Hydraulic Shaft - Thermal Spray - Advice needed

[Jagdeep]

I recently thermal sprayed a hydraulic shaft for a CAT machine OD 90mm.
We machined the worn out area to about 1mm under the size required.
The area was then threaded and was cleaned making sure there was no contamination
We then pre-heated the worn piece to about 70 - 80 degrees
Then sprayed the item with proxon 21031.
After we finished we then grinded the item with coolant and it really looked cool and job complete.
The client the fixed the item and complained about it leaking and after some time the coating came off.
Any advice on how this happened.

I mean if the coating was not right it would have come off will grinding.

I understand that after a part has been thermal coated you can then use a product made by diamanti know as dichtol. They say it seal cracks and small porous holes. can with stand pressure of about 8000 p.s.i.

Any comments, has any one tired this.

[KevGarty]

I'm would doubt that Flame Spray is a good enough process for Hard Chrome Replacement. This is a hard chrome replacement job yeah ?

If it is you would have to go to HVOF rather than conventional flame spray as the bond strength of the proxon powder will only be in the region of 4000 psi where an HVOF hard chrome coating your looking at 9000-10,000 psi.

Flame sprayed coatings generally have around 10% porosity in them the dichtol would help that a great deal but the coating is still generally soft probably around 150-180 HV an HVOF coating would be around 800HV with less than 2% in some cases 1% porosity

Dichtol works very well if applied correctly but this has to be applied before finish machining.

I would think that the coating isnt good enough for the application IF, it is a hard chrome replacement application.

All the best

Kev

[shantanu]

I'm would doubt that Flame Spray is a good enough process for Hard Chrome Replacement. This is a hard chrome replacement job yeah ?

If it is you would have to go to HVOF rather than conventional flame spray as the bond strength of the proxon powder will only be in the region of 4000 psi where an HVOF hard chrome coating your looking at 9000-10,000 psi.

Flame sprayed coatings generally have around 10% porosity in them the dichtol would help that a great deal but the coating is still generally soft probably around 150-180 HV an HVOF coating would be around 800HV with less than 2% in some cases 1% porosity

Dichtol works very well if applied correctly but this has to be applied before finish machining.

I would think that the coating isnt good enough for the application IF, it is a hard chrome replacement application.

All the best

Kev

I would agree with Kev....cannot replace hard chrome with flame spray- specially for hydraulic piston rods of earth moving equipments which generally work under a decent amount of stress and load...
If HVOF is not available, the least you require would be an ARC spray- say a good metcoloy II coating over a well blasted surface with a bond coat of Ni Al. May be a TAFA 95 MXC kind of material may also help

Regards

Shantanu