What is undercut in welding?

What is undercut in welding?

An undercut in welding is a defect that happens during the welding process. It is a groove that forms during welding on the root or base of the welding. Undercut forms due to using more current, high weaving speed, electrode size, improper filler material, arc length, etc. Due to that, welding becomes weaker & its strength decreases. It forms when the weld reduces the cross-sectional thickness of the base metal.

If the part with a such defect is in use for the specific application, then there is a chance of happening failure in the defect portion. It may cause a large accident. These undercut defects are common in the process of butt & fillet weld.


As we know, welding is a promising and effective metal joining process. But due to lack of knowledge and some silly mistakes causes defects in welding. There are several defects in welding, but undercut in welding is one of the ordinary and most dangerous defects that every welder has faced several times. Let’s see their types, causes & prevention with repair techniques.

Types of undercut in welding.

There are two types of undercutting that happen generally.

Internal Undercut

  1. It is also called a root undercut because it happened in the root of the weld.
  2. Internal undercut mainly occurs in butt welds.

External undercut

  1. It is also called a crown undercut because it happened in the toe of the weld.
  2. External undercut mainly occurs in fillet welds.

What are the causes & prevention of welding undercut?

There are the following main factors that will lead to welding undercuts.

High heat

A high amount of current generates high heat during welding. Due to that, parent metal melts & drains into the edge of the weld area. It causes the formation of an undercut in welding.

The operator has to regulate the defined current & voltage to avoid the formation of high heat than the limit during the welding process. 

Over speed       

As we know, during the welding process, high heat generates. If the operator increases the weaving speed causes the weld does not become proper. In some portions, it looks dense but not in others. Due to that, undercut in welding forms.

To prevent this, it requires constant weaving speed during the welding process. It leads to good quality of the weld.

Improper Electrode Angle and size

Improper electrode angle and size are directly proportional to uneven heating and melting of metal. As well as uneven distribution of weld in the welding portion. Due to high heat on the edges, undercut in welding occurs. 

To avoid the welding undercut, proper electrode angle & size requires. The size of the electrode is selected according to the plate thickness. Avoid more contact with the edge of the base metal.

GAS field

During a MAG welding process, if wrong selection in the shielding gas causes the formation of an undercut in welding. Incorrect composition of shielding gas in different welding processes can forms welding undercuts.

To avoid it is necessary to select shielding gas composition as per the material, size & condition.

Improper filler material

The selection of the correct filler material is very important in welding. If the filler material is different from the recommended, then there is a high chance of the formation of an undercut in the welding process. 

So correct filler material as per the base material is important in the welding process to prevent undercut formation.

Arc Length

Arc length is a very important factor in the formation of an undercut. As per the electrode, the arc length gets decided. Variation in the arclength causes undercut defects.

Lack of practical experience and knowledge of welding causes this defect in welding.

How to repair undercut in welding?

Repairing undercut in the industry is the most crucial topic. It depends upon the cost required for repair & benefits due to repair. It is better to avoid it while doing welding. Because it is so costly to repair. That affects the quality & productivity of the product.

Lower than 0.5mm of undercut is acceptable. Weaving & stringer bead techniques are the two methods to do it. Removing this defect with the grinding operation is a common technique. But removing excess weld material can reduce the strength of the weld.


During the welding process, if we do not take care of weaving speed, voltage, current, arc length, shielding gas, electrode angle, size & filler material, then there is a chance of the formation of undercut in welding. The operator should be well aware of this process.

If the part with the welding undercut is imposed by the force, then there is a chance of failure and accidents. So it is necessary to have information about the above points to the operator. 

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