Surface modification of high-speed steel by intense pulsed ion beam irradiation

The structural and phase transformations occurring in the near-surface layers of pre-quenched W6Mo5Cr4V2 high-speed steel (HSS) after intense pulsed ion beam (IPIB) irradiation, as well as their effect on the wear and corrosion resistance of irradiated samples have been investigated. The IPIB consists mainly of Cn+ (30%) and H+ (70%), with high beam current densities of 80–160 A/cm2, an acceleration voltage of 250 kV and pulse duration of 70 ns. Samples were bombarded with 1, 3 and 5 pulses. Electron microscopy and X-ray diffraction revealed that after IPIB irradiation the initial martensite in the near-surface layers of HSS changed into austenite where residual stresses were produced also. Refined crystal grains in the near-surface layer with a thickness up to ∼ 2 μm were also observed. Redistribution and interlace of dislocations in the irradiated samples were generated due to the impact of the shock wave. With increasing number of pulses, liquid-phase dissolution of the M6C carbides occurred in the near-surface layer and nanocrystalline MC was produced. This process results in a decrease of martensite crystal (α-phase), an increase of the austenite (γ-phase) and dispersion of the carbides. The wear resistance of the HSS was improved by a factor of 2 due to the formation of metastable nanocrystalline phases, the presence of residual stresses and the redistribution of dislocations. Corrosion resistance of HSS was also improved after IPIB irradiation.