Boiler

Materials

1950s design steam locomotive boiler , from a Victorian Railways J class

     The pressure vessel in a boiler is usually made of steel (or alloy steel) , or historically of wrought iron. Stainless steel is virtually prohibited (by the ASME Boiler Code) for use in wetted parts of modern boilers , but is used often in superheater sections that will not be exposed to liquid boiler water. In live steam models , copper or brass is often used because it is more easily fabricated in smaller size boilers. Historically , copper was often used for fireboxes (particularly for steam locomotives) , because of its better formability and higher thermal conductivity ; however , in more recent times , the high price of copper often makes this an uneconomic and cheaper substitutes (such as steel) are used instead.
      For much of the victorian "age of steam" , the only material used for boilermaking was the highest grade of wrought iron , with asembly by rivetting . This iron was often obtained from specialist ironworks , such as at cleartor Moor (UK) , noted for the high quality of their rolled plate and its suitability for high-reliability use in critical applications , such as high-pressure boilers. In the 20th century , design practice instead moved towards the use of  steel , which is stronger and cheaper , with welded construction , which is quicker and requires less labour.

Diagram of a water-tube boiler

      Cast Iron may be used for the heating vessel of domestic water heaters. Although such heaters are usually termed "boilers" in some countries , their purpose is usually to produce hot water , not steam and so they run at low pressure and try to avoid actual boiling. The brittleness of cast iron makes it impractical for high pressure steam boilers.

Diagram of a fire-tube boiler