Jet kote III orthogonal spray parameters query

[wangqun72]

Hi everyone!
In order deposited an accepted WC/12Co coating without cracks and delamination, the optimal coating process (OCP) designed by orthogonal test program (L9(34))for high velocity oxy-fuel (HVOF, Jet Kote III) thermal spraying a agglomerated and sintered WC-12Co powder(20~53um) on carbon steel substrate had been designed and would be implemented.
Any advices about the scheme you could give would be greatly appreciated!
By the way, some previous tested results (the spraying parameters provided by equipment manufacture) have been attached.

No. C3H6 O2 Feed rate Spray distance
SCFH SCFH g/min mm
1 120 920 50 170
2 120 1000 55 185
3 120 1080 60 200
4 130 920 55 200
5 130 1000 60 175
6 130 1080 50 185
7 140 920 60 185
8 140 1000 50 200
9 140 1080 55 175
1.Spray gun speed :500mm/s, 2.Step distance:2.5mm

[Gordon]

Hi Wangqun72

First thing that strikes me is "1.Spray gun speed :500mm/s, 2.Step distance:2.5mm" . This I feel is far too slow.

Looking at the photomicrographs in your attached pdf file, you can see the individual deposited layers per spray pass, which are far too thick IMO.

Increase your gun/work piece speeds so that you deposit much less coating thickness per pass. Ideal world, I would look for about 0.0002" (5 micrometres) per pass. You will probably need to consider spraying your test pieces on a rotary fixture to get the speeds. If the actual parts require ladder scan type spraying, then obviously it would be best to replicate same on test pieces, but just try to increase gun speed and maybe reduce spray rate.

This may not be the complete answer to your cracking problem, but I think it will go a long way in reducing the problem.

[Gordon]

Hi Wangqun72

I think some attention to your grit blast preparation would be wise. A lot of retained grit on your samples. I don't think this is a significant influence on your crack problem though.

[rayudugirish]

Hi Wangqun72,
Like Gordon has suggested your working low speeds 500mm/s or 30000mm/min is on the lower side of the parameter. Make it 45000mm/min. Also make your step 4mm instead of 2.5 mm. Powder feed rate 25-30 gms/min instead of 54gms/min.
try to deposit 30-40 microns per pass. Hope this should solve the problem.
Regards
Girish

[k09]

Hi Wangqun72,

As mentioned by both, Gordon and Girish the gun speed does appear to be on the slower side and it would definetly be contributing to the cracks.

I would also like to add the temperature factor here. If you deposit slower, the overall temperature of the component is bound to increase beyond permissible limits. Use a hand held or a remote, not-contact IR Temperature sensor to keep a check on the temperature. Ensure that it doesnot exceed 150-180Deg C (on the safer side).

Ensure that you have continuous but gradual air cooling and that there are no Thermal Shocks (especially sudden cooling after spraying).

From the micrographs, the distance between two parallel cracks is 100microns+. If it is assumed that the seperated layer are individual sprayed layers then you are definetly spraying too much! Increase the gun translation speed and you should see marked difference in the micrographs.

All the best.

Regards

K09

[wangqun72]

Thanks for all the replies!
We have increase the velocity of gun and sample to 90m/min, and the thickness of coating per pass is about 5 microns accoding to your advices.The cracks of coating deposited by jet kote III have been successfully prohibited despite of the fact that the coating is somewhat brittle. The flux of C3H6 and O2 are 135 SCFH and 1020 SCFH.

Wang qun

[k09]

Over heating of particles could lead to coating being more harder and brittle.
slighlty increasing the oxygen for cooling the spray and fine tuning the fuel might help.

Do let us know if you were able to overcome this problem.

Regards
K09