Annealing

Although the term annealing refers to a heat treatment that renders a metal soft by the removal of strain, in P/M, the term is used more broadly and includes the softening of a powder by the removal of interstitials, particularly carbon, nitrogen, and oxygen. Powders that have been work hardened through milling; powders that contain residual amounts of carbon, oxygen, or nitrogen; and powders that contain objectionable surface oxide films are most often subjected to this type of annealing. Examples include sponge and atomized iron powders, carbonyl and electrolytic iron powders, tool steel powders, and ferritic stainless steel powders. In most cases, annealing is performed in a reducing atmosphere, which not only protects the powder against oxidation, but also removes surface and interior oxides. Annealing improves the compressibility and green strength of a powder due to the softening of the metal. Frequently, annealing also produces slightly agglomerated powders with lower apparent densities and further increases in green strength.

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