The structure and orientational relationships of the α' and γ-phases in medium-carbon steels during rotational friction welding were studied using orientation (EBSD) and transmission electron microscopy methods. It is shown that, as a result of phase recrystallization, martensite with interlayers of the γ-phase is formed in the structure of the weld. Based on the type of spectrum of angular deviations of interphase boundaries, the formation of secondary (reverted) austenite was established. It is shown that in this case an orientation relationship close to Bain’s is realized between the α- and γ-phase crystals.