Bowing of a 1 m × 1 m Plate After Butt Welding
A 1,000 mm × 1,000 mm flat panel is made by butt welding two 500 mm × 1,000 mm pieces of 6 mm mild steel plate. The joint is a single-V butt weld with a 60° included angle, 2 mm root gap and 1 mm root face. The welder uses SMAW with 3.2 mm E6013 electrodes at 110 A, welding from one end to the other on the first side, then back gouging and welding the second side. No preheat or preset is used, and the plate is only tack welded at the ends. After cooling, the panel is bowed by about 15 mm across its width, with the weld side concave. The drawing requires flatness within 5 mm over the 1,000 mm length. The panel is needed for a machine guard and cannot be scrapped.
1. Explain the likely causes of the 15 mm bow, referring to heat input, joint design and welding sequence.
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The bow is mainly longitudinal and transverse shrinkage from a large, locally heated weld. A 60° single-V groove in 6 mm plate needs a relatively large volume of weld metal, and the 2 mm root gap adds more shrinkage. Welding from one end to the other concentrates heat and shrinkage progressively along the joint, so the panel bends. Tacking only at the ends leaves the plate free to move during welding. No preset or balanced sequence is used, and the 3.2 mm electrode at 110 A puts a fairly high heat input into thin plate. After cooling, the weld contracts and pulls the weld side concave. The result, about 15 mm, is three times the 5 mm flatness tolerance, so the panel is nonconforming.
2. Describe a revised welding sequence and practical controls that would reduce distortion if a replacement panel is made.
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Use a balanced, low-heat-input sequence. Reduce the groove volume where the WPS allows, for example a smaller included angle or a square butt with suitable backing, but only with a qualified procedure. Use a smaller electrode, such as 2.5 mm, at a lower current, and deposit stringer beads rather than a large weave. Weld in backstep steps of about 100–150 mm, first on one side, then back gouge and weld the second side in the opposite direction. Clamp the panel to a strongback or use copper chill bars, and preset the joint by about 3–5 mm in the opposite direction to the expected bow. Add more tacks along the joint, allow the panel to cool between passes, and check flatness before releasing the clamps. Record the sequence in the WPS.
3. The supervisor suggests heating the centre of the bow with a gas torch and quenching it with water to shrink it back. Evaluate this method and state how you would correct the panel or dispose of it.
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Heating and water quenching can shrink steel locally, but on 6 mm mild steel it creates hard spots, high residual stress and a risk of cracking. Water quenching is not a controlled correction method. Controlled flame straightening may be used with a neutral flame, a temperature around 600–650°C, and slow cooling, supported by wedges or a strongback and checked with a straight edge. Even then, a 15 mm bow may not be recoverable within 5 mm. Because the panel is a machine guard, fit and safety matter. First attempt controlled straightening under a written method and inspect for cracks; if the 5 mm tolerance cannot be met, replace the panel. Do not hide the bow with filler or paint.