wc/co/cr coating
05-05-2009, 02:05 PM,
#1
wc/co/cr coating
Hi
i recently use from WC/Co/Cr 86/10/4 for coating of gate valve with cng hvof gun.i used before from WC/Co 88/12 for that parts and i did not have any problem.but recently with coat included Cr i get very brittle coat in edge and i chek it that i have some crack in surface.
surface grinder said me with new coat the surface is very hard compared with old coat.
what is the problem?
Ehsan
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05-06-2009, 05:40 PM,
#2
RE: wc/co/cr coating
Hi Ehsan

The problem could be due to many things. Difficult to be specific with limited information.

Coating evaluation - metallography and hardness testing would be advisable to check quality of coating and check spray parameters are optimised for your particular powder. Excessive particle heating can lead to breakdown of some WC into unwanted phases that can make the coating harder and more brittle.

what is the coating thickness? Different coatings may have different levels of residual stress. If it is bordering on the coating thickness limit, it may make the coating more tender to grinding.

There may be no problem with your coating and it may purely be down to poor grinding practice for this particular coating.
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05-06-2009, 06:33 PM,
#3
RE: wc/co/cr coating
Hi Gordon
Thickness of coating in edge was about 550-600 micron before grinding.
i should say that all parameters except kind of coat was same(grinding conditins,spray parameters,...) and max temp was 120 C.
Regards
Ehsan
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05-09-2009, 10:23 PM,
#4
RE: wc/co/cr coating
Just a thought but is this edge-boundary masked? Sometimes cracking back onto the surface occurs after 'bridging' onto the masking. Obviously the edge is compromised by this. Maybe I'm barking up the wrong tree but if this is true you need to review the masking?

Cheers

GlenB
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07-17-2009, 01:51 PM,
#5
RE: wc/co/cr coating
WC12Co is normally slightly harder than WC10Co4Cr in my experience. If your grinder saw a difference in hardness, that tells me something was different in your spray parameters or technique. Make sure that you overshoot the edge of the part at least 150 mm before stepping up and returning to make another pass across the part. If the gun is moving too slowly at the edge of the part, you will overheat the coating and cause it to crack.
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07-24-2009, 08:30 AM,
#6
RE: wc/co/cr coating
Hi djewell
You said that wc12co is harder than wc10co4cr but i think when we reduce percentage of co in powder we get more hardness.
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07-24-2009, 01:58 PM,
#7
RE: wc/co/cr coating
(07-24-2009, 08:30 AM)ehsan Wrote: Hi djewell
You said that wc12co is harder than wc10co4cr but i think when we reduce percentage of co in powder we get more hardness.

Correct. Since 86WC10Co4Cr is 86% tungsten carbide and 88WC12Co is 88% tungsten carbide, the 88WC12Co should be harder.
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07-24-2009, 10:50 PM,
#8
RE: wc/co/cr coating
Hi,
HVOF coatings of WC-Co can form glassy phase and more so with the addition of third element like Cr.In such cases the coating surface would be brittle.Thinner coatings in the edges are more prone to glassy phase formation due to higher cooling rate..The heat treatment of the coating can reconvert the the glassy phase into crystalline phase.For further details please check in my book on 'Advanced Thermally Assisted Surface Engineering Process' Kluwer Academic Publishings,MA,USA, now Springer.
ram chattopadhyay
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