Valve ball surface treatment

[seffrican]

I have a large number of metal seated ball valves (ranging from 50mm to 350mm in the digester section of a pulp mill. Product is highly corrosive sulphide black liquor and So2 gas at a ph of 1.5. I was hoping someone could offer some advice on the most cost-effective method of hardfacing the balls. We currently use either CG8M parent material with a hardchrome or stellite surface treatment or polished solid stellite balls. The problem arises when balls need to be refurbished after the surface is damaged. The solid stellite is not a problem but is extremely expensive. On the other balls we've tried re-chroming as well as stellite weld overlays without much success to the point where we just grind the chrome off and use soft PTFE seats (soft seats are liable to 'wiping'). I would imagine the best method would be some sort of parent material heat treatment but i'm worried about the effect on corrosion resistance. We are also in the process of looking at elctroless nickel plating. I'd appreciate it if anyone's got any good ideas.

 

[israelkk]

Hard chrome plating can go up to 1 mm thick. Therefore, you can remove some base material and use the maximum thickness of chrome plating before grinding. Another way is to deposit the chrome or other ceramic material using plasma process to any thickness and hardness you like before grinding.

 

[seffrican]

Thanks for that. How good is the chroming i.t.o. bonding to the substrate? Also, does the plasma process affect the grain structure, i.e. creation of grain boundaries that are less corrosion resistant?

 

[israelkk]

This hard chrome process is used for refurbish worn crank shafts for engine motors (the area where the babbit bearings are rubbibg). 20 years ago I recommended a friend that needed to refurbished 300 crankshafts to use plasma spraying of ceramics on the worn crank shafts. The refurbished crankshts outlasted the original new ones by more the factor of 2. The process doesn't affect the grain structure to my best knowledge. Anyway, the ball mainly see wear and not high compression stresses. I recall that Union Carbide was the company speciallised (inventor) of the plasma spray process.

 

[unclesyd]

There are numerous companies that specialize in the repair and refurbishment of all types of ball valves. You should replace the seat if you coat the ball and regrind.

Here is one company that does this work.

http://www.dkmachine.com/Default.htm

 

[EdStainless]

Have you ever looked at solid ceramic trim kits? I have run many ball valves with ceramic balls and seats. They were made by CoorsTech, I don't know where to go to get them today.

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