Welding Imperfections and Classification – Part 2: Cavities

Cavities in welding are imperfections characterized by the presence of voids or holes within the weld metal or at the interface between the weld and base metal. These voids form when gas or shrinkage prevents full fusion or sound metal consolidation during solidification.

Cavities are among the most common weld discontinuities and can significantly reduce the strength and integrity of welded joints if not properly controlled.

According to ISO 6520-1:2021 and AWS A3.0M/A3.0:2020, cavities are void-type imperfections that form in the weld metal or at the junction between the weld and the base metal. They occur when gas becomes trapped or when the metal shrinks during solidification, creating separate or connected empty spaces such as pores, blowholes, or shrinkage cavities.

Cavities are classified under Group 2 (Reference No. 200) in ISO 6520-1, which includes gas pores, wormholes, shrinkage cavities, and crater pipes.

See:

ISO 6520-1:2021, Clause 4.2 – Group 2: Cavities
AWS A3.0:2020, Definition of “Cavity” – “A void or depression resulting from trapped gas or shrinkage during solidification.”

Types of Cavities in Welding

1. Gas Pores

Small rounded or elongated voids formed by trapped gas during solidification.
The gas can originate from moisture, oil, rust, or contamination on the joint surface or filler material.
Common gases include hydrogen, carbon dioxide, or nitrogen.

Subtypes:

Uniformly distributed porosity: Evenly spread small pores throughout the weld metal.
Cluster porosity: Group of pores localized in one region.
Surface pores: Visible on the weld surface after solidification.

Common causes:

Contaminated base metal or electrode.
Excessive welding speed or inadequate shielding gas.
Moisture in flux or electrode coating.

2. Wormholes (Elongated Gas Cavities)

Elongated or tunnel-like gas cavities formed by gas entrapment during solidification.
Usually appear near the weld surface or within the weld bead.
Common in processes such as flux-cored arc welding or submerged arc welding when flux moisture is present.

Causes:

Excessive moisture or impurities in flux.
Improper current or travel speed leading to turbulence in the molten pool.
Poor gas shielding or flux coverage.

3. Shrinkage Cavities

Irregular voids caused by metal contraction during solidification when insufficient molten metal remains to fill the shrinkage volume.
Typically located at the weld crater or in the centre of thick sections.

Causes:

High cooling rate.
Improper crater filling at arc termination.
Inadequate filler addition or poor joint design.

4. Pipe Cavities (Shrinkage Pipes)

A specific type of shrinkage cavity that forms as a tubular or funnel-shaped void extending from the surface into the weld interior, often found at the end of a weld bead.
Indicative of poor crater filling or rapid solidification.

Causes:

Premature arc break without crater fill.
High welding current or heat input imbalance.

Common Causes of Cavities

Contaminated base metal, filler, or shielding gas.
Excessive welding speed causing gas entrapment.
Moisture in flux, electrodes, or gas lines.
Inadequate shielding gas coverage.
Improper crater filling or rapid cooling.

Prevention Measures

Keep electrodes, flux, and filler materials dry and clean.
Use proper gas flow rate and nozzle angle for shielding.
Maintain adequate preheat to slow cooling for critical materials.
Avoid high travel speeds that prevent gas escape.
Properly fill the crater before arc termination.
Apply post-weld inspection (radiographic or ultrasonic) for detection of internal cavities.

Cavities illustrations

**Gas pore, scattered and cluster porosities :** **gas pore** when the dimension is less than 1.5 mm, and **blow hole** when the pore is greater than 1.5 mm

**Surface breaking porosity – Linear porosity**

**Wormholes** – the nature of the worm hole may vary depending on the welding process, gas type and flow rate.

Designation of Cavities in Accordance with ISO 6520-1

Cavity-type imperfections are grouped under Group No. 2 (Reference No. = 200) in ISO 6520-1.

200 – Cavity (General Term)

A general description for any void, pore, or gas pocket within the weld metal or at the fusion boundary caused by gas entrapment or shrinkage during solidification.

Example designation:
Imperfection ISO 6520-1-200

201 – Gas Cavity

A small rounded or elongated void formed by entrapped gas.

