How to solder a gold wire inside a steel tube for 600¦C
How to solder a gold wire inside a steel tube for 600¦C
(OP)
I need to solder a 0.42 mm diameter gold wire inside and along a stainless steel tube OD/ID 1.5/0.45 mm and 20 mm long, to resist operating temperatures of 600°C. Any suggestion on how to do it?





RE: How to solder a gold wire inside a steel tube for 600¦C
The special circumstances of small diameter and limited clearance warrant consideration of alternate methods. E.g., plate a very thin layer of nickel inside the SS tube (from a Wood’s Nickel solution, using an auxilliary anode wire), since it is much easier to bond to Ni than to SS. Also, plate a thin layer of Ag onto the Au wire. Then, insert the wire into the tube and heat above 962°C under vacuum or inert gas; the Ag will melt and braze the Au to the Ni.
*The prefix B indicates a Brazing filler metal.
RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
Jesus is THE life,
Leonard
RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
There is a major risk in your design due to the large CTE mismatch of fused silica and SS. The mismatch will open the gap between the wire and SS tube as T increases, which is helpful when you braze, but will cause hi stress at the braze joint everytime you go up to your 600C operational temperture. Will the part see many thermal cycles? Stress rupture and/or thermal fatique may be an issue if it does. One option to mitigate this risk is using a relatively thick, ductile metal interlayer between the silica and SS. This wili give you a compliant interlayer to accomodate the CTE mismatch induced stress. You could copper or (ductile) Ni plate the ID of a SS tube with a larger ID than your current design, or plate the wire, or both surfaces. BAg-8 would wet copper or Ni and gold plating well. A stress analysis based on the stress rupture of Cu or Ni at 600C should tell you if it will work. In any case, plating a small ID will be challenging - so maybe plating the wire, or both surfaces, is a better option. Anyone have other insight?
RE: How to solder a gold wire inside a steel tube for 600¦C
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RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
1) Metalize the alumina tube using the Mo-Mn process (if you are unfamiliar with this, I can give you most of the details).
2) Green Nickel Oxide coat on Mo-Mn metalized surface fired in dry H2. This provides a pure Ni surface for brazing.
3) Pure Silver braze the Au plated silica to metallized tube at 1010C in dry H2.
This will give you matched CTE's with reasonably ductile interface.
Jesus is THE life,
Leonard
RE: How to solder a gold wire inside a steel tube for 600¦C
Thanks for details of the application. (I was trying to visualize a corrosive situation where the SS needed grounding & provided mechanical protection)
Due to the thinness of the gold, brazing will create serious long-term alloying problems at 600°C, and most rapidly for the lower melting braze metals. I will mention that it is even necessary to plate a Ni barrier layer to prevent diffusion between Cu and Au in much lower temperature applications, e.g., circuit boards and jewelry.
With the new details and thermal mismatch issue, I suggest not doing any brazing at all; just gold plate the inside of the SS tube. Bare Au to bare Au will provide electrical contact, and w/o a rigid brazed connection, no CTE expansion problem. Note: to plate Au onto SS, first do a Wood’s Ni ‘strike’ onto the SS. You might be able to buy an already gold-plated pin (tube), they are used a lot in the aerospace industry.
RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
If you suggest gold plating the inside of the tube and not doing brazing at all, how will the fiber be fixed to the tube?
RE: How to solder a gold wire inside a steel tube for 600¦C
To preserve your thin coating of gold long-term at 600 F (which I presume is essential to maintain your optic signal), it's advisable to not contact it with any other metal, which is why I suggested plating the tube ID with gold. For tighter contact, just put a little more gold on the ID.
What is the purpose of the SS tube? Could you use graphite (about the only conductor that can tolerate 600 C and not react with gold)?
RE: How to solder a gold wire inside a steel tube for 600¦C
We need a strong attachment, in fact, we don't need an electrical contact. The tube serves just as a ferrule to hold the fiber end in a 600°C environement without damage.
RE: How to solder a gold wire inside a steel tube for 600¦C
I did not read your requirements adequately. On rereading I see that max fiber temperature is 750C.
Is it possible to coat the the fiber a little thicker with Gold? If so, you could shrink fit the coated fiber inside an Invar or Kovar tube. If these materials are not available in tube form, you could make a tube by drilling and reaming round stock to size. If tube is custom made then you could stay with original fiber coated dimensions and size the tube hole for shrink fit using LN2 on the fiber and heat the tube.
Jesus is THE life,
Leonard
RE: How to solder a gold wire inside a steel tube for 600¦C
RE: How to solder a gold wire inside a steel tube for 600¦C
from Thread330-59538:
Unfortunately, molten aluminum is the Universal Solvent for most metals. 309, and about anything else I can think of, will very quickly dissolve. It is hard even to find a ceramic that aluminum will not reduce.
Graphite is OK, used to line the pots where aluminum ore is electrically reduced to the metal.
Although it is academic here, thermal fits won't work at high temperature, they very quickly relax.
What other than aluminum will be present? That is, will there be any oxygen or oxidizing species present (CO2, H2O)?
If there is no partial pressure of oxygen at all then one might search a little further to find if some refractory metal might work.
James Kelly
Jesus is THE life,
Leonard
RE: How to solder a gold wire inside a steel tube for 600¦C
Here is some thermodynamic data for braze elements alloying with the gold (& hence degrading the optical signal), and that further, the deduction that the more easily the braze, the more rapid the alloying with the gold:
The partial molar enthalpies of mixing for some elements in gold (at infinite dilution, i.e., into pure gold, in cal/gram atom):
Ag -4040
Al -30724*
Co +9590
Cu -2780
Fe +6090
Ga -22400
Mn -8500
Ni +5140
---Metallurgical Thermochemistry, 5th Edn., p.393-394 (1979).
A negative value indicates a large thermodynamic driving force for dissolution into gold.
These values also explain why Ni is used as a barrier layer between Au & Cu.
Indium is also a bad actor; Selected Values of the Thermodynamic Properties of Binary Alloys, p. 283 (1973), gives the partial molar enthalpy of mixing for gold (at infinite dilution, i.e., into pure indium, in cal/gram atom): -11200 cal/gram atom.
*agrees with James Kelly's comment in Thread330-59538 about liquid Al being a universal solvent [for metals].
RE: How to solder a gold wire inside a steel tube for 600¦C