add more to "valveexpert":
you should choose 17-4PH HT1150 hardened cage, which is good for high temperature (410 C deg) erosion-corrosion resistance, and also higher strength at high temperature. the seat ring should be solid cobalt/stellite hardened,if you can make the cage seat surface hardened with alloy6, it would be better. this is a super-heated steam application, the steam can cut the metal quickly if it leaks or operating at low flow condition. 410SST is not very good for this severe process.
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