Welding

What is Undercut in Welding?

Undercut in welding refers to a common defect that arises when two or more metal pieces are joined by melting and fusing them. This defect can negatively impact the quality and strength of the weld. However, it is important to understand what undercut is in welding and how it can be avoided.

What is Undercut in Welding?

What is Undercut in Welding

Undercut in welding is a defect where a groove or depression runs along the toe of the weld. The toe of the weld is the junction between the weld metal and the base metal.

Undercut occurs when the base metal has melted and has not been properly filled by the filler metal. This creates a weak spot in the weld that can reduce its load-bearing capacity and cause cracks and failures.

What Causes Undercut in Welding?

Undercut in welding is a defect where a groove or depression runs along the toe of the weld, which is the junction between the weld metal and the base metal.

Undercut occurs when the base metal has melted and has not been properly filled by the filler metal, creating a weak spot in the weld. Undercut can have serious effects on the structural integrity and performance of the welded joint, such as:

Reduced strength

Undercut decreases a joint’s strength by creating a notch effect that concentrates stress at the weld toe. This can lead to fatigue cracking and failure under cyclic loading conditions.

Fatigue cracking is a form of damage that occurs when a material is subjected to repeated or fluctuating stresses that are lower than its ultimate tensile strength. Fatigue cracks usually initiate at stress concentrations, such as weld toes, and propagate until the material fails. Fatigue cracking can cause catastrophic failure of welded structures, such as pipelines, bridges, buildings, etc.

Increased corrosion

Undercut traps water and dirt along the weld toe, which can accelerate corrosion and deterioration of the weld. Corrosion is a chemical or electrochemical reaction between a material and its environment that causes degradation or loss of material.

Corrosion can compromise the durability and longevity of the welded joint, especially in corrosive environments, such as marine, chemical, or industrial applications. Corrosion can also cause secondary damage, such as stress corrosion cracking, hydrogen cracking, intergranular corrosion, or pitting corrosion.

Poor appearance

Undercut affects the aesthetic appeal of the weld by creating an irregular and uneven surface. This can lower the value and quality perception of the welded product, especially in applications where appearance is important, such as furniture, art, or consumer goods. Undercut can also make it difficult to apply coatings or paints on the weld surface, which can further affect the appearance and protection of the weld.

What are the Effects of Undercut in Welding?

Undercut in welding is a common defect that can affect the quality and strength of the weld. It is caused by various factors, such as wrong welding parameters, incorrect electrode and shielding gas selection, and poor welding techniques. It can have serious effects on the structural integrity and performance of the welded joint, such as:

Reduced strength

Undercut decreases a joint’s strength by creating a notch effect that concentrates stress at the weld toe. This can lead to fatigue cracking and failure under cyclic loading conditions.

Fatigue cracking is a form of damage that occurs when a material is subjected to repeated or fluctuating stresses that are lower than its ultimate tensile strength.

Fatigue cracks usually initiate at stress concentrations, such as weld toes, and propagate until the material fails. Fatigue cracking can cause catastrophic failure of welded structures, such as pipelines, bridges, buildings, etc.

Increased corrosion

Undercut traps water and dirt along the weld toe, which can accelerate corrosion and deterioration of the weld. Corrosion is a chemical or electrochemical reaction between a material and its environment that causes degradation or loss of material.

Corrosion can compromise the durability and longevity of the welded joint, especially in corrosive environments, such as marine, chemical, or industrial applications. Corrosion can also cause secondary damage, such as stress corrosion cracking, hydrogen cracking, intergranular corrosion, or pitting corrosion.

Poor appearance

Undercut affects the aesthetic appeal of the weld by creating an irregular and uneven surface. This can lower the value and quality perception of the welded product, especially in applications where appearance is important, such as furniture, art, or consumer goods. Undercut can also make it difficult to apply coatings or paints on the weld surface, which can further affect the appearance and protection of the weld.

How to Prevent Undercut in Welding?

Undercut in welding can be prevented by following some general tips and tricks, such as using the right current, voltage, speed, electrode angle, arc length, etc. However, different welding processes may require different methods to prevent undercut. Here are some specific methods to prevent undercut for some common welding processes:

Shielded Metal Arc Welding (SMAW)

SMAW is also known as stick welding or manual metal arc welding. It uses a consumable electrode coated with flux to create an electric arc between the electrode and the workpiece. To prevent undercut in SMAW, you should use a small diameter electrode with a low hydrogen content, such as E7018 or E6010. You should also use a short arc length and a slight drag angle to ensure good fusion and penetration.

Gas Metal Arc Welding (GMAW)

GMAW is also known as MIG welding or metal inert gas welding. It uses a continuously fed wire electrode and a shielding gas to create an electric arc between the wire and the workpiece.

To prevent undercut in GMAW, you should use a suitable wire diameter and type, such as ER70S-6 or ER308L. You should also use a proper voltage and wire feed speed to ensure a smooth and stable arc. You should also use a correct torch angle and travel speed to avoid excessive heat input and spatter.

Gas Tungsten Arc Welding (GTAW)

GTAW is also known as TIG welding or tungsten inert gas welding. It uses a non-consumable tungsten electrode and a shielding gas to create an electric arc between the electrode and the workpiece.

