What is Undercut in Welding?

Undercut in Welding
Undercut in Welding

Welding is not a child’s play. It requires proper training and dedication to achieve perfection. Yet, even experts can make mistakes. So, as a welder, your primary job should be ensuring you leave nothing. One of the most common welding defects is undercuts in welding. 

So, what is an undercut in welding? An undercut in welding is a groove that develops in the base metal near the weld’s toe or root. It happens when the underfill in welding fails to fill in that grooved area. Grooving is a long and narrow cut in hard material. In an undercut, the edge of a weld gets an inverse U shape. It is the weld’s weakness. This type of defect degrades the strength of the weld and workpieces.

An undercut in welding is a common mistake. It is a mistake that even the most skilled welder can make. But the downside is not only the degradation of welding quality. Undercutting is also costly. Hence, we thought you would like to know what is undercutting in welding and how to prevent or repair it. As you read ahead, you will find various relevant points that would help you to polish your welding skills and achieve perfection. 

Types of Undercut in Welding

Before knowing more about undercuts in welding, we want you to understand that they can show up in two different ways. These are Internal or Root Undercut and the External or Crown Undercut.

Undercut welding types
Undercut welding types

1. Internal or Root Undercut

Internal undercut forms near the root of the weld on the parent metal. It appears as a depression on the sidewall. The root undercut’s radiographic image appears as a dark irregular line offset from the weldment centerline. Internal undercuts can form near the inside of the pipe next to a narrow edge in pipe welding.

2. External or Crown Undercut

Crown undercut or external undercut forms near the toe or crown of the weld area on the parent metal. The crown undercut’s radiography image appears as a dark line on the edge of the welded area.

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What Causes Undercutting in Welding?

Several factors can cause an undercut in welding. Knowing these causes will help in decreasing the chances of making such welding mistakes. Make sure you understand them correctly. 

1. Excessive Amount of Current 

Welding machine
Welding machine

One of the biggest reasons for an undercut in welding is using a very high current. It can make the edges of the parent metal melt due to the vast amounts of heat. The molten metal caused by this heat then drains into the welded area. Thus, it increases the possibility of forming an undercut.

2. Traveling Too Fast

Welding is a steady process and needs the right speed. So, if you hurry or travel too fast, the puddle will not fit correctly. And while you increase the temperature to over-speed, the main metal melts more quickly. 

So, make sure you take your time and use the right temperature. Go for thinner metals and adjust arc speed accordingly. With a little more attention, you can avoid an undercut in welding. 

3. Use of an Improper Gas Field

While performing a MAG (metal active gas) welding, you must be careful about selecting the right gas for the job. A wrong choice, in this case, can cause undercuts. You can easily avoid the complication. First, access the material type and thickness and select the gas accordingly.

4. Incorrect Electrode Size

Welding Mistake
Welding Mistake

Choosing the correct electrode size for welding is a crucial task. Large electrodes generate more heat and melt an excessive amount of metal, leading to undercuts. On the other hand, too small electrodes are incapable of generating an insufficient amount of molten metal. It also leads to welding defects. 

Thus, picking the right size is necessary. However, the rod should also carry high amounts of current without interfering with the molten metal supply or deforming the metal.

5. Excessive Arc Length

Maintaining appropriate arc length in welding is essential. An arc is a gap from the tip of the welding rod to the metal to be welded. The size of the arcs varies with the electrodes. Make sure that your arc does not exceed the diameter of the electrode. 

If the size is bigger than the electrode, it will cause the molten metal to cover a broader area than initially intended. It thus leads to increased undercuts.

6. Improper Angle While Placing the Electrode

Mastering the craft of welding is incomplete without knowing your electrode angle. An improper electrode angle can cause an undercut in the weld because when the electrode is incorrectly angled, it will generate too much or too little molten metal in the welded section. Consequently, it leads to undercuts. 

