Need HELP to obtain less than 1% porosity in HVOF
09-11-2008, 03:46 AM, (This post was last modified: 09-11-2008, 03:55 AM by alvinwlh.)
#1
Need HELP to obtain less than 1% porosity in HVOF
Dear all,

We have just setup a Praxair JP8000 system with 5000 gun. Spray with oxygen and kerosene. Powder is 88%WC 12%Co.

Oxygen: 2000 scfh
Kerosene: 6.0 gph
can get combustion pressure of 104 psi.
Nitrogen (carrier gas): 23 scfh
Powder feed : 6 rpm (76g/min)
Spraying flat coupon with a linear speed of 500mm/sec.
Porosity obtained: 5%

Oxygen: 2000 scfh
Kerosene: 6.0 gph
can get combustion pressure of 104 psi.
Nitrogen (carrier gas): 23 scfh
Powder feed : 3 rpm (38g/min)
Spraying flat coupon with a linear speed of 500mm/sec.
Porosity obtained: 2%

I have tried increasing and reducing the standoff disctance by 1 inch, but no improvement on the porosity.
I have tried increasing the chamber pressure to 108.9 psi by increasing the oxygen and the fuel flow, but no improvement on the porosity.
Standoff: 14"
Any one can give me some adivice on how to improve from 2% to less than 1%
Reply
09-11-2008, 01:36 PM,
#2
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi Alvinwlh

Sign0016 to the Surface Engineering Forum.

Two points I would consider before diving into process parameter adjustments away from the standard recommended ones:

* Check that your metallographic testing procedure is in fact giving you the correct porosity level. These coatings can be difficult to prepare for metallography with pull-out or smearing problems.

* Check that your powder is suitable, ideally optimised for this particular process. If unsure, compare to powder recommended by Praxair for this gun.

Good luck. I'm sure some of the JP expert forum members will comment also Smile
Reply
09-12-2008, 01:47 AM,
#3
RE: Need HELP to obtain less than 1% porosity in HVOF
Thanks Gordon,

Actually the praxair recommended parameter is 76g/min (6rpm), I have reduced it to 3 (rpm).

I don't know whether it is ok to vary by so much? or there is really something wrong with my parameter.

Is there anyone has any idea how to differentiate pull out from porosity?
Reply
09-12-2008, 01:10 PM,
#4
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi Alvinwlh

Quote:Is there anyone has any idea how to differentiate pull out from porosity?
With porosity levels ~ 2 - 5% you should be able to infiltrate these with epoxy resin. Use vacuum impregnation (failing that put a few drops of epoxy resin on the warmed sample (~ 80 -100*C) and then quickly hot/pressure mount in bakelite. Works very well, but can gum up the mounting press if your not careful) on the samples before mounting and polishing, this allows you to detect true porosity when these sites are filled with epoxy. If samples show little impregnation, particularly near coating surface, then suspect pull-out. Usually, these coatings are too dense to allow any noticeable impregnation.

I don't think reducing the spray should do too much harm to coating quality, in fact it will probably improve it, but at the expense of production time and increased use of process gases/fuel.

What powder are you using? and which JP gun barrel?
Reply
09-16-2008, 01:51 AM,
#5
RE: Need HELP to obtain less than 1% porosity in HVOF
Thanks Gordon,

I am using powder 88%WC 12%Co (88% Tungsten carbide 12% Cobolt). 4" Jp gun barrel.

If we reduce the powder spray rate and reduce the linear speed, will it increase the efficiency?
Reply
09-16-2008, 10:26 PM,
#6
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi Alvinwlh

Quote:If we reduce the powder spray rate and reduce the linear speed, will it increase the efficiency?
Reducing spray rate may increase deposit efficiency slightly. Changing speeds and feeds should really have little effect other than to cause problems with overheating if set too low. Process or economic efficiency on the other hand will suffer, with lower productivity and higher gas/energy consumption per job.

Quote:I am using powder 88%WC 12%Co (88% Tungsten carbide 12% Cobolt)
What is the specific make and grade? Is it optimised for this process?
Reply
09-18-2008, 03:06 AM, (This post was last modified: 09-18-2008, 03:27 AM by alvinwlh.)
#7
RE: Need HELP to obtain less than 1% porosity in HVOF
Dear all,

The powder is a requirement from customer, it contains only tungsten carbide and Cobolt.
Their spec is
Thickness - over 2mm
Finish - 130 u Ra
Bond strength 10000 psi
hardness -1400 DPH300
Porosity < 1%
oxides < 2%

I am not sure whether our lab is doing the correct ginding and polishing.
Because there are some differences between our own result and third party result, although the third party claim that theirs might have some smearing effects.
Is there any recomemded steps for polishing this powder?
Image Attached for third party inspection.

