Can brazing work for joining these parts?

My worry is the 350 °C range between max temperature for one of the parts and service temperature for the finished assembly.

I have two parts to join. First is a device that contains various things inside of a steel shell, about half an inch in diameter. It’s not supposed to get heated above 750 °C. Second is a part I get to specify, which I was hoping to make of pure nickel for oxidation resistance and good thermal conductivity. I want to braze the nickel (?) part onto the exterior of the steel shell. The service temperature of the finished item will be 400 °C.

Oven brazing won’t work. The first part is a few inches long and its other end has a much lower maximum temperature limit of only 420 °C.

The stress on the joint can be small. I can machine the parts to fit closely or even make one part capture the other. Probably the nickel (?) part will be a sleeve over the steel device. There is no motion or vibration at high temperature, and just assembly and handling stresses at room temperature to construct a machine. I do want the joint to maintain fairly good thermal conductivity, and need to keep things small, hence the desire to braze.

Can brazing work?

Thanks!

We get silver based brazing calibrated by temperature, from 6% (920°C+/-) down (40% Ag is about 650°C) Your post isn’t clear about whether your worries are about assembly temps, or working temps. for esthetic reasons you may be better off with a threaded assembly using either teflon or another compound. could you give us more detail on pressure etc.
Hand brazing will probably work, high grade nickel will blacken but welds quite well, local temperatures will go well over fusing points though, so if tempering and/or cristallin métal properties are at stake, mecanical assembly may be a good idea.
Another criterea is fatigue, any local weld will temper the steel unless extreme care is taken, I don’t know how the nickel reacts, so if mecanical stress is a factor care must be taken.

I’d think that traditional brazing would overheat your part. Do you have access to GMAW equipment? It’s fast and thermal conductance becomes less of an issue. Get some low temperature filler (some type of silicon bronze), setup the GMAW equipment for brazing, and make your joint quickly.

Seems like a job for Laser welding.

Ooh, or laser brazing!

Cplif, my worries are that assembly and working temps are too close together. I could pick a filler metal for which this means the assembly temp is the problem, or another for which the working temp is the problem. Looking around a little bit, I did not find any filler metals with an assembly temp below 750 and a working temp above 400 °C. Is this because it’s hard to find, or because I’m just not familiar with the subject?

I don’t need a seal and don’t have any fluids involved, if that’s what you mean by pressure. I need a mechanical assembly that is small and lightweight and has decent thermal conductivity. The part with a steel shell is a pre-existing OEM subassembly, and the steel is only 20 or 30 mils thick. I don’t think I can machine it significantly; certainly I can’t thread it. And it’s stamped or deep drawn or something; it isn’t so precisely cylindrical and different ones don’t have a very repeatable diameter, they vary by maybe a mil. I like the idea of filling the gap with metal, in part to handle this variability.

The worry about assembly temperatures is not that I will temper or change crystallinity of either the steel or the nickel. The worry is that the devices that are sealed inside the steel are rated for 750 °C.

Balthisar, I have access to GMAW and a variety of other methods. It is pretty easy to sub this job out to any shop in the US. And I have a fair sized machine and welding shop downstairs right now, with a staff of 20. I don’t know what to look for; that’s the weak link in the chain right now.

I wish I had a better perspective of the subject. I’ve never specified anything besides ordinary arc and TIG welds and E-beam welding, and that was always with handholding. I’m pretty clueless whether this is trivial or practically impossible or somewhere in between!

I would look for a polymer, something like a carburator to engine bloc liaison if you have the space. Maybe a thermo-retractible.
Another solution may be gaining thickness on your steel part, then further machining. Low temperature recharging can be done by projected metalisation (I’m not quite sure of the term in english, molten metal (nearly any fusible metal from gold to carbon steel) projected by airblast through a plasma or gas torch like a nifty spray paint. The base must be sandblasted, then recharged and machined, but the treatment itself is cost reasonable.
I can’t imagine a filler metal with a 750° fusion point supporting 400°C constants with differentials to room temp, just intuition though.

What polymer has 400 °C service temperature?

What do you mean by “with differentials to room temp”?

Thanks!

you could possible thermal braze the parts together.

tinning the added part then spot weld it on to the other so there is minimal risk of over heating the component part.

50/50 silver solder…or a zinc derivative.

just an idea.

( been welding 20 years… i never got to complex into it as far as temps)

Interesting. I could also plate some filler metal (or something, like maybe copper or silver) onto the delicate component part too, to thoroughly wet it before heating it any. Both parts would be tinned that way. Might that help?