Oxy-Fuel Bevel Quality and Arc-Gouged Crack Repair
A CNC oxy-fuel table with a bevel head is cutting 30° bevels for a single-V butt in 50 mm ASTM A36 plate. Ambient plate temperature is 5 °C and patches of the plate are rusty. The operator set the cutting speed about 20% above the tip chart to get through the job faster, ran the cutting oxygen at the top of the regulator range, and left the preheat flame long and bushy. The result: the top edge is rounded and washed, the kerf is noticeably wider than the chart, drag lines lean steeply away from vertical, and hard dross is stuck to the bottom edge. The bevel face is bluish with a rippled texture and file-checks hard, and the root face is 3 mm short of drawing. Later, magnetic particle testing finds a 30 mm transverse crack in an adjacent flange, and the crew proposes to gouge and weld it in one operation without stopping.
1. Match each observed cut defect to its cause and state the setting changes you would make.
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- Rounded, washed top edge and over-wide kerf: too much cutting oxygen pressure and an over-long preheat flame. The excess oxygen burns the top corner before the cut front reaches it. Reduce cutting oxygen to the chart value for the tip and set a short, neutral preheat flame.
- Steeply leaning drag lines and hard adherent dross: cutting speed too high. The cut front lags the torch, so the oxygen jet has to burn through thicker metal and the bottom edge freezes slag. Set speed back to the tip chart for 50 mm plate.
- Ripple and hard face: a combination of the above plus surface rust, which disrupts the cut and quenches the cut face. Wire-brush or grind the cut line, and set the correct standoff (roughly 3–6 mm) rather than the larger gap used here.
- Short root face: a dimensional nonconformance, not a thermal one. The torch height or bevel angle is set wrong; correct the setup and recut that edge rather than welding it out.
2. Can the bevels be welded as cut? What preparation and checks would you require before fit-up?
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Not as cut. Three issues must be dealt with first.
- Geometry. A 3 mm short root face changes the groove shape and the root gap, so the joint no longer matches the WPS. It should be recut or the bevel angle corrected, not filled with weld metal.
- Surface condition. Dross, oxide and slag on the bevel must be ground off to bright metal. Welding over oxide gives slag inclusions and porosity, and on a 50 mm joint those are expensive to find later. Remove the blue hardened layer too, at least near the root.
- Hardening and cracking. An oxy-fuel cut face on 50 mm plate cools quickly and can harden; the hard rippled layer is a poor start for a root pass. Grind back to sound, softer metal and check with a file or hardness tester. If the cut face is suspect, magnetic particle test it before welding, because thermal-cut cracks are not unknown in thick plate.
Cold plate at 5 °C also means the WPS preheat must be applied and measured, not assumed, before the root pass.
3. Describe the correct arc-gouging and weld repair sequence for the 30 mm flange crack, including what the crew's 'gouge and weld in one go' proposal would get wrong.
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Sequence:
- Excavate: carbon-arc gouge from both ends toward the middle, extending 50 mm beyond each visible crack tip, in shallow passes. Use DCEP with the correct electrode size for the current, air pressure about 550–690 kPa, and a 30–45° travel angle.
- Grind: remove 1–2 mm from the gouged surface to take out carbon and any copper picked up from the electrode. This is what the 'one go' proposal skips: welding straight onto a gouged surface risks carbon pickup, hardness and new cracks.
- Verify: magnetic particle test the excavation and confirm the crack is gone. Weld only after it is clean.
- Weld: apply the qualified preheat for 50 mm A36, use a low-hydrogen consumable, and stay within the qualified heat input.
- Inspect: MT after cooling, and again after 48 hours if hydrogen cracking is a concern.
Safety: gouging throws molten metal and noise; use a face shield, hearing protection, leathers, and ventilation or respiratory protection for the fume, and keep the air hose and stinger leads clear of the work.