Delayed HAZ Cracking on a Thick Nozzle Weld
A shop welds a 45 mm thick SA-516 Grade 70 shell course to a P-No. 1 forged nozzle using SMAW with E7018-H4R electrodes, 3.2 mm root and 4.0 mm fill passes. Preheat is specified as 90 °C and measured with a contact thermometer 75 mm from the joint. Ambient temperature is 8 °C, relative humidity 85%, interpass is held at 150 °C. Electrodes come from a holding oven at 120 °C, but each welder carries about 4 kg in an unheated quiver and refills twice per shift; individual rods are exposed for up to 90 minutes. Final PWHT is scheduled ten days after welding. At 60 hours after completion, magnetic particle inspection finds several tight, intermittent linear indications in the coarse-grained heat-affected zone, transverse to the weld. Hardness there is 320–340 HV10 against 165 HV10 in the base metal. Radiography at 24 hours was clean.
1. Diagnose the indications. What mechanism explains them, and why did the radiographic result not reveal the problem?
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These are hydrogen-assisted cold cracks in the coarse-grained HAZ - often called underbead or toe cracks. The evidence fits the mechanism:
- Timing. Indications appeared at 60 hours, not at 24 hours. Hydrogen cracking is delayed: hydrogen diffuses to the high-triaxial-stress region, pressure builds, and cracks nucleate hours or days after the arc has gone.
- Location and orientation. Transverse linear indications in the HAZ beside the fusion line are the classic underbead position, where the coarse grain and rapid cooling give the hardest, least ductile microstructure.
- Hardness. 320–340 HV10 in the HAZ against 165 HV10 in the parent shows an untempered transformation product. On a 45 mm joint at 8 °C ambient with only 90 °C preheat, heat is drawn away fast and the cooling rate is severe.
- Hydrogen source. E7018-H4R guarantees low hydrogen as supplied, not after 90 minutes in an unheated quiver at 85% RH. Moisture pickup recharges the electrode coating, and the coating is the dominant hydrogen carrier.
Radiography was clean for two reasons: the cracks had not formed when the film was taken, and even when present they are tight, planar, and aligned roughly normal to the beam, so they produce little or no density change. Radiography is the wrong tool for this defect; MT or UT is appropriate.
2. What is your immediate disposition of the joint and of other work in the shop?
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Contain first, then treat, then repair - and do not simply reweld at the same parameters.
- Stop and quarantine. Halt welding in the affected area and identify every joint welded in the same period, with the same electrode lot, under the same ambient conditions and the same quiver practice. The defect is a practice defect, not a one-off.
- Inspect wider. MT or UT all suspect joints, including the 100% of the nozzle welds done in that campaign. Record hardness traverses across the HAZ on each.
- Do not PWHT as a rescue. Heat treatment may partially heal or may simply close the cracks and make later detection impossible. The cracks must be found and removed first.
- Excavate. Grind out each indication to sound metal, confirm with MT, and record the excavation depth against the wall thickness and repair limits. If removal exceeds the permitted repair depth, the joint is a cut-out, not a repair.
- Repair under a tightened route. Higher preheat and interpass, controlled electrode exposure, and a documented hydrogen bake-out before cooling if the procedure calls for it.
- Re-inspect after a delay. At least 48 hours after the repair, with MT, before PWHT. The delay is part of the inspection method here, not an inconvenience.
3. Which changes to the WPS and shop controls would you make, and how would you verify that they worked?
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Change both sides of the balance: reduce hydrogen in, and slow cooling out.
- Heat input and preheat. Raise preheat to a level justified by the carbon equivalent of the actual heat and by the 45 mm thickness - 150 °C maintained through the wall rather than 90 °C measured 75 mm away, with the measurement point specified in the WPS and taken near the joint. Cap interpass so the HAZ is tempered but the joint is not overheated.
- Hydrogen control. Reduce quiver load and set a maximum exposure time per rod; use heated quivers; return rods to the oven at the end of the exposure window and do not re-issue them without reconditioning. Confirm that the holding oven is actually at temperature and that electrodes are not being dried beyond the manufacturer's recommendation.
- Post-heat. A hydrogen bake-out around 250 °C for a couple of hours, before the joint cools to ambient, gives hydrogen a route out while the joint is still warm.
- Restraint and sequence. Review the welding sequence to reduce the triaxial restraint at the nozzle, and consider the nozzle-to-shell order.
- Delayed inspection. Move final MT/UT for thick joints to at least 48 hours after welding, and state it in the inspection plan.
Verification: repeat hardness traverses after the change; if HAZ hardness falls clearly below the earlier 320–340 HV10 range and no indications appear at 48 hours on the next three similar joints, the change is working. Track each change with a date and a sample of joints inspected, so effectiveness is measured rather than assumed.