The primary objective of welding science is to produce a joint whose properties are as close as possible to those of the base material. Achieving this is almost impossible unless the weld can reproduce the same thermal conditions and material deformation history that occurred during mill processing. The greatest innovation in welding would therefore be the development of a methodology that would eliminates defects in every weld. Innovation in AI-assisted welding control, adaptive arc sensing, real-time NDT, and automated seam tracking is expected to help approach this high-end defect elimination.

However, due to material variability, phase transformations, technological limitations, human factors, and complex geometries, absolute defect elimination remains impossible, even with the most advanced welding technologies. In other words, achieving completely defect-free fusion welds is practically impossible. Since this goal is unattainable, we must work with the imperfections that remain after welding. Inspection, qualification, and repair processes will therefore continue to be integral parts of the welding quality system.

Weld defects are particularly costly when components are intended for service in fatigue-critical environments. Fatigue performance is extremely sensitive to even small surface or subsurface imperfections, because the discontinuities act as stress concentrators, accelerating crack initiation. This is why weld toe grinding, peening, and fatigue enhancement techniques are often used on fatigue-critical welds.

The primary purpose of the whole NDT industry is to detect, characterize, and size discontinuities or imperfections in welds, and to verify their conformance to specified acceptance criteria. Unfortunately, NDT techniques serve only as verification tools – they cannot prevent imperfections. The responsibility for defect prevention lies with the welding engineer and the welder, who must apply sound procedures, appropriate parameters, and best practices to ensure the weld meets all product and service requirements.

At present and for the foreseeable future, the practical goal is to minimize imperfections and ensure that any discontinuities that remain are within acceptable quality limits. It is pertinent to note that a weld defect only exists when it is defined by the contract documents and the applicable construction code or standard. Current standards that can be used to designate and classify imperfections or defects in fusion welding are:

1. DD CEN ISO/TS 17845:2004: Welding and allied processes – Designation system for imperfections.
2. BS EN ISO 6520-1:2007: Welding and allied processes – Classification of geometric imperfections in metallic materials – Part 1: Fusion welding
3. BS EN ISO 5817:2023: Welding – Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) – Quality levels for imperfections (Welding imperfections – Quality levels for imperfections — acceptance levels), defines acceptance criteria).
4. PD 5500 Specification for unfired fusion welded pressure vessels.
5. AWS D1.1 Structural welding code – Steel.

Number #1 is a draft for development of designation system for imperfections, while #2 is another designation system for imperfections, current and confirmed. Number #2 is of particular interest because the types of imperfection are defined with explanations and illustrations where necessary. Number #3 is used to evaluate the defect acceptance level. The ISO 6520-1 is used together with ISO 5817. However, application standard that forms the basis for the component design can be referred and an example of such standard might be no. #5 and #6. 

Of interest in this article series is ISO 6520-1 which  classifies geometric imperfections in metallic materials.

Terms and definitions

In fusion welding, an imperfection is any discontinuity in the weld or deviation from the intended geometry (ISO 6520-1). A defect is an unacceptable imperfection, and what makes it unacceptable is defined by another standard. For example, ISO 5817 sets the quality levels and tells you what is acceptable, and what becomes a defect. Therefore, in fusion welding, what we really deal with are the 6 main groups or categories of imperfections, namely:

1. Cracks

2. Cavities

3. Solid inclusions

4. Lack of fusion and penetration

5. Imperfect shape and dimension

6. Miscellaneous imperfections

These main group of imperfections are denoted with 3-figure reference number, and the sub-terms are denoted with 4-figure reference number.

Cracks

A crack in welding is a type of imperfection where a fracture or separation occurs in the weld metal or the base metal. It happens when the material fails to withstand the stresses that develop during or after welding. Cracks are among the most serious weld defects because they can lead to structural failure.

Types of Cracks in Welding

1. Hot cracks (solidification cracks)
   • Occur at high temperatures during solidification of the weld metal.
   • Usually form in the centre of the weld bead or along the fusion line.
   • Causes include high sulphur or phosphorus content, excessive heat input, or poor joint design. Example:

2. Cold cracks (hydrogen-induced or delayed cracks)
   • Form after the weld has cooled to room temperature, sometimes hours later.
   • Common in steels, caused by hydrogen, high residual stress, and a hard or brittle microstructure.
   • Often appear in the heat affected zone (HAZ).

