According to ISO 6520-1

This final article concludes the Welding Imperfections and Classification series in accordance with ISO 6520 1, following earlier discussions on cracks, cavities, solid inclusions, lack of fusion and penetration, and imperfect shape and dimensions.

Group 6 Miscellaneous imperfections cover welding related conditions that do not fall within Groups 1 to 5. They commonly arise from handling errors, inadequate cleaning, surface contamination, poor technique, tack welding issues, or post weld finishing activities.

While many appear superficial, they can become initiation points for corrosion, fatigue, cracking, or contamination, particularly in applications that demand strict surface integrity. Their correct identification strengthens inspection accuracy and helps maintain consistent welding quality.

Understanding Miscellaneous Imperfections

These imperfections generally occur outside the primary weld geometry and are often linked to human factors or secondary processes. Typical examples include arc strikes, spatter, grinding and chipping marks, tack weld defects, discoloration, scaling, slag residues, misalignment of opposite runs, and poor fit up.

Although less severe than cracks or fusion related defects, Group 6 imperfections provide valuable insight into workmanship, surface care, and process control. When neglected, they may influence corrosion resistance, cleanliness, alignment accuracy, and the overall appearance of the weld.

A sound understanding of their classification under ISO 6520-1 ensures complete defect recognition and supports consistent quality control across welded fabrication.

ISO 6520 1 assigns these imperfections to Group No 6, with reference numbers beginning at 600.

ISO 6520-1 Group 6 – Miscellaneous Imperfections

600 – Miscellaneous imperfections (General Term): A grouping for all imperfections that do not fall into Groups 1 to 5.

601 – Arc strike (stray arc): Local surface damage caused when the welder initiates an arc outside the joint. Arc strikes alter material microstructure and can create hardness zones or crack initiation points, especially in carbon and alloy steels.

Common causes:

• Accidental arc contact
• Poor electrode handling
• Lack of arc initiation control

602 – Spatter: Globules of molten metal expelled during welding and adhering to parent material or weld metal.

Subcategory:

• 6021 – Tungsten spatter: Tungsten particles detached from a GTAW electrode and deposited on the surface.

Common causes:

• High current in GTAW
• Electrode contamination
• Poor arc stability

603 – Torn surface: Surface tearing caused by breaking off temporary welded attachments, such as strong backs or alignment lugs.

604 – Grinding mark: Local surface damage resulting from grinding operations.

605 – Chipping mark: Surface indentation or gouging caused by chisels or mechanical removal tools.

606 – Underflushing: Excessive grinding that reduces workpiece thickness below acceptable limits.

607 – Tack weld imperfection: Defects originating from defective tack welds.

Subcategories:

• 6071 – Broken run or no penetration
• 6072 – Defective tack overwelded

Common causes:

• Poor tack size or placement
• Lack of fusion in the tack
• Overheating or breaking during welding

608 – Misalignment of opposite runs: Difference between the centrelines of weld runs applied from opposite sides of a joint.

Common causes:

• Incorrect indexing or sequencing
• Poor fit up or distortion

610 – Temper colours (visible oxide film): Visible oxidation from welding heat in the weld zone, common in stainless steels.

Cause

• Formed when stainless steel surfaces reach elevated temperatures during welding.
• Results from inadequate shielding gas coverage or disturbed gas flow.
• Also caused by excessive heat input or insufficient protection during cooling.
• Allows oxygen to react with chromium on the surface, creating visible heat tint.

Consequences

• Reduces chromium content at the surface and weakens corrosion resistance.
• Increases risk of localised corrosion if not removed.
• May introduce contamination concerns in clean industry applications.
• Can create unacceptable surface appearance or roughness.
• Indicates poor process control that may affect overall weld quality.

6101 – Discolouration: Tinted oxide layers in the weld and heat affected zone, often caused by inadequate gas coverage in stainless steel or titanium.

613 – Scaled surface: Heavy oxidation in the weld zone due to excessive heat or inadequate shielding.

614 – Flux residue: Residual flux not fully removed from the weld surface.

615 – Slag residue: Adherent slag left on the weld after insufficient cleaning.

617 – Incorrect root gap for fillet welds: Excessive or insufficient gap between components before welding. Affects throat size, fusion, and penetration.

618 – Swelling: Occurs mainly in light alloys. Caused by prolonged holding time during solidification, leading to bulging or surface deformation.

Summary Memory Cues

Reference	Description	Typical Cause	Detection Method
601	Arc strike	Accidental arcing	VT
602	Spatter	High current, poor technique	VT
6021	Tungsten spatter	Electrode contamination	VT
603	Torn surface	Removing attachments	VT
604	Grinding mark	Excessive grinding	VT
605	Chipping mark	Chiselling or impact tools	VT
606	Underflushing	Overgrinding	VT, thickness gauge
607	Tack weld imperfection	Poor tack welding	VT
6071	Broken tack	Lack of fusion	VT
6072	Overwelded defective tack	Poor placement	VT
608	Misalignment of opposite runs	Distortion or poor sequencing	VT, measuring tools
610	Temper colours	Lack of shielding	VT
6101	Discolouration	Surface oxidation	VT
613	Scaled surface	Excessive heat, oxidation	VT
614	Flux residue	Inadequate cleaning	VT
615	Slag residue	Poor slag removal	VT
617	Incorrect root gap	Poor fit up	VT, gauge
618	Swelling	Prolonged solidification in light alloys	VT

Combined Summary Across Parts 1 to 6

Together, Parts 1 to 6 provide a complete mapping of weld imperfections defined in ISO 6520-1.

