There’s nothing more frustrating than watching your weld pool drop through the workpiece and leave a smoking hole in your project.
I’ve been there more times than I care to admit. Early in my welding career, I burned through countless pieces of sheet metal before I understood what was actually happening.
Welding burn through is a defect where excessive heat penetrates completely through the base metal, creating an unwanted hole or opening in the workpiece. It occurs when heat input exceeds the material’s ability to dissipate heat, typically caused by too much amperage, slow travel speed, large electrode diameter, or poor joint fitup. Burn through is preventable by adjusting welding parameters and using proper technique.
After 15 years of welding and teaching dozens of apprentices, I’ve learned that burn through is rarely mysterious. It’s almost always caused by one of five specific factors.
Let me break down exactly what’s happening and how to fix it.
Burn Through vs Proper Penetration
Before we dive into causes and fixes, it’s important to understand the difference between burn through and proper penetration. These two concepts are often confused, but they’re fundamentally different.
| Characteristic | Burn Through | Proper Penetration |
|---|---|---|
| Definition | Excessive heat that creates a hole | Controlled fusion to proper depth |
| Appearance | Visible hole, missing material, sagging | Smooth reinforcement, complete fusion |
| Root Cause | Too much heat input | Balanced heat and speed |
| Acceptability | Always a defect | Required for sound welds |
| Strength Impact | Severe weakness, potential failure | Full strength achieved |
I see welders chase “more penetration” by cranking up the heat, only to end up with burn through instead. This is especially common when welding thin materials where the margin between proper penetration and burn through is razor-thin.
What Causes Welding Burn Through?
Burn through happens when the welding arc delivers more heat than the base metal can handle before melting completely. After analyzing thousands of weld failures in my career, I’ve identified the primary culprits.
Quick Summary: Burn through is almost always caused by excessive heat input. The five main causes are too much amperage, slow travel speed, oversized electrode, poor joint fitup, and inadequate heat dissipation. Fix any one of these and you’ll eliminate most burn through issues.
1. Excessive Amperage (Too Much Current)
This is the number one cause I see in the shop. Welders crank up the machine to “get better penetration” and end up blasting through the workpiece entirely.
Higher amperage means more heat input per second. When that heat exceeds what the metal can absorb and dissipate, you get burn through.
I’ve seen welders running 140 amps on 20-gauge sheet metal when they should be at 60-70. That’s double the necessary heat.
The fix is simple but requires discipline: start with lower settings and work up only if penetration is inadequate.
2. Slow Travel Speed
Moving too slowly allows heat to build up in one spot. The arc stays in each location longer, dumping more heat into a concentrated area.
I call this “parking on the weld.” The welder essentially stops moving while the arc continues to pump heat into the metal.
This is especially problematic with thin materials that dissipate heat slowly. Each second you linger is more heat going into the workpiece.
3. Electrode or Wire Too Large
A larger diameter electrode or wire carries more current and creates a bigger weld pool. This is great for thick materials but disastrous for thin sheet metal.
Using 3/32-inch stick electrodes on 18-gauge steel is a recipe for burn through. That size electrode is designed for much thicker materials.
The relationship between electrode size and heat input is direct. Bigger electrode equals more heat potential.
4. Poor Joint Fitup (Gaps and Root Opening)
This is a silent killer that many welders overlook. When your joint has a gap, you naturally slow down to fill it, which increases heat input.
I’ve seen this countless times with pipe welders. A slight root opening seems minor, but it forces the welder to weave and pause to fill the gap.
That extra time and motion dumps more heat into an area that’s already compromised by the gap itself.
5. Inadequate Heat Sinking
Some materials and setups don’t dissipate heat effectively. Aluminum, for example, conducts heat away from the weld zone quickly.
But stainless steel holds heat in the weld zone. Without proper backing or heat sinking, that heat continues to build and can cause burn through.
Copper backing bars are one solution. They absorb excess heat and prevent melt-through on the backside.
Other Contributing Factors
Beyond the big five, I also see burn through caused by wrong polarity settings, incorrect shielding gas flow, and improper joint design.
