Hydrogen Cracking After Rapid Cooling
A field crew is welding a repair on a 32 mm thick A516 Gr.70 nozzle using SMAW with E7018 electrodes. The WPS requires 100 C preheat and a maximum interpass of 250 C. Night temperatures drop to 5 C and the crew is behind schedule. They skip preheat verification, weld continuously, and cool the completed repair with a water-soaked rag to allow quick inspection. Twenty-four hours later, magnetic particle testing reveals several tight, linear indications transverse to the weld in the HAZ. The repair is on a vessel that must return to service the next morning.
1. Identify the likely defect mechanism and explain how the crew's actions produced it.
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The transverse linear indications in the HAZ 24 hours after welding are characteristic of hydrogen-assisted cold cracking. Several of the crew's actions contributed. Skipping preheat verification meant the joint may have started at 5 C rather than 100 C, increasing the cooling rate and producing a harder, more crack-susceptible HAZ. Welding continuously without interpass control is less of an issue for cracking than the cooling, but the water-soaked rag is decisive: rapid cooling from the weld temperature produces hard martensitic or bainitic structures and traps hydrogen in a supersaturated state. E7018 is a low-hydrogen electrode, but it still introduces some hydrogen, and the combination of a hard HAZ, residual stress, and diffusible hydrogen is the classic cold-cracking triangle. The 24-hour delay is typical because hydrogen diffuses slowly at ambient temperature.
2. What must be done before the vessel returns to service, and what controls prevent recurrence?
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The vessel cannot return to service as-is. The cracked region must be excavated: grind or gouge out the indications, confirm removal by MT or PT, and re-weld with verified preheat of at least 100 C, controlled interpass temperature, and slow cooling under insulation or a heat blanket. After repair, perform the required NDE, typically MT or UT plus, if the code requires, a delayed inspection after 24 to 48 hours to allow hydrogen to diffuse. Consider a hydrogen bake-out if the procedure allows. Controls: require preheat verification with contact thermometers or thermal crayons before welding, prohibit accelerated cooling, provide insulated cooling for thick sections, and add a hold period before final inspection. Brief the crew that schedule pressure cannot override preheat and cooling requirements.