Surface reactions of SS preventing brazing 3

[n85]

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

 

[cloa]

What's your full surface preparation procedure for these parts?

 

[Ceramicguy]

You're not reducing the tenacious, refractory and non-wetting oxide present on the steel surface. This layer, which forms naturally is what makes stainless stainless. So, you have to find a way to remove disrupt or chemically bond to this thin layer. In the interest of a quick solution, try contacting the following:

Dr. Ron Smith
Materials Resources International
811 W. Fifth Street
Lansdale, PA 19446 USA

(215) 631-7111
(215) 631-7115

http://www.MaterialsResources.com

rsmith@materialsresources.com

He has active alloy solders and brazes that might work as well as an ultrasonic soldering method.

Bruce

http://www.accuratus.com

 

[n85]

I suspected the oxide problem from what I've been reading but I wasn't sure. It is the chromium that creates this passivation layer right? What confuses me is that even with a reducing gas (with hydrogen) I saw no discernible improvement.

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

 

[Ceramicguy]

Chromium is one of the culprits. The Gibbs free energy of formation of the oxide is -697 kJ/moloxygen making it really refractory and difficult to drive the reaction the other way. If you can find an Ellingham diagram is will show you that even at about 800C a partial pressure of oxygen below 10E-30 atmospheres is required to even make the reaction thermodynamically favorable. This doesn't even speak to the kinetics of the reaction. So, blowing a little hydrogen at your solder joint is simply wasting time, money and resources.

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

http://www.accuratus.com

 

[EdStainless]

Have you looked into spot welding the TC junction directly to the SS part? I used to do this for test purposes and it worked well.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[jsokal]

One suggestion if I may, blasting with alumina grit is not going to help, on the contrary, blasting with a non-mettalic oxide can inhibit brazing even more. In fact alumina (aluminum oxide) is a common braze repellant/stop-off. Try using metallic blast like Nicroblast, or even silicon carbide is better than alumina.

John

Denial ain't just a river in Egypt...Mark Twain

 

[n85]

Thanks for the eye-opener jsokal! I was dusting off the workpiece with compressed air but I guess particles could get embedded. Using alumina as a repellent is a new idea for me! What I was attempting to control the solder spread was actually to heat-oxidize the surface, mask and then grit blast only the area I needed to solder.

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

 

[EdStainless]

Spot weld for adhesion and then flow braze for sealing?
We did some braze work and always used virgin SiC grit.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube