Within this high-alloy category of materials, there are extremely diverse chemical compositions, properties, and applications. In general, however, they share one of two broadly defined common denominators: (1) the ability to perform under exceedingly rigorous service conditions or (2) properties (e.g., electrical, thermal, magnetic) that are finely tuned to suit particular requirements. Description (1) could be said to have three subgroups which, when combined with Description (2), would yield the following four groups of alloys:
Most of the alloys in the four groups are high-nickel alloys, with nickel content ranging from 25% to nearly 100%. However, some of the alloys are cobalt-based (or high in cobalt) and a few are iron-based. In virtually all the alloys, other elements—including chromium, niobium, molybdenum, aluminum, and titanium—significantly influence strength or corrosion-resistance.
The alloys in all four groups have significant corrosion resistance by virtue of their chemical compositions. A number of the alloys, however, are specifically intended for use in acids and other industrial chemicals, corrosive waters, commercial food and beverage processing, pollution control, and similar areas where the main criterion for selection is resistance to the environment.
Materials in this group include commercially pure nickel (Nickel 200/UNS NO2200) for handling caustic soda and for food processing. An alloy of nickel with one-third copper (alloy 400/UNS NO4400) is a versatile material that resists brines, sea water, some acids, and many substances found in chemical and petrochemical processing. As environments become more severe, alloys with more complex compositions are required. Oxidizing media such as chemicals containing nitric acid are generally best resisted by nickel alloys having significant amounts of chromium (e.g., alloy 690/UNS NO6690). Nickel-chromium alloys with the additions of copper and molybdenum (such as alloy 20/UNS NO8020) resist both oxidizing and reducing chemicals. Molybdenum also provides resistance to pitting and crevice corrosion. Molybdenum in rather large amounts, such as the 16% in alloy C-276/UNS N10276, produces some of the most corrosion-resistant of the nickel alloys. This element is also important in a versatile and widely-used “6% moly” group of alloys. These alloys are at the low end of nickel content (about 25%) of the corrosion-resistant alloys but their resistance is bolstered by the molybdenum and often by small amounts of nitrogen.
The alloys classified here as heat-resistant alloys are the general-purpose high-temperature alloys as used in a multitude of industrial thermal-processing applications. These uses include equipment (furnace components, racks, baskets, etc.) for heat treatment of other metals and radiant tubes in petrochemical-processing furnaces such as for production of hydrogen and ethylene.
The alloys in this group must have sufficient strength at the operating temperatures and must resist oxidation, carburization, or other attack by the environment, normally a hot gas. All the alloys contain nickel and chromium; most contain significant amounts of iron. Chromium content ranges from about 15% (alloy 600/UNS NO6600) to about 25% (alloy 333/UNS NO6333). For the most part, iron content is inversely proportional to nickel content, which ranges from a high of about 75% (alloy 600/UNS No6000) down to about 32% (alloy 800/UNS NO8800). Some of the alloys contain small amounts of elements such as aluminum (alloy 610/UNS NO6601) and silicon (alloy 330/UNS NO8330) to enhance heat resistance.
Superalloys is a general definition of all the alloy groups. However, the superalloy group discussed here comprises those high-performance alloys at the leading edge of metals technology. Most of the alloys are nickel-based, although some have cobalt or iron as the predominant element. Development of the alloys normally has involved the two-pronged goal of resistance to degradation by high-temperature environments combined with extremely high strength at elevated temperatures. The primary area of application for such alloys is the gas turbine engine, both aircraft and land-based versions. Applications also include compressor blades, vanes, spacers, and discs; shafts; casings and rings; and sheet components such as combustion chambers, ducting, exhaust systems, afterburners, and thrust reversers.
Superalloys general have a basic heat-resistant composition (i.e., a chromium-containing base of nickel, cobalt, or iron) with alloying additions that act to boost specific strength characteristics at intended service temperatures. Often of particular concern are the time-dependent strength factors of creep resistance and rupture life. Some superalloys (e.g., alloy X/UNS N06002, N06625/alloy 625) are solid solution (single phase, not strengthened by heat treatment). The majority, however, are metallurgically complex alloys that are precipitation hardenable (age hardenable). That is, they are strengthened through microstructural precipitation of a second phase during a suitable heat treatment. This heat treatment can double the strength alloy. The heat-treatable alloys can be comparatively simple. For example, alloy 750/UNS N07750 is little more than its solid-solution counterpart of alloy 600/UNS N06600 with additions of aluminum and titanium to cause precipitation hardening. Others of these alloys (e.g., alloy 263/N07263, alloy 718/N07718) may contain specified amounts of ten or twelve elements.
Superalloys can be found throughout the typical gas turbine engine. Applications include compressor blades, vanes, spacers, and discs; turbine blades, vanes, and discs; shafts; casings and rings; and sheet components such as combustion chambers, ducting, exhaust systems, afterburners, and thrust reversers.
The alloys of this group vary widely in chemical composition and properties. They are grouped together because their usefulness depends on physical characteristics—thermal, magnetic, electrical—and not specifically on corrosion-resistance or strength.
A series of nickel-iron alloys in this group have low (alloy 36/UNS K93600) or controlled (alloy 42/UNS K94100, K94610/alloy K) coefficients of thermal expansion. Their applications include measurement standards, glass-to-metal sealing as in lamp bulbs, thermostats, and semi-conductor lead frames. Other nickel-iron alloys are used for their magnetic properties.
Another series (e.g., alloy C/UNS N06003, N06004/alloy B) comprises nickel-chromium electrical resistance alloys that are used as heating elements in applications ranging from domestic appliances to industrial furnaces.
Other alloys in this group are used for thermocouple wire (Alumel/UNS N02016), electronic components (Nickel 205/UNS N02205), and precision springs (alloy 902/UNS N09902).