Using straight polarity (DCEN) when you should use reverse (DCEP) can concentrate heat in the wrong place.
And some joint designs are simply more prone to burn through. Open root butt joints with no backing are inherently risky on thin materials.
How to Prevent Welding Burn Through
Prevention is always better than repair. After dealing with burn through on everything from automotive body panels to exhaust systems, I’ve developed reliable prevention strategies.
1. Use Correct Amperage for Material Thickness
The single most effective prevention method is matching your amperage to your material thickness. Most welders guess and end up too hot.
| Material Thickness | Stick Amperage | MIG Amperage | TIG Amperage |
|---|---|---|---|
| 1/16 inch (1.6mm) | 40-60 amps | 30-50 amps | 30-45 amps |
| 3/32 inch (2.4mm) | 55-80 amps | 45-70 amps | 40-60 amps |
| 1/8 inch (3.2mm) | 75-110 amps | 60-90 amps | 55-80 amps |
| 3/16 inch (4.8mm) | 100-140 amps | 80-130 amps | 75-120 amps |
| 1/4 inch (6.4mm) | 130-180 amps | 120-170 amps | 100-150 amps |
These are starting points. Always begin at the low end and adjust up only if needed.
2. Increase Your Travel Speed
Faster travel means less heat input per inch of weld. This is the easiest adjustment to make and pays immediate dividends.
I recommend increasing travel speed by 20-30% if you’re experiencing burn through. The weld may look slightly different at first, but you’ll avoid the hole.
Practice on scrap to find the sweet spot where you get good penetration without melt-through.
3. Choose the Right Electrode or Wire Size
Smaller diameter electrodes and wires deposit less heat. Match your consumable size to your material thickness.
| Material Thickness | Stick Electrode | MIG Wire | TIG Filler Rod |
|---|---|---|---|
| Under 1/16 inch | 1/16 inch | 0.023 inch | 1/16 inch |
| 1/16 to 1/8 inch | 3/32 inch | 0.030 inch | 1/16 or 3/32 inch |
| 1/8 to 3/16 inch | 1/8 inch | 0.035 inch | 1/8 inch |
| 3/16 inch and up | 5/32 inch | 0.045 inch | 3/32 or 1/8 inch |
4. Improve Joint Fitup
Tight fitup means less filling and less heat input. Grind your joints to close gaps before welding.
Use clamps and tack welds to maintain alignment. A well-prepped joint requires less filler metal and less heat.
I spend more time preparing joints than I do welding. It saves time in the long run by preventing rework.
5. Use Backing Bars or Heat Sinks
Copper or aluminum backing bars absorb excess heat and support the weld pool. This is essential for thin materials.
You can also use wet rags or heat sink paste near the weld area to draw heat away from the joint.
These tools act as a heat insurance policy, giving you more margin for error.
6. Employ Pulse Welding Techniques
Modern welding machines offer pulse settings that alternate between high and low current. This delivers heat in bursts rather than continuously.
Pulse welding is especially effective for thin materials. The peak current achieves penetration while the background current allows cooling.
I’ve seen pulse welding reduce burn through on 20-gauge stainless by nearly 70% compared to standard settings.
7. Use Intermittent Welding
Instead of one continuous weld, use stitch welding or skip welding. This allows heat to dissipate between segments.
This technique is common on long seams where continuous heat input would cause distortion and burn through.
MIG Welding Burn Through Prevention
MIG welding is particularly prone to burn through because the wire feed continuously feeds heat into the workpiece. Here’s what I’ve learned from thousands of MIG welds on thin materials.
Use Short Circuit Transfer
Short circuit transfer operates at lower voltages and wire feed speeds. The metal actually “shorts” across the arc hundreds of times per second.
This results in lower overall heat input compared to spray transfer. For materials under 1/8 inch, short circuit is your best friend.
Reduce Wire Feed Speed
Slower wire feed means less filler metal and less heat. Start around 200-250 inches per minute for 20-gauge steel.
Only increase if you’re not getting adequate penetration. Most MIG welders run too hot, not too cold.
Lower Voltage Settings
Voltage controls arc length and heat. For thin materials, run at the low end of your machine’s recommended range.