This knowledge supports:

• precise inspection and reporting
• accurate acceptance decision making using ISO 5817
• improved weld quality and manufacturing consistency
• better training for welders and inspectors
• standardised communication across engineering teams

The full classification helps ensure that every imperfection is correctly understood and managed, reinforcing safety and reliability in welded structures.

Part 1 – Cracks (Group 1)

Cracks are the most serious weld imperfections due to their ability to propagate under load. They include longitudinal, transverse, radiating, crater, branching, and microcracks. Causes involve high stress, hydrogen, rapid cooling, and improper welding conditions.

Key risk: immediate threat to structural integrity.

Part 2 – Cavities (Group 2)

Cavities include gas pores, porosity, wormholes, shrinkage cavities, crater pipes, and microshrinkage. They arise from trapped gas or inadequate molten metal compensation during solidification.

Key risk: reduced ductility, reduced soundness, onset of fracture.

Part 3 – Solid Inclusions (Group 3)

Slag, oxide, flux, tungsten, copper, and metallic inclusions result from improper cleaning, oxidation, or electrode contamination. They become stress concentration sites and impair toughness.

Key risk: crack initiation and reduced quality.

Part 4 – Lack of Fusion and Penetration (Group 4)

Planar imperfections where weld metal fails to fuse or penetrate. Includes side wall fusion lack, interrun fusion lack, root fusion lack, and incomplete penetration.

Key risk: severe reduction in load carrying capacity, behaves like a crack.

Part 5 – Imperfect Shape and Dimensions (Group 5)

These affect external weld geometry and dimensional accuracy. Include undercut, excess weld metal, excessive penetration, overlap, misalignment, sagging, burn through, root concavity, irregular surface, and incorrect weld dimensions.

Key risk: stress concentration, non-compliance, reduced fatigue life.

Part 6 – Miscellaneous Imperfections (Group 6)

All surface and handling related imperfections not included in Groups 1 to 5. Include arc strikes, spatter, torn surfaces, grinding marks, chipping marks, underflushing, tack weld defects, misalignment of opposite runs, discoloration, scaling, flux or slag residue, incorrect root gaps, and swelling.

Key risk: localised stress raisers, corrosion, contamination, and poor workmanship appearance.

Combined Reference Table for All Six Parts (ISO 6520-1)

This master table consolidates every imperfection group from 100 to 618.

Group	Imperfection Category	Typical Imperfections Included
1	Cracks	Longitudinal, transverse, radiating, crater, branching, microcracks
2	Cavities	Pores, porosity, wormholes, shrinkage cavities, crater pipes, microshrinkage
3	Solid inclusions	Slag, flux, oxide, tungsten, copper, other metallic inclusions
4	Lack of fusion and penetration	Lack of side wall fusion lack, lack of root fusion, lack of inter run fusion, incomplete penetration, spiking
5	Imperfect shape and dimensions	Undercut, overlap, excessive penetration, burn through, misalignment, sagging, incorrect weld dimensions, irregular surface, root concavity
6	Miscellaneous imperfections	Arc strike, spatter, grinding marks, torn surface, tack weld defects, flux or slag residue, discoloration, incorrect root gap, swelling

Frequently Asked Questions (FAQs)

Welding Imperfections and Classification – Part 6: Miscellaneous Imperfections

1. What are miscellaneous imperfections in ISO 6520-1?

They are welding imperfections that do not fit into Groups 1 to 5.
These include surface damage, arc strikes, spatter, tack weld defects, discoloration, residues, and alignment issues.

2. Do miscellaneous imperfections affect weld integrity?

Yes. While many are surface related, they can:
• introduce stress raisers
• trigger corrosion
• reduce cleanliness in hygienic service
• affect dimensional quality
• interfere with inspection

3. Are arc strikes dangerous?

Arc strikes can alter microstructure and create hard spots.
In steels under cyclic loading, they may become fatigue crack starters.

4. What causes spatter and is it always a defect?

Spatter results from unstable metal transfer, high current, or incorrect settings.
It becomes a defect when it adheres to surfaces and affects appearance, quality, or service cleanliness.

5. What makes tack weld imperfections significant?

Defective tack welds create stress concentration points, misalignment, or lack of fusion.
They can be carried into final welds when overwelded.

6. How does discoloration affect stainless steel welds?

Temper colours indicate oxidation and reduced corrosion resistance because chromium is tied up in surface oxides rather than forming a protective passive film.

7. What is underflushing?

Underflushing is overgrinding that reduces material thickness.
This can compromise structural requirements.

8. Why must flux and slag residues be removed?

Residues can mask underlying imperfections, absorb moisture, initiate corrosion, or interfere with subsequent welding.

9. How is misalignment of opposite runs identified?

By comparing the centrelines of weld runs made from opposite sides.
Deviation indicates poor sequencing, distortion, or setup errors.

10. What industries are most sensitive to miscellaneous imperfections?

• Pressure vessels
• Nuclear and power plant fabrication
• Pharmaceutical and food industry
• Aerospace
• Stainless steel pipework and hygienic systems