What Causes Undercut in Welding

Introduction

While welding, it is essential to achieve a strong bond between the pieces. One of the common issues faced during welding is undercut. Undercut is the depression formed at the edge of the welded joint, which reduces the strength of the joint. In this article, we will discuss what causes undercut in welding.

Types of Undercut in Welding

Undercut is a common welding defect that occurs when the molten metal does not properly fill the weld joint, resulting in a groove or depression on the base metal adjacent to the weld toe. Undercut can manifest in different forms, each with its own characteristics and causes. Understanding the types of undercut is crucial in identifying and addressing this welding defect effectively.

1. Linear Undercut

Linear undercut is the most common type of undercut and appears as a continuous groove along the length of the weld bead. It is often caused by improper manipulation of the welding torch or electrode, such as weaving too widely or moving too quickly. Insufficient molten metal deposition leads to inadequate sidewall fusion and the formation of a recessed area.

2. Pitting Undercut

Pitting undercut is characterized by localized depressions or pits within the weld bead. It is usually caused by the presence of contaminants, such as oil, grease, rust, or moisture on the surface of the base metal. When these contaminants vaporize during welding, they create small cavities and result in pitting undercut. Proper cleaning and preparation of the joint can help prevent this type of undercut.

3. Root Undercut

Root undercut occurs at the root of a weld joint, where the base metal meets the root pass. It appears as a groove or depression on the inside surface of the joint. Root undercut is commonly caused by improper joint fit-up or inadequate root penetration. Insufficient heat input or improper welding technique can contribute to this type of undercut. Proper joint preparation and ensuring adequate root penetration are essential in preventing root undercut.

4. Sidewall Undercut

Sidewall undercut is formed along the sidewalls of the weld joint. It appears as a groove or depression running parallel to the weld bead. Sidewall undercut is often caused by improper electrode angle, excessive welding current, or high welding speed. Insufficient sidewall fusion leads to the formation of this type of undercut. Proper manipulation of the welding torch or electrode and optimizing welding parameters can help prevent sidewall undercut.

5. Toe Undercut

Toe undercut occurs at the toe of the weld bead, where it meets the base metal. It appears as a groove or depression at the edge of the weld. Toe undercut can be caused by various factors, including improper electrode angle, excessive welding current, inadequate heat input, or improper joint fit-up. Careful control of welding parameters and ensuring proper joint preparation can help minimize toe undercut.

Causes of Undercut in Welding

Undercut can be caused by various factors, some of which are listed below:

1. Excessive Welding Current

Using excessively high welding current can lead to undercut. When the current is too high, the weld pool becomes excessively fluid, causing the molten metal to flow away from the weld groove. This can result in insufficient fusion and the formation of undercut along the weld toe. Proper adjustment of welding current is crucial to prevent undercut.

2. High Welding Speed

Welding at high speeds can contribute to undercut formation. When the welding speed is too fast, the molten metal does not have sufficient time to flow and fill the groove properly. As a result, undercut can occur along the weld toe. It is important to maintain an appropriate welding speed to ensure proper fusion and minimize undercut.

3. Improper Electrode Angle

The angle at which the welding electrode is held plays a significant role in undercut formation. If the electrode angle is too steep or too shallow, it can lead to inadequate sidewall fusion and the formation of undercut. Maintaining the correct electrode angle ensures proper heat distribution and promotes complete fusion along the weld toe.

4. Incorrect Arc Length

The arc length, which is the distance between the electrode tip and the workpiece, affects the heat input and penetration during welding. An incorrect arc length, either too short or too long, can result in undercut. A short arc length can cause excessive heat concentration, leading to insufficient sidewall fusion and undercut. Conversely, a long arc length may result in insufficient heat input and inadequate fusion. Maintaining an optimal arc length is crucial to prevent undercut.

5. Insufficient Filler Metal

Inadequate deposition of filler metal can contribute to undercut. If there is insufficient filler metal added to the weld pool, it may not fully fill the groove or joint, leaving gaps and resulting in undercut. Proper control of the filler metal deposition and ensuring adequate volume of molten metal in the weld pool is essential to prevent undercut.

6. Joint Misalignment or Poor Fit-Up

Improper joint alignment or poor fit-up can lead to undercut. If the joint parts are misaligned or have gaps between them, it can hinder proper fusion and result in undercut. It is crucial to ensure accurate fit-up and alignment of the joint parts to minimize the risk of undercut formation.

7. Incorrect Shielding Gas

The type and composition of shielding gas used in welding can also affect undercut formation. In some cases, using an incorrect or inadequate shielding gas may result in excessive oxidation, poor protection of the weld pool, and the formation of undercut. It is important to use the appropriate shielding gas for the welding process and material being welded.

8. Poor Welding Technique

Lastly, poor welding technique can contribute to undercut formation. Factors such as improper manipulation of the welding torch, inconsistent travel speed, or erratic movements can result in uneven heat distribution and inadequate fusion, leading to undercut. It is important to maintain proper welding technique and follow established best practices to minimize the risk of undercut.

Preventing Undercut in Welding

Undercut can be prevented by taking the following measures:

1. Proper Joint Preparation

Thoroughly clean the base metal before welding to remove any contaminants, such as oil, grease, rust, or moisture. Properly prepare the joint by ensuring accurate fit-up and appropriate root gap. Good joint preparation helps promote better fusion and reduces the risk of undercut.

2. Control Welding Parameters

Optimize welding parameters, including current, voltage, travel speed, and electrode angle, based on the specific welding process and material being welded. Proper control of these parameters helps ensure adequate heat input and deposition of molten metal, minimizing the chances of undercut formation.

