Brazing Stainless Steel

By dee

Date 11-13-2002 01:02

Louis,
Can you give some more detail?

I suspect you might be heating the work hotter and keeping it hot longer than you should, effectively dissolving alloying elements from the base and filler metals to their detriment. On that account it becomes necessary to know the composition of both, and what heating method you are using for the process.

It may be a problem to change the bronze, considering color variations among different alloys; presuming this is a decorative item it may be a concern, but excluding that, there are a variety of alternatives which begin to liquify and flow at a wide variety of temperature ranges... my impression is that there are fewer options for changing the chemistry of the stainless and the practical differences between them are, themselves, more limited.

Sooooo...... how thick is the brass (technically most often a bronze) layer, how are you heating it, how are you cooling it, how are you applying it, and how,where and when are the cracks developing? Any other detail I might have forgotten to mention you might think add, along with alloy designations, type of flux, etc. would also help.

Good luck with it.

Regards,
d

By Louis

Date 11-13-2002 03:24

Yes, there in lies my problem. I do not have that many details myself. I am using some scrap stainless steel that we had around the shop. I have no idea about its chemical composition or even its number classification. I am using some old brazing rods about 3/32” in diameter. They do have a flux on them, but I have no other info on them either. I am using Welco sprinkle on brazing flux and an Oxy-acetylene torch to do the brazing. The cracking occurs during brazing. Yes we have thought about too much heat being the problem and I tried to cut down on it. It reduced the cracking but did not eliminate it and I have to get it hot enough for the brass to melt. I even tried doing it a little at a time. We let it air cool on its own. We have thought about stresses being introduced due to the engraving. We brazed on a piece that had not been engraved and got the same bad results. We are only using this base metal and rod to try and produce a proto type to use for display and also to find out if it worked. So far we haven’t had any luck. Perhaps it would just be easier if you could suggest a type of SS and brazing rod that would work with the oxy-acetylene torch to do this. The only stipulation that we have is that we would like to use 10ga. or thicker.

By billvanderhoof (****)

Date 11-13-2002 05:00

It could be that a little at a time is the wrong way to go. All at once may be better (less stress from differential heating). You could build a small furnace from a few firebricks to heat in. Get the bricks hot first then put in your piece. Probably flux on the piece as it goes in. Perhaps bronze filings mixed with flux would work. Add heat till it melts then cool as evenly as possible. Oxy acet as a heat source should be ok but something softer like propane air might heat more evenly. I have little experience with stainless but do know that some stainless is adversly affected by long periods at high temps.

If all else fails electroplating copper is easy and may give the effect you want. With a little fiddling you should be able to build something out of a battery charger that would work well enough. Any plating shop that does decorative chrome will be able to do copper (they put it under the chrome)

Good luck- Bill

By Louis

Date 11-13-2002 15:25

Can anyone recommend a type of stainless and brazing rod that will work without the SS cracking when using the oxy-acetylene torch?

By Niekie3 (***)

Date 11-14-2002 18:23

Hi Louis

It is possible that you are experiencing a failure mechanism called liquid metal embrittlement. It would be difficult to say for sure without having a lot more information, but you can try the following test to give you an idea where the problem lies:

1) Heat up you SS plate to a simmilar temperature as you do when brazing, but add nothing to the surface. (In your mind perform the same types of moves with the oxy-acetylene torch as when brazing.) If no cracking occurs here, then the problem is not the thermal stresses per se.
2) Add the flux to the surface while doing as above. If there is no cracking, then it is not due to the flux.
3) Add the brazing while doing as above. If it now cracks, then the problem lies with the addition of the brazing itself. In this case, almost surely liquid metal embrittlement.

Regards
Niekie Jooste

By Louis

Date 11-14-2002 21:45

I am not familiar with this type of embrittelment. I have found some 304 3/16” thick SS to work with and have some other SS number unknown that is about 14ga. I have found I can do whatever I want to the 14ga and it doesn’t crack. I felt like this was due to its thickness. It does not go from red hot to just hot as fast as the thicker metal because the thicker metal acts as a heat sink and draws the heat away from the immediate brazing area. I have brazed on a smaller piece of the 3/16” SS and played the torch on it to keep it from cooling so fast and the piece did not crack, but I feel this is a hit or miss way of doing it. The oven idea may be a possibility but I question whether the brass would coat the part evenly. If I find that it is this embrittlement that you mention what can I do about it?

By dee

Date 11-15-2002 05:00

Louis
If your prep is good to start with and your flux is properly applied try the oven. It wasn't an off-hand suggestion when it was made; it was a right answer.

I doubt you can find it for sale but they do use brazing alloy powders mixed with flux designed specifically for jobs of your nature. If you attempt to experiment and make your own, use a new file or risk unpredictable results... bits of silver solder or bronze are also spaced evenly (considering the volume of the pieces) over a surface wher a full and even flow is desired. The trick is appropriate distribution of heat.

Note also if your steel is heating unevenly or too rapidly it is usually an indication that you have too much heat; a problem with the torch type and size and how it's being handled, not the metal thickness. Air-acetylene (like a plumber would use) is a bit less intense and often used in jewlry and similar brazing aplications. Flame size and pattern is very important and becomes more important as one delves into this kind of advanced and "delicate" kind of work.

I try not to be critical of anyones medium and process plans, but I agree with the electroplating suggestion; use brass instead of Cu if that's what you want. Properly masked, the same ground you used for etching might be used for plating as well. Final finishing will be minimal. We're describing a finished product identical to the one you conceived. It will be possible to fill the etched recesses close to flush and wind up with a smooth, flat surface. Subsequent finishing should be negligible; ten-thousandts of an inch perhaps. By the way, the concept is very creative and I am sure will be extremely interestingn when completed.

You seem to be mixing variables. This compromises the validity of your experimentation. Try to eliminate the variables such as metal thickness, alloys involved, etc. General rsearch into the various techniques successfully used to braze would also help lead you to the solution.

Good luck
d

By Louis

Date 11-15-2002 15:12

All right thanks guys. I am looking into the oven idea I already have fire bricks that we might use to build the oven. I am also looking into purchasing one although I know they are quite expensive. I have also done some research on the SS and found that 304L and 316L can be held at higher temps for longer time periods before becoming embrittled.
Thanks again for all your help, Louis

By Niekie3 (***)

Date 11-16-2002 13:31

The thinner the material, the lower the stresses built up due to uneven heating. These stresses are obviously a contributing factor to the cracking.

If it is liquid metal embrittlement, then lowering the temperature would have a positive effect. Lowering the thermal stresses would also help. All in all, heating it in a controlled way in an oven, as suggested by some of the other posts would help.

If the material you are using is not an L grade (or stabilised) then you could also be sensitizing the material. This in itself will not result in cracking, but will render the material more sensitive to corrosion. This corrosion may be from the flux, but this is purely a guess.

Regards
Niekie Jooste

By Steve 3884 (*)

Date 11-19-2002 22:52

Hello Louis,
Have you considered having someone arc spray the bronze onto the S.S.? After spraying, the excess could be power brushed off or the part could be masked off with thermal tape prior to spraying. You could most likely hold part temp down to under 200°F using that process.
HTH,
Steve