nitride after HVOF
12-03-2018, 08:19 AM,
#1
nitride after HVOF
We have coated some pins in designated areas with 88/12 tungsten powder and ground them to size. After this process the customer has nitrided the pins in areas next to the coating informing us that the heat treater can mask off the coating so it will not be affected by the nitriding, however I received a call from the customer informing me that the coating was coming off after the process.
Could this be a fault of the coating or should the pins be masked off and nitride before we apply the coating?

Thankyou
Michael
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12-03-2018, 10:49 AM, (This post was last modified: 12-03-2018, 10:52 AM by Vadim Verlotski.)
#2
RE: nitride after HVOF
(12-03-2018, 08:19 AM)Michael Mccarthy Wrote: We have coated some pins in designated areas with 88/12 tungsten powder and ground them to size. After this process the customer has nitrided the pins in areas next to the coating informing us that the heat treater can mask off the coating so it will not be affected by the nitriding, however I received a call from the customer informing me that the coating was coming off after the process.
Could this be a fault of the coating or should the pins be masked off and nitride before we apply the coating?

Thankyou
Michael

Hi Michael,
The tungsten-containing coatings (WC-Co, WC-Co-Cr, WC-Cr3C2-Ni and others) must not be heated above 300°C, because otherwise they will flake off after cooling. Due to very high differences in coefficients of thermal expansion of steel and WC-containing coatings, when the layer is cooled from temperature above 350°C, critical compressive stresses occur in the layer which cause the layer to flake off.

So, a nitride of coated parts must lead to the spalling of the coating.

Regards
Vadim

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