Repairing Rejected Stick Welds

A repair is harder than the original weld. The joint is now restrained, the groove is an awkward shape, the access is usually worse, and everyone wants it done fast. A careless repair can leave the structure worse off than the flaw did, with more heat, more stress and a new crack beside the old one. Senior welders treat a repair as its own small job, with its own plan, its own limits and its own inspection.

Repairs on code work are done by qualified welders, under an approved procedure, with the required approvals. Gouging, preheat and welding are not tasks to try on your own. Rules vary by code and contract, so the numbers here are examples from published sources. Your governing documents set the real requirements.

What You Will Be Able to Do

• Work out who needs to approve a repair before it starts.
• Find the real extent of a flaw and mark it.
• Remove a flaw by grinding or gouging, and handle the risks of each.
• Prove the flaw is gone before you reweld.
• Reweld and re-inspect under the right rules.
• Recognize when a repair should stop and go to an engineer.

Before You Touch the Weld

Do not start grinding as soon as you see a reject tag. Answer these questions first:

1. What exactly was rejected? Read the inspection report. Note the flaw, location and size.
2. Who has to approve this? Some repairs are routine under a written procedure. Others need the engineer's approval or the owner's. Cracks and base metal damage often do.
3. Which procedure applies? Find the repair WPS. The original WPS may not fit the shape of the repair groove.
4. Is the welder qualified? Check position, process and thickness for the repair, which may be different from the original weld.
5. What will the repair do to nearby work? Think about coatings, heat-sensitive parts, seals, and anything on the other side of the plate.
6. Is this the first repair at this spot? Many specifications limit repeat repairs in the same place.
7. Why did it fail? If you do not find the cause, the same flaw can come back.

Repair Methods by Flaw

The repair depends on what went wrong. Here is a general guide, and your code has its own list.

Finding the Extent

The flaw you can see is often smaller than the flaw that is there. A crack can run on under the surface, past the point where it seems to end.

Use the methods your code and procedure allow, such as etching, MT or PT, to find the crack ends. An experienced NDT technician can help. For internal flaws, the original UT or RT report gives you a location. Have the technician mark both the length and depth on the steel, with a clear reference point.

Removing the Flaw

Remove the flaw by the method your procedure allows.

Air Carbon Arc Gouging

Air carbon arc gouging melts metal with an arc from a carbon electrode, and a jet of air blows it out. It is the common way to remove long or deep weld flaws, and it takes training to do well.

Published guidance for steel gives these settings:

• Polarity: DC electrode positive.
• Power source: Constant current.
• Air: About 80 to 100 psi of clean, dry air at the torch.
• Jet: Air comes from behind the electrode and sweeps under the tip.
• Angle: Hold a shallow angle. A steep angle can put carbon into the groove.

Poor technique or low airflow leaves carbon in the groove, which makes a hard, brittle layer that can crack. So after gouging, grind the groove to clean metal if a carbon-rich layer formed, and always on crack-sensitive steels. Look for a smooth, bright groove with no dark deposits.

Warning: Gouging throws molten metal a long way and is very loud. Clear combustibles and protect openings. OSHA's fire watch rules call for a watch to stay at least half an hour after hot work, where a fire watch is required. Wear hearing protection and full leathers, and control the fume. OSHA's minimum filter shades are 10 for under 500 amps and 11 for 500 to 1,000 amps.

Removing a Crack

Cracks need special care, because working on them can make them worse.

1. Mark both ends. Use the NDT result, not your eye.
2. Add the margin. Older text of AWS D1.1 calls for removing the crack and sound metal 2 inches beyond each end. Check your current edition.
3. Start outside the crack. Start removing metal beyond the crack end, and work toward the center. If you start in the middle, the heat and the shock can drive the crack longer.
4. Go deep enough. Remove all of the crack depth, and do not stop at the first clean-looking metal.
5. Prove it is gone. Use MT or PT before you weld.

Worked Example: Sizing a Crack Repair

MT shows a crack 3 inches long in a fillet on 1 inch plate. Your job follows a 2 inch rule.

• Crack length: 3 inches
• Margin at each end: 2 inches x 2 = 4 inches
• Total length of the excavation: 7 inches

You mark both ends of the 7 inch zone. You start gouging at each end and work toward the center. Then you grind the groove clean, slope the ends, and call for MT before you reweld.

