AbstractThe microstructural development during rolling is compared in two Hadfield steels, one having low carbon content (0·65 wt-%) and the other high content (1·35 wt-%). Differences in substructure are observed which are due not to small changes in stacking fault energy, but to carbon segregation, which occurs in the low carbon steel (through vacancy diffusion) but not in the high carbon steel. This is demonstrated using Mössbauer spectroscopy and is in agreement with systematic characterisation of microstructures by optical and transmission electron microscopy. In the low carbon steel mixed microstructures are formed which contain intrinsic stacking faults, deformation twins, and brass type shear bands. In the high carbon steel mixed substructures of dislocation tangles, deformation twins, and shear bands (both copper and brass type) are found to develop. In spite of the difference of substructure development during rolling in the two steels, the difference in stacking fault energy is measured to be small (~2 mJ m−2, i.e.<10% of the stacking fault energy).MST/1417