WC-17Co plasma coating
03-15-2007, 05:27 PM,
#1
WC-17Co plasma coating
Good evening? or morning? or day
Does anybody have experience in plasma, (not HVOF) coating of WC-17%Co?
We obtain very brittle coating: the cross- section shows transverse cracks, originated from thermal stresses, I think. At the same time microhardness results are about 700HV, that is below the Spec. requirements: 800-1000 HV
We use the METCO powder 73FNS-1, spray distance- 80 mm, gases helium and hydrogen, deposition rate 20 mkm per pass, total thickness about 250 mkm.
Thank you, volf
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03-16-2007, 01:57 PM,
#2
RE: WC-17Co plasma coating
Hi volf

Really need more information to help you. What plasma gun and parameters used?

Deposition rate 20 mkm (assume that's 20 micrometres per pass) I doubt will be the cause of all of your problems, but possibly a contributory one. I would reduce thickness per pass to something like 5 micrometres if possible.

helium/hydrogen plasma gases???? is this right?
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09-21-2007, 04:37 PM,
#3
RE: WC-17Co plasma coating
Gordon Wrote:helium/hydrogen plasma gases???? is this right?

is the use of H/He in this app. used to increase the plasma gas velocity?

To max out the plasma gas velocity, would you typically choose H/He combination? Also, would that be the typical gas choice when spraying carbide??

I have seen where amerpit 526 (wc-c0 83/17) is best used or "designed" for kerosene guns. So I would assume that you would try and max out the plasma velocity if using a plasma gun. Correct?

thanks
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09-23-2007, 01:55 AM,
#4
RE: WC-17Co plasma coating
Hi Intel55

I have not come across the use of helium/hydrogen (primary/secondary) parameters before, though I suppose its use is possible. I suspect Volf is really referring to Argon/Helium and Argon/Hydrogen.

The higher quality plasma spray carbide coatings usually employ Argon/Helium high energy parameters and as you say this is to achieve higher velocity (not higher particle temperatures).
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09-24-2007, 04:35 AM,
#5
RE: WC-17Co plasma coating
hi every body
I hope i can be helpful even my bad english
in my experience in plasma transfer arc ,we used to mix helium and argon for welding stellite
and in coating carbide (-45 + 15 )we use always azote but sometimes we use argon to have better coating .
size of guns too must concord with the size of the powder used ,if there are different longers of nozzles ,may be that means we have to choose right one
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09-24-2007, 02:19 PM,
#6
RE: WC-17Co plasma coating
Hi Hvofhamid

Quote:we use always azote but sometimes we use argon to have better coating
Azote being the same as nitrogen, had me wondering for a while Happy0193
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09-30-2007, 03:06 PM,
#7
RE: WC-17Co plasma coating
I suggest Volf used to Argon/Hydrogen gas.spraying gun is F4-MB, it's so easy to the microhardness results meet 800-1000 HV,Ar:70 slpm,H2:2.5 slpm,current:700A,Powder carrier Gas:5.0 [slpm];Powder feed rate:45g/min
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05-25-2023, 08:14 AM,
#8
RE: WC-17Co plasma coating
Hi all of the specialist,

Come back to this topic again and really need your profesional suggestion.

Recently we met an issue about this plasma sprayed coating: vertical cracks occurred in coating without any change of the parameters.
The process information is as below:
Gun: F4     Current: 700A   Ar: 70NLPM   H2:  3NLPM   Carrier gas(Argon) :3.5NLPM  Powder:  METCO 73F-NS-2   Powder feederate : 30g/min

The vertical cracks occurred suddenly without any sign of it.
After this, I  optimized the cooling including the distance and pressure, the cracks decrease but still exist;
Now what I observed seems abnormal is that the flame is not stable, and I maintained the powder hopper, powder line and powder injector, it doesn't works.

So do you have any experience about this issue?


Attached Files Thumbnail(s)
   
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05-25-2023, 09:42 AM,
#9
RE: WC-17Co plasma coating
Hi Zhao Yun,

Vertical cracks in a sprayed coating is an indication of critical tensile stresses in the coating. One measure you can take against this is to preheat the steel substrate to around 200°C. Another measure is the thickness reduction of monolayer, which you can achieve by increasing the speed (number of revolutions) of the component.
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05-25-2023, 10:41 AM,
#10
RE: WC-17Co plasma coating
(05-25-2023, 09:42 AM)Vadim Verlotski Wrote: Hi Zhao Yun,

Vertical cracks in a sprayed coating is an indication of critical tensile stresses in the coating. One measure you can take against this is to preheat the steel substrate to around 200°C. Another measure is the thickness reduction of monolayer, which you can achieve by increasing the speed (number of revolutions) of the component.

Thanks, Vadim Verlotski.

The critical tensile stresses comes from two aspects I think:
- temperature of the coating is too high;
- the coefficient of thermal expansion varies greatly;

For the 1st one, optimize the cooling system or the spraying parameters; 
For the 2nd one, preheating works;

Do you think so? And from you experience do you think the flame's trembling or quivering a little bit is normal or not?

I'll try the two measures you mentioned.
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05-25-2023, 01:05 PM,
#11
RE: WC-17Co plasma coating
If you think that the plasma is unstable I would advise that you observe your parameters for fluctuation. You want to see the volts and amps remain steady, as well as your gas flows.
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05-25-2023, 02:14 PM,
#12
RE: WC-17Co plasma coating
(05-25-2023, 10:41 AM)Zhao Yun Wrote:
(05-25-2023, 09:42 AM)Vadim Verlotski Wrote: Hi Zhao Yun,

Vertical cracks in a sprayed coating is an indication of critical tensile stresses in the coating. One measure you can take against this is to preheat the steel substrate to around 200°C. Another measure is the thickness reduction of monolayer, which you can achieve by increasing the speed (number of revolutions) of the component.

Thanks, Vadim Verlotski.

The critical tensile stresses comes from two aspects I think:
- temperature of the coating is too high;
- the coefficient of thermal expansion varies greatly;

For the 1st one, optimize the cooling system or the spraying parameters; 
For the 2nd one, preheating works;

Do you think so? And from you experience do you think the flame's trembling or quivering a little bit is normal or not?

I'll try the two measures you mentioned.

I'm sure that preheating and faster component rotation when coating will solve your problem, I've seen it in practice several times. 
As far as unstable flame is concerned, it can have several causes and it does not necessarily have to be related to the cracking of the layer. The most common cause of an unstable flame is wear of the anode (nozzle) and cathode.
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