It is essential to achieve highest quality of weld such that weld is as good as parent metal. Problems in welds can be classified as :

1. Imperfection: Any deviation from the ideal weld.
2. Defect: An unacceptable imperfection.

Weld acceptance standards

Classification of imperfections according to BS EN ISO 6520-1:

This standard classifies the geometric imperfections in the case of fusion welding, dividing them into six groups:

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

It is important that an imperfection is correctly identified thus allowing for the cause to be identified and actions taken to prevent further occurrence.

Cracks

Definition: An imperfection produced by a local rupture in the solid state, which may arise from the effect of cooling or stresses. Cracks are more significant than other types of imperfection, as their geometry produces a very large stress concentration at the crack tip, making them more likely to cause fracture.

Types of crack:

1. Longitudinal
2. Transverse
3. Radiating (cracks radiating from a common point)
4. Crater
5. Branching(a group of connected cracks originating from a common crack)

These cracks can be situated in the:

1. Weld metal
2. HAZ
3. Parent metal

Exception: Crater cracks are found only in the weld metal.

Depending on their nature, these cracks can be:

1. Hot cracks (i.e. solidification cracks and liquation cracks).
2. Precipitation induced cracks (ie reheat cracks, present in creep resisting steels)
3. Cold cracks (i.e. hydrogen induced cracks).
4. Lamellar tearing

Cavities

Cavities can be of various types as follows : 

Gas Cavity : Formed by entrapped gas.

1. Gas Pore
2. Uniformly distributed porosity.
3. Clustered (Localised Porosity).
4. Linear porosity.
5. Elongated Cavity.
6. Worm-hole.
7. Surface pore.

Shrinkage Cavity : Caused by shrinkage during solidification.

1. Interdendritic Shrinkage.
2. Crater pipe.
3. Interdendritic micro-shrinkage
4. Transgranular micro-shrinkage.

Solid Inclusions

1. Slag Inclusion
2. Flux Inclusion
3. Oxide Inclusion
4. Metallic Inclusion

Lack of fusion and penetration

Lack of fusion

Definition: Lack of union between the weld metal and the parent metal or between the successive layers of weld metal.

1. Lack of side wall fusion.
2. Lack of inter-run fusion.
3. Lack of root fusion.

Lack of Penetration

1. Incomplete penetration.
2. Incomplete root penetration.

Imperfect Shape and Dimensions

1. Undercut.
2. Excess weld metal.
3. Excess penetration.
4. Overlap.
5. Linear misalignment.
6. Angular distortion.
7. Incompletely filled groove.
8. Irregular width.
9. Root concavity.
10. Burn through.

Miscellaneous Imperfections

1. Stray arc.
2. Spatter.
3. Torn surface.
4. Grinding mark.
5. Chipping mark.
6. Underflushing.
7. Misalignment of opposite runs.
8. Temper color.