I typically run 16-17 volts on 20-gauge steel with 0.023 wire. Anything above 18 volts risks burn through.
Use Smaller Wire Diameter
Switch to 0.023 or 0.024 wire instead of standard 0.030 or 0.035. The smaller wire requires less current and produces a smaller, more controllable weld pool.
Short Circuit Transfer: A MIG welding transfer mode where the wire actually contacts the weld pool and “short circuits” hundreds of times per second. This results in lower heat input compared to spray transfer, making it ideal for thin materials and out-of-position welding.
TIG Welding Burn Through Prevention
TIG welding gives you precise control over heat input, which makes it excellent for thin materials. But that control requires skill and attention.
Master Foot Pedal Control
The foot pedal is your heat throttle. Learn to feather it rather than floor it. I treat my foot pedal like a gas pedal in stop-and-go traffic.
Back off the amperage as you move through the weld. You don’t need full power for the entire weld.
Use Pulse Settings
Most modern TIG machines have pulse capability. Set your pulse rate to 2-5 pulses per second for thin materials.
This allows the weld pool to cool slightly between pulses, preventing heat buildup and burn through.
Reduce Amperage and Increase Speed
TIG welding often requires lower amperage than welders expect. I run 50-60 amps for 16-gauge stainless steel.
Combine that with a steady travel speed and you’ll get clean penetration without burn through.
Use Smaller Tungsten and Filler Rod
Smaller tungsten (1/16 or 3/32 inch) concentrates the arc more precisely. Smaller filler rod (1/16 inch) adds less heat to the joint.
This combination gives you maximum control over heat input.
Stick Welding Burn Through Prevention
Stick welding presents unique challenges because you’re dealing with a consumable electrode that changes as you weld. Here’s how I prevent burn through with stick.
Choose Smaller Electrodes
For thin materials, use 3/32 or even 1/16 inch electrodes. The smaller 6011 and 6013 rods run cooler and are less likely to burn through.
I’ve successfully welded 18-gauge sheet metal with 1/16 inch 6013 rods at 45-50 amps.
Run Lower Amperage
Stick welding amperage should be at the bottom of the electrode’s recommended range for thin materials.
For 3/32 inch 6011, that means around 60-70 amps instead of the maximum 90-100.
Use Weaving Techniques
A slight weave distributes heat across a wider area. This prevents heat concentration in one spot.
But don’t weave too wide or you’ll lose penetration and get undercut.
Consider 6013 Electrodes
6013 rods run cooler and freeze faster than 6010 or 7018. They’re more forgiving on thin materials.
I keep a box of 3/32 inch 6013 specifically for sheet metal work under 1/8 inch.
How to Fix Welding Burn Through
Sometimes burn through happens despite your best efforts. When it does, you have options. I’ve repaired hundreds of burn-through defects over the years.
Repair Decision Guide: Small burn throughs (under 1/4 inch) can often be repaired. Larger holes or multiple burn throughs in critical areas usually require replacing the affected piece. When in doubt, cut it out and start over.
Step 1: Assess the Damage
Examine the size and location of the burn through. Small holes in non-critical areas are repairable. Large holes or structural defects may require replacement.
I generally don’t attempt repairs on holes larger than 1/4 inch unless it’s absolutely necessary.
Step 2: Prepare the Area
Grind around the burn through to clean metal. Remove any slag, oxidation, or contaminated material from the edges of the hole.
Bevel the edges slightly if the hole is larger, creating a V-groove that will accept filler metal.
Step 3: Use a Backing Plate
Place a copper backing plate behind the hole. This prevents the weld from falling through and gives you something to weld against.
The copper won’t fuse to the steel, but it will support the weld pool.
Step 4: Fill with Lower Amperage
Reduce your amperage by 20-30% from your original settings. Build up the weld gradually, layer by layer.
Don’t try to fill the hole in one pass. Multiple smaller passes prevent re-burning through.
Step 5: Grind and Inspect
After filling the hole, grind the surface flush. Inspect the repair for complete penetration and lack of defects.
If the repair looks questionable, don’t hesitate to redo it. A bad repair is worse than no repair.
