The control valve user normally specifies the body material. Which is often the same material as the pipe. The most common choices of body material are carbon steel, chrome molybdenum steel and stainless steel.
Carbon steel is the most commonly used for bodies. It handles most non corrosive liquids and gases up to 800oF for continuous service or to 1000oF for occasional service. Carbon steel can be used for most condensate and steam services.
Chrome – moly steel is used for higher temperatures and pressures than carbon steel, including such services as high pressure steam or flashing condensate which requires corrosion and erosion resistance. This is stronger than carbon steel and in some cases is as strong as stainless steel. It costs less than SS but not as corrosion resistance.
SS is specific for higher temperature services (1000oC and above) or in corrosive application.
Special alloys such as Super Duplex, Duplex, Hastalloy B or C, monel, nickel and titanium are also available.
The valve body can be cast, forged, wrought or fabricated. Castings are the first choices in standard sizes and ratings. Forgings are used for smaller sized bodies (generally these are high pressure ratings). Barstock are recommended when the delivery is critical and a casting or forging is not available. Fabricated type is a convenient way to manufacture large angle valves.
Bonnets are manufactured from barstock of the same material as the body. Exception is the smaller low pressure chrome moly valves where SS bonnet is the standard.