Weld imperfections can seriously reduce the integrity of a welded structure. Therefore, prior to service of a welded joint, it is necessary to locate them using NDE techniques, assess their significance, and take action to avoid their re-occurrence.

The acceptance of a certain size and type of defect for a given structure is normally expressed as the defect acceptance standard. This is usually incorporated in application standards or specifications.

All normal weld imperfection acceptance standards totally reject cracks. However, in exceptional circumstances, and subject to the agreement of all parties, cracks may be allowed to remain if it can be demonstrated beyond doubt that they will not lead to failure. This can be difficult to establish and usually involves fracture mechanics measurements and calculations.

It is important to note that the levels of acceptability vary between different applications, and in most cases vary between different standards for the same application. Consequently, when inspecting different jobs it is important to use the applicable standard or specification quoted in the contract.

Once unacceptable weld imperfections have been found, they have to be removed. If the weld imperfection is at the surface, the first consideration is whether it is of a type, which is normally shallow enough to be repaired by superficial dressing. Superficial implies that, after removal of the defect, the remaining material thickness is sufficient not to require the addition of further weld metal.

If the defect is too deep, it must be removed by some means and new weld metal added to ensure a minimum design throat thickness.
Replacing removed metal or weld repair (as in filling an excavation or re-making a weld joint) has to be done in accordance with an approved procedure. The rigor with which this procedure is qualified will depend on the application standard for the job.

In some cases it will be acceptable to use a procedure qualified for making new joints whether filling an excavation or making a complete joint. If the level of reassurance required is higher, the qualification will have to be made using an exact simulation of a welded joint, which is excavated and then refilled using a specified method. In either case, qualification inspection and testing will be required in accordance with the application standard.