Brass backer bars

By G Roberts Date 05-14-2001 21:55

We use quite a bit of copper backing for CJP welds. It conducts heat away from the joint better than alloyed copper. If you are going to have to remove the backing bar, copper could be a good way to go, as cleanup is usually less than after you cut a steel backing bar off, plus you eliminate the step of cutting the backing bar off. The thing you have to watch out for is putting too much heat into the copper and fusing it with you weld. Often we have to let the copper cool down quite a bit between passes. You also want to use the biggest reasonable piece you can fit behind you joint in order to avoid heating it up to the point where it fuses, as the bigger copper has a higher heat sink capacity.

By G.S.Crisi (****)

Date 05-15-2001 14:54

I must confess my ignorance and so I'm asking a question, thanking in advance whoever answers it. Why should copper or brass backing bars promote a better penetration than steel ones?
Giovanni S. Crisi
Sao Paulo - Brazil

By ziggy

Date 05-15-2001 16:34

My question arose from a desire to easily remove the backer bar for this particular structural application rather than better penetration.
The foreman in the field desired to use brass backing material and other ironworkers were highly recommending it.
My hesitation comes from the fact that the D1.1 does not discuss the use of brass for backing material unless you consider the statement in 5.10 ",or similar materials" would apply. Copper is mentioned in 5.10. However, in the C5.10, it states that any material besides steel for a prequalified groove weld needs to be qualified.
The WPS that I created for this particular application (splice welds in W trusses - 2 splices each end - top and bottom chord) calls for steel backing per D1.1.
Although the workers out in the field are preferring brass, I hesitate to deviate from the WPS for the above reasons. In addition, as G Roberts stated, the concern for weld contamination exists with copper or brass. The AWS Welding Handbook, Volume 4, "Metals and their Weldability" considers the risks associated with the use of copper or brass in the welding process.
In the replies I have received thus far as well as the research, brass is ruled out and if we go with copper, we will need to qualify it.
I appreciate any input on this. Thanks.

By RonG

Date 05-15-2001 16:42

We use Copper because of its heat sink properties alot.

I was always led to believe Brass contains undesireable elements. I know it is not as good a heat sink as copper. And I dont like what Zink and Lead does to the air I breath.

By - Date 05-15-2001 17:36

Have you considdered using consumable ceramic backing? If you only have a relatively small job, or you have a lot of different weld configurations, ceramic backing might be the cost effective answer.

Whether you will have to qualify this, I am not sure. Maybe someone else out there can tell us?

Regards
Niekie

By - Date 05-15-2001 17:33

I am not sure if I understand your question properly, so I will give you the "fundamental" answer. Tell me if I am aiming too low!

As with any backing, the idea is to ensure a complete penetration of your weld in a consistent manner because you have something to "weld against". This requires much less skill in achieving full penetration welds than without backing. When using steel backing, the backing strip will fuse into the weld. On the other hand, when using a material that has a very high co-efficient of thermal conductivity, it conducts the heat away fast enough so that the backing material does not melt. You then have the benefits of full penetration without having to remove the backing strip by grinding. Obviously, there are the problems as mentioned by some of the others.

If your question rather revolves around "better" penetration, then I do not believe that you actually get "better" penetration. You just get other benefits than is the case with steel backing.

Regards
Niekie

By RonG

Date 05-15-2001 21:03

If I understand you. The WPS requires a backing (steel) and some one is trying to talk you in to using Brass? And you are wondering if you can do that with out doing a PQR?

NO. I dont think so. Check your essential variables.

By ziggy

Date 05-16-2001 12:28

Yes, others are requesting a variation from the WPS; using copper or brass rather than the specified steel backing.
I checked the essential variables. #35 refers to the omission, but not inclusion, of backing or backgouging.
Since the D1.1 C5.10 refers only to steel in prequalified complete joint penetration groove welds, I would opt that the substitution of the copper backing bar for the steel necessitates a PQR. And thus the welders in the field would need to qualify to that as well.
Many thanks for the help.

By CHGuilford (****)

Date 05-16-2001 13:32

If you have access to the root side of the weld, you would be able to backgouge and repair any lack of fusion or other problems. That would be in compliance with D1.1 -5.9 so that qualification would not needed.

As far as using brass for a backing..I would think that falls into the "or similar" category of 5.10. Either way, if the use of brass causes defects at the root, it would still need repairs. If it doesn't, then great.
CHGuilford