What is undercut in welding?

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Last Updated on October 25, 2021 by weldinghubs

The term “undercut” refers to the section of material that is not welded. It can be difficult for a novice welder to properly account for this when welding, and it’s important to know how an undercut affects your work before you begin.

There are many ways to overcome an undercut: adjusting your welding speed, changing wire feed direction or voltage level, or adding additional filler metal in between passes. You can also try using different welding methods such as gas tungsten arc (GTAW) or metal inert gas (MIG). Understanding what type of techniques will work best for your situation is crucial in order to produce quality welds. In this blog post, I will cover what is undercut in welding? what causes an undercut? and what to do about it.

What is the meaning of undercut in welding?

The grooved space beneath the weld is known as undercut and happens when the metal doesn’t fill in that area. This defect will result in a weak, cracked-prone joint with uneven toes.

When welding, there are many types of cracks that can occur. One type is called undercutting, which often occurs near the toe or root of a weld. If you find these issues with your toes then it’s because they didn’t fill up with enough weld material to stay strong and sound during welding.

Moreover, an undercut in welding is when a weld does not drain all of the hot metal away from the edges. This leaves an incomplete layer on the underside of your welds, which leads to cracking and breaking. To avoid this problem, make sure that you are using enough filler rod material for each pass and try to use fewer passes in order to reduce the number of times you are melting metal.

Welding is not that easy as it appears by reading blogs. When you weld a thing the first time, you have to face many problems like material deformity. The deformity or impairment is an undercut in welding. Any groove that grows at the root or the base of material is an undercut.

Depression is an unwanted lengthy cut in the parent material. Undercutting in welding can be external or internal. Undercut in welding is not a new term; it is something that you can confront at any time. This problem causes a drastic loss in the productivity and money of the welder.

How does undercut affect welding?

When welding, there are many parameters to think about. It has been found that an undercut at the toes of welded joints is one of the most important “weld” geometry parameters. Compared to joints without an undercut, welded joints with an undercut have at least twice the reduction in fatigue strength.

An undercut is a concave radius at the toe of a welded joint, which is opposite to an overcut. When you set up your machine for welding steel and aluminum, it’s important not only to consider different types of wire but also how much negative clearance (undercut) should be used on joints made from materials other than stainless steel.

The undercut is important because it has a direct effect on fatigue strength, which means that the time interval between welds required breaking the joint decreases as negative clearance increases.  “Faults” or slag can accumulate in this concave radius and cause defects such as porosity and lack of fusion when welding steel joints with an overcut.

What are the Causes of Undercutting?

So, what causes an undercut in welding? Usually, undercut develops because of some wrong welding technique. Overheating or inefficient use of welding processes are the primary causes of undercutting. The too-long length of the arc is one of the causes, and we should maintain it about 1/8 inches of the material. For MAG processes, improper gas field selection can also cause the problem. Poor electrode handle, high welding current, and rust are also the root causes of undercutting. When you fail in filling the joint in the right way and metals did not fuse properly, the chances of undercutting increase.

Undercuts in an area result in weak welded material. These undercuts lead to cracking along the toes, which is also a problem in welding.

If you find yourself frequently dealing with the undercut, there is something wrong with your welding technique. You could be overheating the metal as well. A few common causes of undercutting are too much heat in one area or not enough shielding gas to protect the molten weld pool from the atmosphere while it cools and solidifies.

The most common reasons for undercutting are listed below.

Maintaining an arc for too long:

The arc length of your welding rod will vary depending on the type and thickness of the metal you’re working with. For a starting point, make sure that the width or height to which you are applying the electric current is not greater than any other part of what’s being heated up except for where it attaches to whatever material you are using as an electrode.

In order to avoid burning or overheating the metal, you will want to maintain a constant arc. Maintaining an arc for too long can cause an undercut in welding and is not conducive to your end goal of producing high-quality welds that are strong enough for use on heavy machinery.

