On Sat, 13 Oct 2012 16:29:03 -0500, dpb wrote:

On 10/13/2012 4:02 PM, Mike Marlow wrote:
dpb wrote:
...

For some reason I've been unable to figure out the brazing stuck very
nicely on one side ...but ... not
... consistently on the other. ...

So... a couple of thoughts come to mind. One is cleanliness of the surface
to be brazed and the other is the actual integrity of the material. In
order to get a good braze joint, you have to be able to heat the metal to
red hot. If you're not doing that, then you're just flowing molten brazing
rod onto a surface that it won't adhere to. If the surfaces can't be heated
to red hot, perhaps because of corrosion, then same problem - no joint.
...

Ground to shiny surfaces on all faces before starting...heat shouldn't
be problem; rod flows but then on cooling is occasional minute fracture
line along one surface edge. Meanwhile, same technique to best as can
replicate worked perfectly on other edge.


Perhaps your patch is too thick, and it cools at a different rate than
the base metal. The first edge worked good - the patch was
unconstrained. Now the second edge is moving around all over the place
as it heats and cools. Nice to have 2 different brazing metals - one
melting hotter than the other. Braze the one side with the "hot" rod,
then heat the whole thing - base and patch, to just under the plastic
temperature of the hot rod, which will hopefully be high enough to
melt the "cool" rod to braze the other 3 sides - while not melting the
first joint. Then let the whole thing cool slowly and evenly - with no
cracks.

The other way requires having the job set up so gravity hold the patch
in place. Have the patch formed to a close fit. "sweat" a coating of
braze onto the base metal, and while still hot, drop the heated patch
in place, and keep heating untill the braze flows out the edge of the
patch and wets the edges of the patch. You might want to flow a bit
more braze into the edges of the patch to be sure it is fully sealed -
then let cool slowly and evenly - no cracks or pinholes.