TWI Frequently asked questions
The limits for incomplete fusion imperfections in arc welded joints in steel are specified in BS ISO 5817 at three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C(intermediate). For Quality level D (moderate), they are only permitted providing they are intermittent and not surface breaking.
For arc welded joints in aluminium (BS EN 30042), long imperfections are not permitted for all three quality levels. However, for quality levels C and D short imperfections, are permitted but the total length of the imperfections islimited depending on the butt weld or the fillet weld throat thickness.
As shown in the table, limits on the size and quantity of imperfections are also covered in standards for pressure vessels, linepipe, storage vessels etc.
| Application | Code/Standard | Acceptance limit |
|---|
| Steel | BS EN ISO 5817:2003 | Level B and C not permitted.
Level D short imperfections permitted but not surface breaking. |
| Aluminium | BS EN 30042:1994 | Levels B, C, D.
Long imperfections not permitted.
Levels C and D.
Short imperfections permitted. |
| Pressure vessels | PD5500:2006 | Not permitted |
| Storage tanks | BS2654:1989 | Not permitted |
| Pipework | BS2633:1987 | 'l' not greater than 15mm (depending on wall thickness) |
| Line pipe | API 1104:2005 | 'l' not greater than 25mm (less when weld length <300mm) |
