The effect of shock stress amplitude and duration on the hardness and microstructure of Hadfield steel has been investigated over the pressure range of 5–480 kbar. Electron microscopy techniques were used to examine the microstructure and measure the dislocation density of shock loaded specimens. Stacking fault and twin fault probabilities were estimated from x‐ray diffeaction experiments. For a given peak shock stress the time duration of the stress pulse greatly affected the hardness and microstructure of the shocked material. The Hugoniot equation of state of Hadfield steel is reported for shock stresses of 5–500 kbar.