6.4 Casing materials
HP and IP casings, where temperatures over 350°C are encountered, are made from creep-resistant materials. Two different low alloy steels are used; 2.25Cr IMo for temperatures up to 538°C and 0.5Cr 0.5Mo 0.25V for temperatures up to 565°C. The 2.25Q material is slightly weaker and more ductile in creep, but it is also tougher (i.e., less prone to fast fracture at low temperatures) and easier to weld.
Casings in these materaals are cast rather than fabricated because of the difficulty of quality control and likelihood of thermal distortion in the thick sections involved. LP cylinder casings are generally fabricated from carbon steel, although the inner cylinder may be cast from spheroidal graphite iron. Large defects in castings are ground out and repaired by welding, but nevertheless the materials are chosen to have a high tolerance to the presence of defects which may grow during service.
High temperature bolts are either made from low alloy steels designed specially for creep-resistant bolting, for example, Durehete 1055, or from nickel alloys such as Nimonic 80A. The nickel alloys are more creep-resistant and stronger, allowing a more compact flange with better thermal properties; however, they are subject to stress corrosion cracking, particularly if wet or contaminated with sulphur compounds. Nickel alloy bolts are thus used mainly on inner cylinders where the compact flange is most advantageous.
Low temperature bolts on LP casings are made from normal high tensile bolting steels.High temperature wet steam is considerably more erosive than dry steam, consequently HP casings for wet steam machines are either made from 2.25Cr IMo steel with stainless steel cladding at the joint faces or entirely from a higher alloy steel.