To prevent undercut in GTAW, you should use a clean and sharp tungsten electrode with a suitable diameter and tip shape, such as 2% thoriated or 2% lanthanated.

You should also use an appropriate current type and polarity, such as direct current electrode negative (DCEN) or alternating current (AC). You should also use a proper gas flow rate and nozzle size to ensure adequate shielding.

How to Detect and Measure Undercut in Welding?

Undercut in welding is a defect where a groove or depression runs along the toe of the weld, which is the junction between the weld metal and the base metal.

Undercut occurs when the base metal has melted and has not been properly filled by the filler metal, creating a weak spot in the weld. Undercut can affect the quality and strength of the weld, and can lead to cracks and failures under stress or corrosion.

There are various tools and equipment that can help detect and measure undercut in welding, such as welding gauges, inspection lights, liquid penetrant testing, etc. Some of these tools and equipment are:

Welding Gauges

Welding gauges are instruments that can measure various aspects of a weld, such as size, shape, angle, depth, etc. There are different types of welding gauges, such as fillet weld gauges, V-weld gauges, U-weld gauges, etc.

To measure undercut in welding, you can use a V-weld gauge or a U-weld gauge that has a notch or a groove that fits over the weld toe. You can then read the depth of the undercut from the scale on the gauge. 

For example, you can use the Insize 4836 Fillet Weld Gauge or the Insize 4832 Fillet Weld Gauge to measure concave or convex welds up to 25 mm.

Inspection Lights

Inspection lights are devices that can illuminate a weld and reveal its defects, such as cracks, porosity, slag inclusion, etc. There are different types of inspection lights, such as white light, ultraviolet light, infrared light, etc. To detect undercut in welding, you can use a white light or an ultraviolet light that can highlight the groove or depression along the weld toe.

You can then compare the weld with a standard or a reference to determine the severity of the undercut. For example, you can use the LAP Rechargeable LED Inspection Light or the Nebo Lil Larry LED Torch to illuminate the weld area.

Liquid Penetrant Testing

Liquid penetrant testing is a method that can reveal surface defects in a weld by applying a liquid dye that penetrates into the defects and then using a developer that draws out the dye and makes it visible. There are different types of liquid penetrants, such as visible dye penetrants, fluorescent dye penetrants, etc.

To detect an undercut in welding, you can use a visible dye penetrant or a fluorescent dye penetrant that can fill the undercut and then use a developer that can show the outline of the undercut. You can then measure the width and depth of the undercut using a magnifying glass or a ruler. For example, you can use the Magnaflux ZL-60D Fluorescent Penetrant or the Magnaflux SKD-S2 Visible Penetrant to perform liquid penetrant testing on welds.

Conclusion

Undercut in welding is a common defect that can affect the quality and strength of the weld. It is caused by various factors, such as wrong welding parameters, incorrect electrode and shielding gas selection, and poor welding techniques.

It can have serious consequences for the structural integrity and performance of the welded joint, such as reduced strength, increased corrosion, and poor appearance. It can be prevented by following some general tips and tricks, such as using the right current, voltage, speed, electrode angle, arc length, etc.

It can also be prevented by using some specific methods for different welding processes, such as SMAW, GMAW, GTAW, etc. It can be detected and measured by using various tools and equipment, such as welding gauges, inspection lights, liquid penetrant testing, etc.

FAQs

Here are some frequently asked questions about undercut in welding:

How much undercut is acceptable in welding?

The acceptable amount of undercut in welding depends on various factors, such as the type of weld, the material of the base metal and filler metal, the application of the welded joint, etc.

Generally speaking, undercut should be avoided as much as possible in welding because it reduces the strength and quality of the weld. However, some standards and codes may allow a certain amount of undercut depending on the situation.

For example, according to AWS D1.1/D1.1M:2020 Structural Welding Code – Steel, undercut shall not exceed 0.01 inch (0.25 mm) for groove welds and 0.03 inch (0.8 mm) for fillet welds.

How do you fix undercut in welding?

The best way to fix an undercut in welding is to prevent it from happening in the first place by following proper welding procedures and techniques. However, if an undercut has already occurred in welding, it can be fixed by grinding or filling it with filler metal.

Grinding is a method of removing excess or defective material from a weld by using an abrasive tool or machine. Filling is a method of adding filler metal to a weld by using another welding process or technique.

What is the difference between underfill and undercut in welding?

Underfill and undercut are both welding defects that affect the size and shape of the weld. However, underfill is a defect where there is insufficient filler metal at the root or face of the weld.

This creates a concave or hollow appearance in the weld that reduces its cross-sectional area. Underfill can be caused by low current, high travel speed, incorrect electrode angle, etc.

An undercut is a defect where there is a groove or depression along the toe of the weld. This creates a weak spot in the weld that can reduce its load-bearing capacity and cause cracks and failures. Undercut can be caused by high current, Low voltage, high travel speed, incorrect electrode angle, etc.

Thomas James

Thomas James is an experienced auto mechanic who enjoys writing comprehensive guides and offering valuable tips on various car issues.

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