Delayed transverse cracks in a tank seam
A 16 mm thick longitudinal seam in a carbon-steel tank is welded by FCAW with 1.2 mm E71T-1 wire and CO2, using a single-V groove, 60° included angle, 2 mm root face, and 3 mm root gap. The WPS specifies 100–150 A root, 220–260 A fill, 24–28 V, preheat 10°C minimum, interpass max 250°C, and heat input 0.8–1.5 kJ/mm. Shop temperature is 6°C and humidity 88%; the plates were not preheated. The root pass was welded at 260 A, 28 V, 6 mm/s. Two days later, PT and MT reveal three transverse indications 8–18 mm long on the cap and one linear indication at the root toe. Project acceptance criteria reject any crack and any linear indication over 6 mm. Production proposes grinding the visible indications and re-welding without changing the procedure.
1. Calculate the heat input of the root pass and evaluate compliance.
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Heat input = (V × A × 60) / (travel speed in mm/min × 1000). Travel speed = 6 mm/s × 60 = 360 mm/min. Heat input = (28 × 260 × 60) / (360 × 1000) = 1.21 kJ/mm. That is within the WPS range of 0.8–1.5 kJ/mm, so heat input alone is not the violation. The root pass is outside the WPS current range: 260 A vs 100–150 A for the root. Preheat is also violated: the WPS requires 10°C minimum, but shop temperature is 6°C and the plates were not preheated. High humidity and exposed FCAW wire add a moisture source. The pass is therefore non-compliant even though the calculated heat input number looks acceptable.
2. Interpret the indications and explain why grinding only the visible defects and re-welding is inadequate.
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Transverse cracks appearing two days after welding point to hydrogen-assisted cold cracking, favoured by low preheat, high restraint, moisture and the out-of-range root current. The root-toe linear indication may be a crack or lack of fusion; both are rejectable under the stated criteria (any crack, or linear indication over 6 mm). Grinding only what PT/MT shows can leave crack tips under the cap or at the root. Re-welding without preheat, controlled hydrogen and a qualified repair procedure can reintroduce the same condition. The correct judgement is to treat the indications as a systemic cracking problem, not a cosmetic cap defect.
3. Outline the repair and verification sequence.
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Stop production, quarantine the seam, and map all indications with UT/MT before repair. Remove the defects by grinding or arc gouging to sound metal; confirm removal with PT or MT. Preheat to the qualified level, use a qualified repair WPS with root current in the 100–150 A range, fill at 220–260 A, interpass max 250°C, and heat input 0.8–1.5 kJ/mm. Slow-cool and protect the repair. Re-inspect by MT/PT after cooling, and repeat UT/RT as specified after 48 hours to allow delayed cracking to appear. Record the defect map, repair WPS, welder, parameters and NDT results; do not release the tank on visual acceptance alone.