Backing weld / back run
Applied to joint
Supplementary symbol applied to fillet weld
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Description
A weld bead applied to the root side of a groove weld. A 'back weld' is applied after the groove weld is complete (to seal or reinforce the root). A 'backing weld' is applied before the groove weld (to support the root pass). The symbol is the same but placement and sequence differ.
In plain English
A weld on the back side of a groove joint. If you weld the groove from one side, then flip it and put a sealing run on the root side, that is a back weld (or back run). If you put a weld on the root side first, then weld the groove, that is a backing weld -- it supports the root pass. The symbol looks like a concave arc on the opposite side of the reference line from the main groove weld. The sequence (before or after) is usually noted or obvious from the WPS.
Symbol position
Concave arc on the opposite side of the reference line from the main weld symbol.
Size notation
Size may be specified or left to WPS.
Notation examples
Terminology differs slightly. AWS formally distinguishes 'back weld' and 'backing weld' by sequence. ISO uses 'backing run' and 'back run'. The symbol itself is the same.
AWS distinguishes 'back weld' (applied after) from 'backing weld' (applied before) using the same symbol. Sequence determined by welding procedure.
ISO uses the term 'backing run' or 'back run'. Symbol is the same concave arc. Sequence specified.
Common uses
- Sealing root side of single-V groove welds
- Providing root support for open-root groove welds
- Ensuring full penetration from the root side
- Repair of root defects
