Porosity problem - HVOF
08-27-2020, 04:26 AM,
#1
Porosity problem - HVOF
Dear,

my company had a porosity problem in the carbide coatings (tungsten, chromium, tungsten + chromium, etc.) by an air-cooled HVOF system. This problem started recently and we were unable to resolve it. We believed it was a problem with the equipment calibration, but due to the corona virus, it was not possible to bring a technician to our company.

We then decided to purchase a new MEC MDJ2700 water-cooled equipment - propylene.

We used the following parameters to apply the woka3302 powder (WC 10Ni):
O2: 250 lpm, 10 bar
C3H6: 80 lpm, 8 bar
Air: 390 lpm, 9 bar
N2: 20 lpm, 9 bar
Feed rate: 35 g / min (4 rpm)
Distance: 240 mm

When we performed the penetrating liquid test, the result was the same as in the past: no crack, but less than 30 seconds later the piece was COMPLETELY red.

Our compressed air is completely dry and free of impurities (network of activated carbon filters recently added) and the fuel gas cannot be agreed since the equipment has a vaporizer that is kept at 55 degrees celsius.

Do any of you have any idea what's going on?

Sorry for my english, hope you can understand.

Thank you very much for your attention.
Reply
08-27-2020, 07:05 AM,
#2
RE: Porosity problem - HVOF
(08-27-2020, 04:26 AM)thaisrms Wrote: Dear,

my company had a porosity problem in the carbide coatings (tungsten, chromium, tungsten + chromium, etc.) by an air-cooled HVOF system. This problem started recently and we were unable to resolve it. We believed it was a problem with the equipment calibration, but due to the corona virus, it was not possible to bring a technician to our company.

We then decided to purchase a new MEC MDJ2700 water-cooled equipment - propylene.

We used the following parameters to apply the woka3302 powder (WC 10Ni):
O2: 250 lpm, 10 bar
C3H6: 80 lpm, 8 bar
Air: 390 lpm, 9 bar
N2: 20 lpm, 9 bar
Feed rate: 35 g / min (4 rpm)
Distance: 240 mm

When we performed the penetrating liquid test, the result was the same as in the past: no crack, but less than 30 seconds later the piece was COMPLETELY red.

Our compressed air is completely dry and free of impurities (network of activated carbon filters recently added) and the fuel gas cannot be agreed since the equipment has a vaporizer that is kept at 55 degrees celsius.

Do any of you have any idea what's going on?

Sorry for my english, hope you can understand.

Thank you very much for your attention.


Dear 'thaisrms',

I fear that you expect too much from HVOF coatings. Basically, all HVOF coatings are permeable to gases and liquids and your result of the red-white test is completely normal. There are no pores or rough cracks that make the layer permeable, but particle boundaries in the entire coating thickness up to the substrate. Only the very best HVOF/HVAF coatings can master red-white tests, but you cannot create such layers from commercial powders with MEC MDJ2700.

However, there is a worthwhile solution for you: impregnation with special organic sealing oils. There is a lot of information about these sealants in this forum and on the Internet.

Regards
Vadim
Reply
08-27-2020, 11:57 AM,
#3
RE: Porosity problem - HVOF
(08-27-2020, 07:05 AM)Vadim Verlotski Wrote:
(08-27-2020, 04:26 AM)thaisrms Wrote: Dear,

my company had a porosity problem in the carbide coatings (tungsten, chromium, tungsten + chromium, etc.) by an air-cooled HVOF system. This problem started recently and we were unable to resolve it. We believed it was a problem with the equipment calibration, but due to the corona virus, it was not possible to bring a technician to our company.

We then decided to purchase a new MEC MDJ2700 water-cooled equipment - propylene.

We used the following parameters to apply the woka3302 powder (WC 10Ni):
O2: 250 lpm, 10 bar
C3H6: 80 lpm, 8 bar
Air: 390 lpm, 9 bar
N2: 20 lpm, 9 bar
Feed rate: 35 g / min (4 rpm)
Distance: 240 mm

When we performed the penetrating liquid test, the result was the same as in the past: no crack, but less than 30 seconds later the piece was COMPLETELY red.

Our compressed air is completely dry and free of impurities (network of activated carbon filters recently added) and the fuel gas cannot be agreed since the equipment has a vaporizer that is kept at 55 degrees celsius.

Do any of you have any idea what's going on?

Sorry for my english, hope you can understand.

Thank you very much for your attention.


Dear 'thaisrms',

I fear that you expect too much from HVOF coatings. Basically, all HVOF coatings are permeable to gases and liquids and your result of the red-white test is completely normal. There are no pores or rough cracks that make the layer permeable, but particle boundaries in the entire coating thickness up to the substrate. Only the very best HVOF/HVAF coatings can master red-white tests, but you cannot create such layers from commercial powders with MEC MDJ2700.

However, there is a worthwhile solution for you: impregnation with special organic sealing oils. There is a lot of information about these sealants in this forum and on the Internet.

Regards
Vadim

Dear Vadim, how are you?

Thank you for sharing.

Unfortunately I can't attach the photo so you can understand exactly what I'm talking about. The coating I did yesterday turned completely red (exactly the color of the penetrating liquid) in less than 30 seconds. We carried out the test in parallel on a sample that was coated (same material) 1 year ago and until now it is mostly superficial almost white (obviously with reddish spots and areas - but not the color of the penetrating liquid). There is certainly something wrong because the difference is very stark.

