Weld Repair of a Sour-Service Girth Weld with Hardness Concerns
A 20-inch, grade X60 sour-service gas line requires a girth weld repair. The original weld was made with a cellulosic downhill procedure and passed radiography, but ultrasonic testing found a 30 mm lack-of-fusion indication in the fill region. The project specification limits weld and heat-affected-zone hardness to 250 HV10 for sour service and requires repairs to be made with a qualified repair procedure. The proposed repair is to grind out the indication from the outside and re-weld with the same cellulosic procedure, using the same preheat of 80 °C. The repair welder is qualified for the original procedure but has not made a repair weld in sour service. Ambient temperature is 25 °C and humidity is high.
1. Evaluate the proposed repair procedure against sour-service requirements and identify the technical risks the inspector should raise.
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The proposed repair has three technical risks that the inspector should raise before approval.
First, hardness. Sour service limits hardness to 250 HV10 because hard microstructures are susceptible to sulfide stress cracking. A cellulosic downhill repair with only 80 °C preheat on a 20-inch X60 weld produces fast cooling and a hard heat-affected zone, particularly on the repair sidewalls where the base metal is reheated. The inspector should require hardness testing of the repair procedure qualification - traverse across the weld, HAZ, and base metal - and should question whether 80 °C preheat is adequate to keep the HAZ below 250 HV10. A higher preheat or a different procedure may be needed.
Second, repair procedure qualification. The original WPS qualifies production welding, not repair welding. Repair welding differs in constraint, heat sink, and pass sequence, and the specification requires a qualified repair procedure. The inspector should require a procedure qualification test that reproduces the repair geometry and confirms both mechanical properties and hardness.
Third, welder qualification. The welder is qualified for the original procedure but not for repair welding in sour service. Repair welding is a distinct skill and the qualification should cover it.
Additional risks: high humidity with a cellulosic electrode raises hydrogen and moisture concerns, so electrode storage and handling should be verified; and the grinding operation must fully remove the indication, confirmed by MT or PT, before re-welding.
2. What pre-weld and post-weld controls should the inspector impose, and how should the repair be documented?
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Pre-weld controls:
- Confirm the indication is fully removed by grinding, verified with MT or PT and by measuring remaining wall thickness against the minimum allowed.
- Verify the repair procedure qualification includes hardness testing and meets the 250 HV10 limit; if not, require qualification before proceeding.
- Verify the welder's repair qualification for sour service.
- Control electrode storage and exposure: cellulosic electrodes absorb moisture, and high humidity makes this critical. Require electrodes from a sealed container or a controlled oven, and limit exposure time.
- Apply and measure preheat per the qualified procedure, with a re-check if welding is interrupted.
- Protect the joint from wind and rain; humidity alone does not prevent welding, but moisture on the joint and in the electrode does.
Post-weld controls:
- NDT of the repair - radiography and, where the geometry allows, ultrasonic testing - to confirm the defect is gone and no new defects were introduced.
- Hardness testing of the repair area if the procedure qualification or specification requires it on production welds.
- Visual inspection for surface cracking, especially at the repair toe.
- Documentation: repair location, extent of excavation, procedure and welder used, NDT results, hardness results, and preheat/interpass records. The repair should be traceable in the as-built record so future integrity assessments know the weld is a repair.
The judgement is that a repair in sour service is not just a repeat of production welding; it carries higher hardness and hydrogen risk and must be qualified and controlled accordingly.