Slide 002: Weld Defect Prevention & Repair

Slide Visual

Slide Overview

This slide covers the root causes of common weld defects, how to prevent them through proper technique and parameter selection, and the correct procedure for repairing defective welds.

Instruction Notes

Prevention is Better Than Repair

Most weld defects are caused by one of four root categories: 1. Contamination: Dirty base metal, oily filler, wet electrodes 2. Wrong parameters: Incorrect voltage, amperage, travel speed 3. Poor technique: Wrong torch angle, inconsistent speed, improper filler addition 4. Bad fit-up: Excessive or insufficient gap, misaligned joints

Defect Prevention Matrix

Defect	Root Cause	Prevention
Porosity	Gas contamination, dirty metal, wind	Clean to bright metal, check gas flow, shield from drafts
Undercut	Too much heat at weld toe	Reduce voltage/amps, adjust torch angle, slow down at edges
Lack of fusion	Insufficient heat, wrong angle	Increase amps, direct arc into root, ensure both plates are heated
Burn-through	Too much heat for thickness	Reduce amps, increase travel speed, use pulse, add heat sink
Cracking	Rapid cooling, high carbon, restraint	Preheat if required, slow cool, reduce restraint, crater fill
Excessive spatter (MIG)	Voltage/WFS imbalance, wrong polarity	Balance V and WFS, verify DCEP, clean nozzle
Tungsten contamination (TIG)	Touching pool/filler	Maintain arc length, practice dip technique
Distortion	Excessive heat input, inadequate clamping	Minimize passes, tack sequence, clamp, backstep technique

Weld Repair Procedure

When a defect is found, the repair must remove ALL of the defective material:

Step 1: Identify the extent - Mark the start and end of the defect with soapstone - For surface defects: use a magnifier to determine true extent - For suspected subsurface defects: the entire questionable area must be removed

Step 2: Remove the defect - Use an angle grinder with a thin grinding disc or flap disc - Grind out the defective weld metal AND a small amount of sound metal on each side - Create a groove (U or V shape) that can be re-welded - The groove bottom must be clean, bright metal with no traces of the original defect - For cracks: grind at least ¼" beyond each visible end of the crack

Step 3: Prepare for re-weld - Wire brush and clean the ground area - Verify the groove shape allows electrode/torch access to the root - If necessary, apply preheat (for crack repairs on high-carbon or thick material)

Step 4: Re-weld - Use the correct parameters — often the repair weld needs LOWER heat input than the original (to prevent the same problem) - Ensure tie-in to the sound weld metal at each end of the repair - Use crater fill technique at the end

Step 5: Re-inspect - Clean and visually inspect the repair weld - Apply the same acceptance criteria as for the original weld - If the repair fails inspection, repeat the process — do NOT stack multiple repairs on top of each other without grinding

Common Repair Mistakes

• Welding over a defect without grinding it out — this TRAPS the defect and makes it worse
• Not grinding deep enough — leaving traces of the crack root
• Using too much heat on the repair — causing new defects (burn-through, distortion)
• Not blending the repair smoothly into the original weld — creating stress concentrations

Key Talking Points

1. Prevention through proper preparation and parameters costs far less than repair
2. The #1 rule of weld repair: grind it out COMPLETELY before re-welding
3. Cracks must be ground ¼" beyond the visible end — they often extend further than they appear
4. A repair weld is held to the same quality standard as the original
5. Multiple failed repairs on the same location weaken the base metal — get help before the third attempt

Learning Objectives (Concept Check)

• [ ] Link 5 common defects to their root causes and prevention methods
• [ ] Demonstrate the correct weld repair procedure (mark, grind, clean, re-weld, re-inspect)
• [ ] Explain why welding over a defect is never acceptable

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Last Updated: 2026-03-19