Which material is the adecuated for demineralized water and superheated steam in check and gate valves.
Demin water has nothing in it that is dissolved to saturation.  Therefore almost anything is soluble in it. Imagin the crusty old engineer telling me: "Boy, it's HUNGRY for minerals"
Typically 300-series stainless steels are the material of choice for demin/DI water services.  This application (565psi@333F) is out of the reach of a TFE-Lined valve, but that would be a good choice at gentler temps and pressures.
Marklobo: If tungsten carbide parts are being eroded, I suggest you should take a serious look at your velocity and trim design.  Basic anticavitation trim >CONTAINS< cavitation, managing the bubbles so that they implode where they wil not cause damage.  Severe-service anticavitation trims such as the Valtek Channelstream, CCI Drag, Masoneilan VRT and others all manage the velocity of the flow so that the fluid pressure never drops to the vapor pressure so cavitation will not happen.  Velocity-control trim would seem to be indicated for your application. 

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