What is Undercutting in Welding?

Last Updated on May 2022


Undercutting is one of the most common problems affecting welders. In welding, undercutting refers to when the cross sectional thickness of base material is reduced by a weld. Undercutting often reduces the overall strength of a weld as well as work pieces.

Did you know that even the most experienced welders can be victims of undercutting? You don’t have to be a novice welder for undercutting to occur, it affects everyone and has several causes, which we will discuss in this read.

Want to know more about undercutting and how to prevent the problem from occurring? Read on as we provide a detailed explanation about the common problem of undercutting in welding.

What causes Undercutting?

Undercutting is public enemy number one for welders. It reduces welding quality and often ends up wasting time and resources. So, since we have established that undercutting is bad for welds, what are some of the main causes?

  • Maintaining an excessive long arc length
  • Incorrect selection of gas shield
  • Attempting to weld too quickly with a fairly high current
  • Welding on rusty surfaces or without properly preparing the surface before welding
  • Welding at improper electrode angles

An undercut will occur when the base material and weld material fail to fuse properly when welding. Undercutting of steel has for the longest time been a major issue when welding, and has resulted to massive losses and delay of project completion dates. Failing to fix an undercutting problem more often than not weakens the weld and increases the chances of cracking over time.

Ways to Prevent Undercutting

Work at Medium Speed

Fast welding is a common cause of undercuts. When you weld too quickly, the base metal tends to become molten metal, and on most occasions, fails to return to its initial shape as a result of rapid solidification. When the molten metal solidifies too quick, it consequently creates a depression along the welds edges.  Preferably, you should try welding at a medium pace since slow welding rarely yields positive outcomes.

Work at the Right Heat Input

Among the most common causes of undercutting when welding is usually the use of excessively high heat when operating close to the free edges. Such action normally melts the main metal. As a result, it is crucial to check on the weld current and keep it low enough to allow you to maintain a moderate arc speed. It is advisable to check your welding temperature as you approach thinner and free edges.

 Check your Weaving Technique

Weaving helps to regulate the heat in a weld puddle; to prevent undercutting, be sure to pause each of the welds sides. Fast weaving more often than not results to undercutting. If you are using a weaving technique, ensure you hold the duct carefully at the groove`s sidewall. You should also avoid working on dirty surfaces as it can lead to fusion deficiency.  Always aim to clean your weld surfaces thoroughly before and after welding.

Observe the Electrode Size and Angle

Understanding the appropriate welding angles is critical in avoiding errors in your welding operations. If heat is being added to free edges at the wrong angles, chances are an undercut will be produced. It is also advisable to refrain from using electrodes that are larger than needed. If the molten metal becomes excessively thicker than the plate, then an undercut will be produced.

Check on your Welding Position

You have to be extra careful when welding vertically or horizontally. Welding in a wrong position more often than not leads to insufficient metal fillings on the connecting edges, and eventually an undercut. Welding flat helps prevent undercutting depending on the type of welding project you undertake.

Check your Selection of Gas Protection

Undercuts are commonly caused by the wrongful selection of gas protection during welding projects. The right composition should be based on source derivatives, material type and thickness. For instance, using carbon dioxide with inert gases provides the right results when carbon steel welding. It is advisable to increase the shielding gas flow in your regulator to successfully eliminate the chances of producing undercuts.

Operate on the Right Travel Speed

Travel speed is an important consideration when welding. Always keep track of the amount of weave used and observe the weld speed. Avoid holding the electrode close to the vertical plate when you want to make horizontal fillet welds.

Go for a Multi-Run Weld Technique

The multi-run weld technique is crucial in reducing the amount of undercuts and lower segments during welding. Using this technique helps you to generate great resources and also improve the mechanical properties.

Fixing or Cutting Undercutting Problems

An undercut is not easy to correct; however, fixing is not too hard either. You have to examine factors such as the length of the undercut. Placing a stringer bead (entire length) of the weld`s undercut. Alternatively, you can also chose to blend the undercut with a metal grinder for you to get a preferred weld profile. If an undercut is not larger than 0.5mm, it most probably doesn’t warrant a repair. If welding on a low-carbon steel surface, the undercuts can remain uncorrected if they are 1mm deep (but not more).

What does Internal Undercutting Mean?

Not too many articles focus on internal undercutting since external undercutting is a lot more common. A root undercut, popularly known as internal undercutting, normally occurs on a butt joint weld close to the base metal. These welds occur at the root of the weld.

When pipe welding, internal undercuts can form close to the inside of the pipe along a narrow edge. To fix internal undercuts, you will need to use weld beads and position them inside the weld area. The beads then form inclusions to repair the weld.

You should pay close attention when welding roots or narrow edged base materials when welding to avoid the occurrence of internal undercuts.


While some undercuts can be easy to repair and overcome, others can have lasting impacts and negatively affect the progression of welding projects. Industrial welding for example structural welding requires quality welds for structure and buildings to remain sturdy. The occurrence of undercuts can affect the stability and strength of a building if not identified during the inspection process. Fixing undercuts can also derail the completion of the project as repairs consume both time and resources.

It is advisable to weld in moderate speed and to also check on your use of heat to prevent the creation of depressions and gaps on the weld. The use of poor welding techniques can also lead to the occurrence of undercuts.

Since an undercut occurs when there lacks gaps in connection between different metals when welding, it can be avoided by following the right procedure. As we stated in the earlier section, even expert welders can be victims of undercutting if they fail to weld at the right pace or if they position their electrodes at incorrect angles.

Another important precautionary measure that is often assumed is preparing the welding surface. You need to thoroughly clean the surface to reduce the chances of undercuts occurring. Remember, these undercuts tend to weaken your welds and lead to cracking over time, which can negatively affect your profile, especially if you weld professionally.

Luckily, not all undercuts warrant repairs, at least not if they are below 0.5 mm. Be sure to check on the location and extent of undercuts to determine whether repair is needed, and if so, the best way to make them.

Are you likely to experience undercutting as a beginner? Yes, chances are you might produce undercuts as you learn your way around tools and different welding speeds. This is why beginners are always advised to hone their skills on small scale DIY projects before targeting the big welding projects. Starting small gives you the confidence and skill to move up the DIY ladder and take up more challenging projects.

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