Formation of cementite in tempered Fe-Co-C alloys

In Fe-Co-C alloys, undesirable grain coarsening results from the specific austenite orientation variants that form after the γ → α → γ transformations. Tempering of martensite before reheating prevents austenite returning to its original orientation and also limits grain coarsening. However, the reasons for this are unclear. It may be assumed that some differences between cementite formed in tempered and rapidly heated alloys may cause the variation in the final austenite structure. In the present work the orientation relationships between cementite and martensite in two tempered Fe-Co-C alloys have been studied using microbeam electron diffraction in a transmission electron microscope. In both alloys after short-term (rapid heating at 100°C s-1followed by quench) and long-term (1 and 3 h) tempering treatments the orientation relationships were shown to obey the Isaichev orientation relationships: (1̄03)c//(110)α, [010]c//[11̄1̄]α, [311]c0.91° from [11̄1]α. However, after rapid tempering, only one carbide variant was found in each α crystal, while after long-term tempering, up to three variants were present. This might account for the observed crystallographic reversibility in rapidly heated alloys, contrary to the multiplication of γ variants formed from the long-term tempered martensite.