Discontinuity in the weld.

Scary Weld Discontinuity | A Practical Guide to Perfect Weld

In Understanding Weld Discontinuities by Matthew BehlenLeave a Comment

Introduction

Weld discontinuities are defects or irregularities in a weld that can compromise the strength and integrity of the weldment. They can be caused by a variety of factors, including poor welding technique, incorrect welding parameters, or the use of contaminated materials.

Weld discontinuities can be classified into two main types: geometric discontinuities and metallurgical discontinuities. Geometric discontinuities are defects in the weld geometry, such as cracks, porosity, and undercuts. Metallurgical discontinuities are defects in the microstructure of the weld, such as inclusions, segregation, and hardness variations.

Weld discontinuities can have a significant impact on the performance of a weldment. They can reduce the strength and toughness of the weld, and they can also make the weldment more susceptible to corrosion and fatigue. In some cases, weld discontinuities can even lead to catastrophic failure of the weldment.

It is important to be able to identify and understand weld discontinuities in order to ensure the safety and reliability of weldments. This article provides an overview of the different types of weld discontinuities, how they are caused, and how they can be prevented and repaired.

Weld Discontinuity Definition

A weld discontinuity is any imperfection in a weld that can adversely affect the properties of the weld or the structure in which it is used. Weld discontinuities can be caused by a variety of factors, including:

  • Improper welding procedures
  • Inadequate welding materials
  • Poor weld joint design
  • Contamination
  • Mechanical damage

Weld discontinuities can be classified into two main categories:

  • Structural discontinuities
  • Metallurgical discontinuities

Structural discontinuities are those that can affect the strength or integrity of the weld. Metallurgical discontinuities are those that do not affect the strength of the weld, but can cause corrosion or other problems.  This is why it is very important to perform excellent electrode and filler metal identification for the joint being welded.

Weld discontinuities can be detected by visual inspection, radiographic inspection, ultrasonic inspection, or magnetic particle inspection.

The prevention of weld discontinuities is essential to ensure the quality of the weld and the safety of the structure in which it is used. Welding procedures should be followed carefully, welding materials should be of good quality, and weld joint design should be appropriate for the application. Contamination should be avoided, and welds should be protected from mechanical damage.

Types of Weld Discontinuities

Weld discontinuities can be classified into two main categories: geometric discontinuities and metallurgical discontinuities.

Geometric discontinuities are those that affect the geometry of the weld, such as cracks, porosity, and undercut.

Metallurgical discontinuities are those that affect the metallurgical properties of the weld, such as inclusions, segregation, and lamination tears.

The following is a list of the most common types of weld discontinuities:

  • Cracks
  • Porosity
  • Undercut
  • Incomplete Fusion of Lack of Fusion
  • Overlap or Cold Lap
  • Lamination
  • Inclusions (Slag Inclusion)

For more information on the different types of weld discontinuities, please refer to the following resources:

Discontinuity in the weld.

Lack of Fusion discontinuity located within the two weld passes.

Consequences of Weld Discontinuities

Weld discontinuities can have a number of consequences, including:

Reduced strength and toughness of the weldment
Increased susceptibility to corrosion and cracking
Reduced fatigue life
Increased stress concentrations
Reduced aesthetics

In some cases, weld discontinuities can also lead to catastrophic failure of the weldment.

Detection of Weld Discontinuities

The detection of weld discontinuities is an important part of the welding process. Weld discontinuities can reduce the strength and integrity of a weld, and they can also lead to leaks or other problems. There are a number of different methods that can be used to detect weld discontinuities, including:

  • Visual inspection
  • Magnetic particle inspection
  • Liquid penetrant inspection
  • Radiographic inspection
  • Ultrasonic inspection

The choice of inspection method will depend on the type of weld discontinuity that is suspected, the size of the weld, the code requirements, the Engineer of Record, and the environmental conditions.

Visual inspection is the simplest and least expensive method of detecting weld discontinuities that is usually performed by an AWS CWI. It involves visually inspecting the weld for any defects, such as cracks, porosity, or slag inclusions.

Magnetic particle inspection is a non-destructive testing method that uses magnetic fields to detect discontinuities in ferromagnetic materials. The weld is first magnetized, and then a fine powder is sprinkled over the surface. The powder will collect in areas where there are discontinuities, creating a visible indication of the defect.

Liquid penetrant inspection is a non-destructive testing method that uses a liquid penetrant to detect discontinuities in the surface of a material. The penetrant is applied to the surface of the weld, and then allowed to penetrate into any discontinuities. The penetrant is then removed, and a developer is applied to the surface. The developer will cause the penetrant to bleed out of the discontinuities, creating a visible indication of the defect.

Radiographic inspection is a non-destructive testing method that uses X-rays or gamma rays to create an image of the weld. The weld is placed between an X-ray source and a film or detector. The X-rays or gamma rays will pass through the weld, and the film or detector will record the image of the weld. Discontinuities in the weld will appear as dark areas on the film or detector.

Ultrasonic inspection is a non-destructive testing method that uses ultrasonic waves to detect discontinuities in a material. The ultrasonic waves are directed at the weld, and the reflected waves are analyzed to identify any discontinuities. Discontinuities in the weld will cause a change in the reflected waves, which can be detected by the ultrasonic inspection equipment.

The detection of weld discontinuities is an important part of the welding process. By using the appropriate inspection method, weld discontinuities can be identified and repaired before they cause any problems.

Prevention of Weld Discontinuities

There are a number of things that can be done to prevent weld discontinuities, including:

  • Using qualified welders and welding procedures
  • Ensuring that the base metal is clean and free of defects
  • Using the correct welding parameters
  • Monitoring the weld process closely
  • Inspecting the welds after they have been completed

By following these steps, it is possible to significantly reduce the risk of weld discontinuities occurring.

Repair of Weld Discontinuities

There are a variety of methods available for repairing weld discontinuities. The most common methods include:

  • Mechanically removing and re-welding
  • Thermally removing and re-welding
  • Adding weld
  • Use of Backing Strips (copper or steel)
  • Carbon Arc Gouging and re-welding

The best method for repairing a particular weld discontinuity will depend on the type of discontinuity, the size of the discontinuity, and the location of the discontinuity.

Re-welding is the most common method of repairing weld discontinuities. It involves melting the weld metal and re-welding the discontinuity. Re-welding is a good option for repairing small discontinuities that are not located in a critical area.

Backing bars are used to support the weld metal during welding. They are typically made of steel or aluminum and are placed behind the weld joint. Backing bars can help to prevent undercuts and other weld defects.

Groove welds are used to repair discontinuities that are located in the root of a weld joint. Groove welds are typically made with a backing bar to support the weld metal.

Backing strips are used to support the weld metal during welding. They are typically made of either the parent base metal or designed to not fuse like copper backing bars welding mild steel. Backing strips can help to prevent lack of fusion and other weld defects.

Backing rings are used to support the weld metal during welding. They are typically made of steel or aluminum and are placed around the weld joint. Backing rings can help to prevent lack of fusion and other weld defects like incomplete penetration.

Conclusion

Weld discontinuities are a common occurrence in welding, but they can be prevented and repaired. By understanding the different types of weld discontinuities, how they are caused, and how to identify them, you can take steps to prevent them from occurring in your own welds. If weld discontinuities do occur, you can repair them using the appropriate methods.

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