Parameters of Welding

3. Use Proper Welding Technique

Adopt suitable welding techniques for the specific joint configuration and welding process. Maintain a consistent travel speed and avoid excessive weaving or oscillation, as it can lead to inconsistent heat distribution and potential undercut. Follow recommended welding procedures to achieve optimal weld quality.

4. Adequate Electrode Manipulation

Proper manipulation of the welding electrode is crucial in preventing undercut. Maintain a steady arc length and ensure consistent contact between the electrode and the base metal. Avoid excessive push or pull angles that can contribute to inadequate sidewall fusion and potential undercut.

5. Preheat when Required

In certain cases, preheating the base metal before welding can help minimize the risk of undercut. Preheating reduces the temperature gradient between the weld zone and the surrounding metal, allowing for better control of heat distribution and reducing the chances of undercut formation.

6. Perform Adequate Root Penetration

Ensure proper root penetration to avoid root undercut. Adjust welding parameters and technique to achieve adequate fusion at the root of the joint. Employ suitable joint design and fit-up to facilitate proper root penetration and minimize the chances of undercut.

7. Inspect and Evaluate Welds

Regularly inspect the welds to identify any signs of undercut or other defects. Perform visual inspections, and if necessary, utilize non-destructive testing methods to evaluate the integrity of the weld joints. Promptly address any identified undercut issues to prevent further propagation and potential failure.

Exploring the Permissible Limit of Undercut in Welding

While it is generally desirable to minimize undercut, there is an acceptable amount of undercut that can be tolerated based on industry standards and requirements.

The acceptable amount of undercut can vary depending on several factors, including the specific welding application, the material being welded, and the applicable welding codes or standards. Here are some general guidelines regarding the acceptable amount of undercut:

1. Welding Codes and Standards

Welding codes and standards provide guidelines and specifications for acceptable weld quality. They often define the maximum allowable undercut depth or percentage based on the welding process, material type, and joint configuration. These standards ensure that the weld joint meets the required strength and integrity criteria. It is essential to refer to the relevant welding codes and standards to determine the acceptable amount of undercut for a specific welding application.

2. Application and Structural Requirements

The acceptable amount of undercut can also depend on the intended application and structural requirements of the welded component. In some cases, minor undercut may be acceptable if it does not compromise the functionality, strength, or appearance of the weld joint.

However, in critical applications or structural components, even minimal undercut may be considered unacceptable. The specific requirements of the application and the judgment of the responsible engineer or inspector play a significant role in determining the acceptable amount of undercut.

3. Visual Inspection Criteria

Visual inspection is commonly used to assess the quality of welds, including the presence of undercut. Visual inspection criteria, such as those outlined in welding standards or inspection guidelines, can provide guidance on the acceptable limits of undercut. Typically, the depth or width of undercut is measured and compared to the specified limits. If the undercut falls within the acceptable range, it may be considered within the acceptable amount.

4. Weld Joint Considerations

The type of weld joint and its intended purpose can influence the acceptable amount of undercut. For example, in fillet welds, slight undercut along the toe of the weld may be tolerated as long as it does not compromise the overall strength of the joint. On the other hand, in groove welds or welds subjected to higher stress or load, the acceptable amount of undercut may be more restricted to ensure adequate fusion and structural integrity.

It is important to note that while there may be acceptable limits for undercut, it is generally desirable to minimize undercut as much as possible to ensure optimum weld quality and strength. Welders should strive for proper welding technique, parameter control, and joint preparation to minimize the occurrence of undercut and produce high-quality welds.

How to remove Undercuts For A Bad Weld?

Undercuts in welding can compromise the integrity and strength of a weld joint. When a bad weld with undercuts is identified, it is essential to take corrective measures to remove the undercuts and ensure a sound weld. Here are some tips on removing undercuts for a bad weld:

1. Grind and Fill

One common method to remove undercuts is by grinding down the affected area using an angle grinder or other suitable grinding tools. Care should be taken to remove the undercut completely without grinding too much of the base material. After grinding, the undercut area can be filled by re-welding or using suitable filler material. This process helps to fill the groove or depression caused by the undercut and creates a smooth and solid weld.

2. Build-Up Welding

If the undercut is extensive or deep, it may be necessary to perform build-up welding to restore the proper shape and dimensions of the weld joint. Build-up welding involves depositing additional weld metal in the undercut area to fill the groove and create a uniform surface. This process requires careful control of welding parameters, including heat input and filler material selection, to ensure proper fusion and minimize the risk of new defects.

3. Back Gouging

In some cases, particularly for thicker materials, back gouging can be an effective method to remove undercuts. Back gouging involves removing the weld metal from the backside of the joint using grinding or gouging tools. By removing the excess weld metal, the undercut is eliminated, and a clean surface is obtained. However, this method should be applied with caution, considering the material thickness, joint design, and structural requirements.

4. Re-welding

In situations where the undercut is minimal or shallow, re-welding the affected area may be sufficient to remove the undercut. The welder should carefully prepare the joint by cleaning and beveling the edges, ensuring proper fit-up, and adjusting welding parameters to achieve complete fusion and proper reinforcement. Re-welding allows for the filling of the undercut and the creation of a sound weld joint.

It is important to note that removing undercuts from a bad weld requires skill, experience, and knowledge of proper welding techniques. It is recommended to consult with qualified welders or welding inspectors to ensure the appropriate corrective actions are taken. Additionally, preventive measures should be implemented to minimize the occurrence of undercuts, such as proper joint preparation, suitable welding parameters, and adequate quality control during the welding process.