Crystallography of formation of cementite under tempering of carbon martensite

The crystallography of cementite precipitation from carbon martensite is considered using hypothesis of regular iron-atoms' displacements. Series of lattice deformations is generated, and it is shown that minimal deformations are very close to invariant-plane strain. Main crystallographic parameters of martensite-cementite transformation are calculated using phenomenological theory of martensite (Wechsler-Lieberman-Read model). Results obtained for energetically favourable conditions explain presence of Isaichev orientation relationships (OR) and Petch OR as consequences of the two distinct lattice deformations. From the same reasons, the Isaichev OR are more favourable than the Petch OR, and, as a result, the former OR are realized predominantly. Calculated indexes for habit planes are in good agreement with experimentally determined ones.