Succesful sodering of stainless steel with standard electrical solder

[chrisforpower]

My dad and I recently discovered that by cleaning it with phosphoric acid successful soldering can be achieved using standard electrical solder (60% tin, 40% lead, rosin core) and a soldering gun. By applying a drop of phosphoric acid and immediately following it by solder and heat a bond was successfully made between the stainless steel screw and a wire. When the strength of the bond was tested the wire failed before the solder did. Is their a factor that would prevent the use of this technique in the construction of a fusor?

[Chris Trent]

Have you tried lead free solder yet?

Also Check this thread:

viewtopic.php?f=6&t=2917&hilit=Lead+Solder#p12427

[jcs78227]

Congratulations on being innovative and trying a technique you were unsure of.

I would be wary of trying to use any soldering process (whatever the method, material, flux, etc.) for jobs where you are joining SS. Along with the difficulty in maintaining a uniform heat to allow the solder to flow evenly (uneven heating also raises problems with the SS alloy, potentially creating weak points in your base metal--metal creep), you also have to contend with the issues of micro- and macroscopic warping, difficulty in judging the strength and vacuum readiness of the solder joints over the entirety of a given joint, etc. Welding doesn't guarantee you won't introduce weaknesses that might influence creep, but at least you don't have large, uncontrolled discrepancies in temperature over repeated attempts.

Given the cost of the materials that you will be using (e.g. SS chamber, conflats, etc.), using a substandard method of joining the pieces undermines your investment. TIG welding is difficult to master, but from an engineering perspective, it's worth it to make the final product far more durable and not built on a technique meant for small scale joining operations. Welding is vastly different from soldering and affords the benefits of joining the base metals directly. There really seems to be no substitute for a quality weld operation, preferably TIG.

[Richard Hester]

Taking all these factors into account, I have two feedthroughs that I will be attempting each to mate to a flange using soft solder. The most promising is a large feedthrough (probably intended for a transformer or a large capacitor) that has a mating ring already tinned. I will probably try soldering this one to a properly prepared ISO63 flange. I also have another, smaller feedthrough with a copper mating ring that I will try soldering to a brass KF50 flange. Proper design of the mating surfaces goes a long way towrds a long-lasting solder joint, same as in TIG welding and silver soldering.

Having said that, I wouldn't try to use solder on larger components like a vacuum chamber. TIG for me, in that application.

BTW, thanks for verifying the effectiveness of the phosphoric acid flux. I was intending to try that when I had the time to get my hands on some of the flux. I'll probably be using tin-silver-copper no-lead solder.

.

[John Futter]

Yes
phosphoric acid should work

I soft solder SS all the time and I use killed spirits of salts or put another way saturated zinc chloride solution --easy to make --muriatic acid with pure zinc, excess zinc ie keep putting zinc in until no more dissolves.

make both surfaces wet with it and tin using ordinary 60/40 solder

I have used this down to mid ten to the minus 9 millibar with no bad results

[jcs78227]

I meant to add that I don't think anything disastrous would come of using solder, especially if one had taken the time to do it the best way possible to get clean results. The nature of the structures involved coupled (no pun intended) with the fact you are working with negative pressures would make a catastrophic failure unlikely to result in a great deal of harm.

Also, there is definitely a place for solder, I just thought that it's application for large pieces would prove more failure prone and frustrating than just having them TIG welded.

-Jonathan

[Carl Willis]

Hi Chris,

Interesting results with the phosphoric acid. Just be aware that purpose-formulated acid flux is probably sold somewhere locally and it is highly effective on stainless. I do a lot of soldering using Aladdin 450 lead-free solid-core silver solder and Laco liquid acid flux. I recommend both those products.

Soft solder conveniently joins stainless wire, plumbing, and fittings sized below about 2" diameter, for assemblies that are not going to get hot in service. Feedthroughs, tubing seals, and ad-hoc QF plumbing are where I tend to use it. Larger joints become progressively more difficult to heat and cool uniformly to produce clean, leak-free fillets. On the 6" chamber scale, hand soldering is an exceedingly frustrating and unreliable technique that I would dissuade anyone from adopting. I also discourage soldering on fusor cathode grids, if anyone has that idea rolling around in their head.

-Carl

[Richard Hester]

I did some nosing around, and some of the commercial SS flux use phosphoric acid. Other sites recommend using the phosphoric acid rather than a chloride flux with SS. I'll be trying some test pieces for sure before I touch any of the good stuff. Some pre-tinned lap joints should be a good test.

[derekm]

Forget the joint properties , TIG welding is just more fun to do...
There's just something magical about the quiet blue green plasma glow with a pool of molten metal beneath it. There's just a very low whisper of the gas... And there's something really apropos about using a plasma to create a plasma....

[Doug Coulter]

Yes, it's not hard to solder SS with the right flux, and I use some stuff McMaster sells for the purpose.
I use it for example to soft solder SS capillary tubing into copper tubing as part of my gas flow stuff.

The issue isn't that, it's that solder isn't strong at room temp, and is extremely weak at 100c, and parts of my fusor get hotter than that. The joint will creep under any small force.

I see lines from it on my mass spectrometer when there's any in the tank and it's warm - not even hot.

Brazing works well on SS too, if you get the right alloy silver solder (McMaster) that contains nickel (avoid Cd in vacuum systems). I couldn't use it in the above application as brazing heat makes the inside of the capillary tubing oxidize and plugs it up.