind it out and re-tack.
- Leave the clamps in place until at least the root and second pass are complete.
2. The trainee wants to fix the problem by simply turning the current up to 140 A. Judge whether this will solve the incomplete penetration, and state what the likely result would be at the ends.
Read the worked answer
Raising the current will not fix a zero root gap. Incomplete penetration at the centre is a joint-geometry problem, not a heat problem: with no gap and square edges, the arc cannot reach the root, so more current only makes a wider, deeper cap without reaching the back of the joint. It may also increase the risk of slag entrapment and undercut at the toes.
At the ends, where the gap is already 2.5 mm, the extra current will make burn-through much more likely, because the larger gap plus higher heat input melts through the thin root section. So the same change makes one defect worse while failing to cure the other.
The correct answer is to fix the fit-up, keep the current in the recommended range for a 3.2 mm electrode (roughly 90-130 A on this class of set), and use a modest root gap with a slightly slower travel speed for the root pass.
3. After re-fit-up, the trainee runs the root pass and finds a small blowhole at the start of the run. List two probable causes and one check the trainee should make before the next pass.
Read the worked answer
Two probable causes:
- Arc strike on the plate away from the joint, or starting the arc in a cold, dirty spot, so the first metal is contaminated and gas is trapped.
- Damp or damaged electrode coating, or a coating that has absorbed moisture, which introduces hydrogen into the weld pool.
A third possibility is that the root face was not cleaned of mill scale or rust before welding.
Before the next pass the trainee should check the electrode: look for a cracked or damp coating and, if in doubt, bake or replace the electrode. Also strike the arc just ahead of the start point and back-step into the joint, and clean the joint faces to bright metal. Grind out the blowhole before depositing the next pass so it is not buried.