This article continues the Welding Imperfections and Classification series in accordance with ISO 6520 1, following earlier parts on Cracks, Cavities, Solid Inclusions, and Lack of Fusion and Penetration.

Part 5 examines Group 5 imperfections: Imperfect shape and dimensional deviations.  These discontinuities relate to the external form, geometry, and dimensional accuracy of the weld.

Imperfect shape and dimensional defects may not always be as critical as planar imperfections, but they strongly influence weld fatigue life, stress distribution, service performance, and code compliance.

Many geometric imperfections become initiation points for cracking or reduce effective throat thickness. ISO 6520-1 places these discontinuities under Group No. 5, with reference numbers beginning at 500.

ISO 6520 1 Group 5 – Imperfect Shape and Dimensions

500 – Imperfect shape: General term for any deviation in the external shape or geometry of a weld surface.

501 – Undercut: An irregular groove at the weld toe, either in the parent material or previous weld metal. The latter can result in slag becoming trapped in the undercut region.

Subcategories:

• 5011 – Continuous undercut: Undercut of significant uninterrupted length

• 5012 – Intermittent undercut: Short, discontinuous undercut segments.

• 5013 – Shrinkage grooves: Undercuts formed on both sides of the root run.

• 5014 – Inter run undercut: Undercut along the longitudinal direction between weld runs.

• 5015 – Local intermittent undercut: Irregularly spaced, short undercut segments on the side or on the surface of the weld runs

Undercut is common in arc and gas welding. In single sided pipe welds, undercut may also be found at the bore surface.  It is also seen on the vertical face of fillet welds made in the horizontal vertical position.

Common causes:

• A wide spreading arc caused by high arc voltage combined with insufficient fill from low current or high travel speed is the usual cause.
• High heat input leading to toe erosion
• Incorrect electrode angle
• Poor weaving technique by the welder
• Excessive or High welding current can also lead to undercut, usually because it forces a high travel speed to avoid overfilling the joint.

502 – Excess weld metal: Excessive buildup of weld metal on the face of the weld (overfill). Often appears on butt welds.

The term weld reinforcement is used extensively in the ASME/AWS specifications, but it is avoided in Europe as it implies that excess metal contributes to the strength of the welded joint. This is rarely the case.

Common causes:
• Poor welder technique in manual welding
• Poor parameter selection in mechanised welding – there may be too much filler metal for the travel speed used.

503 – Excessive convexity: Fillet weld reinforcement is excessively convex. It can be described as weld metal lying outside the plane joining the weld toes.

 This feature is also covered by the definition for excess weld metal  

Note that the term ‘reinforcement’, although used extensively in the ASME/AWS specifications is avoided in Europe as it implies that excess metal contributes to the strength of the welded joint. This is rarely the case.

Common causes

• Travel speed too slow, allowing excess weld metal to pile up.
• Welding current too low, producing a cold weld with poor wetting.
• Excessive filler metal deposition for the joint size.
• Poor electrode or torch manipulation that does not allow the weld pool to spread.
• Incorrect torch or electrode angle that restricts proper side wall fusion and bead flattening.
• Low heat input that prevents the weld metal from flowing smoothly into the joint edges.

504 – Excessive penetration: Excessive reinforcement at the weld root. This applies to weld made from one side.

With pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion and/or corrosion problems.

Subcategories:

• 5041 – Local excessive penetration
• 5042 – Continuous excessive penetration
• 5043 – Excessive melt through

Common causes:

• Welding current set too high for the joint thickness.
• Travel speed too slow, allowing excess heat to concentrate at the root.
• Root gap too large due to poor fit up.
• Root face too small or missing, leaving little material to control penetration.
• Incorrect joint preparation that creates an overly open root.
• Excessive heat input from weaving or pausing too long at the root.
• Improper torch or electrode angle that directs heat too deeply into the joint.

505 – Incorrect weld toe: Deviation in the expected weld toe geometry.

Subcategories:

• 5051 – Incorrect weld toe angle: Toe angle too small (i.e. reduction of α)
• 5052 – Incorrect weld toe radius: Toe radius too small

These imperfections increase stress concentration at the toe.

506 – Overlap: Weld metal flows over the parent material surface without fusing to it. Occurs in both fillet and butt welds and is considered a serious imperfection.

Subcategories:

• 5061 – Toe overlap
• 5062 – Root overlap

Overlap is considered a serious imperfection because it behaves like an unfused notch.

Common causes:

• Poor manipulation of the electrode or welding gun.
• A large and cold weld pool that allows gravity to distort the weld before it solidifies.
• Tightly adherent oxides or scale on the metal surface that prevent proper fusion.
• Lack of fusion between the weld metal and the parent metal leading to overlap.

