Toe cracking in a restrained S690QL support
An S690QL support structure, 40 mm thick, is welded in a shop at 8 °C ambient. The qualified WPS requires preheat 150 °C, heat input 1.5–2.0 kJ/mm, interpass 200 °C maximum and hydrogen-controlled consumables. The welder measured preheat only after a two-hour delay and recorded 100 °C, then welded at 1.2 kJ/mm because the root gap was tight. On the second side, interpass reached 230 °C. The joint is highly restrained, with a full-penetration tee and a box-section closing plate. The steel certificate gives CEV 0.58, and the consumable packet had been open for three days. At 24 h, magnetic particle inspection found toe cracks on 6 of 14 welds. The shop wants to grind out the cracks, weld them with the same WPS and hydrotest the following day.
1. Explain the cracking mechanism and rank the contributing factors.
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Mechanism. Toe cracks appearing at 24 h in 40 mm S690QL with CEV 0.58 should be treated as hydrogen-assisted cold cracks until proven otherwise. Three conditions coincide: a susceptible HAZ from rapid cooling, diffusible hydrogen, and high restraint.
Ranking of factors:
- Preheat too low: 100 °C instead of 150 °C, and measured after a two-hour delay, so the actual temperature at the arc was probably lower.
- Heat input too low: 1.2 kJ/mm against a qualified 1.5–2.0 kJ/mm increases the cooling rate and HAZ hardness.
- Hydrogen source: the opened packet has absorbed moisture; the consumable is no longer hydrogen-controlled in practice.
- Restraint: the full-penetration tee and closing plate concentrate residual stress at the toe.
- Interpass too high: 230 °C exceeds the 200 °C maximum and can degrade HAZ toughness, but it is not the primary cause of cold cracking.
Grinding and immediate rewelding with the same WPS would not remove hydrogen or correct the procedure.
2. Define the repair procedure and the inspection timing.
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Do not hydrotest. First remove the cracks completely by grinding or arc gouging, then confirm removal by MT or PT. A crack that runs under the weld may need sectioning.
Repair controls:
- Bake the consumables at the manufacturer's recommended temperature, typically 300–350 °C for 1–2 h, and hold them in a heated quiver.
- Preheat the repair area to 150–175 °C, measured adjacent to the excavation before welding and checked during the repair.
- Weld with a qualified low-hydrogen procedure at 1.5–2.0 kJ/mm and interpass ≤200 °C.
- Apply a hydrogen bake-out at 200–250 °C for 2–4 h if the qualified procedure requires it, then cool slowly.
- Inspect by MT/PT after 48 h, not 24 h, and by UT for subsurface defects.
If cracking recurs, review the procedure and whether the steel and restraint are suitable for the design.
3. What production controls would prevent recurrence?
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Use hydrogen-controlled consumables with certificates and control baking and holding. Measure preheat and interpass at the joint, not at the shop floor, using contact thermocouples or temperature-indicating crayons rather than an infrared gun on shiny plate. Enforce the heat-input range with calibrated equipment. Train welders on S690QL behaviour, especially the need to keep within the qualified window. Add a hold point before welding and a second inspection hold point after 48 h. Check that the WPS is valid for the actual thickness, joint type and restraint. The key lesson is that the WPS was adequate but the execution fell outside it.