Heat Treating of Nickel and Nickel Alloys,Heat Treating of Nickel and Nickel Alloys,Heat Treating of Nickel and Nickel Alloys
NICKEL AND THE NICKEL ALLOYS are, in some ways, easier to heat treat than many of the iron-base alloys that
depend on carbon-related microstructural changes to achieve desired properties. Nickel is an austenite former, and in the
nickel and high-nickel family of alloys, no allotropic phase transformations occur--the alloys are austenitic from the
melting temperature down to absolute zero. While some precipitates may form such as carbides and the γ' hardening
phase, these do not change the basic austenitic-type structure of the matrix. (See the article "Heat Treating of Superalloys"
in this Volume for discussions on the formation of carbides and γ' precipitates.)
Because nickel is found in nature as nickel sulfide and nickel oxide ores, it has a natural tendency to combine with sulfur
and/or oxygen. One of the most important factors to consider when heat-treating nickel or the nickel alloys is to minimize
exposure to sulfur, whether in solid form (such as lubricants, grease, or temperature-indicating sticks) or in gaseous form
(such as SO2 or H2S). When embrittlement by sulfur occurs, there are no techniques that can be used to reclaim the
affected material; the contaminated area must be either removed, such as by grinding, or scrapped.
Because nickel has a very low solubility for carbon in the solid state, it does not readily carburize. For this reason the
nickel-chromium alloys, most notably Inconel 600, are used as fixtures in carburizing furnaces. Almost all heat treatment
methods used with nickel and the nickel alloys are employed either to soften them, such as annealing, or to increase their
strength, such as age hardening.

Types of Heat Treatment

Nickel and the nickel alloys may be subjected to one or more of six principal types of heat treatment, depending on
chemical composition, fabrication requirements, and intended service. These methods include:
· Annealing. A heat treatment designed to produce a recrystallized grain structure and softening in workhardened
alloys. Annealing usually requires temperatures between 705 and 1205 °C (1300 and 2200
°F), depending on alloy composition and degree of work hardening
· Solution annealing. A high-temperature anneal (1150 to 1315 °C, or 2100 to 2400 °F) of certain nickel
alloys to put carbides in solid solution and produce a coarse grain size for enhanced stress-rupture
properties
· Stress relieving. A heat treatment used to remove or reduce stresses in work-hardened non-agehardenable
alloys without producing a recrystallized grain structure. Stress-relieving temperatures for
nickel and nickel alloys range from 425 to 870 °C (800 to 1600 °F), depending on alloy composition
and degree of work hardening
· Stress equalizing. A low-temperature heat treatment used to balance stresses in cold-worked material
without an appreciable decrease in the mechanical strength produced by cold working
· Solution treating. A high-temperature heat treatment designed to put age-hardening constituents into
solid solution. Normally applied to age-hardenable materials before the aging treatment
· Age hardening (precipitation hardening). A treatment performed at intermediate temperatures (425 to
870 °C, or 800 to 1600 °F) on certain alloys to develop maximum strength by precipitation of a
dispersed phase throughout the matrix

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