inconsistant dye check
03-12-2008, 04:22 AM,
#1
inconsistant dye check
[/color]we coat and grind a lot of chrome carbide and have been getting some weird dye checks after grinding. sometimes the coating checks all white, sometimes it is partially white with splotches of red and other times it bleeds red like metallizing would. we have checked our gas and hoses for contamination, double checked our spray distance. we are using the jp-5000 kerosene fueled hvof system and sulzer metco powder. any suggestions as to what could be causing this? also, our inconel 718 seems to be coming out too brittle i believe due to oxidation. what are your thoughts on this issue and your suggested per pass build up spraying this powder? same torch

your help is appreciated.
Allen
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03-12-2008, 05:09 PM,
#2
RE: inconsistant dye check
Hi Allen

Sign0016 to the Surface Engineering Forum.

Can you see any difference in the ground finish between the coatings that showed very different dye penetrant test results?

I think you will need to evaluate the coatings by metallography to determine whats going on. Dye penetrant test may be picking up differences in coating porosity levels or it may just be inconsistancies within that test.

What problems are you getting with the inconel 718 coating to make you think it is too brittle? If oxidation/overheating powder is a problem you may need to adjust parameters to give cooler spray stream or select an alternative coarser grade 718 powder. Generally, I would aim at about 0.0005" (0.13 mm) build up per pass of the gun.
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03-12-2008, 11:15 PM,
#3
RE: inconsistant dye check
Gordon,
Thanks for your reply.

There is no visual difference. At first i though it was post-spray contamination by greasy straps or hands contacting the coated areas. That would tend to "stain" the part and would be visable after grinding. The coating would look spattered sometimes or just on one end of the part after dye check. Very frustrating as this does not pass our inspection and causes untimely delays.
Could it be somehow i'm not getting a consistant supply or atomization and combustion of the kerosene? I will get my parameters together and pass those along. I have to admit, the coating industry has passed me by a long time ago.

We believe we have coating that turns up hard because it grinds hard and we have chipping around the edges of keyways and the edges of the coating is cracked. We use inconel a lot in our business and we have had issues on the coating on the i.d. of parts also.

On another subject, do you believe there is a considerable difference between the hvof inconels and the twin wire arc sprayed inconels? In particular, the bond strengths. We seem to have better luck with the arc process, especially when applied to the i.d.

Your reply is appreciated.
Allen
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03-13-2008, 03:44 PM,
#4
RE: inconsistant dye check
Hi Allen

Generally, HVOF sprayed inconel type alloys should produce denser, cleaner and higher strength coatings than normal arc spraying. I stress here though, that the powders need to be carefully selected and optimised for the particular HVOF process. Say using Diamalloy 1006 through the JP5000 may not produce the same coating as spraying with the diamond jet. I would try a Praxair or other equivalent powder (chemically) with a slightly coaser particle size.

This thread may be of interest porous-machined-surface-on-part-lower-inner-diameter
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03-14-2008, 02:04 AM,
#5
RE: inconsistant dye check
gordon, can you explain why the 718 wire has a higher bond strength than the hvof? and has anyone ever reproduced the 6000-7000 bond strength that the tafa book states? i have heard rumors that even tafa's mother company, praxiar, tried but coul not produce the 6000 they say is possible.

we spray the tafa powder when coating inconel 718. i'm sure it is the optimal powder for this gun and yet we do not have the results we are looking for. we have started to really monitor and record our stand-off distance to try and isolate the reason for the inconsistant dye check. what simple tweak to my parameters should i try first?

you don't know how much i appreciate your help. i'm starting to get back into the coating industry after 15 years away from the action.

respectfully,
allen
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03-19-2008, 01:51 PM,
#6
RE: inconsistant dye check
Hi Allen

Inconel 718 wire is a nickel chromium iron base alloy probably not too different to the old
traditional arc wire bond coat materials (Metco 470AW) so arc spray coating should have a pretty good and reliable bond. Not sure on claims for Tafa/Praxair HVOF bond strengths, but I quote from the first generation air cooled Metco Diamond jet process manual on Diamalloy 1006 (Inconel 718)

Quote:Diamalloy 1006 is a dense, well-bonded nickel/chromium/iron base coat. Typical coatings exhibit microhardness in the range Rc 29-34 and microhardness DPH300 320-380. The coatings are characterized by a dense (negligible porosity), high tensile bond strength (greater than 11000 psi (76 MPa) and a high thickness limitation (greater than 0.100" (2.5 mm). The Diamalloy 1006 coatings resemble a wrought superalloy in that they can be machined, milled, drilled and tapped. Excellent finishes (8-12 microinches aa) can be obtained by grinding
Yes, probably somewhat hyped up Happy0193

Anyway, unless the coating bond strengths are not adequate for application ie the coatings are falling off or lifting in process or service, I think bond strengths are probably more a distraction away from your problem.

To be honest, before going down the road of HVOF parameter development, I think you need to first establish how good and consistent your dye penetrant test really is, and to evaluate your coatings using metallography and cross-reference to dye test results. I would certainly not recommend trying to optimise the HVOF process purely on dye penetrant testing alone.
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