Delayed Cracking in Thick Quenched-and-Tempered Steel
An A514 quenched-and-tempered steel beam flange, 1.5 in thick, is welded in the field at 35°F ambient. The WPS calls for SMAW with E11018 low-hydrogen electrodes, a 150°F preheat, and a 400°F maximum interpass. The steel certificate shows C 0.18, Mn 0.90, Si 0.25, Cr 0.50, Mo 0.20, V 0.05, Ni 0.40, Cu 0.20. Electrodes were taken from a heated rod oven at 250°F and used over a four-hour shift. No postweld heat treatment is specified. At 72 hours after completion, magnetic particle inspection reveals several tight, interbead cracks in the coarse-grained heat-affected zone, parallel to the fusion line. The contractor asks whether the preheat was adequate and whether the electrodes caused the cracking.
1. Calculate the carbon equivalent and evaluate the 150°F preheat.
Read the worked answer
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.
CE = 0.18 + 0.90/6 + (0.50 + 0.20 + 0.05)/5 + (0.40 + 0.20)/15
CE = 0.18 + 0.15 + 0.15 + 0.04 = 0.52.
A CE of about 0.52 in 1.5 in thick A514 at 35°F ambient is a high-restraint, fast-cooling condition. The 150°F preheat is below what AWS D1.1 preheat selection would typically require for this CE and thickness, and it is especially low against a cold ambient. The preheat should be increased to the code table value or a qualified minimum, commonly on the order of 250°F for this condition, while staying below the steel supplier’s maximum interpass. Preheat alone is not the only issue, but 150°F is not adequate here.
2. Identify the cracking mechanism and explain why the cracks are in the HAZ rather than the weld metal.
Read the worked answer
The pattern-tight interbead cracks appearing after 72 hours, parallel to the fusion line, in the coarse-grained HAZ-is consistent with hydrogen-assisted delayed cracking. Hydrogen from moisture or electrode coating enters the weld, diffuses into the HAZ, and is trapped in a hard, martensitic coarse-grained region under high restraint. The HAZ cools faster than the weld metal and can transform to a hard microstructure that is sensitive to hydrogen. The weld metal may be lower-carbon and more ferritic, so it can tolerate hydrogen better. The cracks are delayed because hydrogen needs time to diffuse and concentrate. The 250°F holding oven is a holding temperature, not a baking temperature; E11018 electrodes must be properly baked before holding, so electrode condition remains a likely contributing source.
3. Recommend corrective action and verification.
Read the worked answer
- Remove the cracked metal completely by grinding or air-carbon-arc gouging to sound metal; confirm with MT or PT before repair.
- Re-bake the E11018 electrodes per the manufacturer’s recommendation (typically 700–800°F for low-hydrogen electrodes) and hold them at 250°F in a closed oven; issue only in heated quivers.
- Raise preheat to the qualified minimum, roughly 250°F for this condition, and maintain it through the joint; control interpass to the supplier’s maximum, often 400°F for A514.
- Protect the joint from wind and moisture, use temp sticks, and control cooling. Consider a postweld hydrogen-release treatment if permitted by the steel supplier.
- Qualify the revised WPS and inspect the repaired area after a 48-hour delay using MT or UT; do not accept based on immediate inspection.