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Welding Undercut – Types & Tips To Prevent

Welding is more challenging than it looks. In the beginning, you would have to deal with various ilk of deformities that will weaken your joint. Although it has several defects that affect the quality of the weld, it can be done perfectly if you have experience.

Welding undercut is the most common defect, but it is a severe deformity that occurs in the weld and weakens the quality of the joint’s strength. This blog will discuss the causes, effects, and remedies of welding undercut. So let’s move below to have an insight into it.

What Is Undercut In Welding?

Welding undercut is the defect that occurs when the weld puddle fuses with the workpiece area that has not melted yet and causes the groove to appear. It appears along the welding bead with the appearance of a trough. It has enough dug space to collect dirt and water and keeps holding on. This space puddle weakens the joint as it is already thinner in the welding area; it simultaneously holds water for a more extended period, resulting in the premature breaking of the weld.

Let’s take an instance to understand it more precisely. Supposedly, you have to weld three plates of metal with a thickness of 9mm. You’d want to make the area of the weldment and its surrounding accordingly near 9mm. But there, an undercut appears, unfortunately. It’ll reduce the thickness of the weld to less than 9mm near the root or toe of the weld, making it vulnerable when it loads something. Those areas with undercut will break earlier in any loading case.

Types of Undercut

The undercut is classified into two types: External and Internal undercut. Both are slightly different from each other. Let’s have a look at them.

1. External Undercut

The undercut that appears in welding is known as an external undercut. This happens as the area is closer to the weld gun, and it appears on the outside surface. It is sometimes called a toe undercut due to its formation at the joint toe.

2. Internal Undercut

Internal undercut doesn’t appear at the surface of the weld; that’s why it’s called internal. It is sometimes also called a root undercut due to its formation at the root of the weld.

Tips To Prevent Undercut In Welding

Below are some tips for adopting to prevent undercutting in your welding:

  1. Reduce the current to prevent over-fusion due to overheating.
  2. Adjust your voltage according to the nature of the metal to get a clean final look of welding.
  3. Always be patient when welding, as it’ll enhance the speed of the factors in welding, making it more vulnerable.
  4. Always ask experienced welders or consult a chart before deciding on the filler metals for your welding.
  5. Angle matters a lot in welding. You must adjust it according to the filler metal and the workpiece to provide perfect results.
  6. Electrode size must be according to the size of your workpiece. Larger electrodes make a pool of weld that will overflow, so it’s advisable to choose the measure following the size of the workpiece.
  7. Shielding gas must be according to the nature of the filler and the metal piece.
  8. Keep your hand steady while welding. Feel free and weave at high speed.
  9. Arc length is negotiable as it will help you reduce the spatter and the resistance in welding.
  10. Always clean the dirt and the grease from the metal to be welded before performing so act on it.

Main Causes of Undercut

Undercut mostly happens due to three main reasons. It can be the wrong selection of the welding parameters, electrode and shielding piece, and the welding technique used in the process.

Let’s dive deeper into the factors that are included in them and can contribute to undercutting.

1. Wrong Selection of Welding Parameters

Welding parameters mainly consist of current, voltage, and travel speed. Current matters a lot in it as if exceeded from the required limit, it’ll melt down the workpiece. Similarly, the arc voltage works on the same principle and lets more heat enter the metal if it is too high. The more heat produced, the more will be the metal pool. As the metal pool increases, it will overflow and leave gaps, resulting in undercuts at the weld. The third-factor contribution is travel speed. The travel speed affects the welding process too furiously. If the rate is too fast, it will not allow the molten pool to spread all over the metal but will solidify before it. It will result in the appearance of undercutting.

2. Electrode & Shielding

Electrode and shielding can always be addressed in the welding process as they both dominate the workpiece. The electrode is chosen as per nature, size, and angle effects as the primary effect of the welding process. That’s why selecting the electrode is always advisable after analyzing the nature of your metal workpiece. Every metal piece goes with q specific kind of electrode. If the electrode angle is set accordingly, it will prevent more entrance of heat, leading to the improvised melting of one side of the metal than the other. It is simultaneously resulting in the formation of the undercut.

In contrast, the other ends remain intact and solid. In this manner, the electrode size affects the welding. Once you’ve your angle set, ensure the size fits the workpiece size. As if the size enlarges than the workpiece. It will result in more metal pools, just like above, and cause the formation of undercutting at one of the edges.

Shielding gas has a significant effect as it’s the factor that prevents any environmental interference. If the mixture of the gas is not chosen wisely, its pressure can cause much damage.

3. Welding Technique

If you are professional enough to tackle all the above-given causes, you can overcome the welding technique with no wonder. The welding technique comprises the edge preparation and the length of the arc. You must maintain both of these to move on without resulting in an undercut to your weld. The workpiece’s practice includes removing any debris, dirt, or grease piled at the workpiece. You have to clean these before welding so as not to cause any resistance during the process. Now, when it comes to the arc length, it must not be in excess. Just as the saying, “Excess of everything is bad,” it can lead to undercutting.

What Is The Acceptable Amount of Undercut?

The acceptable range of the undercut depends on several factors, including the material nature, its continuity, joint strength, and the appearance of the weld.

In most cases, the welders finding any undercut below 0.5mm or 0.2 Inc. consider it safe, but it is not acceptable if it exceeds this limit.

This standard is also determined by the American welding society (AWS). It states that any undercut above 1/16 is not acceptable, while the ratio of 1/32 is in the safe zone.

Removing Undercuts For a Bad Weld

Once you’ve decided that undercut is unacceptable, you can remove it through different methods. Usually, the undercuts are narrow and long. In this case, the welders apply the strategy of weaving and stringing. The weaving strategy is used when the undercut is longer and wider. This helps in the cleanness of the joint. Suppose you’re not thinking of doing any of these two. You can move to the grinding one. In this method, the undercut part is ground off slightly. It helps in its removal, but this can only be done when the joint is thick enough to bear the grinding. If it is the thing, don’t try this, as it will make the joint delicate and vulnerable.


Welding is the most adventurous task due to its new emerging problems and solutions. If you deal with one, the other can pop up, and it keeps happening. But at the same time. You have the answer to deal with them. Undercutting is not a very dangerous defect on your weld, but as you know, you can’t compromise on the joint quality; you have to deal with it not to get rejected. We have covered all the basics and remedies of the undercutting in this blog, but if you still have some queries, let us know in the comments below.

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