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Inconel
alloys are oxidation and corrosion resistant materials well suited for service
in extreme environments. Inconel forms a thick, stable, passivating oxide layer
when heated, protecting the surface from further attack. Inconel retains strength
over a wide temperature range, making it attractive for high temperature applications
where aluminum and steel would succumb to creep as a result of thermally-induced
crystal vacancies (Arrhenius equation). Inconel's high temperature strength
is developed by solid solution strengthening or precipitation strengthening,
depending on the alloy. In age hardening or precipitation strengthening varieties,
small amounts of niobium combine with nickel to form the intermetallic compound
Ni3Nb or gamma prime (γ'). Gamma prime forms small cubic crystals that
inhibit slip and creep effectively at elevated temperatures.
Inconel is a difficult metal to shape and machine using traditional techniques due to rapid work hardening. After the first machining pass, work hardening tends to elastically deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened Inconels such as 718 are machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. Alternatively, the majority of the machining can be performed with the workpiece in a solutionised form, with only the final steps being performed after age-hardening. External threads are machined using a lathe to "single point" the threads, or by rolling the threads using a screw machine. Holes with internal threads are made by welding or brazing threaded inserts made of stainless steel. Cutting of plate is often done with a waterjet cutter.
Inconel is often encountered in extreme environments. It is common in gas turbine blades, seals, and combustors, as well as turbocharger rotors and seals, high temperature fasteners, chemical processing and pressure vessels, heat exchanger tubing, natural gas progessing with contaminants such as H2S and CO2, firearm sound suppressor blast baffles, and Formula One exhaust systems.
North American Aviation constructed the skin of the X-15 rocket plane out of an Inconel alloy known as "Inconel X".