The crystallography and nucleation of pearlite

By choice of a suitable iron—manganese—carbon alloy it has been possible to study pearlite nodules growing in austenite, without the austenite transforming on cooling to room temperature. Thin foil electron microscopy has been used to examine the orientation relations between cementite, ferrite and austenite as well as morphological aspects of the transformation. It is shown that one of the classical ferrite—cementite orientation relations found in pearlite (Pitsch—Petch) arises when the pearlite colonies nucleate on ‘clean’ austenite grain boundaries. The other familiar relation (Bagaryatski) arises when the colonies nucleate on pre-existing hyper-eutectoid cementite layers at the austenite grain boundaries. Some observations are made on the mode of nucleation of the pearlite nodules.