All welds contain discontinuities. Even an expert welder can make mistake while welding. But the discontinuities become defect when it crosses the standard acceptance level. A flaw or discontinuities can be accepted or rejected based on welding prequalified standards.
These standards are set by expert inspectors or few welding quality control organizations like AWS (American Welding Society). To maintain the quality of the weld and make the project acceptable all over the world, these standards are followed very seriously.
Types of Discontinuities
Discontinuities can be either volumetric or planar. They can be internal or external defect as well.
- Volumetric: Discontinuities in three dimension (length, width and thickness) are called volumetric discontinuity. Such as slag and porosity.
- Planar: Two dimensional defects are planar. For example: lack of fusion, cracks.
- Internal: Discontinuities inside the weldment in internal damage. These type of defects can not be inspected by visual inspection. NDT like radiography or ultrasonic testing can detect the problems. For example: Lack of fusion, internal porosity etc.
- External: Discontinuities outside the metal or surface area of weldment is external damage. This type of defects can be detected by visual inspection or dye penetration. Example: Over-lap and spatter.
Flaws Or Discontinuities
We can classify the defects in following category. They are-
- Porosity
- Cracks
- Incomplete fusion
- Incomplete penetration
- Undercut
- Overlap
- Spatter
- Distortion
- Inclusion
- Under-fill
- Burn-through
- And excessive reinforcement
Lets discuss them briefly-
1. Porosity
- Description: Porosity in welding refers to the presence of small cavities or voids within the weldment. These voids are usually gas pockets trapped during the solidification of the molten weld metal. Porosity can be linear, localized, uniformly distributed or tubular in nature. Porosity can compromise the integrity and strength of the weld, making it essential to address and prevent.
- Causes: Contaminants such as moisture, oil, grease, or rust on the base metal can be held responsible for porosity. Also, improper shielding gas, or incorrect welding parameters are another reasons.
2. Cracks:
- Description: Cracks in welding refer to fractures or discontinuities in the weld metal. It is a serious defect as it can deform the weld structure. These cracks can compromise the structural integrity of the welded joint, leading to potential failure under stress. Welding cracks can manifest in various forms, including longitudinal, transverse, or crater cracks. Longitudinal is parallel to weld, transverse is at 90 degree to the weld and crater may be a star like in shape.
- Causes: Rapid cooling, incorrect termination of arc, excessive heat input, excessive joint restraint, high hydrogen levels, brittle parent metal, improper joint design, mismatched filler metal or stress concentrations are reasons for crack on weld.
3.Incomplete Fusion:
- Description: Incomplete fusion in welding occurs when there is a failure to achieve proper bonding between the weld metal and the base metal. This results in a lack of fusion at the interface, compromising the integrity of the weld joint. Incomplete fusion can cause
- Lack on integrity between base and filler metal at the root
- A lack of interfusion between base metals and filler metal adjacent layer
- A lack of fusion between parent and filler metal at the side of them
- Causes: Insufficient heat, improper joint preparation, incorrect electrode angle, mismatched weld perimeter or incorrect welding technique and incorrect joint design are responsible for fusion problem..
4. Incomplete Penetration:
- Description: Weld metal doesn’t fully penetrate the joint. Incomplete penetration in welding occurs when the weld metal does not fully extend through the thickness of the joint, leaving a gap or unfused area at the root of the weld. This can compromise the strength and integrity of the welded joint.
- Causes: Insufficient heat, inappropriate weld process, improper joint design, or incorrect welding parameters.
5. Undercut:
- Description: Undercutting is a type of weld defect characterized by a groove or depression along the base metal near the toe of the weld. It typically occurs on the sides of the weld or fusion face of the weld. It can compromise the structural integrity of the joint. Undercut is often identified by its V-shaped appearance.
- Causes: Excessive current, high travel speed, improper torch manipulation, excessive arc length or incorrect weld technique adoption can be held responsible for the penetration issue.
6. Overlap/Over-Roll:
- Description: Overlap is a welding defect characterized by the extension of weld metal beyond the toe of the previously deposited weld bead. It results in a layered appearance, where one weld bead partially covers the previous one. Overlap can weaken the weld joint.
- Causes: Excessive welding current, slow travel speed, or wrong torch angle can be the reasons of this problem.
7. Spatter:
- Description: Spatter refers to the small, unwanted droplets of molten metal that can be expelled during the welding process. These droplets can land on surrounding surfaces and create undesirable splatter. Spatter is not only aesthetically undesirable but can also lead to issues such as porosity and reduced weld quality if the droplets solidify within the weld or on adjacent materials.
- Causes: Excessive current input, incorrect shielding gas, or poor quality filler metal can be the reasons.
8. Welding Distortion:
- Description: Welding distortion refers to the deformation or misalignment of metal structures that occurs during or after the welding process. This distortion can manifest as bending, warping, or twisting of the welded components, and it is a common challenge in welding that can affect the dimensional accuracy and appearance of the final product.
- Causes: Uneven heating and cooling, improper fixturing, or inadequate tack welding can be the the reasons.
9. Inclusions:
- Description: Inclusions in welding refer to foreign materials or contaminants, trapped within the weld metal during the welding process. These inclusion can be metallic like tungsten inclusion or non-metallic like Sulphide & Oxide. Metallic inclusion can take various form including Linear, isolated or grouped and non-metallic are results of chemical reaction. Inclusions can negatively impact the integrity and strength of the weld as they can be volumetric or internal damage in nature.
- Causes: Contaminated base metal, improper cleaning, incorrect weld parameter or poor quality welding consumables are the reasons behind inclusion.
10. Underfill:
- Description: Underfill in welding refers to a condition where the weld metal does not adequately fill the groove or joint, leaving a depression or gap at the root of the weld. This one is external damage and can be easily detected by visual inspection. This incomplete filling can compromise the strength and integrity of the weld joint.
- Causes: Inadequate filler metal deposition, high travel speed, high welding current input, or improper joint preparation can be the reasons.
11. Burn-Through:
- Description: Burn-through in welding refers to the excessive penetration of the weld metal, resulting in a hole or opening across the base metal. This condition occurs when the heat input is too high or when welding thin materials, and it can compromise the integrity and strength of the weld joint. It is also called met-through in welding application.
- Causes: Excessive heat, high welding current, or thin base metal can be the reasons.
12. Excess Reinforcement:
- Description: Excess reinforcement in welding refers to a welding defect where the weld bead extends beyond the specified dimensions, creating an additional convexity or bead on the weld joint. It is external damage and can be inspected visually.
- Causes: Excessive current input and slow travel speed can be responsible for the problem.
How To Test The Defects
The easiest way is to visually looking for the defects. There are also few non-destructive methods dye penetration, magnetic particle inspection, radiographic testing, ultrasonic testing and few more testing techniques. I have written separate article on different tests to qualify weld quality. Check it out now.
Lastly
Welding is a technical work and like every technical job it may have some defects. You must know which defect are acceptable and which are not and to what extend? So be cautious about the weld quality when you are working in a big welding project.
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