Removing flux on stainless steel

[Robert1998]

Hello. I need to braze some stainless parts. some of them are 1,5mm and others 4mm. I´m using 40% silver rods and a white powder as flux. I don´t know the composition of the flux, but works OK in stainless. The question is, I have seen people quenching the part just after brazing it while is hot, to remove the flux. Is that the correct procedure? I´m not sure because that can cuase too much stresss in the joint and theses parts are intended to work in vacuum, so they have to be air tight. Most of the joints are T joints.

Sometime, I have let the part to cool down, and then, put into water for 1 or 2 hours to soften the flux, but I got some stains, so, not sure what to do.

Thank you

best regards

 

[hotponyshoes]

You won't harm a (good) braze by quenching it.
A 2-stage quench would be more effective,
Allow long enough for the braze to set completely solid. Then dunk it into hot water while it's still warm and swirl it around to remove the majority of the flux.
If you have the hot water hot enough then a second dunk into cold water will shock off the majority of the remainder

 

[hotponyshoes]

I'd think the stains are probably due to heating the stainless rather than flux staining (although it could be either)
I'd try electropolishing for that. There are a few guids on here on how to try it on the cheap but a dedicated machine (bought or homemade) makes it easier if you are in production volumes

 

[Robert1998]

Thanks for your asnwer. Ok, I´ll do it that way.

Another question. I´m joining this 2 parts





Using silver, it usually split it too much. i mean, the brazing area is bigger than I need. Usually that is not a problem, but yes in this case. The silver CAN´T touch the red area. Any advice?

 

[optima21]

if its tenacity No.5 flux, (used for sliver soldering stainless steels and works up to 900c I think a caustic soda solution is recommended, but that didnt work for me, so I used a weak sulphuric acid solution, this worked fine for me, but a citric acid solution is an alternative (and readily available.

borax may also be used as a flux for brazing and silver soldering, but never tried it, so not sure if suitable for stainless.

if you're wanting to join the two flanges together, do you have access to a tig welder as that may be an alternative.
or is it a flange in a tube? if its the second, I'd use a ring of solder around the joint (0.5mm wire may be fine ), then coat that in flux and then heat it up, that way you get a consistent ring and when it is at the correct temperature, the solder/brazing wire will just flow into the seam.

 

[500e]

If you get temp right it will capillary round the joint with minimum spread.
Don't go spreading the flux to far.

 

[voipio]

Solder is not the best thing to use on stainless steel. I second the use of TIG, if possible, and to purge the inside of the tube with shielding gas. If the parts are close fitting, then autogenous TIG welds should be possible (i.e. no filler required).

 

[Robert1998]

Thanks for your answers. I have TIG welder buuuut I lack the skills to make such joint

 

[optima21]

Solder is not the best thing to use on stainless steel. I second the use of TIG, if possible, and to purge the inside of the tube with shielding gas. If the parts are close fitting, then autogenous TIG welds should be possible (i.e. no filler required).

yeah I'd go with that too, and my skills aren't too good, but I did this on my car exhaust recently using pulse tig, and it went surprisingly well. the tube in 38mm o/d 1.0mm wall thickness, the flange was alot thicker. they were tacked and then autogenous tig welded.

 

[Robert1998]

Ok. I'll make a try

 

[Robert1998]

Ok, I´m going to try with tig and no filler. The question is, the surface of the ring (the red painted) have to remain absolutely flat. How should I weld it to avoid deformation due heat?
Should I go for a continous weld till complete the circle or welding sections, for example opposite?

 

[Robert1998]

Aaaand another question. The small parts of this project (a oil difussion vacuum pump) I tried with tig but they are too small so I´m brazing them.




in this picture you can see the change of color in the stainless steel. Is normal? It gets dark gray. Electropolishing is out of bufget, because I only need for this project. It is 304 Stainless steel. How can I remove this layer, Soaking it in diluted hydrochloric acid maybe? HCL don´t disolves silver.