Monel as Gasket Material?

Yes, but try to avoid to locate two dissimilar metals in contact together, so I prefer to use stainless steel gasket.

As the process involved is not described, I would follow the stainless steel recommendation to avoid galvanic activity. I suggest that the stainless sheet be annealed before cutting the gasket out.

SCS

I would stick with using a the same material for the gasket.

Take a look at the Kolsterising process www.kolsterising.info for surface hardening (68 HRc) of both the ball and the gasket. Super Duplex stainless steels are an ideal candidate and you will not the corrosion resitance of the material. Also no dimensional change to treated componets.

You state that the application is for a Ball Valve.
I advise that you check with the valve vendor on what replacement gasket material they recommend (require). If you do not use an approved replacement part and there is a failure you may find that you have voided the valve warrenty and you (your company) will be responsible.

As someone else said, it would be necessary to define the service, and environments, fluid composition, to know if your materials selection is correct. Monel can be anodic to Duplex Steel.

regards

Vitt