Casings

Among the high-temperature alloys, nickel-base alloys are the most widely used. As a result, they are often found in aerospace engine and power generation turbine components, as well as in petrochemical, food processing, nuclear reactor, and pollution control equipment. Nickel-base alloys can be strengthened by two methods: through solid solution strengthening or by being hardened through intermetallic compound precipitation in fcc matrix. Alloys such as INCONEL® 625 and Hastelloy® X are solid solution strengthened. These solid solution hardened alloys may get additional strengthening from carbide precipitation. Alloys such as INCONEL® 718, however, are precipitation strengthened. A third class of nickel-base superalloys, typified by MA-754, is strengthened by dispersion of inert particles such as yttria (Y2O3), and in some cases with γ´ (gamma prime) precipitation (MA-6000E). Nickel-base alloys are available in both cast and wrought forms. Highly alloyed compositions, such as Rene 95, Udimet 720, and IN100, are produced by powder metallurgy followed by forging. For the above wrought alloys and for cast alloys (Rene 80 and Mar-M247), the strengthening agent is γ’ precipitate. For INCONEL® 718, γ˝ (gamma double prime) is the primary strengthening agent. Alloys that contain niobium, titanium, and aluminum, such as INCONEL 725, are strengthened by both γ´ and γ˝ precipitates.

Cobalt-base superalloys possess superior corrosion resistance at temperatures above 2000˚ F (1093˚ C) and find application in hotter sections of gas turbines and combustor parts. Available in cast or wrought iron form, cobalt-base superalloys are characterized by a solid solution strengthened (by iron, chromium, and tungsten), by an austenitic (face centered cubic or fcc) matrix, in which a small quantity of carbides (of titanium, tantalum, hafnium, and niobium) is precipitated. Thus, they rely on carbides, rather than γ´ precipitates, for strengthening, and they exhibit better weldability and thermal fatigue resistance than nickel-base alloys. Cast alloys, such as Stellite 31, are used in the hot sections (blades and vanes) of gas turbines. Wrought alloys, such as Haynes 25, are produced as sheet, and are often used in combustor parts.

Iron-nickel-base superalloys are similar to wrought austenitic stainless steels, except for the addition of γ´ strengthening agent. They have the lowest elevated temperature strength among the three groups of superalloys, and are generally used in the wrought condition in gas turbine disks and blades. Most wrought alloys contain high levels of chromium, which provide corrosion resistance. They owe their high-temperature strength to solid solution hardening (hardening produced by solute atoms dissolved in the alloy matrix) or precipitation hardening (hardening produced by precipitate particles). Alloys such as Haynes 556 and 19-9 DL are solid solution strengthened with molybdenum, tungsten, titanium, and niobium. Alloys such as A286 and Incoloy 909 are precipitation hardened. The most common precipitates are γ´, (Ni3 [Al, Ti]) (e.g., A286), and γ˝, (Ni3Nb) (e.g., Incoloy 909). Another group of iron-nickel-base alloys contains high carbon content and is strengthened by carbides, nitrides, and solid solution strengthening. A group of alloys, based on Fe-Ni-Co and strengthened by γ’, combines high strength with a low thermal expansion coefficient (e.g., Incoloy 903, 907, and 909) and finds application in shafts, rings, and casings for gas turbines.