Microstructural evolution and mechanical response of nanocrystalline and ultrafine-grained steel obtained by mechanical milling

High dense bulk specimens of 0.55% C steel with ferrite grain sizes in the nanocrystalline and ultrafine regimes have been produced by a warm consolidation process from ball-milled iron powder and subsequent heat treatments. The resulting microstructure was investigated and related to the mechanical behavior studied by uniaxial compression tests. It was found that the samples containing nanocrystalline ferrite presented very high strength with low ductility. As the ferrite grain size increased to the ultrafine regime, larger cementite particles appeared inside ferrite grains and along the grain boundaries. As a consequence, the amount of strain hardening and total plastic strain increased and the maximum strength diminished. With a consolidation temperature of 500 °C and a heat treatment temperature of 775 °C, an average ferritic grain size of 0.85 μm was obtained. This steel showed a yield strength of 1070 MPa and strain hardening up to a plastic strain of 31%.