Acceptable Levels of Undercut in Welding

Welding is a process of joining two or more metal components by melting a filler metal. During welding, various welding defects occur, with undercut being a common one.

This article will explore the concept of undercut, its impacts, and acceptable undercut levels in different situations.

Understanding the Undercut Phenomenon

Undercut, technically known as “undercutting” or simply “undercut,” is a welding defect that occurs when the weld metal fails to fuse properly with the base metal along the weld toe. This leads to the formation of a groove or depression in the base metal, compromising the structural integrity of the weld. 

Welding undercut occurs due to various factors, including:

  1. Excessive welding current or voltage leading to overheating.
  2. Improper welding technique, such as incorrect travel speed or electrode angle.
  3. Inadequate filler material deposition.
  4. Poor fit-up between the base metals.

Hazards of Undercut in Welding

Excessive undercut can have serious consequences on the structural integrity of the welded joint. It reduces the effective cross-sectional area of the base metal, which can lead to stress concentration in the area of undercut. This, in turn, can increase the risk of cracking and failure of the welded joint.

Additionally, excessive undercut can affect the aesthetic quality of the welding joint, diminishing the visual appeal of the weld seam.

Factors Determining Acceptability of Undercut

The acceptability of undercut in welding depends on several factors, including the material type, welding process, and the applicable standards or specifications. Different materials and welding processes have different tolerances for undercut, and the applicable standards or specifications often provide specific guidelines on the acceptable levels of undercut.

Acceptable Undercut Standards

In many cases, the acceptability of undercut is governed by industry standards or specifications. For example, the GB50236-98 standard in China classifies undercut into three levels:

  1. Level 1 – No undercut is allowed.
  2. Level 2 – The undercut depth should not exceed 0.05δ (where δ is the thickness of the base metal) and the continuous length should not exceed 100 mm. Additionally, the total length of undercut on both sides of the weld should not exceed 10% of the total weld length.
  3. Level 3 – The undercut depth should not exceed 0.1δ and the length is not specifically limited.

The acceptable level of undercut depends on the specific requirements of the welding joint, the application scenario, and standard regulations. Generally, undercut should be minimized to ensure the quality and performance of the welding joint. 

Remedial Measures for Undercut

To mitigate the risk of undercut, it is essential to adhere to proper welding parameters and techniques. 

This includes selecting the appropriate welding current, arc length, welding speed, and electrode angle based on the material type and thickness. Additionally, proper training and certification of welders is crucial to ensure that they are capable of executing the welding process correctly and minimizing the risk of undercut.

Evaluation and Inspection Methods

Inspecting welds for undercut involves visual examination, supplemented by non-destructive testing (NDT) methods such as:

  1. Visual inspection: Assessing weld profile and dimensions to identify undercut.
  2. Ultrasonic testing (UT): Detecting internal defects, including undercut, using high-frequency sound waves.
  3. Radiographic testing (RT): Imaging internal weld features to evaluate undercut severity and extent.
  4. Magnetic particle testing (MT) and dye penetrant testing (PT): Detecting surface-breaking defects, including undercut, using magnetic fields or fluorescent dyes.


In conclusion, the acceptability of undercut in welding is a matter that requires careful consideration and adherence to applicable standards or specifications. Effective inspection, evaluation, and corrective measures are integral to preventing and addressing undercut defects, thereby ensuring the reliability and performance of welded components and structures.

If you are looking for a reliable partner for your sheet metal welding needs, BOYI is the right choice. We invite you to contact us today to discuss your project and let us demonstrate our capabilities and commitment to excellence. Together, we can achieve your manufacturing goals and create a lasting partnership.

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