Almost 100% except insulation and lagging, material in steam boiler construction is made from steel. Methods of forming shape are also described in requirement of ASME BPV Section I and Section II (American Society of Mechanical Engineer Boiler and Pressure Vessel.
The following below is the methods of forming shape of steel:
Casting is the method of forming steel by molten a metal in a certain place which has been designed and permit it to be solid shape
Forging is the method of forming steel by using pressure and impact to form steel as per design.
Rolling is the methods of forming steel by roll process either cold roll or hot roll.