507 – Linear misalignment: Components are offset but their surfaces remain parallel.

Subcategories:

• 5071 – linear misalignment between plates, pieces are plates
• 5072 – linear misalignment between tubes, pieces are tubes

508 – Angular misalignment: Components are misaligned such that their surfaces are not parallel or at the intended angle.

Common causes (507 and 508):

• Poor component fit up before welding.
• Variations in the shape or thickness of components, such as out of roundness in pipe.
• Tack welds breaking during welding, allowing components to shift.
• Uneven or incorrect tack welds that allow one side to pull more than the other.
• Movement of components during welding leading to misalignment.
• Excessive heat input on one side of the joint, causing distortion and rotation.
• Incorrect clamping or inadequate fixturing that fails to hold components in position.
• Welding sequence that causes uneven shrinkage on one side of the joint.

509 – Sagging: Weld metal collapse due to gravity during deposition.

Subcategories:

• 5091 – Sagging in horizontal position
• 5092 – Sagging in flat or overhead position
• 5093 – Sagging in fillet weld
• 5094 – Sagging or melting at the weld edge

510 – Burn through: Collapse of the weld pool resulting in a hole in the weld.
Common in thin materials or when excessive heat is applied.

Common causes:

• Excessive heat input during welding.
• Welding current set too high for the joint thickness.
• Travel speed too slow, allowing the weld pool to overheat and drop out.
• Poor fit up, especially excessive root gap.
• Incorrect root face or joint preparation that leaves too little supporting material.
• Holding the arc too long in one spot, causing localised overheating.

511 – Incompletely filled groove: A continuous or intermittent longitudinal channel on the weld surface due to insufficient filler deposition.

Resulting weld thickness becomes less than the design requirement, increasing the risk of failure.

Common causes

• Insufficient filler metal deposited in the joint.
• Welding current set too low.
• Wire feed speed set too low in mechanised or MIG processes.
• Travel speed too high, preventing proper filling of the groove.

512 – Excessive asymmetry of fillet weld: One leg of the fillet weld is significantly larger than the other.

513 – Irregular width: excessive variation in width of the weld, usually caused by inconsistent travel speed.

514 – Irregular surface: Excessive roughness of the weld surface.

515 – Root concavity: A shallow groove at the weld root caused by shrinkage or root melting during subsequent passes. (Closely related to shrinkage grooves in 5013.)

Common causes

• Melting of the root pass during the second pass.
• Poor joint preparation, particularly a root gap that is too small or too large.
• High travel speed that does not allow enough filler metal to support the root.
• TIG welding conditions where precise control is required and small errors in gap or heat input lead to concavity

516 – Root porosity: Porous formation at the weld root due to bubbling of molten metal during solidification.

517 – Poor restart: A local surface irregularity where welding was stopped and restarted.

Subcategories:

• 5171 – Defective restart in capping run
• 5172 – Defective restart in root run

Note: 518 &519 not listed

520 – Excessive distortion: Dimensional deviation due to weld shrinkage and distortion.

521 – Incorrect weld dimensions: Deviation from required weld dimensions as specified in WPS or drawings.

Subcategories:

5211 – Excessive weld thickness: weld thickness is too large

5212 – Excessive weld width: weld width is too large

5213 – Insufficient throat thickness: actual throat thickness of the fillet weld is too small

5214 – Excessive throat thickness: actual throat thickness of the fillet weld is too large

Common causes of 521 – Incorrect weld dimensions

• Failure to follow the WPS or drawing requirements.
• Poor fit up leading to unplanned weld size changes.
• Inconsistent travel speed or heat input.
• Incorrect electrode or wire size selection.
• Poor welder control or technique.
• Excessive reinforcement due to overfilling.
• Underfilling caused by high travel speed or low current.
• Incorrect welding position or torch angle.

Common causes of 5211 – Excessive weld thickness

• Excessive filler metal deposition.
• Welding current set too high for the joint.
• Travel speed too slow.
• Poor control during multi pass welding.

Common causes of 5212 – Excessive weld width

• Excessive weaving by the welder.
• High heat input that allows the pool to spread too far.
• Incorrect torch or electrode angle.
• Over preparation of the joint creating a wide groove.
• Slow travel speed allowing weld metal to spread.

Common causes of 5213 – Insufficient throat thickness

• Travel speed too fast for the required fillet size.
• Welding current too low, producing a small deposit.
• Incorrect joint fit up such as a tight fillet angle.
• Poor torch angle that limits penetration.
• Use of an electrode or wire too small for the required fillet size.