   
Reply
09-18-2008, 08:00 PM,
#8
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi Alvinwlh

First, I hope you don't mind, but I have converted your original bmp image ~ 250KB to a JPG image ~ 11KB. Makes a big difference when considering server speed/bandwidth and memory Smile

It not really possible to tell from your image whether this is smeared or not. The coating density/porosity level looks inline with what you would expect though. Just goes to show importance of interpretation and methods of metallography. Useful tool, but so easy to get wrong.

I think the most important step for your metallographic preparation would be epoxy impregnation of your samples to determine whether your apparent porosity is pull-out or real. If you get no impregnation, chances are that your real porosity level is below the !% mark. Polishing procedure - well the majority of my experience is with manual polishing, so metallographers experience and skill play a significant part as well as procedure. I use silk clothe for diamond polishing and a very short final polish on soft clothe with colloidal silica just to give a slight relief effect (viewing microstructure between steps to check progression).

The specific powder you are using for this job will have a big effect on the quality of coating. Are you using a praxair or similar powder optimised for this process or are you using an unknown possibly customer supplied or specified?
Reply
09-19-2008, 02:02 AM,
#9
RE: Need HELP to obtain less than 1% porosity in HVOF
(09-18-2008, 08:00 PM)Gordon Wrote: First, I hope you don't mind, but I have converted your original bmp image ~ 250KB to a JPG image ~ 11KB. Makes a big difference when considering server speed/bandwidth and memory Smile

No Problem Smile

(09-18-2008, 08:00 PM)Gordon Wrote: I think the most important step for your metallographic preparation would be epoxy impregnation of your samples to determine whether your apparent porosity is pull-out or real. If you get no impregnation, chances are that your real porosity level is below the !% mark.

Thanks I will ask the lab engineer to try out.

(09-18-2008, 08:00 PM)Gordon Wrote: Polishing procedure - well the majority of my experience is with manual polishing, so metallographers experience and skill play a significant part as well as procedure. I use silk clothe for diamond polishing and a very short final polish on soft clothe with colloidal silica just to give a slight relief effect (viewing microstructure between steps to check progression).

They are using some struers grinder to gind and polish. But the rotation speed, force and duration is standard to most coating. I think recently they just realized that the different in setting will contributes to different results.

(09-18-2008, 08:00 PM)Gordon Wrote: The specific powder you are using for this job will have a big effect on the quality of coating. Are you using a praxair or similar powder optimised for this process or are you using an unknown possibly customer supplied or specified?

ya, we are using Praxair powder. Actually, I have a good support from Hs Stack although I am not using HS powders. Will think to switch from Praxair to HS.
Reply
10-01-2008, 11:52 AM,
#10
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi all,
I have one question.
I don't know, how to set rotation rate of the polishing disk and fixed support(mount) on the automatic polisher.
Is rotation rate of the polishing disk same fixed support?

regards,

william
Regards, William
Reply
10-01-2008, 09:53 PM,
#11
RE: Need HELP to obtain less than 1% porosity in HVOF
Hi William

(10-01-2008, 11:52 AM)William Wrote: Hi all,
I have one question.
I don't know, how to set rotation rate of the polishing disk and fixed support(mount) on the automatic polisher.
Is rotation rate of the polishing disk same fixed support?

regards,

william

Don't think I can help too much, but I think it will be very dependant on your specific polishing equipment. Some will have fix disk rotation speed and head speeds/direction, while others have variable everything Happy0193 You will need to experiment to see which settings give the best results for your particular samples.
Reply
10-11-2008, 04:16 PM,
#12
RE: Need HELP to obtain less than 1% porosity in HVOF
(09-11-2008, 03:46 AM)alvinwlh Wrote: Dear all,

We have just setup a Praxair JP8000 system with 5000 gun. Spray with oxygen and kerosene. Powder is 88%WC 12%Co.

Oxygen: 2000 scfh
Kerosene: 6.0 gph
can get combustion pressure of 104 psi.
Nitrogen (carrier gas): 23 scfh
Powder feed : 6 rpm (76g/min)
Spraying flat coupon with a linear speed of 500mm/sec.
Porosity obtained: 5%

Oxygen: 2000 scfh
Kerosene: 6.0 gph
can get combustion pressure of 104 psi.
Nitrogen (carrier gas): 23 scfh
Powder feed : 3 rpm (38g/min)
Spraying flat coupon with a linear speed of 500mm/sec.
Porosity obtained: 2%

I have tried increasing and reducing the standoff disctance by 1 inch, but no improvement on the porosity.
I have tried increasing the chamber pressure to 108.9 psi by increasing the oxygen and the fuel flow, but no improvement on the porosity.
Standoff: 14"
Any one can give me some adivice on how to improve from 2% to less than 1%

hi powder feed make it 100g/mn
test it ,it can help
Reply




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