Hello, i have an issue with brazing of the same part with two different flux types on the induction braze of cylindrical lap joint. Material is 304L sst per QQ-S-736. Our braze filler is having trouble wetting the joint on a part that we otherwise have never had a problem with. Cleaning parts are standard, with a ultrasonic soapy wash, alcohol, and air dry. The part that holds the braze ring turns black early and the filler does not pull through like normal. The mechanical fit checks out. Our standard use Ultra-flux is not expired and all processing is standard. We have switched to a Black Flux (boron-based I think) that seems to do the trick in protecting the part and letting the filler wet case metal. But I do not understand the source of the problem.
The vendor was changed recently and am concerned about their cleaning method. But as we clean the parts extensively ourselves I don't beieve this is the problem. Is there a metallurgical constituent that promotes the oxide/carbide formation and still have the stainless steel meet the spec?
Is there a cleaning method/chemical of machined stainless steel that leaves a resilient residue on there that we are just not getting off? I believe the vendor uses lacquer thinner. Passivation of the part is not required per our drawing.
I am writing because this is I problem I have noted for a few years. It starts with one part, we troubleshoot, and the the problem mysteriously disappears. Has anyone out there had this experience?
Thanks,
Casey
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