Hydrogen Bake-Out Timing Under Production Pressure
A pressure-vessel fabricator is welding 25 mm thick carbon steel with a consumable that can introduce hydrogen. The qualified procedure requires a post-weld hydrogen bake-out at 250 degrees Celsius for four hours, to be started within two hours of completing the weld. Production is behind schedule, and a supervisor proposes letting the finished weld cool overnight and performing the bake-out the next morning, arguing that the weld will be inspected by ultrasonic testing anyway and that any hydrogen will have escaped by then. The client specification references a hydrogen control requirement and does not permit deviation without engineering approval.
1. Assess the supervisor's proposal against the purpose of the hydrogen bake-out requirement.
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The bake-out is a hydrogen control measure, not a substitute for inspection. Its purpose is to drive diffusible hydrogen out of the weld and heat-affected zone before it can accumulate at discontinuities and cause delayed cracking. The timing requirement exists because hydrogen diffuses and can concentrate at stress raisers within hours; waiting overnight allows that diffusion and concentration to occur, which defeats the purpose of the treatment.
The supervisor's argument that ultrasonic testing will catch any problem is flawed. Ultrasonic testing performed before the bake-out may miss hydrogen-induced cracks that have not yet formed, and testing after the delay does not undo the exposure. The requirement is a process control, not a final inspection gate.
The proposal is a deviation from the qualified procedure and the client specification. It cannot be accepted without engineering approval, and the coordinator should not treat it as a scheduling decision.
2. What should the welding coordinator do, and what records should be created?
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The coordinator should reject the proposal as written and hold the weld for the required bake-out within the specified window. If the schedule pressure is real, the coordinator can escalate to engineering for a formal deviation request, but should not authorize the change unilaterally. Engineering may require additional measures, such as extended bake-out time or additional inspection, before accepting any deviation.
The coordinator should also review why the schedule slipped and whether the bake-out window can be protected on future welds, for example by scheduling the treatment immediately after welding and assigning a dedicated furnace slot.
Records should include the weld identification, the time welding was completed, the time the bake-out started and ended, the furnace temperature records, and the disposition of any deviation request. If a deviation is approved, the approval, its conditions, and the additional inspection results must be documented and retained with the weld history.