Brazing SB-75 tubes to 304 Stainless

Date 01-05-2004 18:45

We are Torch Brazing SB-75-122 copper tubes to 304 Stainless Steel manifolds using Bag-1 brazing wire. We have been experiencing some failures of some of the brazed joints after a short time in operation. The failed joints seem to have inadequate bonding of the tube to manifold. The tubes are only 18ga. and the manifold thickness varies from 1/4" to 1/2" thick. The joint is a socket type lap joint with the tubes projecting through the wall of the manifold. The units work in a steam and water enviroment with maximum design temperatures of 350°F.
Would appreciate any input as to how to possibly correct the problems and input from anyone that may have experienced similar failures. Anything at all will be appreciated.

Hojo
108th Inf

By MBSims

Date 01-06-2004 03:00

I have seen similar failures in air cooling heat exchangers where 304 SS pipe was used as a header and the copper tubes were brazed into drilled holes in the header. This seems to be a poor joint design and could be improved by having a swaged stub on the header to get better heat transfer by making more of a socket-type lap joint than the T-joint configuration. This is much more expensive than just drilling holes though. Why does the header need to be SS? Why not a copper or copper-nickel pipe?

Marty

By Hojo

Date 01-06-2004 13:02

Marty,
Thanks for the input. The manifolds are relatively small in Ø (1-1/2" - 2-1/2"Ø) with either 3/8"Ø or 1/2"Ø tubes brazed into them in a staggered pattern with spacing of 0.160" to 0.200" between tubes.

Hojo

By jwright650 (*****)

Date 01-06-2004 15:03

I don't know anything about your situation, I'll just throw out a question, would it be feasible to use short stubs of stainless tubing to weld to the stainless manifold, then braze your copper tubes to the stainless tubes that are welded to the manifold? It might be possible to get a joint that is stronger.
just a thought,
John Wright

By MBSims

Date 01-06-2004 23:46

I would still look at using a 90-10 CuNi pipe instead of 304 stainless for the header. You would get much better results on the braze joints.

Marty