Wrong Gas field selection:

Some gas welds are better for certain situations than others, so it’s important to know which one is right. Watch out for undercuts as well; they’re the leading cause of failed MAG welding jobs and will waste your time if you don’t identify them early on in production!

Gas selection may seem insignificant at first glance but could be a matter-of-fact deal-breaker when dealing with different types of metalwork. For instance, some gases like argon or helium might not be compatible with all metals while other gases such as oxygen can only handle low heat input due to their reactivity. Inaccurate gas selection should always come before any actual manufacturing process begins–find the best fit now instead of being stuck doing costly repairs later on.

To avoid this from happening, first assess the material type and thickness. Then identify the correct gas composition accordingly. For carbon steel welding for example; a mixture of gases can be used to create an excellent result- in particular one made up of carbon dioxide with inert gases such as argon or helium which helps produce great welds without any corrosion on your work!

To successfully complete these tasks you’ll need to ensure that you have all the right equipment at hand including shielding gasses like CO2 mixed with either Argon (Ar) or Helium(He). Carbon Steel Welding needs more than just heat therefore so it’s important not only to be able to use proper tools but also to understand what makes successful welds possible too: by using combinations of different types of shielding gas with the right amount, you can achieve a perfect weld every time.

Weld too quickly:

Many people think that the higher you set your welding temperature, the faster and better it will weld. However, this is not always true as using high temperatures can lead to an increased risk of contamination or even fire should flammable gases be present in the area where they are working.

Increased heat speeds up metal melting rates which may seem like a good thing but when there’s no oxygen around (outside air) for example then any gas fumes could easily ignite from too much heat coming off of what someone was trying to meltdown – causing big problems! So if you’re going to weld outside, make sure that all potential sources have been eliminated so they don’t interfere with whatever material is being worked on at hand.

Be careful not to overheat the metal. A little bit of time and attention will help you avoid an undercut by using a lower current and arc speed for thin metals.

Wrong electrode angle:

Here are some of the most important things you need to know if you want to master welding. Knowing your electrode angle and size is essential, so here are a few tips:

  •  Check your electrode size before you start welding. They come in different sizes, and if the wrong one is used it may damage both the welder’s equipment as well as the material being welded. If they’re too small for a large area, their electrical capacity will be limited which could lead to undercutting on thin materials or overrunning (spreading of the current) on thicker ones; this can cause issues with a number of factors including arc stability, puddle control, and heat input rate.
  • The optimum angle at which to hold an electrode varies depending on what kind of metal you are welding; hotter metals require a sharper angle than cooler metals so make sure that when you are using tungsten electrodes they have been heated to the correct temperature for your application.
  • The angle helps determine how much metal you remove from a joint during welding. If it is too shallow, there may be insufficient penetration of the weld; if it is too steep, then all or part of the electrode will go past one side of the joint and undercut that section
  • To help prevent this problem, make sure that both electrodes are long enough so that they can reach across the entire width of materials being joined with little overlap on either side. This minimizes any chance for an undercut and maximizes efficiency by keeping heat input constant throughout each pass.
  • For thinner metals like sheet steel where overrunning might become more prevalent due to its low electrical capacity (which causes arc instability), use a more stable tungsten electrode.
  • For thicker metals like a round bar, you may need to use a less stable tungsten electrode because the resistance of these thick materials is too low and causes arc instability if used with a high capacity (stable) electrode.

What is the prevention of undercutting?

So, how can we prevent undercuts? We can reduce the undercutting problems by applying some techniques and through proper and careful work. Some methods of prevention we will discuss here.

01. Proper heat input:

As we know, high heat is a cause of undercutting when we are working near the edges. High heating causes the parent metal to melt. So the reduced heat can prevent the loss. Reduced welding current and reduced arc speed are beneficial for the prevention of undercut. Near the edges and at thin corners, the reduced temperature is favorable.

02. Medium speed:

Undercuts also came into being because of fast welding speed. When the welding occurs rapidly, parent metal melts and will not return to its original shape that causes the deformity of metal. We should maintain the welding speed at medium because being slow will not get a high yield as the balance is significant in everything.

