Porosity and Slag in an E7018 Pressure Vessel Seam
A 12 mm thick SA516 Gr 70 pressure-vessel shell seam is welded with 4.0 mm E7018 electrodes on a 400 A AC transformer. The electrodes were opened three days earlier and kept in an open rack; shop humidity is 80% and the monsoon season is on. The welder sets 110 A, uses a short arc and a stringer technique, and does not clean between passes beyond a light brush. Radiography reports scattered rounded porosity and linear slag inclusions; a root bend test fails. The welder says the current is 'within the range printed on the packet' and blames the steel. The vessel is due for hydrotest the next day, and the production engineer wants to 'hydrotest it and see'. The inspector must decide whether to accept, repair, or re-test.
1. Assess the likely causes of the porosity and slag inclusions, separating electrode-storage effects from current/technique effects.
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Two defect types have different causes. Rounded porosity is gas trapped in the weld. The strongest clue is the opened E7018 kept in an open rack at 80% relative humidity. E7018 is a low-hydrogen electrode, but its coating absorbs moisture; moisture in the arc introduces hydrogen, and porosity can also come from rust, oil, paint or damp plate. Slag inclusions are solid non-metallic material trapped between passes. They point to poor interpass cleaning, low current, incorrect electrode angle, or a stringer technique that leaves slag at the weld toe.
The 110 A current is at the low end for 4 mm E7018, so penetration is shallow and slag is not floated out. Light brushing between passes is not enough; slag must be chipped and the surface ground clean. Both the storage and the technique therefore contribute, but the wet electrode is the main hydrogen/porosity risk.
2. State the correct storage, re-drying and current practice for E7018, and explain why 'within the printed range' is not sufficient.
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E7018 must be kept dry. Store it in a sealed container or holding oven at about 100–150 °C. If it has absorbed moisture, re-dry at 250–300 °C for 1–2 h, following the manufacturer's data. Do not exceed the recommended baking temperature or the coating may be damaged. Electrodes exposed to the atmosphere for several hours, especially in monsoon humidity, should be re-dried or discarded.
For 4 mm E7018, a typical current range is about 130–170 A, with the exact value from the electrode maker and the qualified procedure. 'Within the printed range' is not sufficient because the range assumes dry electrodes, correct polarity, clean plate, stable arc and the right position. A low current within the range can still give cold laps and slag. Use a calibrated meter, check the return lead, keep the arc length short, clean each pass by chipping, wire brushing and grinding, and use a controlled stringer or slight weave that wets both walls.
3. As inspector, specify the repair and re-inspection sequence and state whether hydrotest can proceed.
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Reject the weld. Radiography shows porosity and linear slag, and the bend test has failed. The indications must be assessed against the acceptance standard, but linear slag and a failed bend are normally unacceptable. The inspector should not allow hydrotest to proceed on the unrepaired vessel. Hydrotest is a strength and leak test, not a substitute for radiography, and it can be dangerous if known defects are left in a pressure boundary.
Repair sequence: mark the defects from the radiograph, grind or gouge out to sound metal, confirm removal by magnetic particle or dye penetrant inspection, re-weld with a qualified procedure using re-dried E7018 and the correct current, then re-radiograph the repair. If production bend testing is required, repeat it. Only after NDT acceptance should the hydrotest be done, with the correct test pressure and safety precautions. The production engineer's 'hydrotest it and see' approach is not acceptable.