HVOF Coating Problems

[Ajay29]

Hello All,
We are facing some issues on tungsten carbide coated parts sprayed using HVOF Powder Coating and after grinding the area with diamond wheel, we are observing patches in Liquid Penetrant Test. can you all suggest me that, what's the problem behind this, like its a coating problem or grinding problem. Please do share your thoughts as well, with your answers.

Regards,
Ajay

[Vadim Verlotski]

Check out this thread:

https://www.gordonengland.co.uk/sef/Thre...en-Carbide

[Ajay29]

It's not pitting Vadim Sir, as it is not directly visible, the patches only comes after doing the Liquid penetrant Test and i have attached photos for the same.

Regards,
Ajay

[Stephen Booth]

good morning Ajay29

Your picture is not showing: What powder brand, type, etc what gun are you using, parameters etc.. give us some details.

The link from Vadim-Verlotski is very good, try all the solutions, and do a search on the website.

possible ideas on this are:

1. Powder not mixed well before spraying, the WC particles are very heavy

2. Dust collector is not working well, and dust from over spray brought into the flame

3. Powder has excessive fine particles and needs to remove the dust with a classifier

4. and other problems with powder feeder, hoses, static etc
hope that helps.... as a start

[Ajay29]

This is the image and the patches i was talking about

[Stephen Booth]

can you give us the information also on what gun, powder, parameters

also what was the coating thickness after spray, before grinding, and after grinding

[Ajay29]

Coating Thickness after Spraying - 400 Microns
After Grinding Thickness - 200 - 220 Microns

[loriolo]

Hi Ajoy29
As I could see in the picture, the porosity is near the areas when it can be some turbulences in the HVOF flame, and the coating can be affected of this turbulences, if the piece is all as in the picture, try to do the spray in 2 times, masking once a time the zones to be coated, and do some change in the HVOF gun direction, and if the piece is only the cylindrical zone,  try to coat it first