Common causes of 5214 – Excessive throat thickness

• Overfilling of the fillet due to slow travel speed.
• Welding current too high increasing penetration.
• Use of a larger electrode or wire than specified.
• Welder technique that pushes excess metal into the root.
• Incorrect interpretation of fillet size during production.

Summary of Causes, Detection Methods, and Preventive Measures

A. Common Causes of Shape and Dimensional Imperfections

• Incorrect travel speed
• Inconsistent heat input
• Poor torch or electrode angle
• Excess or insufficient filler metal
• Gravity effects in positional welding
• Poor fit up or misalignment
• Inadequate joint preparation
• Incorrect welding sequence
• Lack of welder control or manipulation
• Overly wide weaving
• Low or high current settings

B. Detection Methods

• Visual Testing – primary tool for most shape imperfections
• Profile and fillet weld gauges – assess throat, leg length, convexity
• Calipers, rulers, straight edges – measure width, thickness, alignment
• Radiographic Testing – useful for penetration and root shape issues
• Ultrasonic Testing – reveals shape related indications at the root
• Phased Array UT – more detailed evaluation of root and toe conditions

C. Preventive Measures

• Follow WPS for heat input, travel speed, and bead placement
• Maintain correct electrode or torch angle
• Ensure proper joint preparation and alignment
• Avoid excessive weaving or erratic manipulation
• Control filler metal deposition rate
• Use appropriate fixtures or clamps to prevent distortion
• Ensure proper inter run cleaning and bead sequencing
• Maintain stable arc conditions and correct polarity
• Avoid overheating, excessive gaps, and poor fit up

Designation Examples (ISO 6520-1)

• Imperfection ISO 6520-1 5011 – Continuous undercut
• Imperfection ISO 6520-1 5042 – Continuous excessive penetration
• Imperfection ISO 6520-1 5061 – Toe overlap
• Imperfection ISO 6520-1 515 – Root concavity
• Imperfection ISO 6520-1 5213 – Insufficient throat thickness

Letter Codes for Part 5 (Ga series)

Imperfect Shape and Dimensions — Letter Codes

These letter codes identify the mechanism or nature of each shape or dimensional imperfection.

(Note that this letter codes are not contained in ISO 6520-1)

Letter Code	Imperfection (ISO 6520 1)	Typical Mechanism
Ga	Undercut (501)	Arc spread, high travel speed, toe erosion
Gb	Excess weld metal (502)	Excess filler deposition or slow travel
Gc	Excessive convexity (503)	Low heat input, poor wetting, slow travel
Gd	Excessive penetration (504)	High current, large root gap, slow travel
Ge	Incorrect weld toe geometry (505)	Poor manipulation or torch angle
Gf	Overlap (506)	Cold, fluid weld pool flowing over unfused surface
Gg	Linear misalignment (507)	Poor fit up, inadequate fixturing
Gh	Angular misalignment (508)	Uneven shrinkage or incorrect tack placement
Gi	Sagging (509)	Gravity effects and insufficient weld pool support
Gj	Burn through (510)	Excess heat, thin material, low travel speed
Gk	Incompletely filled groove (511)	Insufficient filler or high travel speed
Gl	Fillet weld asymmetry (512)	Poor positioning or inconsistent leg deposition
Gm	Irregular width (513)	Variable travel speed or weaving
Gn	Irregular surface (514)	Poor arc stability or contamination
Go	Root concavity (515)	Shrinkage, poor root control, incorrect heat input
Gp	Root porosity (516)	Gas bubbling at root during solidification
Gq	Poor restart (517)	Improper arc re striking or bead overlap
Gr	Excessive distortion (520)	Uneven heat input or incorrect sequence
Gs	Incorrect weld dimensions (521)	Poor control of bead size, incorrect parameters

Designation Examples (ISO 6520 1 Combined with Ga Letter Codes)

Format:
Imperfection ISO 6520 1 – [Number] G[Letter]
Number = specific Group 5 imperfection
Letter = mechanism identifier

These examples follow the same style you used for cracks (Ea), cavities (Fa), and inclusions.

1. Undercut caused by high travel speed

Imperfection ISO 6520 1 5011 Ga
Meaning: Continuous undercut formed by excessive travel speed and toe erosion.

2. Excess weld metal due to overfilling

Imperfection ISO 6520 1 502 Gb
Meaning: Excessive face buildup caused by excess filler deposition.

3. Excessive convexity from low heat input

Imperfection ISO 6520 1 503 Gc
Meaning: Overly convex fillet profile resulting from insufficient heat input and poor spread.