03. Proper Size and Angle of Electrode:

The right angle the right will be the welding because they play a crucial role in welding techniques. If a technician works at the wrong angle and applying high heat to it, the undercut will occur in a minute. Also, the perfect electrode length plays a significant role in smooth welding. If the electrode is too long according to the thickness of the plate, then the undercut is compulsory.

04. Accurate Weaving:

Regulating heat on the welding peddle is done by weaving. If a welder is performing at a fast speed, an undercut can be the cause. If we hold the duct at the side of the groove, then it will be correct. Thorough cleaning of the surface will be better for perfect welding.

05. Selection of Gas Protection:

Wrong gas composition is one of the major causes of undercuts. We have to choose the correct concentration of the gas with the parent material and its thickness. If we want to weld carbon steel, the usage of carbon dioxide with inert gas will be of a good result.

06. Right Welding Position:

The right welding position is a challenging and complex process. It’s important to take the time to learn so you don’t waste valuable resources. There are three main positions for MIG, Tungsten Inert Gas (TIG) or Shielded Metal Arc Welding (SMAW). They are: Flat Position; Vertical Down Position; and Horizontal Undercut Angle Position.

An incorrect welding position can lead to an undercut or flame cutting. Remember, you should always weld in a flat position to avoid the issue of an undercut. Welding is usually done on horizontal and vertical surfaces, but sometimes there are not enough metal edges for it to connect with.

07. Multi-Run Welding Technique:

Understanding how welding works can help you avoid problems like an undercut. The Multi-Run Welding Technique lessens the risk of getting undercuts in your project by doing multiple operations at once with grinding for finishing touches afterward. This technique includes two or more passes of welds on the same set-up, which are then ground to create a finished appearance.

How do you repair an undercut Weld?

After all this discussion, the question arises is that how could we fix or repair the undercuts in welding. Curing an undercut is quite significant although, it slows you down. If we do not fix it, then it causes problems in further processing. In industries, it is a vital question whether the undercut should be cured or not because of the additional costs. The main point is that you should have precautions for preventing the undercuts in your work.

The very small undercuts of 0.5mm do not need fixing, and for low carbon steel, the undercut of 10mm can be fixable. Stringer beads are useable for removing the undercuts; we have to place them along the whole length of the external undercut. We can also blend the pieces of undercuts by using of grinder to remove them. In AWS standards, they say,

“You can remove the segments of weld metal or base metal using different processes. These may include fabricating, pulverizing, scratching, or cutting. You have to do it in such a manner that there is no need of excavating weld metal or base metal. You can perform Oxygen excavation for use on components as-rolled steels. You must scratch out the undesired portions without substantial pulverization of the base metal. The surface of sheets to be weld must be clean. You can pile up Weld metal to fill the interstices or crevices.

So, we can understand, there is an acceptable limit of undercuts in the welding technique, upon which we can compromise.

Many times the undercutting is caused because of the inexperienced and untrained technician of welding. The goal for any welder is to weld fool-free work because undercuts are the deep cause that decreases the integrity of welding. It is okay to work slow but be perfect. More projects you have done with undercuts do not mean good productivity at all. Work attentively; practice your working skills often, and do not be scared of slow work. Every good project takes time for a good appearance.

How much undercut is allowed?

The undercut limit in welding is dependent on the thickness of the metal being welded. For metals less than one inch thick, you can have a very small amount of cutoff between exposed welding and fillet welds to avoid difficulties.

Actually, the “1/32 inch undercut is allowed for every 12″ that D1.1 allows. The metallurgical specification states NO undercut greater than 1/32″ may be used on metal less than 1-inches thick or for welds of any length up to 2-inches. For metal more than or equal to 1-inch thickness, an undercut not exceeding 1/ 16” may be used for joints of any length up to 2 inches long in material thicker than or equal to one inch.

As a practical matter then, in materials less than one inch thick the cutoff between exposed welding and fillet weld can be very small and thus give rise to difficulty and potential defects where tolerances are close; however, with all joints, the undercut must be less than or equal to the allowable limit.