3. Crater cracks
   • Small cracks that form at the end of a weld bead when the arc is stopped abruptly.
   • Caused by improper termination of welding current or insufficient filler material in the crater.

4. Underbead cracks
   • Occur below the weld bead in the HAZ, often due to hydrogen and high restraint.

5. Throat and root cracks
   • Appear in the weld throat (centre) or at the weld root (bottom), often due to high stress concentration or poor penetration.

Common Causes

• Excessive restraint during welding or high residual stress.
• Improper preheating or post-weld heat treatment.
• High cooling rates/Rapid cooling or uneven contraction
• Contaminants such as hydrogen, moisture, oil, or rust.
• Poor joint design or incorrect welding parameters.

Prevention Measures

• Use low-hydrogen electrodes or filler materials.
• Preheat and control cooling rate for susceptible materials.
• Ensure clean surfaces before welding.
• Apply proper joint design and welding sequence to reduce stress.
• Use post-weld heat treatment (PWHT) where necessary.

Designation of cracks in accordance with ISO 6520-1

Cracking phenomena occurring during or after welding are designated by letters. When a full description of cracking is necessary,  a combination of the classification number and letters is recommended.

Group No. 1  Cracks  (Reference no. = 100)

100 – Crack

A general term describing any local rupture in the solidified or solid-state metal caused by stress or cooling effects. This is the parent category for all other crack types.

• 1001 – Microcrack:  A very fine crack that is visible only under a microscope. These tiny discontinuities may occur within grains or along grain boundaries and can grow under repeated stress.

101 – Longitudinal Crack

A crack that runs essentially parallel to the axis of the weld. It may form in different zones depending on stress concentration or solidification behaviour.

Sub-categories based on location:

• 1011 – In the weld metal: The crack lies within the deposited weld material itself, often due to solidification shrinkage.
• 1012 – At the weld junction: The crack is located along the fusion line, where weld metal meets parent metal. It is often associated with poor fusion or metallurgical incompatibility.
• 1013 – In the Heat Affected Zone (HAZ): The crack appears just outside the weld bead, in the area affected by heat but not melted. It may result from hardening or stress concentration.
• 1014 – In the parent material: The crack extends into the unaffected base metal beyond the HAZ, usually from high residual stress or pre-existing flaws.

Note: The code “1” refers to the HAZ region.

Learning tip:
Think of cracks as a family.

• The “100” level defines what a crack is.
• The “1000-series” describes its scale (micro or macro).
• The “101-series” shows direction and position relative to the weld.

102 – Transverse Crack

A transverse crack runs essentially across the axis of the weld, that is, perpendicular to the weld direction. It usually develops due to high tensile stresses during cooling or from poor control of welding sequence and restraint.

Typical causes:

• Shrinkage stress across the weld
• Inadequate preheating of thick sections
• Excessive hardness or brittleness in the heat affected zone

Sub-categories based on location:

• 1021 – In the weld metal: The crack passes through the deposited weld material itself, generally due to poor ductility during solidification.
• 1023 – In the HAZ: The crack appears just outside the weld bead, often linked to high hardness or hydrogen-induced cracking.
• 1024 – In the parent material: The crack extends into the unaffected base metal, usually caused by residual stresses or brittle base metal properties.

Key point:
Transverse cracks are serious because they cross the weld axis and can rapidly propagate through the joint under load. However, this depends on the direction of load application.

Learning tip:

• Transverse cracks cut across the weld axis, think of them as “crossways cracks.”

103 – Radiating Cracks

Radiating cracks spread outward from a single point, forming a pattern similar to spokes of a wheel. They usually occur when local stresses or solidification shrinkage concentrate around a defect such as a crater or inclusion.