4. Local excessive penetration due to high current

Imperfection ISO 6520 1 5041 Gd
Meaning: Localised excessive root penetration caused by excessive current.

5. Incorrect weld toe angle from poor manipulation

Imperfection ISO 6520 1 5051 Ge
Meaning: Toe angle too small due to poor torch handling.

6. Toe overlap caused by cold, unfused metal flow

Imperfection ISO 6520 1 5061 Gf
Meaning: Unfused weld metal extending over the base metal.

7. Linear misalignment caused by poor fit up

Imperfection ISO 6520 1 5071 Gg
Meaning: Misalignment between plate edges due to inadequate preparation.

8. Angular misalignment from uneven shrinkage

Imperfection ISO 6520 1 508 Gh
Meaning: Angular deviation caused by asymmetric heat input or poor tack welding.

9. Sagging in fillet weld from gravity effects

Imperfection ISO 6520 1 5093 Gi
Meaning: Fillet weld sagging due to insufficient pool support.

10. Burn through caused by excessive heat on thin material

Imperfection ISO 6520 1 510 Gj
Meaning: Hole formed at the root due to overheating.

11. Incompletely filled groove caused by high travel speed

Imperfection ISO 6520 1 511 Gk
Meaning: Groove not fully filled because travel speed was too fast.

12. Excessive asymmetry of fillet weld from poor positioning

Imperfection ISO 6520 1 512 Gl
Meaning: Unequal fillet legs due to poor joint access or torch angle.

13. Irregular width from inconsistent travel speed

Imperfection ISO 6520 1 513 Gm
Meaning: Bead width variation caused by inconsistent welder movement.

14. Irregular surface due to unstable arc

Imperfection ISO 6520 1 514 Gn
Meaning: Rough bead surface caused by arc instability or contamination.

15. Root concavity due to shrinkage

Imperfection ISO 6520 1 515 Go
Meaning: Shrinkage induced groove at root.

16. Root porosity from gas entrapment at the root

Imperfection ISO 6520 1 516 Gp
Meaning: Porous formation at root from poor gas protection.

17. Poor restart caused by incorrect arc striking technique

Imperfection ISO 6520 1 5171 Gq
Meaning: Restart defect in capping run due to poor overlap and arc control.

18. Excessive distortion from uneven heat input

Imperfection ISO 6520 1 520 Gr
Meaning: Dimensional deviation caused by asymmetric shrinkage.

19. Insufficient throat thickness due to fast travel speed

Imperfection ISO 6520 1 5213 Gs
Meaning: Actual throat thickness smaller than required due to fast travel and low deposition.

Summary Memory Cues

Reference	Description	Typical Cause	Detection Method
500	Imperfect shape (general)	Poor technique or manipulation	VT
501	Undercut	High travel speed, high arc voltage, poor angle	VT, profile gauge
5011	Continuous undercut	Consistent toe erosion	VT
5012	Intermittent undercut	Unsteady manipulation	VT
5013	Shrinkage grooves	Shrinkage adjacent to root run	VT
5014	Inter run undercut	Poor bead placement between runs	VT
5015	Local intermittent undercut	Irregular excessive heat or poor torch control	VT
502	Excess weld metal	Excess filler or slow travel	VT, profile gauge
503	Excessive convexity	Low heat, slow travel, poor spreading	VT, fillet gauge
504	Excessive penetration	High current, large root gap	VT, RT
5041	Local excessive penetration	Local overheating at root	VT, RT
5042	Continuous excessive penetration	Excess heat input	RT
5043	Excessive melt through	Weld pool collapse	VT, RT
505	Incorrect weld toe	Incorrect torch angle	VT
5051	Incorrect toe angle	Poor manipulation	VT
5052	Incorrect toe radius	Sharp transition due to poor control	VT
506	Overlap	Cold, unfused metal flowing over edge	VT
5061	Toe overlap	Poor wetting at weld toe	VT
5062	Root overlap	Lack of root fusion	VT
507	Linear misalignment	Poor fit up, distortion	VT, straight edge
5071	Misalignment (plates)	Incorrect prep or tack sequence	VT
5072	Misalignment (tubes)	Out of roundness, poor fit up	VT
508	Angular misalignment	Uneven shrinkage, poor tacks	VT, angle gauge
509	Sagging	Gravity effects, low viscosity	VT
5091	Sagging (horizontal)	Poor support of molten pool	VT
5092	Sagging (flat or overhead)	Excess heat input	VT
5093	Sagging in fillet	Pool collapse in fillet profile	VT
5094	Edge sagging	Melt back at weld edge	VT
510	Burn through	Excess heat, thin material	VT
511	Incompletely filled groove	Insufficient filler or high travel speed	VT, profile gauge
512	Excessive asymmetry (fillet)	Poor positioning or access	VT, fillet gauge
513	Irregular width	Inconsistent travel speed	VT
514	Irregular surface	Arc instability, contamination	VT
515	Root concavity	Shrinkage or poor root control	VT, RT
516	Root porosity	Gas entrapment at root	RT, VT
517	Poor restart	Poor arc re striking	VT
5171	Restart defect (cap)	Inadequate overlap	VT
5172	Restart defect (root)	Poor restart in open root	VT
520	Excessive distortion	Uneven heat distribution	VT, dimensional check
521	Incorrect weld dimensions	Poor control, improper parameters	VT, gauges
5211	Excessive thickness	Overfilling	VT
5212	Excessive width	Excess weaving, high heat	VT
5213	Insufficient throat thickness	Fast travel, low current	VT, fillet gauge
5214	Excessive throat thickness	Over deposition or high current	VT, fillet gauge