It’s always best to speak with your supervisor before proceeding when there is uncertainty as they will know what limits apply in certain circumstances.

How is weld undercut measured?

Undercut measured with Pit Gauges may also be used to help measure the effectiveness of a Weld. In one instance, I was inspecting Fillet Welds on an offshore platform and found that some welders were not fully penetrating all the way through a metal plate being welded. Undercut measurement showed this problem very quickly!

The process of measuring undercut with Pit Gauges is very simple. The Dial Indicator on the tool can be oriented either to measure in-line with the weld or transverse to it, depending on which direction users want to measure from.

A good rule of thumb for undercut measurement using a Pit Gauge is that when measuring Fillet Welds, Undercuts should not go beyond the recommended 0.5 mm depth, and it must also be combined with a minimum of 1/32 inch thickness to avoid interference issues. The same goes for any undercut that exceeds one inch deep on three inches worth of structure; only those within ranges will be accepted as a normal practice is followed regarding such an issue. The greater the undercut measurement, the less likely that weld will hold up to vibration and shock loads without cracking.”

Undercut in welding can have many causes which may vary depending on the type of welding being performed. However, there are a few factors that may contribute to undercut and should be considered when examining welds for this defect:

– Lack of penetration in the molten metal due to improper electrode angle.

– Insufficient wire feed speed (more often seen with GMAW).

– Inadequate arc length (more often seen with GMAW).

– Insufficient welding wire feed pressure.

– Lack of weld pool control.

When examining a weld for undercut, it is best to inspect the opposite side of the weld as well; any undercut on one side should be reflected in undercuts on the other. Additionally, when measuring undercut, it is best to measure from the opposite side of where the weld was made.

All these factors should be considered when inspecting for undercuts in welding and can provide clues as to what might be causing a problem with this defect.”

What are the differences between external undercuts and internal undercuts?

Undercuts are one of the serious problems faced in welding. It is so common and unpredictable that an experienced and skillful welder can also experience it. There are two types of undercuts that are internal undercuts and external undercuts.

  • Internal Undercuts

Now, you have to understand the term internal undercuts in welding. They usually develop near the base or root of the weld on the parent material, and they appear as dejection on the sidewall. They are also known as root undercut. In testing, they will be like a dark line around the center of welded area. Mostly it can be seen inside the pipe in pipe welding. This type of undercutting can also be present inside the weld.

  • External Undercuts

The other type of undercut is the external undercut in welding. These undercuts usually develop next to the crown or toe of the welded area on the parent metal. These undercuts are also called crown undercuts. In testing, they will appear as a dark line on the edge of the weld area. This problem is not healthy for welding as it makes the metal more susceptible to cracks.


Above all points leads us that undercut usually formed when there is no space between the two metals we have to join. These undercutting processes in welding make the material weak and result in defective cracks. Excessive heat, more current at thinner edges, faster speed, and wrong gas are the substantial causes of the undercuts. There are many preventive measures to avoid undercutting in the process that had discussed. The low current is a key for avoiding undercutting in the process. To some extent, undercutting can be acceptable, but too much undercutting does not.

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What is the acceptable limit of undercut?

According to standards, any undergo should not be more than 0.5mm and cannot interfere with the thickness of any section. More than 1 / 32 inches of undercutting is not up to standard, so if anyone has 1/16 inch of undercuts in work, then you do not have to worry.

Compare Undercut Vs. Underfill

Many people are confused about the two different terms of welding that are undercut and underfill. Undercut develops in the base metal contiguous to the weld, and the underfill usually appears in the weld metal. Underfill only appears on grooves, but the undercuts develop on both grooved areas and fillets.

What are the Effects of undercutting?

Many people do not know the bad aspects of undercut and do not understand the importance of not having undercuts. In the welding technique, undercut decreases the strength and potency of the welded material. The deepness of the undercut decreases the cross-sectional region of the parent metal.

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