If the crack had looked 3 inches long by eye and you had simply taken 3 inches out, the crack ends would likely still be in the steel. The repair weld would then pull them open.

Preparing the Groove

The groove you leave behind has to be weldable.

• Keep it open enough. You need room to reach the bottom with the rod and to see the sidewalls.
• Slope the ends. A square-ended pit is hard to fuse. Give the ends a gradual ramp.
• Round the bottom. A U shape is easier to fuse than a sharp V.
• Clean it. Remove slag, carbon and scale, and clear out any moisture.
• Check it. Run the required NDT, and do not reweld until it is released.

If the groove goes deeper than expected, or if it reaches through the wall, stop. The approved plan may not cover that, and you may need backing or a different procedure.

Rewelding

A repair area is usually more restrained than the original joint, so it is at higher risk of cracking.

1. Preheat. Use the temperature from the repair WPS. Heat a wide area, not only the groove, and check the temperature where the procedure says to.
2. Use controlled rods. Take low-hydrogen rods from a controlled oven, and use the size the procedure calls for.
3. Weld in stringers. Use small, well-placed beads. They help you keep heat input and shrinkage down.
4. Fill in order. Work from the bottom up, and tie each bead into the sidewall.
5. Keep the interpass temperature in range. Measure it, and do not let it drop below the minimum or rise above the maximum.
6. Clean each pass. Check each one as you go.
7. Finish to the original profile. The repair should match the required size and shape, and blend smoothly.
8. Control the cooling. Follow any post-weld requirement. A repair after post-weld heat treatment may need heat treatment again.

Re-Inspecting

A repair is not done until it is inspected and accepted. Inspect it at least the way the original was inspected. Your specification may require more.

• Use the same criteria and the same or stricter methods. A repair found by UT is proven by UT, not by a visual look.
• Include the area around the repair. Heat from repair welding can create a flaw beside it.
• Mind the timing. On certain high-strength steels, D1.1 ties acceptance to an inspection at least 48 hours after welding. That wait restarts with the repair.
• Update the records. Record the repair number, welder, procedure, rods, preheat and inspection reports.

When to Stop

Knowing when not to repair is part of the job.

Repeat repairs add more heat and more stress, and they widen the heat-affected zone. If a repair fails twice, stop and ask why. There may be a problem with the steel, the joint design or the procedure, and no third attempt will fix that.

Planning a Repair for a Crew

When you are leading, write a short plan. It takes only a few minutes.

Finally, ask why the weld failed. One repair fixes one weld. If moist rods, poor fit-up or a bad habit caused it, fix that too, or the next reject is already on its way.

Common Mistakes

• Starting before approval. A repair without authority can make the whole joint nonconforming.
• Gouging from the middle of a crack. You can drive the crack longer.
• Leaving carbon in the groove. It makes a hard layer that can crack.
• Skipping preheat because the area is small. A small repair cools fast and is highly restrained.
• Rewelding before NDT proves the flaw is gone. The crack stays under the new weld.
• Trying a third repair at the same spot. Ask why it keeps failing.

Check Your Understanding

MT shows a 5 inch crack, and your job follows a 2 inch rule. How long is the excavation, and where do you start?

9 inches. Start beyond each end of the crack and work toward the center.

A welder finishes gouging and sees dark deposits along the sides of the groove. What should happen next?

Grind the groove down to clean metal. Dark deposits can mean a carbon-rich layer, which is hard and can crack.

A repair area was preheated, welded and looks good. The job requires UT on the original weld. Is a visual check enough?

No. Inspect the repair at least the way the original was inspected, so it needs UT, along with a look at the area around it.

A repair at one location has failed twice. The foreman says to try again with a hotter preheat. What is your reply?

Stop and find out why it keeps failing. Repeat repairs add heat and stress, and an engineering review may be needed before anyone welds there again.

Key Takeaways

• Treat a repair as its own job, with approval, a procedure and a plan.
• Find the true extent of a flaw before you remove it.
• For cracks, remove past both ends and work toward the center.
• After gouging, leave clean metal with no carbon, and prove the flaw is gone.
• Preheat and weld the repair with care, because it is highly restrained.
• Re-inspect at least as strictly as the original, and stop if repairs keep failing.