Peel of coating during pressure testing

[raviswanadha]

Stellite 6 coating with HVOF/Plasma on Ball Valves.

Firstly i wish all the members of the forum A Happy & Prosperous New Year 2011.
coming to the problem we have been doing stellite 6 coating with HVOF/Plasma on Ball Valves. Initially we did not see any problem in coating and also the pressure testing of balls.
recently we sprayed stellite 6 on 300mm balls and lapped them and did not find any problem. But when the balls we subjected to testing we found a chip of the coating.
could you explain any reasons for the coating to chip off during pressure testing and not during lapping process.
i am attaching a couple of photos for your reference.
regards
   
   

[djewell]

I don't see any surface roughness on the ball where the coating peeled off. If it is true, as you say, that the failure occurred after lapping, then I would suggest that your grit blasting step was skipped or done poorly. For a piece of coating as large as you show in these pictures to come off, then you had poor bonding to the base metal, and it was held in place by the rest of the coating. Then, during testing, pressure got under the coating and lifted it off.

[kschewe]

I agree with the above. You can also take the sharp edges off before spray. That might help.

[k09]

Like djewell said, blasting quality surely could be the main factor but what could be the reason for both balls to peel off at the same location?
Was this the side that was actually subjected to pressurized fluid or is the delamination irrespective of the side that was pressurized?
What was the coating thickness?

From the extent of delamination, poor surface preparation seems to be the culprit. But there could be other reasons why this has occured at the exact same location for both balls.

Regards
K09

[ceciliaerickson]

I wish all the members of the forum A Happy & Prosperous New Year 2011.
I think the reason for both balls to peel off at the same location could be the blasting quality and the side of pressurized fluid subject.

[kschewe]

was there a old coating remaining on it? Maybe it was hard chromed before and not all of it was removed

[Gordon]

Hi raviswanadha

Looking at the images, I can't help but think the coating was removed during finishing and not during testing.