Closing Note

Imperfect shape and dimensional deviations may appear superficial compared to internal imperfections, but their influence on weld performance is significant. These discontinuities affect stress distribution, fatigue resistance, corrosion behaviour, and compliance with design and code requirements. Many, such as undercut, overlap, excessive penetration, or misalignment, act as geometric stress raisers and can initiate cracks under service loads.

A clear understanding of the mechanisms behind Group 5 imperfections, along with proper classification using ISO 6520 1, enables welding inspectors, engineers, and fabricators to correctly identify, document, and prevent these shape related defects. Effective control of weld geometry is essential to achieving consistent, reliable, and code compliant welded structures.

More lack of fusion and penetration illustrations

Frequently Asked Questions (FAQs)

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

1. What are imperfect shape and dimensional imperfections in welding?

These are deviations from the intended weld geometry or size, such as undercut, overlap, excessive reinforcement, misalignment, or incorrect throat thickness. They affect the external appearance and dimensions of the weld rather than its internal soundness.

2. Why are shape and dimensional imperfections important if they are not internal defects?

Although they are not planar cracks or fusion defects, geometric imperfections influence:
• stress concentration
• fatigue performance
• corrosion behaviour
• fluid flow characteristics
• compliance with design requirements

Some, like overlap or undercut, behave similarly to crack initiators.

3. Which standard classifies imperfect shape and dimensions?

ISO 6520-1, Group 5, classifies imperfect shape and dimensional deviations using reference numbers 500 to 521.

4. What is the difference between excess weld metal and excessive convexity?

• Excess weld metal (502): Excessive buildup on the weld face of a butt weld.
• Excessive convexity (503): Overly convex profile in a fillet weld.

Both add unnecessary metal without improving strength and may reduce fatigue life.

5. What causes undercut, and why is it critical?

Undercut is caused by high arc voltage, incorrect electrode angle, high heat input, or excessive travel speed.
It is critical because it reduces effective throat thickness and produces a sharp notch that increases fatigue cracking risk.

6. What is overlap and why is it considered a serious defect?

Overlap occurs when weld metal flows over the parent material surface without fusing to it.
It behaves like an unfused notch and significantly increases the likelihood of crack initiation.

7. How is excessive penetration different from melt through?

• Excessive penetration (504): Too much reinforcement at the root.
• Melt through (5043): Weld metal falls through the joint, forming an opening or sagging blister.
Both stem from excessive heat or poor root control.

8. What causes misalignment in welded joints?

Misalignment results from poor fit up, uneven tack welds, distortion during welding, inadequate clamping, or incorrect welding sequence.

9. How can burn through be prevented in thin materials?

Use lower heat input, higher travel speed, correct root face, closer control of arc positioning, and smaller electrodes or wires.

10. What are the most effective inspection methods for shape and dimensional defects?

• Visual Testing (VT)
• Weld gauges (fillet gauges, profile gauges)
• Straight edges and calipers
• Radiography or UT for root shape issues
Most Group 5 imperfections are surface visible and easily assessed with VT.

11. Which imperfections in Group 5 most affect structural integrity?

The most critical are:
• undercut (501)
• overlap (506)
• excessive penetration or melt through (504)
• root concavity (515)
• misalignment (507, 508)

These directly affect load paths, fatigue life, and weld throat strength.

12. Are these imperfections acceptable under ISO 5817?

Acceptance depends on the quality level (B, C, D).
Some imperfections like overlap and burn through are rarely acceptable at any level.