Surface reactions of SS preventing brazing
Surface reactions of SS preventing brazing
(OP)
Pardon my possibly silly question but the following is a subject where I have little experience.
Due to other process limitations I have been forced to choose brazing/soldering to join thermocouple measuring points to stainless steel fittings (the thermocouples need to be gorunded).
A high-silver solder used together with a flux intended for stainless steel seems to be a workable solution, but unfortunately the part geometry makes it impossible to ckean all the flux from the joint - I am expecting this to create reliability issues due to corrosion.
I have been trying to solder with electric induction heating and a jet of inert gas onto the hot area but I have not been successful. The solder (a high-silver material for good workability) is balling up and does not wet any of the parts. Also there is visible discoloration of the parts even under the gas jet. The same problem is apparent with 95%nitrogen/5%hydrogen mix, argon and 97% argon/3% hydrogen mix as shielding gases.
What could be the cause of the problem? impurities in the gases? Incorrect purging of gas lines? Incorrect cleaning? Gas picking up contaminants? Impurities in the metal leaking and damaging the surface?
Any guidelines towards the correct direction would be useful! Anyone here has experience with flux-less brazing?
Regards,
n85
Due to other process limitations I have been forced to choose brazing/soldering to join thermocouple measuring points to stainless steel fittings (the thermocouples need to be gorunded).
A high-silver solder used together with a flux intended for stainless steel seems to be a workable solution, but unfortunately the part geometry makes it impossible to ckean all the flux from the joint - I am expecting this to create reliability issues due to corrosion.
I have been trying to solder with electric induction heating and a jet of inert gas onto the hot area but I have not been successful. The solder (a high-silver material for good workability) is balling up and does not wet any of the parts. Also there is visible discoloration of the parts even under the gas jet. The same problem is apparent with 95%nitrogen/5%hydrogen mix, argon and 97% argon/3% hydrogen mix as shielding gases.
What could be the cause of the problem? impurities in the gases? Incorrect purging of gas lines? Incorrect cleaning? Gas picking up contaminants? Impurities in the metal leaking and damaging the surface?
Any guidelines towards the correct direction would be useful! Anyone here has experience with flux-less brazing?
Regards,
n85





RE: Surface reactions of SS preventing brazing
RE: Surface reactions of SS preventing brazing
Dr. Ron Smith
Materials Resources International
811 W. Fifth Street
Lansdale, PA 19446 USA
(215) 631-7111
(215) 631-7115
www.MaterialsResources.com
rsmith@materialsresources.com
He has active alloy solders and brazes that might work as well as an ultrasonic soldering method.
Bruce
www.accuratus.com
RE: Surface reactions of SS preventing brazing
I will definitely look into the active soldering alloys. Do these contain reactive metals that reduce the oxide layer?
Surface preparation for the samples I did consisted of a thermal drying/degreasing process (the thermocouple is a mineral-insulated cable)followed by sandblasting with alumina grits. The attempts at brazing were done within a few minutes (less than 5).
Regards,
n85
RE: Surface reactions of SS preventing brazing
The active alloy materials contain reactive metals like titanium and zirconium that act as very aggressive oxygen scavengers to reduce the stable oxide layers thus permitting wetting of the faying surfaces. Add some ultrasonic energy to help disrupt the oxide layers and voila, a good solder joint at reasonable temperatures and atmospheres. I'm sure Dr. Smith can give you a much better description of the joining mechanisms when you call him.
Best regards.
Bruce
www.accuratus.com
RE: Surface reactions of SS preventing brazing
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Plymouth Tube
RE: Surface reactions of SS preventing brazing
John
Denial ain't just a river in Egypt...Mark Twain
RE: Surface reactions of SS preventing brazing
Edstainless, I have actually used the technique you suggest in the past and it works very nicely but the requirement here is that the thermocouple end is sealed at the same time - the thermocouple is mineral insulated metal clad with stainless steel cladding too.
n85
RE: Surface reactions of SS preventing brazing
We did some braze work and always used virgin SiC grit.
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Plymouth Tube