Sub-categories based on location:

• 1031 – In the weld metal:
  Radiating cracks within the weld bead, often starting from the crater at the end of a weld run.
• 1033 – In the HAZ:
  Radiating cracks extending into the heat affected region surrounding the weld.
• 1034 – In the PM: Radiating cracks that spread into the unaffected base metal, often from a stress concentration point.

Note:
Small radiating cracks are also known as “star cracks”, commonly found at weld craters or overlapping bead intersections.

Learning tip:

• Radiating cracks spread outward from one point, think of them as “star or burst cracks.”

104 – Crater Crack

A crater crack forms in the crater at the end of a weld bead when the molten pool solidifies too quickly or the welder fails to fill the crater before breaking the arc. Crater cracks are caused by shrinkage stresses that develop as the final molten pool cools and contracts.

Sub-types based on direction of cracking:

• 1045 – Longitudinal crater crack: Runs parallel to the weld axis within the crater.
• 1046 – Transverse crater crack: Runs across the weld axis at the crater end.
• 1047 – Radiating crater crack (Star crack): Spreads outward from the crater centre in multiple directions, forming a star-like pattern.

Learning point:
Crater cracks are common at the end of a weld run and can be prevented by gradually reducing current or filling the crater before arc termination.

105 – Group of Disconnected Cracks

A group of disconnected cracks refers to several separate cracks occurring near each other, running in any direction but not physically linked together. They can result from cyclic thermal stresses, solidification shrinkage, or poor control of weld parameters.

Sub-categories based on location:

• 1051 – In the weld metal: Disconnected cracks dispersed within the deposited weld.
• 1053 – In the HAZ: Isolated cracks within the thermally affected region.
• 1054 – In the PM: Small separated cracks formed beyond the HAZ due to residual or external stress.

Key difference:
Unlike branching or radiating cracks, disconnected cracks do not share a common origin.

106 – Branching Crack

A branching crack is a main crack that splits into smaller connected cracks, all stemming from a single primary crack. This pattern distinguishes it from disconnected cracks (105) and radiating cracks (103).

Sub-categories based on location:

• 1061 – In the weld metal (WM): The main crack within the weld bead branches into smaller connected cracks.
• 1063 – In the HAZ: A primary crack extends into the HAZ, forming smaller branches due to localized stress.
• 1064 – In the PM: A crack that originates in the base metal and divides into several secondary cracks.

Learning point:
Branching cracks indicate progressive crack growth under repeated or complex stress conditions.
They often suggest metal fatigue or microstructural brittleness.

Summary memory cues:

• 104 – Crater crack: End of weld, caused by poor crater filling.
• 105 – Disconnected cracks: Separate, unrelated cracks.
• 106 – Branching crack: One main crack splitting into several branches.

Designation System (According to ISO 6520-1)

When a designation is required for an imperfection, it follows a standard structure that identifies:

1. The type of imperfection
2. The reference standard
3. The group and serial number from ISO 6520-1

General Format: Imperfection ISO 6520-1-XXX

Where:

• Imperfection = the name of the defect (for example, Crack, Porosity, Lack of fusion)
• ISO 6520-1 = the reference standard for classification of welding imperfections
• XXX = the numerical code corresponding to the specific defect type

Example

For a crack (Group No. 1, reference 100):

Designation: Imperfection ISO 6520-1-100

If more specific subtypes are identified, the same rule applies:

• Microcrack: Imperfection ISO 6520-1-1001
• Longitudinal crack in HAZ: Imperfection ISO 6520-1-1013
• Radiating crack in weld metal: Imperfection ISO 6520-1-1031
• Crater crack (radiating): Imperfection ISO 6520-1-1047

Learning tip

Think of the designation as a three-part address: [Defect name] [Standard number] [Defect code]. It helps trace any weld imperfection to its exact classification and location as defined in ISO 6520-1.

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

When it is necessary to fully describe a type of cracking, you combine:

• The numerical code from ISO 6520-1 (which shows the position and form of the crack),
  and
• The letter code from the supplementary crack table below (which shows the mechanism or cause of cracking).

