A 500 × 400 mm Welded Angle Frame That Came Out Twisted
A fabricator is making a rectangular frame 500 mm × 400 mm outside, from 25 mm × 25 mm × 3 mm mild steel angle, using a 200 A AC arc welding set, E6013 electrodes of 3.15 mm, at 90 A. The four corners are square butt joints and are welded with a single pass on the outside only. After cooling, the frame is twisted: one diagonal is 648 mm and the other 642 mm, and the 90° corners open to about 87°. At one corner there is undercut along the toe, and slag is trapped between passes at another. The welder suggests increasing current to 130 A to get deeper penetration and welding all four corners in one continuous run.
1. Explain, with reference to the 6 mm diagonal difference and the 87° corners, how the welding sequence produced the distortion.
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A 500 mm × 400 mm frame has a true diagonal of about 640 mm. The readings of 648 mm and 642 mm show that the frame is out of square, and the 87° corners show angular distortion.
Each outside corner weld shrinks transversely and pulls the two legs together. Because the welds were made singly on one side and in one direction, the shrinkage was not balanced. The corners rotated as the weld cooled, shortening one diagonal and lengthening the other by about 3 mm each. The single pass also puts the weld at the outside fibre, which is exactly where it causes the corner to close. Balanced welding on opposite corners, with tack welds and a jig, would have opposed these shrinkage forces.
2. Evaluate the proposal to raise the current from 90 A to 130 A and to weld continuously. Include the electrode size and its normal current range.
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A 3.15 mm E6013 electrode normally runs at about 100–140 A. At 90 A the arc is cold, fusion is poor and slag is likely to be trapped, which fits the reported slag inclusion. Raising to 130 A will improve fusion, but on 3 mm angle it is at the top of the range and can cause undercut and burn-through, especially at the toe.
Continuous welding of all four corners will increase heat input and make the distortion worse. A better setting is about 100–110 A, with a short arc, correct electrode angle, and complete slag removal between passes. The undercut and slag are technique and cleaning problems, not simply a current problem.
3. Give a practical welding sequence and post-weld correction for the frame.
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Tack-weld all four corners first and check the diagonals before welding. Weld diagonally opposite corners alternately, using short runs and a back-step sequence, and allow each corner to cool before the next. Weld one side, turn the frame, and weld the opposite side so shrinkage is balanced. Clean slag after every pass and inspect the toes for undercut.
After cooling, measure both diagonals. If the frame is still out of square, correct it by clamping across the long diagonal and applying local heat, or use a press - not by adding more weld. Re-check the 90° corners with a try square and repair any undercut or slag before painting.