*Inconel alloys* are oxidation and corrosion resistant materials well suited
for service in extreme environments. When heated, Inconel forms a thick,
stable, passivating oxide layer protecting the surface from further attack.
Inconel retains strength over a wide temperature range, attractive for high
temperature applications where aluminum and steel would succumb to creep as
a result of thermally-induced crystal vacancies (see 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.
Gamma prime forms small cubic crystals that inhibit slip and creep
effectively at elevated temperatures.
A nickel-chromium-molybdenum alloy with an addition of niobium that acts
with the molybdenum to stiffen the alloy's matrix and thereby provide high
strength without a strengthening heat treatment. The alloy resists a wide
range of severely corrosive environments and is especially resistant to
pitting and crevice corrosion. Used in chemical processing, aerospace and
marine engineering, pollution-control equipment, and nuclear reactors.
Inconel is a difficult metal to shape. 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.
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, steam generators in nuclear pressurized water
reactors, natural gas progressing with contaminants such as H2S and CO2,
firearm sound suppressor blast baffles, and Formula One exhaust systems. Inconel is increasingly used in the boilers of waste incinerators.
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