THE EFFECT OF HIGH-ENERGY ELECTRON-BEAM IRRADIATION ON MICROSTRUCTURAL MODIFICATION OF A HIGH-SPEED STEEL ROLL

The purpose of this study is to investigate the microstructural modifi cation in a high-speed steel (HSS) roll irradiated with an accelerated high-energy electron beam. The HSS roll samples were irradiated at th e beam travel speeds of 2.5 to 25 mm/s using an electron accelerator ( 1.4 MeV). The microstructure was examined with a scanning electron mic roscope (SEM) capable of in situ fracture testing and simultaneous mea surement of the apparent fracture toughness. Irradiation changed the m atrix phase from tempered martensite to a mixture of retained austenit e and martensite. Coarse primary carbides were partially or completely dissolved, depending on the heat input. Irradiation greatly improved the fracture properties because of the presence of retained austenite, which could retard crack propagation, although hardness was decreased . Occasional interior quench cracks were found in the heat-affected re gion. Appropriate processing methods, such as pre- or postirradiation, were suggested. A heat transfer analysis of the irradiated surface la yer was also carried out to elucidate the influence of the irradiation parameters on the microstructure.