Plating with Stainless Steel

No-- the term 'stainless' is not really appropriate here. It is a non-specific term notating a corrosion resistant ALLOY of chrome, steel, nickel and other elements. There are MANY stainless steel alloys, each with different compositions and manufacturing procedures-solution annealing, hardening,etc.

What you may mean is CHROME PLATING.

YES- high nickel steel can be chrome plated, but so can mild steel, which is low cost and much easier to fabricate into small intricate shapes by many methods.

We would usually use 304 or 316 stainless.

Would chrome plate withstand repeated cycling to 850°C?

I understand your point about mild steel being cheaper but the Kovar steel is specifically formulated to match the thermal expansion of alumina ceramic so using Kovar means that the jigging remains accurate at the brazing temperature.

Net search has hit here. Not being conversant with the high temperature applicateion of plated- had to resort to it.

Look at FAQ#9

In general, a hard chromium surface is consistent against static temperature loads up to 400°C, assuming that the component can be heated up slowly. After 400°C static load, a gradual loss of hardness and after 700°C a rapid loss of hardness of the chromium coating occurs. However, many parameters influence temperature resistance and can possibly change it. To some extent, hard chrome coatings are successfully used for temperatures over 1000°C. We will be happy to provide additional details upon request.

Did you think of some sort of spray deposition (say plasma spray etc of ceramics) on the base metal? I would have said of some high temperature antiseizes, including bonded one (MoS2) available from Molykote, but then these may contaminate your liquid puddle.

Chrome looks possible, that's a good find. We ramp the braze temperature up fairly slowly & cool naturally without quenching.

Spray coats will, I think, be too inaccurate for our small jigs. These can be maybe 12mm diameter.

Anti-seize coatings generally work by increasing lubricity & would, as you have suggested, contaminate the braze joint.

Hard Chrome can work at 800ºC. It works in IC engines.

How about release coatings?

http://pdf.directindustry.com/pdf/walter-international/spatter-block-weld-spatter-release/26411-19217.html

We use a stop-off coating which does the same job but have found that this is not reliable.

Plasma spray of 316 and I suppose 304 stainless steel powders are established technologies here in the USA. For a complex small configuration I think a skilled operator and a hand held spray gun might work well. Obviously go for as dense a coating as you can. Some experimentation with grit blasting to prepare the surface for plasma spray might be needed so the coating will stay attached during cutting loadings.

I would accurately machine the brazing jig part on a good set of datum surfaces that could be masked for spraying. Then put the part back into the same datums on the machining fixture and finish the resulting surfaces to a planned thickness.

In my engineering work about 10 years ago I was well involved with a company in Dayton, Ohio called APS Materials. They are still in the same business and are very technically capable. Check out their website and you'll get some idea of what can be done: http://apsmaterials.com/

If you can't find a shop in the UK with similar capabilities, perhaps one that does work for aerospace companies or turbine manufacturers you might even try dealing across the pond.

Ed Weldon

That's an interesting site, thanks for that. I hadn't realised that plasma spray coatings could be as thin as 0.001".

I'll see if I can find any UK companies offering this service.

I'd think the Nickel steel substrate should be sufficient...

The problem with the nickel steel substrate is that the jig needs to touch the component parts to align them but there is then a danger that the nickel steel jig will end up brazed to the components.

Some of our parts may be only 0.25mm thick so it is not possible to jig them away from the braze joint.

If the jigs get re-used, what about ceramic jigs?

bwire & Andy, I'd be a bit nervous about using ceramic jigs or coatings, we use ceramic jigs in other areas &, although they withstand high temperatures they start to break down over time.

But it depends on how often they can be re-used and the price of a replacement surely.....once a shape is designed, new ones cost very little if I remember correctly. The stuff we used to use was really hard and really strong.....

Ceramic expands very little when hot if I remember correctly......that might even improve the quality of the brazing....

All true, alumina ceramic parts are extremely expensive in small quantities but the expansion rate matches our other components. Glass ceramic parts are more reasonable although still expensive but the expansion is twice that of our other parts.

The Kovar steel jigs are much cheaper to make & last for ever if I can only overcome the problem of the braze material sticking to them.

You would likely be better off with a flame-spray coating. Contact www.eutectic.com for more specifics.

I've sent an enquiry to a UK company who offer both plasma & flame sprayed coatings so I will see what they suggest.

Why not try to talk to a company that does metal spraying. They could put a thin layer of 316 st st or even tungsten carbide on the component

You should also look into thin hard coatings (PVD for instance). Some of them have high oxidation temperatures and able to withstand such temperatures, they are very hard and resilient, if applied right. They are relatively cheap, can be applied very thin (few microns range) and are pretty inert. I succesfully used some TiAlN (there are many recipes) in coating WC ejection pins in aluminum die casting (~700 C). But, again you'll have to choose an appropriate coating with high oxidation temperature, not the commercial TiN that every basement shop offers nowadays. Talk to companies like Balzers-Oerlikon, IonBond or others like that, which have a lot of experience.

A couple of approaches you might consider. You might put the finished kovar fixture through an oxidizing treatment, the same treatment it is exposed to during glass-metal sealing. The oxide will act as a brazing stop, but will break down eventually in your reducing atmosphere. When that happens, you must re-oxidize it. A second approach is to build your fixtures out of kovar and spot weld small stainless inserts at the locations you need to protect from the liquid braze. 300 series stainless spot welds very easily to kovar.

You're right about the oxide, we use it to seal to glass optics but I think the oxide would be reduced every time we braze & may well be reduced during the brazing operation leaving bare steel exposed to the braze. The brazing conditions are not too dissimilar to the stoving conditions we use to remove oxides from Kovar parts before assembly.

We have used stainless inserts to stop the braze but it is becoming more difficult as the assemblies get smaller & more intricate.

You would need to use a much thicker oxide layer than what you use for glass to metal seals. It would be a sacrificial layer that would get thinner with each braze cycle. With 10% hydrogen, it might last a few cycles. Stainless inserts spot welded to kovar works well and is easy to sizes in the 0.1 inch region. We make lots of fixtures this way. If you have proper welding discipline and equipment, you can go much smaller. We currently do spotweld fixtures with 0.01 inch features for medical applications.

Nigh --

Unitek spot welders

http://www.unitekequipment.muc.miyachi.com/

have been used extensively in the USA by orthodontists among others for miniature welding of stainless steel components. Older analog Unitek welders show up here a lot on ebay although I fear the shipping cost from USA machinery surplus sources to the UK may be prohibitive. Seems like since they joined with Myachi they have backed away from supporting the older equipment. So information is hard to come by.

Perhaps some investigation into the dental world in your country might turn up useful leads. Nowadays the dentists deal with outside labs a lot to build fixtures. I suspect these people might be very approachable for someone who arrives, cash in hand, looking to buy old unautomated weldon equipment.

Ed Weldon

We have a couple of small spot welders similar to the Unitek, I can't remember their name offhand but we use them to weld down to 0.025mm thick material.

We could use the stainless strip method but I was hoping to find an all over process. The plasm/flame spray method suggested in earlier posts looks like it has some promise, I'll see how that pans out.