What are turbine casings made of for non superheated steam applications?

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Material choice for turbine casings is driven by service temperature, pressure, and the need for a part that can be cast into a large, complex shape while withstanding vibration and thermal cycling. For non-superheated steam, temperatures are moderate, so the casing doesn’t require the extreme high-temperature strength of some steels or the specialized alloys used for superheated steam. Cast carbon steel offers a practical balance: it has enough strength and toughness at these temperatures, good fatigue resistance to cope with the steady loads and start–stop cycles, and it can be produced as robust castings at a reasonable cost. It also supports reliable machining and assembly after casting. Aluminum would falter at the operating temperatures, and while cast iron can be used in some designs, it is typically more brittle and less forgiving under thermal cycling than carbon steel. Cast carbon steel thus aligns well with the demands of non-superheated steam turbine casings.