□ Weld backing
Weld backing · Soldadura en plano · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “V-groove joint with ceramic backing in symbol tail” states a weld backing requirement.
What controls
Callout indicating that the root of the joint is backed from the opposite side: with a metal backing strip that remains in place, with a ceramic backing that is removed afterward, or with back purging gas. It serves to support the weld pool on the first pass and achieve a sound root while welding from one side only. It is a process condition: it defines how the joint is made, not a characteristic measured on the finished part, except when the backing is permanent and becomes part of the assembly.
How to verify
Before welding, the backing fit-up is checked and, where purging is used, the residual oxygen level with the appropriate meter. Afterward, when the backing is removed, the root is visually inspected from the back side: it must be fused and free of grooves. If the backing is permanent and covers the root, internal soundness can only be demonstrated by volumetric testing or must be covered by the qualified procedure.
Common mistakes when interpreting it
- Treating the backing as an optional aid for the welder: if it is on the drawing, it is part of the joint and of the procedure.
- Leaving in a backing that was supposed to be removed, or removing one that was permanent, without checking.
- Confusing backing with back gouging: backing supports the first pass, back gouging removes material from the root so it can be rewelded from the back.
How to get it in the workshop / if it comes out of tolerance
Check whether the backing is permanent or removed: it changes the final geometry, the weight and sometimes whether the root can be inspected afterward. With ceramic backing, fit it tightly against the back face so there is no gap; if the pool runs behind the backing, the root comes out irregular. With back purging on stainless steels, purge before starting and keep the gas flowing until the root has cooled.
Lack of fusion at the root despite the backing: there is usually a gap between the backing and the plate, or the root opening is smaller than specified. On stainless steel, an oxidized, rough root (sugaring) indicates insufficient purging: increase the purge time and check for air leaks. A misaligned permanent backing leaves an internal step that can act as a stress raiser and cause problems in fluid circuits.
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