Letter Code	Description	Typical Mechanism
Ea	Hot crack	Forms at high temperature during welding solidification
Eb	Solidification crack	Caused by segregation during weld pool solidification
Ec	Liquation crack	Occurs in partially melted zone (HAZ)
Ed	Precipitation induced crack	Due to precipitate formation on cooling
Ee	Age hardening crack	From ageing or overageing of alloy
Ef	Cold crack	Develops after weld cools, often with hydrogen
Eg	Ductility-dip crack	Occurs in brittle temperature range
Eh	Shrinkage crack	Caused by contraction stresses during cooling
Ei	Hydrogen-induced crack	Caused by dissolved hydrogen in the metal
Ej	Lamellar tearing	Caused by through-thickness strain in rolled plate
Ek	Toe crack	Appears at weld toe, stress concentration point
El	Ageing induced crack (nitrogen diffusion)	Due to strain ageing after welding

General format: Imperfection ISO 6520-1-[Number] E[Letter]

Where:

• ISO 6520-1-[Number] = the classification of the crack type (for example 100 = crack, 101 = longitudinal crack)
• E[Letter] = the additional code describing the origin or mechanism of cracking

Example 1


• Crack type: Longitudinal crack in weld metal.

• Cause: Hot cracking (solidification during welding).

• Full description:  Imperfection ISO 6520-1-1011 Ea.

• Meaning: Longitudinal crack in weld metal caused by hot cracking during welding.

Example 2

• Crack type: Transverse crack in HAZ

• Cause: Hydrogen-induced cracking

• Full description: Imperfection ISO 6520-1-1023 Ei

• Meaning: Transverse crack in the heat-affected zone produced by hydrogen embrittlement.

Example 3

• Crack type: Radiating crack (star crack) in weld crater

• Cause: Shrinkage cracking during solidification

• Full description: Imperfection ISO 6520-1-1047 Eh

• Meaning: Radiating crater crack caused by shrinkage stress.

Example 4

• Crack type: Branching crack in weld metal

• Cause: Cold cracking after welding

• Full description: Imperfection ISO 6520-1-1061 Ef

• Meaning: Branching crack in weld metal caused by cold cracking during cooling.

Example 5

Learning tip:

• The number (100–106 etc.) tells you where and what shape the crack is.
• The letter (Ea–El) tells you why or how it formed.
• When combined, they give a complete engineering description suitable for welding reports, WPS qualification, and inspection documentation.

Frequently Asked Questions (FAQs)

Welding Imperfections and Classification – Part 1: Cracks

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1. Why is achieving a completely defect-free weld practically impossible?

Absolute defect elimination in fusion welding is not possible because welding involves complex thermal cycles, phase transformations, and stress fields that differ from those of mill-processed materials. Variations in material composition, human operation, geometry, and process limitations all contribute to the formation of imperfections.

2. What is the main goal of welding science if defect-free welds are unattainable?

The goal is to produce joints whose properties are as close as possible to the base material, while minimizing imperfections and ensuring that any remaining discontinuities are within acceptable quality limits defined by applicable standards.

3. What role will modern technology play in reducing weld defects?

Emerging technologies such as AI-assisted welding control, adaptive arc sensing, real-time non-destructive testing (NDT), and automated seam tracking are expected to significantly reduce the occurrence of defects by optimizing welding parameters and detecting anomalies as they form.

4. What is the difference between an imperfection and a defect?

• Imperfection: Any discontinuity or deviation from the intended weld geometry (for example, porosity, crack, undercut).
• Defect: An imperfection that exceeds the acceptance criteria defined by a standard such as ISO 5817 or a construction code like AWS D1.1.

Thus, an imperfection becomes a defect only when it is unacceptable according to the governing specification.

5. What are the six main groups of imperfections in fusion welding according to ISO 6520-1?

1. Cracks
2. Cavities
3. Solid inclusions
4. Lack of fusion and penetration
5. Imperfect shape and dimension
6. Miscellaneous imperfections

Each group contains subcategories represented by three- and four-digit reference numbers.

6. Why are cracks considered the most serious type of weld imperfection?

Cracks represent actual fractures in the metal, leading to a complete loss of continuity and structural integrity. They act as stress concentrators and can propagate under load or fatigue, causing catastrophic failure if not detected and repaired.

