Welding is a process that joins metals together to create strong structures. But if the job isn’t done properly, defects can appear in the weld. These welding defects not only weaken the metal but can also lead to expensive repairs, rework, and even accidents.
In this guide, we’ll focus on four of the most common welding defects:
- Porosity
- Undercut
- Cracks
- Burn-through
We’ll explain what they are, why they happen, and how to avoid them in your welding projects.
What Are Welding Defects?
Welding defects are imperfections in the weld joint that reduce its strength, appearance, or performance. Some defects are visible, like cracks, while others are hidden inside the weld, like porosity.
A good weld should:
- Be free from holes, gaps, and cracks
- Have smooth, even beads
- Fully fuse the base metals without burning through
Defects are usually caused by wrong techniques, poor preparation, or bad conditions. Let’s go through the most common ones.
Porosity in Welding
What Is Porosity?
Porosity happens when tiny gas bubbles get trapped in the weld metal. These bubbles look like holes or pits and weaken the weld.
Causes of Porosity
- Welding in a drafty area (wind blows away shielding gas).
- Contaminated base metal (rust, oil, paint, or grease).
- Wrong shielding gas flow or gas leaks.
- Damp electrodes or filler rods.
How to Avoid Porosity
- Clean the metal before welding (remove oil, rust, and moisture).
- Store electrodes and filler rods in a dry place.
- Check shielding gas settings and hoses for leaks.
- Avoid welding in windy or wet conditions.
Undercut in Welding
What Is Undercut?
Undercut is a groove or notch along the edge of the weld bead. It looks like the base metal is cut away, leaving the joint weak.
Causes of Undercut
- Using too high current or voltage.
- Moving the electrode too fast.
- Incorrect welding angle.
- Poor control of heat input.
How to Avoid Undercut
- Reduce welding speed to allow proper metal filling.
- Use correct current/voltage settings.
- Keep the electrode angle between 5–15°.
- Maintain a steady hand and even movement.
Cracks in Welding
What Are Cracks?
Cracks are one of the most serious welding defects. They can form in the weld metal or heat-affected zone. Cracks can be surface-level or hidden inside the weld.
Types of Cracks
- Hot Cracks – Form at high temperatures during solidification.
- Cold Cracks – Appear after welding due to stress or cooling.
- Crater Cracks – Form at the end of a weld bead when the arc is stopped too quickly.
Causes of Cracks
- Poor joint design or preparation.
- High levels of stress in the metal.
- Rapid cooling (especially in thick metals).
- Using incorrect filler material.
How to Avoid Cracks
- Preheat thick metals before welding.
- Use proper filler material that matches the base metal.
- Control cooling by covering hot welds to slow down temperature drop.
- Fill the crater at the end of each weld bead before stopping.
Burn-through in Welding
What Is Burn-through?
Burn-through happens when the weld completely melts through the base metal, leaving a hole. This is common when welding thin sheets of metal.
Causes of Burn-through
- Using too high current or heat.
- Staying too long in one spot.
- Thin or weak base metal.
How to Avoid Burn-through
- Use lower amperage for thin metals.
- Move quickly and evenly along the weld.
- Use smaller electrodes for thin materials.
- Back up thin metals with a copper plate to absorb extra heat.
Quick Reference Table
| Defect | What It Looks Like | Main Causes | Prevention Tips |
|---|---|---|---|
| Porosity | Holes, pits, bubbles in weld | Contamination, gas leaks, damp rods | Clean metal, dry rods, correct gas flow, avoid wind |
| Undercut | Groove along weld edges | High heat, fast travel, wrong angle | Lower current, slow down, correct electrode angle |
| Cracks | Visible or hidden fractures | Stress, poor prep, wrong filler, cooling | Preheat, use correct filler, control cooling, fill craters |
| Burn-through | Hole in base metal | Too much heat, thin material, slow move | Lower amperage, move faster, use backing copper plate |
Real-Life Example
In a car repair workshop, a welder was fixing a thin exhaust pipe but used the same amperage as for thick steel. The result? The pipe burned through, leaving holes instead of a smooth weld.
After lowering the current and using a backing copper plate, the welds came out strong and clean. This shows how adjusting settings based on material thickness makes all the difference.
Extra Tips to Prevent Welding Defects
- Always read the welding procedure specification (WPS) before starting.
- Practice on scrap metal before welding the actual part.
- Keep tools and workspace clean.
- Inspect welds regularly and repair small defects immediately.
- Never rush control and patience create strong welds.
Conclusion
Welding defects like porosity, undercut, cracks, and burn-through can weaken your project and waste time and money. But with the right technique’s clean preparation, correct heat settings, proper angles, and attention to detail you can avoid these problems and create strong, reliable welds.
A good welder is not just someone who joins metals it’s someone who prevents defects before they even appear.
FAQs About Welding Defects
Can I fix porosity after welding?
Yes, small porosity can be removed by grinding out the defect and re-welding the area.
What’s worse undercut or porosity?
Both are bad, but undercut is usually more dangerous because it weakens the joint edges.
Both are bad, but undercut is usually more dangerous because it weakens the joint edges.
This usually happens because of high stress or wrong filler material. Check your joint design and filler compatibility.
Is burn-through only a beginner’s mistake?
No. Even professionals can burn through thin metals if amperage is too high. It requires skill and control.