Sub-categories based on appearance:

2011 – Gas pore: gas cavity of essentially spherical form.

2012 – Uniformly distributed porosity: Gas pores distributed fairly uniformly throughout the weld metal; differs from clustered porosity (2013), where pores are localized, and linear porosity (2014), where pores are aligned along the weld.

2013 – Clustered pores: A group of gas pores located close together in an irregular or random pattern within the weld metal.

2014 – Linear porosity: Series of gas pores arranged in a row, approximately parallel to the weld axis.

2015 – Elongated cavity: Large, non-spherical cavity oriented with its major axis approximately parallel to the weld axis.

2016 – Wormhole: Tubular or elongated gas cavity formed in the weld metal by release of entrapped gas.

Porosity or worm hole in weld, Defect of welding process Flux Cored Arc Welding (FCAW)

Common causes of wormhole: Gas entrapment due to flux decomposition or poor shielding.

The shape and position of wormholes depend on the solidification pattern and the origin of the gas. They commonly occur in clusters arranged in a herringbone pattern, and some may extend to or break the weld surface.

2017 – Surface pore: Single gas pore that breaks the surface of the weld.
2018 – Surface porosity: One or more gas pores visible at the surface of the weld; consists of single or multiple surface-breaking cavities.

Detection methods: Radiography or visual inspection for surface pores.

Example designation:
Imperfection ISO 6520-1-2011 (Gas pore)

202 – Shrinkage cavity: Cavity formed by metal contraction during solidification. In other words, a cavity formed due to insufficient metal to compensate for contraction during solidification.

Sub-categories based on appearance:

2021 – Interdendritic shrinkage: Elongated shrinkage cavity, sometimes containing entrapped gas, formed between dendrites during solidification; generally oriented perpendicular to the weld face.

2024 – Crater pipe: Shrinkage cavity formed at the end of a weld run and not removed or filled before subsequent welding. In other words, a funnel-shaped shrinkage cavity at the end of a weld bead, typically resulting from improper crater filling.

2025 – End crater pipe: Open crater at the weld end containing a hole that reduces the weld cross-section.

203 – Micro-shrinkage: Shrinkage cavity visible only under a microscope.

2031 – Interdendritic microshrinkage: Elongated microshrinkage cavity formed between dendrites during solidification, generally following grain boundaries.

2032 – Transgranular microshrinkage: Elongated microshrinkage cavity extending across grains during solidification.

Imperfection ISO 6520-1-203 (Micro-shrinkage)

Full Description of Cavities (ISO 6520-1 with Letter Codes)

Following the Crack trend, cavity descriptions can be expanded by combining the numerical code from ISO 6520-1 with a letter code that defines the mechanism or cause of cavity formation. Note that this is not included in the ISO 6520-1 standard.

General Format

Imperfection ISO 6520-1-[Number] F[Letter]

Where:

[Number] = The ISO 6520-1 classification number for the cavity (e.g., 2011, 2024, 2031).
F[Letter] = The additional code describing the origin or mechanism of the cavity.

Letter Codes for Cavities

Letter Code	Cavity Type	Typical Mechanism
Fa	Gas pore (2011)	Gas entrapment during solidification.
Fb	Uniformly distributed porosity (2012)	Even gas dispersion in the weld pool.
Fc	Clustered pores (2013)	Localized gas accumulation due to poor gas shielding.
Fd	Linear porosity (2014)	Aligned gas entrapment along the weld path.
Fe	Elongated cavity (2015)	Irregular gas release along solidification lines.
Ff	Wormhole (2016)	Gas channel formation during solidification.
Fg	Surface pore (2017)	Gas release that opens at the weld surface.
Fh	Surface porosity (2018)	Gas escape causing multiple surface-breaking pores.
Fi	Shrinkage cavity (202)	Metal contraction during solidification.
Fj	Interdendritic shrinkage (2021)	Shrinkage between dendrites during cooling.
Fk	Crater pipe (2024)	Incomplete crater filling at weld termination.
Fl	End crater pipe (2025)	Open crater left unfilled at weld end.
Fm	Micro-shrinkage (203)	Localized solidification shrinkage at micro level.
Fn	Interdendritic microshrinkage (2031)	Shrinkage between dendrites within grains.
Fo	Transgranular microshrinkage (2032)	Shrinkage across grain boundaries.
Fp	Blowhole (Unclassified)	Trapped gas unable to escape before solidification.