7. What are the main types of cracks in fusion welds?

• Hot cracks (solidification cracks): Form during cooling from the molten state.
• Cold cracks (hydrogen-induced cracks): Appear after cooling, often in steels.
• Crater cracks: Occur at the weld end due to abrupt arc stoppage.
• Underbead cracks: Develop below the weld bead, often from hydrogen or restraint.
• Throat and root cracks: Found in the weld centre or bottom, caused by stress concentration.

8. What are the common causes of cracking in welding?

• Excessive restraint or high residual stress
• Rapid cooling or lack of preheat
• Hydrogen contamination from moisture or surface impurities
• Poor joint design or incorrect parameters
• Inadequate post-weld heat treatment
• Level of heat input

9. How can crack formation be prevented?

• Use low-hydrogen consumables
• Preheat the base metal where required
• Keep surfaces clean and dry
• Control heat input and cooling rate
• Apply post-weld heat treatment (PWHT) where necessary

10. What standard defines how welding imperfections are classified?

BS EN ISO 6520-1:2007 defines the classification of geometric imperfections in metallic materials for fusion welding. It provides the numbering system (e.g., 100 for cracks, 201 for pores, etc.) and descriptive definitions.

11. What standard defines the acceptance criteria for imperfections?

BS EN ISO 5817:2023 specifies the quality levels (B, C, D) for imperfections in fusion-welded joints. It determines which imperfections are acceptable and which are rejectable based on weld type, service condition, and inspection level.

12. What does a typical designation of a weld imperfection look like?

A standard designation includes the defect name, reference standard, and numerical code:

Format: Imperfection ISO 6520-1-XXX

Example:

• Imperfection ISO 6520-1-1013 = Longitudinal crack in HAZ
• Imperfection ISO 6520-1-1047 = Radiating crater crack

13. What if a full description of cracking is needed?

When a more detailed explanation is required, letters are added to describe the crack mechanism or cause.

Format: Imperfection ISO 6520-1-[Number] E[Letter]

Examples:

• Imperfection ISO 6520-1-1011 Ea → Longitudinal crack in weld metal caused by hot cracking.
• Imperfection ISO 6520-1-1023 Ei → Transverse crack in HAZ caused by hydrogen-induced cracking.
• Imperfection ISO 6520-1-1047 Eh → Crater crack caused by shrinkage stresses.

15. Why is inspection and NDT still essential even with advanced welding control?

Even the most advanced technologies cannot eliminate imperfections. Non-destructive testing (NDT) is necessary to detect, characterize, and size discontinuities, and to verify if they meet specified acceptance criteria.

16. What is the ultimate purpose of welding inspection and acceptance criteria?

To ensure that welds are fit for service, meaning that imperfections that remain after welding do not compromise safety, performance, or durability of the structure or pressure component.

17. Why are weld imperfections critical in fatigue service conditions?

In cyclic or fatigue loading, even small imperfections act as stress raisers, accelerating crack initiation and growth. This is why toe grinding, TIG dressing, and peening are used to improve fatigue life of critical welds.

18. Who is responsible for defect prevention?

NDT personnel can only detect imperfections. Prevention is the responsibility of the welding engineer and the welder, who must apply correct procedures, qualified parameters, and sound workmanship practices.

19. Which standards are typically used together for classifying and accepting weld imperfections?

• ISO 6520-1 → Classification of imperfections
• ISO 5817 → Quality and acceptance levels
  The two are often applied together: ISO 6520-1 identifies the type, ISO 5817 decides if it is acceptable.

20. What is the practical objective of modern welding quality systems?

Not to achieve perfect welds, but to ensure that:

• Imperfections are minimized
• Any remaining discontinuities are within allowable limits
• Welds remain safe, reliable, and serviceable throughout their design life.

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Read more…

Welding Imperfections and Classification – Part 2: Cavities

Welding Imperfections and Classification – Part 3: Solid inclusions

Welding Imperfections and Classification – Part 4: Lack of Fusion and Penetration

Welding Imperfections and Classification – Part 5: Imperfect Shape and Dimensions