Frequently Asked Questions
What causes burn through in welding?
Burn through is caused by excessive heat input that exceeds the base metal’s ability to dissipate heat. The main causes include too much amperage, slow travel speed, oversized electrode or wire, poor joint fitup with gaps, and inadequate heat sinking. The most common culprit I see is welders running their machine too hot for the material thickness.
How do you prevent burn through when welding thin metal?
Preventing burn through on thin metal requires reducing heat input. Use lower amperage settings, increase travel speed, select smaller diameter electrodes or wire, ensure tight joint fitup with no gaps, and use backing bars or heat sinks. For very thin materials under 1/16 inch, consider TIG welding with pulse settings or short-circuit MIG transfer for maximum control.
What is the difference between burn through and excessive penetration?
Burn through occurs when heat penetrates completely through the workpiece creating a hole, while excessive penetration refers to weld penetration that exceeds specifications but doesn’t create an opening. Burn through is always a defect that creates a void in the material. Excessive penetration may or may not be rejectable depending on code requirements, but burn through always requires repair or replacement.
Why does my MIG welder keep burning through?
MIG welders commonly burn through due to excessive wire feed speed and voltage settings. The continuous wire feeding creates constant heat input that quickly overwhelms thin materials. Switch to short circuit transfer mode, reduce wire feed speed to 200-250 IPM for thin materials, lower voltage to 16-18 volts, and use 0.023 or 0.024 diameter wire instead of standard sizes.
How do you fix burn through in welding?
To fix burn through, first assess if repair is practical. For small holes under 1/4 inch, grind the area clean, bevel the edges, place a copper backing plate behind the hole, then fill with reduced amperage using multiple smaller passes. For larger holes or critical structural areas, cutting out and replacing the affected section is usually safer and more reliable than attempting repair.
What amperage should I use to prevent burn through?
Amperage depends on material thickness and welding process. For 1/8 inch steel, use approximately 75-110 amps with stick welding, 60-90 amps with MIG, or 55-80 amps with TIG. Always start at the low end of the recommended range and only increase if penetration is inadequate. When in doubt, err on the side of too little amperage rather than too much.
Can you weld over a burn through?
Yes, you can weld over a burn through if you prepare the area properly and use reduced amperage. The key is to clean the area thoroughly, bevel the edges of the hole, use a copper backing plate to support the weld pool, and build up the repair gradually with multiple lower-amperage passes. Never try to fill a burn through hole in a single pass or you will likely re-burn through.
Does travel speed affect burn through?
Travel speed significantly affects burn through risk. Slower travel allows more heat to accumulate in each spot, increasing the likelihood of burn through. Faster travel reduces heat input per inch of weld and helps prevent melt-through. If you’re experiencing burn through, try increasing your travel speed by 20-30 percent while maintaining proper arc length and technique.
What electrode size prevents burn through?
Smaller diameter electrodes help prevent burn through by reducing heat input. For materials under 1/8 inch, use 3/32 inch or smaller stick electrodes. For very thin materials under 1/16 inch, consider 1/16 inch electrodes. For MIG welding, use 0.023 or 0.024 inch wire for thin materials instead of standard 0.030 or 0.035 sizes. Always match electrode size to material thickness.
How do you weld sheet metal without burning through?
Welding sheet metal successfully requires minimizing heat input. Use the smallest practical wire or electrode size, set amperage at the low end of recommendations, maintain steady travel speed without pausing, use short circuit MIG transfer or TIG with pulse settings, ensure perfect joint fitup with no gaps, and consider using copper backing bars or heat sinks. Practice on scrap pieces of the same thickness before welding your actual workpiece.
Final Thoughts
Burn through is frustrating, but it’s also predictable and preventable. After years of dealing with this defect, I’ve learned that it almost always comes down to heat management.
Start cold and work up. Use smaller consumables for thin materials. Improve your joint fitup. And don’t be afraid to slow down and prepare properly before striking an arc.
The best welders I know are the ones who take the time to set up correctly. A few extra minutes of preparation can save hours of repair work.
Remember: you can always add more heat, but you can’t take it back once you’ve burned through.