Some of our customers have complained of accelerated abrasive wear, low hardness and that the coating is "absorbing oil".
Reply
08-27-2020, 12:33 PM,
#4
RE: Porosity problem - HVOF
(08-27-2020, 11:57 AM)thaisrms Wrote:
(08-27-2020, 07:05 AM)Vadim Verlotski Wrote:
(08-27-2020, 04:26 AM)thaisrms Wrote: Dear,

my company had a porosity problem in the carbide coatings (tungsten, chromium, tungsten + chromium, etc.) by an air-cooled HVOF system. This problem started recently and we were unable to resolve it. We believed it was a problem with the equipment calibration, but due to the corona virus, it was not possible to bring a technician to our company.

We then decided to purchase a new MEC MDJ2700 water-cooled equipment - propylene.

We used the following parameters to apply the woka3302 powder (WC 10Ni):
O2: 250 lpm, 10 bar
C3H6: 80 lpm, 8 bar
Air: 390 lpm, 9 bar
N2: 20 lpm, 9 bar
Feed rate: 35 g / min (4 rpm)
Distance: 240 mm

When we performed the penetrating liquid test, the result was the same as in the past: no crack, but less than 30 seconds later the piece was COMPLETELY red.

Our compressed air is completely dry and free of impurities (network of activated carbon filters recently added) and the fuel gas cannot be agreed since the equipment has a vaporizer that is kept at 55 degrees celsius.

Do any of you have any idea what's going on?

Sorry for my english, hope you can understand.

Thank you very much for your attention.


Dear 'thaisrms',

I fear that you expect too much from HVOF coatings. Basically, all HVOF coatings are permeable to gases and liquids and your result of the red-white test is completely normal. There are no pores or rough cracks that make the layer permeable, but particle boundaries in the entire coating thickness up to the substrate. Only the very best HVOF/HVAF coatings can master red-white tests, but you cannot create such layers from commercial powders with MEC MDJ2700.

However, there is a worthwhile solution for you: impregnation with special organic sealing oils. There is a lot of information about these sealants in this forum and on the Internet.

Regards
Vadim

Dear Vadim, how are you?

Thank you for sharing.

Unfortunately I can't attach the photo so you can understand exactly what I'm talking about. The coating I did yesterday turned completely red (exactly the color of the penetrating liquid) in less than 30 seconds. We carried out the test in parallel on a sample that was coated (same material) 1 year ago and until now it is mostly superficial almost white (obviously with reddish spots and areas - but not the color of the penetrating liquid). There is certainly something wrong because the difference is very stark.

Some of our customers have complained of accelerated abrasive wear, low hardness and that the coating is "absorbing oil".

Well, that's different. If your layer is very permeable and also wears out quickly, then the layer quality is actually poor. There can be several reasons for poor layer cohesion: spray parameters that are too cold, spray distance too large, powder too coarse and so on. Sometimes you can orientate yourself on the brightness of the coating stain: the lighter the stain, the better the cohesion of the coating.
Reply
08-27-2020, 01:09 PM,
#5
RE: Porosity problem - HVOF
There are other methods for testing a coating's quality which I would expect that you should be using, but you seem very focused on the dye test. You mentioned that your customers have complained about low hardness, which is one characteristic that you should be checking yourself. What does the microstructure look like? Are testing the bond strength?
Reply
08-28-2020, 12:44 AM, (This post was last modified: 08-28-2020, 12:46 AM by thaisrms.)
#6
RE: Porosity problem - HVOF
Dear Vadim,
I don't understand what do you mean by "the bright of the coating stain". Are you talking about the color of the as sprayed layer?

Dear Lemster68,
I understand that other methods are better for evaluating coating layers and 2 coatings of our company were sensed to metallographic analysis (the results should be done by next week due to corona vĂ­rus issues). The reason why we are insisting on this method is because our client do them as well. They do a qualitative compare with a reference. And by the way, we don't have any laboratorial equipment in our facilities so we are trying to evaluate the coatings with what we have.

Today we figured out that our fuel line have oil remnants from filling the cylinders. We installed filters before and after the gases painel and no more liquid oil were identified. We did another coating with the same parameters listed before and tomorrow we are going to finish it and do the penetrant liquid test again.

We are hoping that the reason for poor cohesion, low hardness and high porosity is the oil that can be cooling the flame. What do you think about it?

Thank you both for sharing those informations.
Reply
08-28-2020, 06:52 AM,
#7
RE: Porosity problem - HVOF
Dear thaisrms,
A spray spot is a luminous spot on the sprayed part that can be seen directly under the torch in the process of spraying. The luminosity of a spot indicates the temperature of the particles at the moment of their contact with the substrate. The higher the luminosity of the deposition spot, the higher the temperature of the particles and the better the cohesion of the layer. In your case, the temperature is clearly insufficient, so you need to increase the brightness of the spraying spot by changing the spraying parameters.

The presence of oil in the fuel line cannot influence the spraying result, as this oil burns with the fuel. Another thing is the oil in the cooling air line. In this case, the oil is very harmful.

Regards
Vadim
Reply




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