Undercutting Welding: A Welder's Dirty Little Secret

welder setting up plasma cutter

Welding Buddy Experts

May 27, 2022
Poor welding or other causes resulting in undercuts

You're a welder, welded for years, and very good at your trade — but there's one little secret that you keep hidden away from everyone. A dirty little secret that only welders know about: undercutting.

If you're not familiar with undercutting in welding, it simply refers to how the weld penetrates too deeply into the metal, causing an uneven or jagged edge which can be a big problem. In addition, it can weaken the overall strength of the weld metal and make it more likely to break.

There's no need to worry about struggling with undercuts because you can still fix them! This article looks at undercuts, their causes, and an easy technique to fix it in a weld. 

How to Spot Undercuts

There's no need to get too under the weather about undercuts. Although if you're welding and you see them cropping up on the edges of your base metal, it's not a good sign. 

Do you know what an undercut looks like? To find out, grab a flashlight and shine it on your weld. If you see a shadow at the center of the cut, that's an undercut. Next, take a straight edge or metal and dab it onto the weld to see if it stops. If it slips, you're all clear. 

You can also use an undercut welding gauge to help you prevent undercut in your welds. By using an undercut welding gauge, you can ensure that your welds meet the required standards accepted by inspectors.

Why You Messed Up in Welding Your Metal

Messing up while welding your metal is perfectly normal, even for professionals! After all, even the best welders have had their fair share of undercutting welding disasters. However, to avoid this costly mistake from happening again, you need to know exactly why you messed up. Here are a few possibilities:

Poor Welding Techniques

Let's start with weld speed. When you weld too fast, you don't give the rod or electrode enough time to fill in. As a result, you have an uneven weld bead that will not hold up well under pressure. The key is to take your time and let the molten metal flow evenly into the joint.

Next, there's poor weaving. This is when you don't maintain a consistent arc while welding, which can cause the heat to be distributed unevenly, resulting in an imperfection known as an undercut. To avoid this, make sure you hold the electrode at the proper angle and keep a steady hand.

Poor Work Angle

The correct electrode angle should be about 30-60 degrees. Your work angle will also depend on your weld's thickness, type, and size.

It's not a matter of filler material or technique alone. There's also the issue of incorrect angles. When your electrode angle is improper, you will create too much or too little molten metal, causing undercuts on your weld.

Incorrect Filler Material Used

Poor filler metal wetting can also cause the formation of porosity. Incomplete wetting leads to a dry interface and solidification cracking. On the other hand, when filler metal wetting is too good, it can cause the filler metal to be ejected from the weld pool. Thus, incorrect or poor filler material used equals the poor quality of cuts.

Not Enough Filler Material

As any welder knows, you need the right amount of filler material for your weld. Too little filler material can cause undercutting, while too much can lead to a messy and ineffective weld. The key is to strike a balance and deposit enough filler material in the weld area.

High Current

Due to high heat, a very high current can over melt the metal. Then, when the molten metal goes into the weld, it forms undercuts. So, if you are avoiding undercuts, don't go overboard with your voltage. Otherwise, you might end up with a less-than-perfect weld.

High Voltage

When voltage is high, the electrode melts into big ole' beads, baby. Your electrode has no choice but to move side to side when that happens, which results in a low fusion of the electrode with the filling material. Imagine all that electrode mixin' with the molten metal that creates undercuts. It's a sight to behold — and it's not good. 

Long Arc Length

Sometimes, it all comes down to arc size. If the arc is too large, it will cover a broader area than initially intended, increasing undercuts. That's why it's important to ensure the arc is no larger than the electrode's diameter. So keep your arc in check, and avoid those pesky undercuts.

Dirty Metal Plate

Metal plates can be difficult to weld with if they're dirty. The deposit of paint, dust, or rust on the surface makes a proper fusion between metal and welding groove impossible because it interferes with its metallic properties. It makes the task more challenging than usual!

Wrong Shielding Gas

Your ruined cut is that you didn't properly mix your shielding gas. Shielding gas is essential for clean cuts —- if it's not properly mixed, you may have an undercut. 

The gas helps to transfer air away from the welding tool and metal plate, preventing contamination. Without a good shielding gas mix, you could have some serious interference with metal properties. 

undercutting welding - Example of a perfect weld using a lay wire technique

How to Repair Undercuts With Tig Lay Wire Technique

Grinding won't fix the undercutting problem. Undercuts are gaps, so your welded area lacks filling material. So, even if you ground the surface, you'll have to fill in the holes to mend your weld. Going back to square one by getting a new metal plate will not help you either. Instead, you need to know quick remedies like the lay wire technique as a welder. 

Tig lay wire technique is one of many ways to get rid of undercuts. You create a puddle and then do a C-crest or half-moon motion with the filler wire. Make sure you pause on the top toes and fill in the gaps.


As a welder, you will run into undercut welds from time to time. It's important to know how to fix these nasty undercuts and understand the causes because a small welding defect will challenge the overall safety of the welded structure. However, with proper techniques and the correct setup of your welding machine, undercuts can be minimized significantly. If you're looking for more helpful welding reads, check out Weldingbuddy today!

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