Usage Example (Full Designation Format)

Gas pore due to entrapped gas: Imperfection ISO 6520-1-2011 Fa
Clustered porosity: Imperfection ISO 6520-1-2013 Fc
Wormhole from flux moisture: Imperfection ISO 6520-1-2016 Ff
Shrinkage cavity: Imperfection ISO 6520-1-202 Fi
Crater pipe: Imperfection ISO 6520-1-2024 Fk
Microshrinkage: Imperfection ISO 6520-1-203 Fm
Blowhole: Imperfection ISO 6520-1-205 Fp

Frequently Asked Questions (FAQs)

Welding Imperfections and Classification – Part 2: Cavities

Q1. What is the main difference between gas porosity and shrinkage cavities?

Gas porosity results from trapped gas bubbles during solidification, while shrinkage cavities form due to metal contraction when there is not enough molten metal to fill the space created as the weld solidifies. Gas pores are typically rounded, whereas shrinkage cavities are irregular or elongated in shape.

Q2. How do blowholes differ from ordinary gas pores?

A blowhole is a large, isolated gas cavity often visible to the naked eye or on radiographs. In contrast, gas pores are smaller and may occur singly, in clusters, or in linear formations. Both result from gas entrapment, but blowholes represent a more severe form of gas porosity.

Q3. What causes wormholes in welding?

Wormholes occur when gas becomes trapped and travels through the molten weld pool, forming tubular or tunnel-shaped cavities. They are commonly caused by moisture in flux, improper shielding, or excessive contaminants that generate gas during solidification.

Q4. Why do shrinkage cavities often form at the end of weld runs?

At the end of a weld bead, if the molten crater is not properly filled before the arc is broken, metal contraction during cooling leaves a void known as a crater pipe. This defect can be avoided by gradually reducing the welding current or filling the crater before arc termination.

Q5. Can porosity always be completely eliminated?

No, it cannot be completely eliminated, but it can be minimized. Proper cleaning of materials, dry electrodes and flux, adequate gas shielding, and controlled welding parameters can significantly reduce porosity formation.

Q6. Which inspection methods are best for detecting cavities?

Radiographic Testing (RT): Ideal for detecting internal gas pores, blowholes, and shrinkage cavities.
Ultrasonic Testing (UT): Useful for identifying larger internal voids in thicker materials.
Visual and Dye Penetrant Testing (PT): Effective for detecting surface-breaking pores or surface porosity.

Q7. How do cavities affect weld strength and performance?

Cavities act as stress concentrators, reducing the effective cross-section of the weld and potentially initiating cracks under cyclic or tensile loading. Large or interconnected cavities can lead to leakage in pressure vessels or structural failure in load-bearing welds.

Q8. What are the acceptable limits for porosity in welds?

Acceptance criteria depend on the applicable standard (e.g., ISO 5817, ASME Section IX, or EN 13445) and the weld quality level. Generally, small, isolated pores may be acceptable, while clustered or aligned porosity is more restrictive due to its impact on joint strength.

Q9. How can moisture and contamination cause cavities?

Moisture, oil, rust, or paint on the base metal or electrode coating release hydrogen and other gases when heated. These gases become trapped in the molten pool, forming gas pores or wormholes during solidification. Clean, dry materials and proper storage of consumables are essential to prevent this.

Q10. How are cavities designated in inspection reports?

Cavities are identified using their ISO 6520-1 designation number (for form and location) and, when applicable, a letter code indicating their cause.
For example:

Gas pore caused by entrapped gas: Imperfection ISO 6520-1-2011 Fa
Crater pipe due to incomplete crater fill: Imperfection ISO 6520-1-2024 Fk

Summary Note

Cavities are gas or shrinkage-related voids that reduce weld soundness. Understanding their formation, classification, and inspection requirements under ISO 6520-1 and ISO 5817 allows accurate identification, reporting, and control to maintain weld quality and structural integrity.