Removable (temporary) backing
Applied to joint
Supplementary symbol applied to fillet weld
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Description
A supplementary symbol indicating that a temporary backing strip, bar, or ceramic backing is used during welding and then removed after the weld is complete. Symbol contains 'MR' (material, removable).
In plain English
Same idea as permanent backing, but you remove it after welding. Could be a steel strip you gouge out, a copper chill bar that does not fuse and pops off, or ceramic backing tape that you peel away. The symbol is a rectangle with 'MR' (M for material, R for removable). Used when a permanent backing would create problems -- corrosion crevice, fatigue concern, or the design does not allow extra material on the root side. Ceramic backing tape is very popular for single-sided pipe and structural welds.
Symbol position
Rectangle with 'MR' on the root side of the weld symbol.
Size notation
Backing type and material noted in the tail or on the drawing.
Common uses
- Fatigue-critical joints where permanent backing is prohibited
- Corrosive environments where crevices are unacceptable
- Single-sided welds requiring flush root finish
- Pipe welds using ceramic backing tape
