Transverse Indications on a Cyclically Loaded Girder
100% RT is required on CJP groove welds in a pedestrian bridge fabricated from 1/2 in A709 Gr. 50; the contract classifies the structure as cyclically loaded. Four welds on girder G-2 were completed on Friday afternoon by the same welder, using the qualified FCAW-G WPS with 150 °F minimum preheat; the shop was about 40 °F and the doors were open. Monday's films show, in three of the four welds, a transverse indication about 3/8 in long in the final pass near the weld toe. The RT contractor calls them transverse cracks. The fourth weld is clean. The welder's repair rate over two years is 2% and he has 11 other welds on this job. The supervisor wants the night shift to gouge out the indicated areas, reweld and re-shoot RT before morning so G-2 ships Wednesday.
1. Do you authorise the night-shift gouge and reweld? What must be resolved before any arc is struck on the repair?
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I would not authorise it as described.
- Cracks are rejectable in every structural code regardless of length, and three out of four welds in one shift is a pattern, not an accident.
- RT is a poor classifier of tight planar defects. A 3/8 in transverse indication near the toe could be a crack, toe lack of fusion, or a slag line. Confirm with UT or MT before the excavation is planned.
- Quarantine G-2, notify the engineer of record (the member is cyclically loaded and fatigue-sensitive), and start a repair plan and repair WPS.
- Find the cause before welding. If the cause is hydrogen, preheat loss or a parameter outside the WPS, the repair on the same night repeats the defect and destroys the evidence.
- Expand inspection: 100% RT on the 11 other welds this welder made in the same window, plus review of anything welded on that shift by others in the same conditions.
Shipping Wednesday is a commercial decision for the PM, made with these facts, not a reason to skip them.
2. List the specific root-cause checks you would run and the data you want back.
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Work backwards from the conditions of that Friday afternoon:
- Preheat: was it measured with a contact pyrometer at the joint, and re-checked before the final pass after the shift break? A 1/2 in plate at 40 °F with open doors cools fast; a torch pass at the start is not preheat at the final pass.
- Interpass: was 350 °F maximum respected? Thermocouple or crayon records, if any.
- Consumables: wire coil left open on the machine over the weekend, lot certificates, condition of the feeder and liner.
- Gas: flow measured at the nozzle under welding conditions, leaks, and the effect of open doors and fans on shielding.
- Electrical: current and voltage measured at the stinger against the WPS range with a calibrated meter, not the dial.
- Technique: final pass profile (a narrow, high-crown pass in heavy restraint is a classic crack site), crater treatment, arc strikes, strongback restraint.
- Environment: joint faces dry and free of oil, frost or condensation at 40 °F.
Rank them: a cause that explains three of four welds on one afternoon is more likely environmental or parameter-driven than random.
3. Set out the repair and release sequence, including the timing of final NDT.
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Sequence:
- Characterise the defect (UT or MT) and record its length, depth and location before excavation.
- Excavate by air-carbon-arc gouging followed by grinding to bright metal, with the excavation long enough to give a 45-degree side slope at each end.
- MT the excavation 100% to prove complete removal. Re-welding an incompletely removed crack is the most common repair failure.
- Preheat to the WPS minimum, or higher if the engineer directs, and hold it through the repair; control interpass.
- Reweld to the qualified WPS, preferably with the same procedure controls that were missing on Friday, and record the parameters.
- Hold the repair before final RT. A film shot the next morning can be clean and crack later; a hydrogen hold of about 48 hours before final inspection is the prudent route on 1/2 in restrained steel.
- Final RT of the repaired zone, plus UT if the engineer requires it for a fatigue-classified member.
- Report the cause and the corrective action to the engineer, and only then release G-2.
Expect the Wednesday ship date to move; say so immediately rather than at the end of the week.