Cracking

Cracks found within weld zones may be divided into two broad categories:

1. Weld process cracks: attributable to the weld process itself.
2. Service induced cracks: attributable to some external influence during service such as vibration or cyclic thermal stresses.

Weld process cracks are categorised/termed in many ways, but there are essentially only four crack types caused through welding:

1. Solidification cracks.
2. Hydrogen induced cracks (HIC).
3. Lamellar tearing.
4. Re-heat cracks.

There are also many ways to categorise/term service induced cracks. The following list identifies the main types:

1. Brittle fracture.
2. Ductile fracture.
3. Fatigue fracture.
4. Creep failure.
5. Stress corrosion cracking.
6. Hydrogen cracking induced by corrosion.

Cracks may also be termed in relation to their direction or shape:

• Longitudinal with the weld axis
• Transverse with the weld axis
• Branched
• Multi-directional
• Chevron

Terminology used may also indicate the location of the crack(s) relative to the weld:

• Edge
• HAZ
• Centreline
• Crater
• Fusion zone
• Underbead
• Parent metal

Cracks may be smooth or jagged in profile. Some cracks have branches, some are more multi-directional and some occur intermittently.