Welded Frame Distortion and Disposition
A workshop in Bongaigaon is fabricating 20 mild-steel frames from 50 mm × 50 mm × 3 mm square hollow sections. Each frame is 1200 mm × 800 mm, mitred at the corners and joined by full fillet welds using a 200 A MIG set with 0.8 mm wire and CO2 shielding. After welding the first three frames, the diagonal measurements differ by 8–10 mm, the frames rock on the surface table, and one corner fillet shows undercut and porosity. The customer requires diagonals within ±2 mm and flatness within 2 mm. The shop has no press, no stress-relieving oven, and only a wooden fixture table with clamps. Twenty frames are due in seven days. The welder proposes to 'pull the diagonals straight' after welding and to increase current to speed up production. The supervisor wants to know whether to accept the first three frames, rework them, or scrap them.
1. Diagnose the likely causes of the 8–10 mm diagonal error, rocking, undercut and porosity. Rank the causes and explain which are process or engineering issues rather than welder skill.
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Ranked causes: (1) fit-up and fixturing - the wooden table and clamps do not hold the frame square, and the mitred corners can gap; (2) welding sequence and heat input - welding all four corners consecutively on one side causes unequal shrinkage and twist; (3) parameters and technique - excessive current, long arc or wrong travel angle cause undercut; (4) contamination - mill scale, oil, moisture or poor gas coverage cause porosity. The 8–10 mm diagonal difference and rocking indicate residual stress and twist, not just poor measuring. Undercut and porosity are parameter and cleanliness issues. These are mostly engineering and process controls: fixture stiffness, tacking, balanced sequence and a written welding procedure. Retraining the welder alone will not fix a frame that is free to move during cooling.
2. Propose a practical welding and fixturing plan, without a press or stress-relieving oven, to hold diagonals within ±2 mm for the remaining frames. Include a pre-set and inspection hold points.
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Build a stiff steel fixture with corner stops, flat pads and diagonal clamps; the wooden table is not adequate. Clean the joint area, set a small root gap, and tack the four corners. Check both diagonals before welding and pre-set them equal or 1–2 mm opposite to the expected shrinkage. Weld with a balanced backstep sequence: weld opposite corners in short runs, alternate sides, and allow each joint to cool before the next. Use the qualified WPS parameters, for example about 130–150 A with 0.8 mm wire and 12–15 L/min shielding gas; do not increase current to gain speed. Keep temporary diagonal braces until the frame cools. Hold points: after tacking, after 50% of welding, and after cooling. Inspect diagonals, flatness on a surface table, weld size, undercut and porosity. The first frame becomes the approved first article.
3. Disposition the first three frames and respond to the welder's proposals to increase current and pull the diagonals straight. What quality gate would you set for the remaining 17?
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Disposition: reject the first three. Check for kinked or buckled tubes; if the sections are sound, cut one weld at each affected corner, realign under clamps, re-tack, check diagonals and re-weld to procedure. If a tube is permanently deformed, scrap that frame. Do not pull the diagonals straight after welding: that induces high residual stress, cracks and joint distortion. Do not increase current: higher heat input will worsen distortion and undercut. Quality gate for the remaining 17: fixture and first-article approval, in-process diagonal checks at tack and after cooling, final dimensional inspection, visual inspection of all welds, and dye-penetrant testing of a sample of critical corner welds. With one day for the fixture and first article, one day to rework the three frames and five days for the remaining 17, the seven-day target is feasible if production starts only after first-article approval.