Hardness Test in TC coated surface
10-11-2009, 08:50 AM,
Hardness Test in TC coated surface
I am working for an oil & gas company. We have had some issues with big size ball valves passing in our new platform. The Ball is made up of forged carbon steel overlayed with Inconnel 625 and contact area being tungsten carbide coated using HVOF process. The seat is inconnel 625 which is also coated with Tungsten carbide on the contact area.

The composition of TC coating is 86%WC-10%Co-4%Cr.

Qualifications tests (Boond test, bend test, micrography, hardness) were performed as part of qualification prior to actual work with satisfactory results. The hardness was >1050 HV during qualification.

Could anyone revert if it is possible to carryout hardness test on the TC coating on the ball itself to know the actual values. We could not obtain satisfactory response from the valve manufacturer.

Being such high hardness is it possible / necessary to maintain a minimum hardness differential between the mating parts if both are TC coated.
10-11-2009, 12:52 PM,
RE: Hardness Test in TC coated surface
I assume that the issues are premature wear?

You can test the component coating but it probably won't be possible by NDT. Depending on the actual coating thickness remaining (after grinding/superfiner finishing) you may be able to get some indicative values with a superficial hardness tester, however more realistic results would be obtained taking a section/piece from the ball valve and getting a metallurgical lab to do micros. Obviously this could be undesirable/costly especially if it �scraps� a ball valve. Try the former first. Your applicator should continually check their coating and all test-pieces should be as close /indicative to the actually ball valve component setup as possible.

Any issues usually result in the applicator tightening up their production process and inspection. The coating quality/integrity usually miraculously returns. You could get them to run some samples on a scrap or dummy ball valve to check that their processes is good on the test-piece and the actual job setup?

I believe that the bend test is now a redundant and futile procedure.

Perhaps Gordon could add to this?

10-11-2009, 03:08 PM,
RE: Hardness Test in TC coated surface
Hi Vishy

Sign0016 to the Surface Engineering Forum.

Could you let us know what the issues are with the coatings?

Checking the actual coating as per the original qualification/production testing (which would have been applied to test coupons) would require destructive testing. The >1050 HV hardness value probably would have been from cross-sectional micro-hardness test usually HV/0.3. Some times 'macro' surface hardness is done as well on test pieces with suitable coating thickness (>250 - 300 um typically needed for this coating) using Superficial Rockwell HR-15N. These hardness tests applied to the surface of a working coating should really be considered as destructive. If you have an area of coating away from the working area, then this may be feasible. The use of non-marking portable hardness testing devices could be tried, but the problem then is correlating the results with the standard methods. Even relating a surface HV/0.3 with cross-section HV/0.3 hardness value could be problematic.

Generally, these coatings will suffer detrimentally low hardness values only if the coating is sub-standard with regard to density and inter-particle bonding. This may well show itself in poor finishing capability of the coating. Coatings with a relative low hardness value, but retaining good density and bonding very often will out perform the same coating with higher hardness value. This really should tell you that hardness is only one of many indicators to coating quality/performance, so avoid judging coating on this one criteria alone.

Unfortunately, there is not much non-destructive testing that can be done, particularly with regard to direct correlation to qualification tests. I would consider may be lightly polishing a small area for metallographic examination using a potable microscope.

Quote:Being such high hardness is it possible / necessary to maintain a minimum hardness differential between the mating parts if both are TC coated.

The two WC-10%Co-4%Cr coatings should really have the same properties. One coating having a different hardness to the other would indicate one coating being sub-standard quality.

Quote:I believe that the bend test is now a redundant and futile procedure.

Hi GlenB

Yes, bend tests do appear to be a thing of the past. People do not like test results with no numerical value Happy0193

I still believe that bend tests can be very useful and informative if applied in the right manner.
10-12-2009, 07:39 AM,
RE: Hardness Test in TC coated surface
Hi Gordon,
These are very new valves that are passing. The TC coating thickness is around 400 Microns on both ball & the seat.

We are investigating to find out the exact reason for the leaks as these valves were tested satisfactorily in the shop & witnessed. During our warranty shut down, we had partial access to see the valve and nothing abnormal turned out.

We have to conduct the detailed examination once we dismantle the valve for repair / investigation during our next shut down. I will keep your recommendation of using potable microscope in mind.

Thanks and Regards
10-12-2009, 02:46 PM,
RE: Hardness Test in TC coated surface
Hi Vishy

Good luck and let us know how you get on Smile

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