Microstructure and wear resistance of high-speed steel treated with intense pulsed ion beam

The structural and phase transformations occurring in the near-surface layers of pre-quenched W6Mo5Cr4V2 high-speed steel (HSS) subjected to intense pulsed ion beam (IPIB) melting have been investigated. The effect of IPIB irradiation on wear resistance of the HSS has also been studied. The IPIB consists mainly of Cn+ (70%) and H+ (30%), with a high beam current density of 80 A/cm2, 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 have revealed that after IPIB irradiation the initial martensite in the near-surface layer of HSS changed into austenite and residual stresses were produced. Refined crystal grains in the near-surface layer having a thickness up to ∼2 μm were also observed. Redistribution and interlace of dislocations in the irradiated samples were generated under the impact of the shock wave. With increasing number of pulses, gradual liquid-phase dissolution of the M6C carbides occurred in the near-surface layer and produced nanocrystalline MC. This process results in the decrease of martensite crystal (α-phase) and increase of the austenite (γ-phase) content and the dispersion of carbides. The wear resistance of the HSS was improved by a factor of 2, which is explained by the formation of metastable nanocrystalline phases as well as by the presence of residual stresses and the redistribution dislocations.