ENP can be applied on the ball of the ball valves for reducing the friction between ball and seat and reducing the torque values for ease of valve operation or corrosion protection (In case of ball base material is not high corrosion resistance).
Some operator companies has experienced with removal of ENP during the valve operation. Is there any way to apply more robust ENP on the base material to avoid losing ENP during the valve operation?
Electroless nickel plating is not really suitable as a wear protection layer because of its low hardness and high coefficient of friction. If you really want to have a nickel plating for the application, then chemically nickel (Ni-P) is much more suitable. This layer must be heat treated at about 400°C and then it gets a hardness of over 800HV.
Even harder and more wear-resistant are the layers based on tungsten carbide, which are applied by means of thermal spraying.
Thanks Vadim. Very good answer. How many micrones of this layer do you recommend? What is the hardness of this layer?
Chemical nickel makes sense with a thickness of 10-20 microns. This layer has a very uniform thickness and is very smooth (does not need to be polished). The layer of WC-Co or WC-Co-Cr sprayed by HVOF (high velocity oxygen flame) should be 200-300 microns thick including 50-100 microns oversize for grinding and polishing. Hardness of good HVOF coatings WC-Co and WC-Co-Cr reaches 1400 HV.
Thanks Vadim. What would be the hardness of chemical nickel?
I had different experiences with the thickness of tungsten carbide.
Total specification has specified 400 micron thickness of tungsten carbide overlay on ball and metal seat contact surface of ball valves . However, I have experienced 150 micron tungsten carbide HVOF in Norwegian offshore industry with clients like Statoil and Lundin. I guess 400 micron is when the TC is applied through weldoverlay maybe?
Maximum hardness of chemical nickel is achieved after heat treatment at 400°C. According to various literature sources, this hardness is between 900 and 1100 HV. Galvanic chrome has similar hardness, but unlike the chemical nickel chromium layer has a cracked network and is permeable.
HVOF layers of WC-Co and WC-Co-Cr should not be thicker than 200-300 microns. A higher thickness means not only higher coating costs, but also higher stresses in the layer, which can cause spalling. In addition, higher layer thickness brings no advantages in operation.
Regarding the maximum thickness of T.C, I also know that end users try to avoid higher thickness than 150 micron to avoid porposity. I have seen and touched some of the balls in 22Cr duplex material which are overlaid by T.C. The ball was machined with very low rougness and mirror face so I could move my hand and touching different parts of the ball very quickly. However, I could not move my fingers quickly on the parts with TC overlay. TC overlay was applied through HVOF and gave a colour of light gray on the shiny colour and well machined 22Cr duplex ball.
As a suggestion, hard steel against hard steel and well lubricated has a low COF near 0.08 using available greases / oils.
E.G. 4140 can be nitrated to high hardness of 60+ HRC which process is not very expensive. See the link. MS to MS is similar, but you have to be careful regarding load capacity and to choose different hardness between the two materials.
https://engineeringlibrary.org/reference/coefficient-of-friction
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