The early stages of aging/tempering of ferrous martensites have been studied over a wide range of carbon contents by numerous experimental techniques. A kinetic framework is presented which allows a correlation of many diverse observations. Three general regimes of structural change termed relaxation, aging, and tempering can be further subdivided, thus identifying two stages of aging (A1 and A2) which precede the conventional stages of tempering (T1-T3). The first stage of aging involves clustering of carbon atoms in c-oriented octahedral sites, while the second stage is associated with a modulated tweed microstructure containing high-carbon regions which may be interpreted as either advanced carbon clusters or coherent transition carbides. Alloy compositions are proposed to better correlate future complementary experiments required to determine these unusual structures more precisely.
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