Fabrication, Welding-Related Damage, and Repair
Fabrication, damage, and repair
Fabrication quality depends on joint preparation, fit-up, root gap, alignment, restraint, cleaning, and sequence. Poor fit-up can cause incomplete fusion, excessive distortion, or residual stress. Distortion control includes balanced welding, back-step sequence, fixtures, and controlled heat input. Weld discontinuities include porosity, slag, lack of fusion, incomplete penetration, undercut, and cracks. Cracks are never acceptable without evaluation; the cause may be hydrogen, restraint, solidification, fatigue, or corrosion.
Welding-related damage includes cold cracking, hot cracking, lamellar tearing, sensitization, corrosion at weld seams, fatigue at toes, and creep damage in high-temperature service. Repair begins with removing the defect completely - usually by grinding or arc gouging - and confirming removal with PT or MT. Arc gouging must not leave carbon contamination on the prepared surface. The repair uses a qualified WPS, proper preheat, and, where required, PWHT. Post-repair NDE and documentation close the loop. Replacing a defect with weld metal without excavation is not a repair.
Traps
- Welding over a crack or slag without excavation.
- Skipping NDE after defect removal.
- Using a repair procedure that is not qualified for the thickness or position.
- Ignoring the root cause, so the defect recurs.
- Forgetting PWHT or hardness checks after repair of hardenable steel.
Worked rule
Follow the repair sequence: locate, excavate, verify removal, qualify the repair procedure, preheat, weld, inspect, and document. If the defect is a crack, determine whether it is fabrication-related or service-induced before repairing, because the repair may need a design or fitness-for-service review.