How to Prevent Undercutting

After knowing what is undercut in welding, we hope you have also learned how an undercut occurs. Undercuts not only decrease durability but are also costly. Therefore, knowing how to prevent them will help you to achieve perfection. The following points on how to avoid undercut in welding will assist you in learning the methods. 

1. Use Accurate Heat

Welding Heat
Welding Heat

We have already mentioned how high heat can cause an undercut in welding. Accurate heat input should therefore be one of your foremost priorities. Determining how much temperature you will need depends on the type of filling material you will be using. So, reduce the welding current. Consequently, you also have to reduce the arc speed. By using accurate heat, you will get the desired welding result. 

2. Correct the Speed

Hurrying through the welding process is another cause of undercuts. Fast welding melts the base metal and makes a molten metal that cannot be returned to its initial shape due to rapid solidification. Thus, it creates a depression along the edges. So, keep a medium and steady pace while welding metals. Don’t be too fast or too slow, since both speeds can cause an undercut in welding. 

3. Use the Correct Electrode Angle

To perform an error-free welding operation, you must know the proper angles of welding. Heating the free edges at the wrong angle can cause an undercut in welding. Therefore, you should always prefer to direct the heat to the edges and areas with thicker components compared to the free edges.

4. Use the Correct Electrode Size

Electrodes are available in different sizes. And what size you should use would vary accordingly with the type of welding material and the arc size. Therefore, you should avoid using electrodes that are too large. A larger electrode would create molten metal that is too large for the plate’s thickness. It would make an undercut. 

5. Use the Correct Technique

Using the correct weaving technique would ensure you get the perfect weld without any undercut. Weaving is to regulate heat in a weld puddle. Using this technique is simple. You just have to pause at each side of the puddle, which allows for beads’ build-up. Thus, it minimizes undercuts from forming. When using the weaving technique, make sure you hold the duct in the groove’s sidewall.

6. Use the Appropriate Shielding Gas

Using the appropriate shielding gas guards your molting metal against exposure to oxygen and other gases. If the molten metal gets into close contact with air in any way, it could lead to undercuts. Therefore, the mixture of gases in the shield must be appropriate for the metal type and thickness. For example, carbon dioxide with inert gases works exceptionally well for carbon steel welding.

7. Correct Your Position and Angle 

We have already mentioned how using your electrode at an incorrect angle while you are welding can significantly increase the chance of undercutting. So, maintain the properly required arc for welding anything. Also, correct your position to avoid any further complications. 

8. Clear the Surface

Perfect Undercut
Perfect Undercut

Deposits like rust, paint, moisture, and oil create welding defects. This debris on the metal surface interferes with fusion. Therefore, before welding, clean your welding surface thoroughly to avoid any chance of welding undercut. 

How to Repair Undercut

Undercutting is a common mistake. Fixing undercuts is costly and also results in a reduction in speed and productivity. But this is avoidable. So, best is to try and avoid it by following the tips on how to prevent undercutting. 

Undercuts measuring 0.5 mm deep or lower do not need fixing. You can simply avoid these. Undercuts measuring 10mm can also be fixed. Below, we will learn how to fix an undercut depending on the types mentioned before. 

1. How to Fix External Undercuts

Fixing an external undercut is easy. You can correct it by placing a stringer bead along the entire length of the undercut. If need be, using a grinder to grind and achieve the desired weld profile is also an option. 

2. How to Fix Internal Undercuts

You can fix an internal undercut using a weld bead. For this, you have to place a weld bead within the weld area. These beads will form inclusions in the weld, fixing the flaw.

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Mastering these skills requires practice and dedication. With this, you will know which angle to use, which is the right selection of gas, and what speed to maintain. Undercutting is a common mistake. Almost all welders, even the most skilled ones, make these welding flaws. So, if your welding has got one, don’t get disheartened.

Anyone new in welding must learn about the fundamental welding defects to avoid those.  Knowing what is undercut in welding, thus, will help you understand the problem. Make sure you take preventive measures to stop it from happening.