Hi Arturas

Quote:Hi
Today we checked the gun . And I think that the reason of those unreacted powders is that powders adhere to the inside surface of long air cap during the spraying time then start growth to the particular size while hot gas stream detach it and deposit on the surface.
But why???
This air cap is new
Arturas

An unstable powder feed (pulsing) or possibly using unsuitable gas/fuel flows and or powder for equipment/hardware selection.

Hi Gordon,
Is it possible that the problem relatet to carrier gas - nitrogen flow? After we decreased rate of nitrogen : flometers readings-25(acording book) to 15 units this problem absolutely dissapeared.

Regards
Arturas

Hi Arturas

Quote:Hi Gordon,
Is it possible that the problem relatet to carrier gas - nitrogen flow? After we decreased rate of nitrogen : flometers readings-25(acording book) to 15 units this problem absolutely dissapeared.

Regards
Arturas

Yes

Only problem is to make sure that this parameter change is not just compensating for other faults in your set-up.

That seems a big change from recommended parameters. Is the powder one recommended by your equipment supplier? I'm trying to establish is whether your HVOF set-up has been optimised for this particular powder. If you are using an alternative/equivalent powder to the one your equipment supplier recommends, then parameter optimisation/changes may be required. Alternative/equivalent powder may look the same and display the same overall particle size range, but from my experience can produce very different spraying and coating characteristics. So take care, cheap powder can sometimes (not always though) be a very expensive choice when things go wrong.

Hi ,
Thanks a lot for all answers
Just one more questions After spraying surface we paint with thick sealer layer and there is a problem with diamond wheel during grinding. Sealer " how it in english - glue, stick" on wheel and it is necessary to clean wheel each time after removing of first layer from coating. Is it possible to clean coating surface with acetone or another kind of solution
Arturas

Hi Arturas

When you apply the sealer to your coating, leave it sufficient time to soak in and then remove any excess sealer from the surface before it cures or sets using absorbent cloth. There is little point to leaving a surface film of sealer if you are going to machine it off.

Gordon Wrote:Hi Arturas

Quote:This problem still exist for me. You suggest to apply self fluxing coatings. I thought also that it is one of the best solution , unfortunately coating (NiCrBSi, NiCrBSi+WC) wears out too quickly ( in three month). Till now I don?t understand it.

If you want to try fused NiCrBSi+WC coatings again, try HVOF spray with a 50% WC content powder (graded for HVOF like Diamalloy 2002), you will find these coatings easier to fuse and superior quality than the typical flame spray and fused coatings. Abrasion resistance will not be as good as HVOF WC/CoCr, but should be much stronger/tougher and will be very much more resistant to the type of failure you are experiencing.

Quote:What is the differents among linseed oil, phenolic, anaerobic type sealers?

Suggest you obtain technical bulletins on sealers from your suppliers (Sulzer Metco etc.)

Quote:Coating thickness per pass was about 10 -15 micrometres

Suggest around 5 micrometres coating thickness per pass for HVOF WC/CoCr is best.

Quote:What permissible delay between blasting of substrate and spraying WC/Co/Cr?

Best practice is to spray immediately after grit blasting. Permissible delay depends very much on the environmental conditions. Typical procedures tend to suggest 4 hours maximum.

Quote:What size of aluminium oxide is optimal for blasting?

Grit number 20 (-10 +30 mesh) roughly about 1 mm across

Quote:Surface of plunger will be prepared by grinding. Question : Is there any differens how to prepare surface by turning or by grinding?

Substrate preparation? shouldn?t make much difference. After coating? by grinding only.

Quote:Is it ok that coating thickness would be between 0.15-0.20mm on side (after grinding)?

This sounds ok, but it does depend on the permissible the wear limit.

Quote:What about cooling of substrate surface by air during the spraying time ? Which solution is better to cool or to stop spraying and wait several minutes?

It would be best to be able to spray without pause, using air-cooling to control temperature if needed. Reducing the coating thickness per pass (as mentioned above) will also help even out the temperature distribution.

Quote:Preheating temperature after blasting with the flame up to 100C - is it enough? Preheating must be done only for surface or in all volume?

100 C should be fine. Higher temperatures could be used, but care must be taken not to oxidise the surface. Ideally preheat should be even throughout work piece, not just surface.

All this discussion seems to find a coating process to solve a certain case as always. I've would gladly inform all my knowledge, as long as I have all details.
To the the extent of info so far may be TS is not the method to consider.
br bhe