[2023] Welding Undercut Defect – What It Is And How To Avoid It

Last Updated on Jan 21, 2023

The term “undercut” describes an unintentional groove that runs parallel to the bead of the weld. It is a lengthy trough-like structure formed within the base metal near the weldment’s edge, which greatly deteriorates weld-up joints. Undercuts result in a material deficiency in critical locations, which causes the joints to fall weak.

On a microscopic level, tiny cracks are visible inside the damaged surface, indicating that a structural failure is imminent if these microcracks are not fixed. This ultimately causes the weld quality to go down, becoming a safety hazard for workers.

Additionally, undercuts also weaken joints by retaining water and debris in trough-like pockets. The trapped water increases corrosion in an already fragile location, resulting in the structure failing mechanically prematurely.

Types of Weld Undercutting Defects

Now that we have a fair idea of what an undercut is, it is time to discuss the two different types of undercuts you might have to address—each with its own set of causes and ways of remedy.

External Undercuts

External undercuts (AKA Crown Undercuts) form towards the Crown (or toe) of the weld up region on the host metal. This undercut will appear on the front of the joint in a fillet weld and can clearly be seen as a dark line through a radiographic test. Since the front surface is much closer to the weld gun, these types of undercuts are more common.

Root Undercuts

Undercuts that develop at the butt-joint weld near the base metal are known as internal or root undercuts. Root undercuts may also be found located right next to the weld’s roof as well as towards the inside of the weld.

These undercuts are repaired using weld beads which are placed in the weld region to create inclusions in the weld, hence correcting the fault.

The 5 Main Causes of  Welding Undercuts

Electrode Size

Electrodes are available in different shapes and sizes, each serving a different functional purpose. Larger electrodes tend to increase the deposited metal and hence are more likely to form undercuts. Similarly, if the electrode is small in size, the deposited molted becomes less (inadequate), which leads to defects forming on the surface of the metal.

Too much heat

Typically caused by high currents, excessive heat generation can cause the rapid solidification of metal, which interferes with its curing and results in an undercut.

Poor Technique

Welding involves many techniques, such as multi-pass and weaving. If these techniques are poorly executed, i.e., if the distribution of heat is uneven, you will cause the curing to be uneven as well. If you want to avoid the formation of undercuts, you must prevent uneven curing from occurring by holding your electrode properly while welding.

Arc length

The gap between the electrode’s tip and the weld’s surface is called arc lengths. Understand that there are four primary welding positions, namely: vertical, flat, horizontal, and overhead—each resulting in a different variation of arc length. Wider arc lengths tend to form larger deposits of molten metal on your welded material which, eventually, results in undercuts.

Shielding Gas Contamination

Shielding gas is utilized in insulating the molten steel from molecular infiltrations. Gases including nitrogen and oxygen are possible agents that may permeate the operation, ultimately lowering porosity.

Unfortunately, certain contaminants are harder to hide. The shielded contaminants typically disrupt the metal fusion process since they affect the metallurgical characteristics of the material. Hence, there are larger risks of undercuts emerging in the weakened shielding gas.

How to prevent Undercuts in Welding – top 6 tips

Undercuts in metal structures are a safety hazard to anyone operating/ dealing with metal structures that contain them. Below are six tips you should follow to prevent such undercuts from forming during welding in the first place:

Use the correct amount of current and voltage.

Long, consistent undercuts may be indicative of the current being too high. If you observe such undercuts, lowering the current will help. Additionally, keep readjusting your voltage levels until you reach the welding sweet spot where no undercuts appear, and you get a finish smooth alongside an even bead profile.

Take your time; do not weld in a hurry

When you weld in a hurry, you don’t give your metal enough time to mix. This causes the premature solidifying of your metal that eventually results in the formation of undercuts. The solution is to weld slowly and give your metal ample time to mix and solidify.

Only use compatible fillers

Different filler materials are appropriate for different kinds of welding. When incorrect or inappropriate fillers are used, undercuts may take form. To avoid this, consult an expert welder or use a chart to pick the right filler material you should use.

Angles matter

Incorrect angles cause unequal distribution of fillers in the weld pool. So, you must always maintain your welding angle such that there are no observable differences along either side of the bead. During stick welding, the electrode is supposed to be directed at the molten pool. While it will depend on case to case, the ideal welding angle is usually within a 10-to-30-degree range.

Choose the Appropriate Electrode

Even while you hold your electrodes at the appropriate position, you might still be getting undercuts in your welding due to your choice of electrodes. Understand that some electrodes have a predisposition to forming undercuts. So, it is really important to choose your electrode carefully.

The EXX10/ 11 types of electrodes are particularly known for their aggressive digging arcs. Using these electrodes will leave your welds highly prone to undercutting. So, make sure you use it only when it is required.

Eyes on the road

While welding, it is really important that you’re able to see how what you’re doing so that you can ensure that you’re doing things right and so you can quickly remedy any errors you make along the way. To facilitate this, it is never a waste of money to invest in a good welding helmet.

Welding helmets (especially the most recent ones) are particularly optimized to balance visibility with eye protection. So, to ensure proper welding, you should definitely get your hands on this remarkably useful piece of equipment.

Acceptable Degree of Undercut

The AWS D1.1 Code dictates that any undercutting smaller than 1/32″ is to be considered innocuous, even if it spans the entire course of the welding. If it exceeds this, you must keep a close check on the examination. The maximum is 1/16″. An undercut that is even a bit deeper than 1/16″ invalidates the component.

Removing Undercuts from a bad weld

Once an undercut is judged to be undesirable, there are some options for fitting it. Undercuts, like other cavity-based faults such as pores or face craters, may be repaired with fresh weld beads.

Due to the fact that undercuts are frequently small and lengthy, it is preferable that they be filled using stringing or weaving. Now welders may choose between these two procedures according to the undercut width (broader parts need weaving).

One method is to fully grind the fault away using an angle grinder. This approach is only possible if the joint is sufficiently thick to tolerate the strength reduction. Experts sometimes apply a filling layer to weaker weldments after they’ve ground them to recover the initial level of thickness.