Avoiding Diffusion Welding in SS under Vac'

[Nick Clark]

I have 2 surfaces of Stainless Steel (316) at high temperature ~600C in vacuum and under a high pressure. These surfaces need to release from each other after the pressure is removed and the tempeture reduced. They stick together, I believe by diffusion welding. Elsewhere in the design HVOF Alumina has been applied successfully (at 800C). However this is a very expensive process, is there any alternative that might be used eg Wet Hydrogen Heat Treatment to form thicker Chrome Oxide layer to reduce this effect.

Nick

[hapewoe]

Hi, Nick,
unfortunately the chromium content in 316 SS is not high enough to form a heavy enough chrome oxide layer. You are right, HVOF spraying of Alumina is very expensive.
For your application it may be sufficient to paint one of the surfaces with alumina powder
(nano powder if possible) in water with 4% gelatine ( e.g. carrageenan) and 40% powder.
Dry this paint at about 150 to 200?C. This will keep the two surfaces apart during the service.
Another way would be to use AQUALON STR kolloid-suspension in isopropyl-alcohol from PRODHAG
This suspension is used in moulds to protect metallic inserts and copper moulds in continuous casting machines. The alcoholic suspension dries in warm air, other than the same suspension in water.
regards
hapewoe

[Gordon]

Hi Nick

to the Surface Engineering Forum.

Hapewoe's suggestions are certainly worthy of consideration.

HVOF alumina coatings seem a bit extreme here. Most thermal spray ceramic coatings used in this type of application can be applied using simple powder flame spray techniques using relatively cheap grey alumina. The coatings tend to be more open structured and friable compared to plasma or HVOF and probably much better suited to this type of application and cheaper.

[Nick Clark]

We did try Graphite coating (using aquadag) but this did not help although the bonding was a little easier to release. We have Alumina powder nm scale but were not conviced of the sticking prospects.

We have redesigned one of the surfaces to be a SiN ball. Hopefully this will solve the problem.

Thanks for your Help

Nick

[Gordon]

Thanks for the update Nick.

[Nick Clark]

Ok so the Si Nitride ball broke under half the load required. I'm back to coating the Stainless Steel??ball. A??little plastic deformation of the stainless substrate ball is likely under load This lack of this probababy cased the SiN failure as well as a poor design (my fault) leading to high stress points at attachement (no FEA resource). On the Stainless ball will the Plasma coating be sufficient ? HVOF Alumina is more than??2 x the price for than Plasma Zirconia.

Thanks for any advice.

Nick

PS as a poor Physicist pretending to be an Engineer are there any books giving a working overview of the many surface coating techniques ?

[Gordon]

Hi Nick

A plasma sprayed ceramic may be worth a try. If HVOF alumina coatings perform well, then I would consider using the same with plasma spray. Under these conditions I think any ceramic coating is going to crack. The secret will be to apply very thin coatings, just enough for coverage. Hopefully, any cracking will be in a perpendicular direction to the substrate surface, which may not sound good, but may actually act as a stress reducer and make the coating less likely to suffer spalling.

There are a number of books around on surface engineering processes, but not having read any of them recently, would not be able to recommend any specific one, particularly when you consider the cost of some of them. As you may have seen, I am trying to put something together for use on this forum, but so far it is only a brief introduction to the subject, with the detail of the various processes needing to be added. See this thread and "Surface Engineering in a Nutshell"

[hapewoe]

Hi Nick

A plasma sprayed ceramic may be worth a try. If HVOF alumina coatings perform well, then I would consider using the same with plasma spray. Under these conditions I think any ceramic coating is going to crack. The secret will be to apply very thin coatings, just enough for coverage. Hopefully, any cracking will be in a perpendicular direction to the substrate surface, which may not sound good, but may actually act as a stress reducer and make the coating less likely to suffer spalling.

There are a number of books around on surface engineering processes, but not having read any of them recently, would not be able to recommend any specific one, particularly when you consider the cost of some of them. As you may have seen, I am trying to put something together for use on this forum, but so far it is only a brief introduction to the subject, with the detail of the various processes needing to be added. See this thread and "Surface Engineering in a Nutshell"

Hi Nick,
Iwas out for some days and read the last comment from Gordon only today (03.03.07)
Your description of the problem sounds as if you have repetitive
impressions to make with a tool in ball form. How many impressions and what is the diameter of the ball ?
It may be worth to use a harder material for the ball.
options: hot working steel or weld overlay with a hard material instead of spray coating.
Material selection depends on the process conditions.
regards
hapewoe