Interpretation of accurate orientation relationships developed during pearlite formation in a hypereutectoid steel

Crystallographic aspects of the austenite-to-pearlite reaction were studied for Hadfield steel containing 1.1 % C and 12.3 % Mn. Specimens isothermally transformed at 650 °C (923 K) were examined via transmission electron microscopy. Microdiffraction patterns were analyzed, and accurate orientation relationships (O.R.s within about 1°) were determined for lamellae of ferrite and cementite within pearlite colonies. Additionally, O.R.s were collected for both of the pearlitic phases and the adjacent austenite grains. Nearly 90 accurately measured O.R.s were compared with standard O.R.s in the literature. Deviations from exact O.R.s were reported as axis/angle pairs. The magnitude of angular deviations was used as the criterion to estimate relative interfacial energies. Proeutectoid cementite crystals formed at austenite grain boundaries; these crystals adopted O.R.s closer to standard O.R.s on one side of the boundary (referred to as the γ 1 grain) as compared with the other grain (γ 2 ). Subsequent nucleation of ferrite occurred at comparatively high-energy cementite/γ 2 interfaces as the precursor to cooperative growth of pearlite into γ 2 . Within lamellar pearlite colonies, a near-Pitsch-Petch O.R. was most-commonly adopted between pearlitic phases. Most angular deviations from exact Pitsch-Petch O.R. were about two to eight degrees. For about half the observations, ferrite crystals adopted a near-standard O.R. with respect to both pearlitic cementite and γ 2 . Otherwise, ferrite adopted a near-standard O.R. with respect to cementite only. Pearlite/austenite growth interfaces displayed characteristics typically associated with semicoherent and incoherent interfaces. In this study, pearlite colonies grew into the so-called γ 2 grains because ferrite nucleated at cementite/γ 2 interfaces rather than at the cementite/γ 1 interfaces. Multiplication of lamellae in pearlite colonies occurred via branching; however, evidence of side-by-side nucleation of crystals of both pearlitic phases is reported. • Accurate orientation relationships from nearly 30 pearlite colonies were determined. • For most colonies, the contribution of adjacent austenite grains was assessed. • Ferrite nucleated at high-energy cementite/austenite interfaces prior to pearlite formation. • Growth of pearlite occurred despite evidence of coherency at the pearlite/austenite interface. • Not all pearlite colonies were bicrystals.