MIL STD 22D Split backing ring.

By cwi_john Date 08-16-2009 15:06 Edited 08-16-2009 15:13

MIL STD 22D states that the split backing ring shall be welded prior to final insertion into pipe. MY question is WHY, I know you must meet the requirements for diametrical clearance in the counter bore. when examined by x-Ray any discontinuities found in the welded split backing are not considered for rejection if not found in the root.
Why even weld the split in the ring if x-ray shows complete fusion. Thanks for your help.

By 803056

Date 08-17-2009 18:35

Some welders end up blowing a hole through the root and split in the backing ring if the split ring isn't welded first. Air is poor backing in all cases, i.e., it promotes oxidation and it doesn't support molten weld metal very well.

The use of the backing ring is to protect the root of the weld from oxygen. It does the job provided there are no breaks in the continuity of the backing ring.

AWS D1.1 has similar requirements regarding backing bars where splices are required to be CJP welds.

Best regards - Al

By cwi_john Date 08-17-2009 21:33

Thank you, I should have included that their is a purge on this SS pipe. which would prevent oxidation of the root as well. the contractor uses this type of ring so they can get out of the 100% coverage of RT now they are required to have 60% coverage, I personally don't see the need to weld the ring prior to insertion, but it is a requirement. I have review some film of these joints and their is complete fusion their must be some reason for this requirement. thanks.

By kipman

Date 08-18-2009 12:09

John,
Most codes do not contain commentary sections (as do some of the AWS codes) that help the code user to understand the intent of the code writers. So any guess as to why there is a particular requirement is just that - a guess. As Al mentions, AWS D1.1 also does require that any splice in backing be welded with a CJP weld (i.e. no split backing allowed) and it does have specific commentary (at C5-10) as to why this requirement exists. This may or may not explain what the MIL-STD-22 writers had in mind - there is no way to know.
Mankenberg