An experimental re-evaluation of the Smith/Hillert and Hull/Mehl models of the pearlite transformation has been made by application of transmission electron microscopy techniques to suitable alloy systems. The observed behavior of the ferrite:cementite lamellar interface shows the strict crystallographic adherence predicted by the Hull/Mehl model. However, the presence of "direction steps" at this boundary allows the necessary geometric freedom for pearlite to develop by the Hillert branching mechanism, which, until now, has been associated with unimportant ferrite:cementite crystallography. It is found that the crystallography at the pearlite:austenite interface plays a much expanded role in determining the mechanism of growth and morphological development of the pearlite colony. Based on these observations, a new model with extensive deference to the ferrite:cementite:austenite crystallographic relationship is proposed for the pearlite transformation. Note: Abstract extracted from PDF file via OCR.