Tungsten Carbide is widely used in valve industry on the ball and seat contact of a metal seat ball valves to avoid galling and friction between the seat and ball. Some of the ball valves in offshore industry are used in sea water services. The question is that is it possible to apply Tungsten Carbide on the ball and seat of the metal seat ball valves in sea water service? Does Tungsten Carbide can be corroded in contact with sea water?
Problem is a little different. The wear resistant coatings, based on tungsten carbide, are absolutely stable in seawater, but they are not really dense. For this reason, the base material may corrode under the layer and the layer will peel off. The problem is, so, not the corrosion resistance of the layer, but its tightness.
In order to avoid underflowing corrosion of the base material, stainless steels are used for this purpose. In such cases, the layer must only protect against wear and its porosity is no longer critical.
sorry, I do not have enough information about the corrosion issue yet
I agree with Vadim Verlotski. However we had tried to deposit dense WCCoCr coating but not tested for corrosion. Pl refer Article High performance thermal-sprayed WC-10Co-4Cr coatings in nar...
Hi Vadim and Satish,
Thanks for your input on this question. I do not understand what you mean by dense hardfacing? Could you please explain to me more?
I like to provide more information about tungsten carbide coating and alternative solutions for hard facing of the seat and ball in metal seated ball valves. Tungsten Carbide is very erosion resisitance compound which is deposited on the ball and seat in different thicknesses. As an example, some end users such as Total may specify 400 micron thickness and others like Statoil may require 150-200 micron thickness maximum. I have heared that increases the thickness of Tungsten Carbide may create porosity and possibility of failure of T.C coating. But I do not have detail knowledge about that. I had experience with another Swidish end user who was worry about low thickness of 150 micron on the plug of a plug valve and possibility of peeling off the coating layer.
Practically, What we do in the industry to apply other hard facing alloys such as Ultimet or Tribally on the ball and seat of the metal seat ball valves in sea water services. Statoil had experienced with peeling off Ultimet especially in cases when the valve was actuated and the speed of openning and closing of the valve were fast.
Thanks very much for the input. Now the question is that what parameter defines the density of Tungsten Carbide? Is the the percentage of tungsten carbide? Just please be informed that we normally use 86% tungsten carbide and the supplier should apply tests such as bonding, hardness, etc.
And the other question is that if the T.C is not really dense, why it is not problem to apply that on materials which are not used in sea water services?
Look at this article that clearly states thet T.C has poor corrosion resistance in the sea water services.
http://etheses.whiterose.ac.uk/7320/1/Thesis%20final.pdf
Hi Karan,
Many of our clients make a mistake about the coats of WC-Co-Cr; WC-Co and WC-Cr3C2-Ni, because they think that these layers are similar to the sintered hard metals with similar compositions. This idea is absolutely wrong. Hard metals are actually dense and very strength because they are metallically bound. In contrast, the coatings are weak (factor 10 less strength than sintered hard metals) and generally not gas-tight despite a low porosity
Hi Vadim.
Thanks very much for your explaination. I am intwerested in NiCrBSi alloys. I have seen plasma overlay of Stellite in the valve industry which provides many advantages such as low stress in the coating, more accurate thickness, higher hardness, etc. However, I do not know why Tungsten Carbide coating through HVOF is widely used in valve industry for many years. I am aware of Cr-C and Cr-Oxide hardfacing but have not experienced and seen them in valve industry.
We apply Tungsten Carbide on the metal faces and we do not expect gas tight leakage rate. However, some manufacturers claim that they can achive gas tight in small sizes valves through extra lapping process.
That is interesting information that hard face metals as coating do not provide the same hardness as hard metals. Hardness test is one of the tests we expect our sub suppliers to implememt to make sure that hard faced tungsten carbide has adequate thickness.
By the way my email is [email protected]
I appreciate your input to my question.
Regards,
Karan
The reason why HVOF/HVAF coatings of WC-Co and WC-Co-Cr are applied to large valves is simple, these are relatively easy to handle. You can coat virtually finished part, then grind and polish and it's done. If your valve is made of stainless steel, the permeable HVOF layer will also work in corrosive seawater for a while.
With NiCrBSi it is certainly more difficult, because the part has to be warmed to 1050°C to melt the layer. The larger the valve, the less attractive is this heat treatment; for large parts, it only works in very large and expensive protective gas furnaces (parts up to 20 kg can still be heated with acetylene-oxygen burner in air).
The advantage of our layer ChroSiMol is that the heat treatment after plasma coating can only be carried out at 800-900°C in air. This is much easier for large parts than 1050°C under inert gas. I attach the presentation of three of our new layers.
Thanks Vadim for the information.
Actually I have seen tungsten carbide to be applied on all metal seated ball valves in all sizes except for those in corrosive sea water services. We accepted to apply tungsten carbide on metal seat ball valves with 25Cr super duplex internals in the past project since the sea water was non-corrosive which means that the amont of oxygen in the sea water was less than 20 particle per billion (20ppb).
Just want to conclude that T.C coat on the ball and seat surface of metal seat ball valves is very common in oil industry worldwide. I have not seen any alternative hardfacing materials on the ball and seat of metal seat ball valves which is introduced in the recent conferences about valves.
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