Blemish in flame sprayed self fluxing nickel based coating

[etelatal]

PIC

Hi guys, this is a flame sprayed Nickel boron coating that has been post treated in a sintering furnace. This weird blemish appeared during sintering (not sure if the image is visible). The powder is metco 16C, the surface has been grit blasted before spraying. Base material is duplex steel. Any ideas as to what this might be or what is could be the cause of it?

br.
Olli

[Vadim Verlotski]

PIC

Hi guys, this is a flame sprayed Nickel boron coating that has been post treated in a sintering furnace. This weird blemish appeared during sintering (not sure if the image is visible). The powder is metco 16C, the surface has been grit blasted before spraying. Base material is duplex steel. Any ideas as to what this might be or what is could be the cause of it?

br.
Olli

Hi Olli,
The dark spots can have different causes: impurities in the powder or in the powder feeder, residual oil on the surface, impurities in the oven and so on. But, in my opinion, you have a different, more important problem with this coating than the stains: your layer is not properly sintered (not melted). Such layers are heated under protective gas or vacuum at 1050-1080°C to get a melt without oxidation (if you have an furnace treatment and not local remelting using a burner or inductor). A well-melted furnace-layer looks white metallic, smooth and shiny. Your coating is rough, porous and oxidized and this indicates wrong process management.
The stains would not be a problem if your layer had melted properly.

Regards
Vadim

[etelatal]

PIC

Hi guys, this is a flame sprayed Nickel boron coating that has been post treated in a sintering furnace. This weird blemish appeared during sintering (not sure if the image is visible). The powder is metco 16C, the surface has been grit blasted before spraying. Base material is duplex steel. Any ideas as to what this might be or what is could be the cause of it?

br.
Olli

Hi Olli,
The dark spots can have different causes: impurities in the powder or in the powder feeder, residual oil on the surface, impurities in the oven and so on. But, in my opinion, you have a different, more important problem with this coating than the stains: your layer is not properly sintered (not melted). Such layers are heated under protective gas or vacuum at 1050-1080°C to get a melt without oxidation (if you have an furnace treatment and not local remelting using a burner or inductor). A well-melted furnace-layer looks white metallic, smooth and shiny. Your coating is rough, porous and oxidized and this indicates wrong process management.
The stains would not be a problem if your layer had melted properly.

Regards
Vadim

Yes, you are correct there are several more severe issues in the coating in question. We are looking into sintering in a different atmosphere for better results. However, this was the first time these blemishes appeared. The surfaces will be ground so the surface quality is not an issue. Thank you for your input!
/Olli

[Vadim Verlotski]

PIC

Hi guys, this is a flame sprayed Nickel boron coating that has been post treated in a sintering furnace. This weird blemish appeared during sintering (not sure if the image is visible). The powder is metco 16C, the surface has been grit blasted before spraying. Base material is duplex steel. Any ideas as to what this might be or what is could be the cause of it?

br.
Olli

Hi Olli,
The dark spots can have different causes: impurities in the powder or in the powder feeder, residual oil on the surface, impurities in the oven and so on. But, in my opinion, you have a different, more important problem with this coating than the stains: your layer is not properly sintered (not melted). Such layers are heated under protective gas or vacuum at 1050-1080°C to get a melt without oxidation (if you have an furnace treatment and not local remelting using a burner or inductor). A well-melted furnace-layer looks white metallic, smooth and shiny. Your coating is rough, porous and oxidized and this indicates wrong process management.
The stains would not be a problem if your layer had melted properly.

Regards
Vadim

Yes, you are correct there are several more severe issues in the coating in question. We are looking into sintering in a different atmosphere for better results. However, this was the first time these blemishes appeared. The surfaces will be ground so the surface quality is not an issue. Thank you for your input!
/Olli

Olli,
the problem is not that the surface of the unmelted coating is rough, but in the poor layer quality (a rough oxidized surface is only an indication of this). If your furnace has too high a partial pressure of oxygen, you cannot achieve the good layer quality at all. The matter of sintering is also made more difficult by the stainless steel substrate. The surfaces of steels with a high chromium content oxidize during heating in the oven with the formation of Cr2O3 and this thin oxide layer prevents a metallurgical bond between the NiCrBSi-coating and the substrate. The coatings of self-flowing NiCrBSi alloys work very well on non-alloyed steels, but on stainless steels it is a challenge (local remelting using a burner in air is not possible at all and an oxygen partial pressure of less than 10-4 Pa is required for remelting in the furnace) .
However, with a high-quality remelted NiCrBSi-coating, the use of stainless steels is superfluous and you can safely use the carbon steels: the layer takes on the function of wear protection as well as the function of corrosion protection.

[etelatal]

PIC

Hi guys, this is a flame sprayed Nickel boron coating that has been post treated in a sintering furnace. This weird blemish appeared during sintering (not sure if the image is visible). The powder is metco 16C, the surface has been grit blasted before spraying. Base material is duplex steel. Any ideas as to what this might be or what is could be the cause of it?

br.
Olli

Hi Olli,
The dark spots can have different causes: impurities in the powder or in the powder feeder, residual oil on the surface, impurities in the oven and so on. But, in my opinion, you have a different, more important problem with this coating than the stains: your layer is not properly sintered (not melted). Such layers are heated under protective gas or vacuum at 1050-1080°C to get a melt without oxidation (if you have an furnace treatment and not local remelting using a burner or inductor). A well-melted furnace-layer looks white metallic, smooth and shiny. Your coating is rough, porous and oxidized and this indicates wrong process management.
The stains would not be a problem if your layer had melted properly.

Regards
Vadim

Yes, you are correct there are several more severe issues in the coating in question. We are looking into sintering in a different atmosphere for better results. However, this was the first time these blemishes appeared. The surfaces will be ground so the surface quality is not an issue. Thank you for your input!
/Olli

Olli,
the problem is not that the surface of the unmelted coating is rough, but in the poor layer quality (a rough oxidized surface is only an indication of this). If your furnace has too high a partial pressure of oxygen, you cannot achieve the good layer quality at all. The matter of sintering is also made more difficult by the stainless steel substrate. The surfaces of steels with a high chromium content oxidize during heating in the oven with the formation of Cr2O3 and this thin oxide layer prevents a metallurgical bond between the NiCrBSi-coating and the substrate. The coatings of self-flowing NiCrBSi alloys work very well on non-alloyed steels, but on stainless steels it is a challenge (local remelting using a burner in air is not possible at all and an oxygen partial pressure of less than 10-4 Pa is required for remelting in the furnace) .
However, with a high-quality remelted NiCrBSi-coating, the use of stainless steels is superfluous and you can safely use the carbon steels: the layer takes on the function of wear protection as well as the function of corrosion protection.

Yes, you are very much in line with what we've found and been told about this coating process. We will make some changes and see how it goes. Thank you very much for your input.