4.4 Welding discontinuties

Why you should be very careful with the choice of words:

Welding discontinuities are conditions that exist in the weldment which may or may not be accepted. Meaning these little "defects" on the welds may still be accepted, if it falls beyond the code acceptance criterias, so be very careful with using the terms discontinuities and defects.

There are also many other words in use on the market such as Flaws, Imperfection, Indications.

Welding defects are the discontinuities which are not acceptable by the inspector in reference to Company design requirements, ASME code requirements or AWS code requirements, whichever takes precedence.

For pressure vessels, you could find the defect acceptance criteria on latest edition of ASME Section VIII Div 1 in the Appendixes and for pipes in ASME Section B31.3 in the Appendixes. Most of these defects will be measured based on the result of NDE testing such as RT, UT, MT and PT. However an inspector may reject a weld based on visual inspection if it obviously contains reject-able discontinuities.

According to API Inspection codes, NDE shall be carried out by the Examiner which is basically the certified NDE technician. Results of the NDE shall be reviewed by the Inspector for acceptance at his discretion.

Types of discontinuities

Excessive Overlap

- When weld bead extends too much beyond the weld toe. 

Excessive Undercut

- When weld bead do not fully cover the beads

Excessive Weld Reinforcement

- Height of weld bead is too high, presents unnecessary stresses to the join. 

Inclusion 

- Inclusion of impurities

Porosity

- Inclusion of porous air sacks/bubbles

Arc Strike

- Spatter like effect caused by electrode scratching when it does not starts due to cold weather.

Cracking 

- Usually caused by presence of water when welding, water breaks down to its elemental form causing causes hydrogen induced cracking. 

Crater crack

- Always pull the weld away from direction of weld when completing, this prevents crater cracks from forming. 

Lack of fusion

- Temperature is too low during passes, the binding between the metals is incomplete. Can occur between passes, or between base/filler metal.

Lack of penetration

- Too fast welding,  and low temperature resulting in inability to penetrate into the root gap,as such not meeting desired weld size.