Steam turbine cases and steam valves are typically made from 12% chrome steel. It's a weldable alloy (just barely), and holds up very well to a steam environment (both strength at elevated temperatures, and corrosion resistance). I've never seen 12% chrome steel offered in bar-stock (only castings) - but that's not to say it doesn't exist in bar-stock form.
Using carbon-steel hydraulic cylinders for stem - unless you can completely dry & oil the cylinder walls after each use - you are going to corrode & pit the surfaces. Unprotected carbon steel is simply not going to last. One thing steam does very well (even low energy steam) is completely strip oil films from metal. For an occasional use model engine, I would recommend 304 or 316 stainless bores - a machinable & weldable alloy, but as you know, not exactly wear resistant. A 400 series martensitic stainless would probably work better, but 400 series stainless is not exactly user friendly (it's heat treatable, and not weldable without accomplishing annealing & re-heat-treat) - and it's quite expensive. . .
