Microstructure of Flash Processed Steel Characterized by Electron Backscatter Diffraction

Flash processing is a new heat treatment process being developed to produce steel with relatively high strength and ductility. It involves rapidly heating steel sheet or strip to a temperature in the austenite range and quenching; the entire thermal cycle takes place within 15 seconds. The resulting microstructure is fine and difficult to resolve using standard metallographic techniques. In this investigation, electron backscatter diffraction was used to measure the grain size, grain orientations, and phase fractions in AISI 8620 samples flash processed to a series of different maximum temperatures. The combination of high strength with moderate ductility obtained by flash processing arises from a refined martensitic microstructure. The morphology of the microstructure depends upon the maximum processing temperature; a lower maximum temperature appears to produce a finer prior austenite grain size and an equiaxed martensite structure whereas a higher maximum processing temperature yields a more conventional lath martensite morphology.