Heat affected zone (HAZ) softening in two armour steels (high hard and flash processed) was investigated by monitoring phase transformations during simulated welding thermal cycles. The high hard steel was produced by conventional thermomechanical processing followed by quench and tempering. Flash processed steel was produced by rapid heating and cooling of a spheroidised steel to produce a mixed microstructure. Heat affected zone softening in high hard steel was observed on heating to a peak temperature below Ac1 due to overtempering of the original microstructure. In flash processed steel, the softening was associated with allotriomorphic ferrite formation when heated to a peak temperature between Ac1 and Ac3. The above results demonstrate the importance of initial microstructure on HAZ on phase transformation transients and associated softening in armour steels.