Fused coating problem

[sprayloud]

Hey,

We sprayed-fused roll with NiCrBSi + tungsten carbide.
After some testing we got this working surface full of tiny holes.

(links dead)

Have you ever seen that ? What could be the cause ?

[Vadim Verlotski]

Unable to open image

[sprayloud]

Unable to open image

Here's alternative link:
https://i.imgur.com/g25xZX2.png?1

[Vadim Verlotski]

Unable to open image

Here's alternative link:
https://i.imgur.com/g25xZX2.png?1

It seems to be from overheating or too long heating on air. If the coated component has too long heating at air, come problems with oxidation. The holes are formed, for example, by oxidation of tungsten carbide (WO3 was evaporated).
The coated parts with NiCrBSi-WC must be heated more quickly to 1050°C as the same parts with only NiCrBSi layer.

[sprayloud]

Unable to open image

Here's alternative link:
https://i.imgur.com/g25xZX2.png?1

It seems to be from overheating or too long heating on air. If the coated component has too long heating at air, come problems with oxidation. The holes are formed, for example, by oxidation of tungsten carbide (WO3 was evaporated).
The coated parts with NiCrBSi-WC must be heated more quickly to 1050°C as the same parts with only NiCrBSi layer.

Maybe i wasn't clear enough in the beginning. Those holes originated after the roll was tested in working conditions. After fusing and machining the surface was smooth with no irregularities. I would expect some wear scars on the surface but not those kind of "holes".

[Vadim Verlotski]

[/quote]

Maybe i wasn't clear enough in the beginning. Those holes originated after the roll was tested in working conditions. After fusing and machining the surface was smooth with no irregularities. I would expect some wear scars on the surface but not those kind of "holes".

[/quote]
Then there is corrosion. Typical pitting corrosion, for example in salt water or acid looks like this.

[sprayloud]

Thanks Vadim, it really looks like pitting corrosion. I guess NiCrBSi is just not suitable in this case.

[loriolo]

Hi Corund
I worked many year doing qualification of coating on plunger for glass industry, with NiCrBSi 50% Wc, and there was a lot of problems, I could see after grinding and some others after the plunger worked.
1.porosity, caused by the humidity in the compressed air, it was necessary to add a special drier catalyzer, because even if the air was exsiccated with an air drier, it wasn’t enough
2 After the piece worked, there was some holes or bubbles, it was caused by the substrate material, which had some impurities inside that burned with the high temperature of the fusion, someone came up during grinding, others as holes after working.
So pay attention to use a good quality material, and with the compressed air, during sand blasting and maybe used in the spray gun.

[KevinS]

If the surface is ground diamond is needed or you will get elongated pitting.