Flash processing of an AISI8620 steel sheet, which involves rapid heating and cooling with an overall process duration of <10 s, produced a steel microstructure with a high tensile strength and good ductility similar to that of advanced high strength steels. Flash processed steel [ultimate tensile strength (UTS): 1694 MPa, elongation: 7·1], showed at least 7 higher UTS and 30 greater elongation than published results on martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5·1). The underlying microstructure was characterised with optical, scanning electron, transmission electron microscopy as well as hardness mapping. A complex distribution of bainitic and martensite microstructures with carbides was observed. A mechanism for the above microstructure evolution is proposed.

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 Ac 1 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 Ac 1 and Ac 3 . The above results demonstrate the importance of initial microstructure on HAZ on phase transformation transients and associated softening in armour steels.

Flash processed (FP) steels have shown potential for armour applications based on the response to ballistic loading. In this research, weldability of these steels was studied and compared to commercial high hard (HH) armour steel. Simulated heat affected zone (HAZ) microstructures in these steels were investigated by comprehensive microscopy and hardness mapping. HAZ softening in HH steel was observed on heating to a peak temperature below Ac 1 due to overtempering of original microstructure. In FP steel, the softening was associated with allotriomorphic ferrite formation when heated to a peak temperature between Ac 1 and Ac 3 . The transformation kinetics, softening, and hardening characteristics are rationalised based on changes in prior austenite grain size, cooling rate and underlying chemical heterogeneity.

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