Posted by Gordon England on April 25, 2003 at 02:42:23:
In Reply to: chromium carbide posted by Jiri Sefcik on April 24, 2003 at 12:59:42:
: Dear Sir,
: I`d like to ask about plasma sprayed chromium carbide coatings.
: How are the main principles how to get coatings with suitable properities. Especialy low porosity
: and adequate hardness (about 850 HV0.3).
: I have tried spray samples onto part (diameter 540mm) and I gave low hardness (450 HV0.3). I have
: tried change the process parameters (primary/secondary gas flow, spraying distance) but the results
: are not adequate. I think that it is due to decarburization of carbide component during spraying.
: Process parameters:
: -plasma gun: GTV 309MB
: -plasma current: 500 AMPS
: -prim.gas Ar flow: 52 slpm
: -sec.gas H2 flow: 8.5 slpm
: powder: Cr3C2+25NiCr 5-45micro m
: carrier gas Ar flow: 4.2 slpm
: part RPM: 35 rpm
: trverse speed: 4mm/s
: spraying distance: 80mm, 10mm, 120mm
: Regards..............Jiri
: jirisefcik@seznam.cz
:
Hi Jiri
Have you look at the coating microstructure? What is the porosity, non-melted particle and carbide content? This will give you a clue to what is happening. What is the make and grade of powder you are using?
If you are suffering from decarburization, the powder particles are being overheated. To correct this there are a number of things you can do (seperately or in combination):
reduce secondary hydrogen plasma gas flow
increase primary argon flow
reduce plasma current
Change nozzle to narrower bore size
Also powder feed rate, carrier gas flow,injector, spray distance and auxillary cooling will also have their effects.
Are you using the manufacturers recommended spray parameters for your particular powder/equipment. These may not always be the optimum for your particular application, but should provide a standard level of quality. If you are using unknowns, then you need to do more development or change the powder to a recommended grade for the equipment with published parameters.
Regards Gordon
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