Soldering or brazing stainless wire

I'm trying to solder .020" stainless wire to .009" stainless wire. I can't have a resultant joint larger than .020" diameter. I've been
grinding off 1/2 the thickness of the .020" wire and creating a lap joint.
I've used "soft solder" (96% Tin, 4% Silver, 220C or 430F melt) and a standard Weller electronics soldering iron, and have created joints that fail at 8 lbs pull test. The metals pull away from the solder.
I've used "hard solder" (which some folks consider brazing) which is 70% Silver, 20% Copper, 10% Zinc, (melt point 747C or 1390F). I've soldered this with a minute butane torch. These joints fail (also at 8lbs (coincidentally?)) because the .009" wire breaks near the soldered connection; the solder joint itself remains intact.
I'm guessing that I'm overheating the .009" stainless wire and it's weakening the metal. It breaks repeatedly in the same place, about 1mm from the solder joint.
Any suggestions?
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snipped-for-privacy@sbcglobal.net wrote:

I'd have a longer section of the wire heated. Then since you are into metal work, the wire must go back through anneal and temper routines to no longer be brittle.
Martin
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Martin Eastburn, Barbara Eastburn
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Could you explain further, Martin? I don't understand your reply...
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What are the anneal and temper routines for stainless?
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snipped-for-privacy@sbcglobal.net wrote:

What is the stainless steel type you are using? Chances are it is 300 series (austenitic type) which cannot not be heat treated , only work hardened. In this case, if it is spring temper wire you could fully anneal at about 1900F and rapid air cool it, but since it is not heat treatable there is no tempering. Heating the wire to solder it is not going to make the wire brittle. Heating it to braze it may make it brittle if you cool it very slowly due to the precipitation of chromium carbides. This however is highly unlikely considering the cooling rate in air of such a small cross section. Most likely the brazing will simply anneal it. In this case, if you are using a spring temper wire you will drastically reduce the ultimate strength, especially in very small diameter wires which can easily achieve strengths over 300ksi.
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snipped-for-privacy@sbcglobal.net wrote:

Why not just increase the length of overlap and continue to use the soft solder. Make sure you abrade the stainless in the brazing area because some stainless wires have a coating. If you find the solder is not flowing well you may need a more aggressive flux.
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Close inspection of the failed soft solder joint shows it just seems to peel away. I suppose extra length might add to the pull force, but I'm skeptical. I have been sanding the stainless wire thoroughly, then fluxing. Anyone know if there's a chart out there of pull forces I should expect with soft solder?
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snipped-for-privacy@sbcglobal.net wrote:

soldered
Just a question about the design requirement (and expectation) of the joint. By my calculation, the 0.009" diameter wire has a cross-sectional area of 0.000064 square inches. An 8 pound failure load for the 0.009" diameter wire equates to 125,000 psi.
I am presuming that you are soldering an overlapped joint of nearly coaxial wires and then pulling the wires in tension (one end the 0.020" wire and the other end the 0.009" wire). For this type of joint, the failure will normally be adjacent to the soldered joint when a "matching" strength joint is made. Any clarifications for this situation?
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snipped-for-privacy@sbcglobal.net wrote:

soldered
Just a question about the design requirement (and expectation) of the joint. By my calculation, the 0.009" diameter wire has a cross-sectional area of 0.000064 square inches. An 8 pound failure load for the 0.009" diameter wire equates to 125,000 psi.
I am presuming that you are soldering an overlapped joint of nearly coaxial wires and then pulling the wires in tension (one end the 0.020" wire and the other end the 0.009" wire). For this type of joint, the failure will normally be adjacent to the soldered joint when a "matching" strength joint is made. Any clarifications for this situation?
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