Materials selection advice.

[Skinjive]

Looking to upgrade a downhole sampling tool, micro valve and I'm a bit lost.

The valve steam are currently corroding and were not upgrading when the rest of the tool was upgraded to inconel 718.

Im having to select a material that will withstand H02 and h2s at pressure, the tool is too be rated to 20k. Also the stem is making a metal to metal seal with the valve seat (inconel 718). So ideally im wanting to pick a metal slightly weaker then inconel 718, as i do not want the valve seat damaged through galling and damaging the seal. Id much rather the valves be replaced periodically.

So far looked at hastelloy, inconel 725, inconel 625 plus, monel k-500. Inconel 725 seams to be the one to go for at this stage. Its HRC rating is slightly lower but contains similar characteristics.

Questions
1) if i just upgrade the stem to match inconel 718, will there be a high chance of galling?
2) If i select the slightly weaker inconel 725, will i still be as susceptable to galling as 718?
3) What coatings may be advantageous to reduce galling on the metal to metal seal? BUT not be too severe as to damage the seat its sealing against?
4) Are there any coatings for use with either of the inconels that will help with galling AND corrosion resistance? (FYI there is a threaded end to the stem, so im not sure if some coatings cant be used)

Any help would be highly appreciated, this is my first materials project and i don't want to screw the pootch!

Thanks

Freddie

 

[EdStainless]

I like the idea of 725 or an age hardenable 625 version.
Could you coat with Tungsten disulfide? It would act as a solid lube and help suppress galling.

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Plymouth Tube

 

[metengr]

Go with the 625. You really don't need the added (high) strength that is optimal for 725.