HVOF Grit Embeddment Help!

[Carguy1968]

Hello All!

This is my first time posting, but have read many of the forums. I am reaching out to you to engage everyone’s opinion on a defect I am currently experiencing.

First off, we are spraying cylindrical parts using HVOF. Some are sprayed with a tungsten carbide blend, others are an iron blend. Nonetheless both materials are producing the same defect.

The defect occurs after grit blasting and HVOF spraying a workpiece. The coating produced has a series of small bumps in it. After some investigation it was concluded that these bumps were actually pieces of grit being coated over. After gritblasting the work pieces undergo a blow off process with compressed air. During finishing of the coating, i.e. grinding, these pieces would fall out causing small voids. These voids were confirmed with non destructive testing.

It appears odd because one day it was fine and the next this issue appeared. Grit supplier and all parameters are the same including no changes in workpiece material. Does anyone know what may be causing this embedment or steps to correct such?

Thanks much in advance!
-C

[Vadim Verlotski]

(01-10-2018 08:34 PM)Carguy1968 Wrote:  Hello All!

This is my first time posting, but have read many of the forums. I am reaching out to you to engage everyone’s opinion on a defect I am currently experiencing.

First off, we are spraying cylindrical parts using HVOF. Some are sprayed with a tungsten carbide blend, others are an iron blend. Nonetheless both materials are producing the same defect.

The defect occurs after grit blasting and HVOF spraying a workpiece. The coating produced has a series of small bumps in it. After some investigation it was concluded that these bumps were actually pieces of grit being coated over. After gritblasting the work pieces undergo a blow off process with compressed air. During finishing of the coating, i.e. grinding, these pieces would fall out causing small voids. These voids were confirmed with non destructive testing.

It appears odd because one day it was fine and the next this issue appeared. Grit supplier and all parameters are the same including no changes in workpiece material. Does anyone know what may be causing this embedment or steps to correct such?

Thanks much in advance!
-C

It can have many causes. We need images of the defect and microscopy of the cross-section to this place to say more. Maybe just the filter of grit blaster was not changed too long.

[loriolo]

Hi Carguy
As Vadim wrote, is necessary to tell more about your process, because the trouble could be quite infinite with indefinite notices, I don’t think that problem is on the blasting operation, there are many others as follow:
Powder sticking inside the barrel
Defective base material
Wrong parameters setting
Wrong spray system or components

[syam]

What is the exact surface profile requirement for HVOF process. Can anyone provide the ASME standards or is it based on the Metal powder specification.
Also anything regarding this is mentioned in ASME B-46.1.


Regards
SG

[Taz]

Hi Carguy,

What type of grit blasting that you use?
What are the grit blast size are you using?

[syam]

(01-17-2018 01:15 AM)Taz Wrote:  Hi Carguy,

What type of grit blasting that you use?
What are the grit blast size are you using?

we are using aluminum oxide grit

[Artyom]

Hello Carguy!

You can try to perform grit blasting using a HVOF gun as follows:
1) Pour a white alumina oxide F120 (100-125 microns) or F240 (2-45 microns) into a powder hopper. It's very convenient to use a couple of hoppers - the one for the spraying powder and the second for alumina.
2) Set up standard spraying parameters - for instance, parameters which you use for tungsten carbide coating.
3) Carry out the standard grit blasting of a part (blowing with compressed air is not required), then carry out grit blasting using a HVOF gun - 2 passes, no more isn't required.
Supersonic jet with alumina oxide particles will remove all remaining grit and will pre-heat the surface for better bonding of the coating. Temperature of the HVOF jet is not enough for melting of alumina oxide so there will be no foreign particles at the substrate-coating boundary. We are using this technique on the liquid-fueled HVOF K2 gun (GTV). Just one note: nozzle wear slightly increases, but if you have a gun with axial feeding, wear rate will be very slow.

Kind regards,
Artyom

[demionkrishan]

Check your grit blasting machine first. Is there any air pressure leakage? Have you seen any difference? Check your blasting gun ID worn? How much roughness are you getting after blasting? I prefer 250-300 ra If all good.
Remove all grit from your blasting machine, empty it. Buy white alumina 20 mesh from another supplier and see the difference. I faced same issue previously.