Pulsed electron-beam melting of high-speed steel: structural phase transformations and wear resistance

The structural and phase transformations occurring in the near-surface layers of pre-quenched high-speed steel subjected to pulsed electron beam melting have been investigated. Melting was induced by a low-energy (20–30 keV), high-current electron beam with a pulse duration of 2.5 μs and an energy density ranging from 3 to 18 J/cm2. Using electron microscopy and X-ray diffraction it has been revealed that with increasing beam energy density gradual liquid-phase dissolution of initial globular M6C carbide particles occurs in the near-surface layer of thickness up to ∼1 μm. This process is accompanied by the formation of martensite crystals (α-phase) and an increase of residual austenite (γ-phase) content. When the carbide particles are completely dissolved, martensitic transformation is suppressed. In this case, a non-misoriented structure is formed consisting predominantly of submicrometer cells of γ-phase separated by nanosized carbide interlayers. Irradiation of cutting tools (drills) in a mode corresponding to an abrupt decrease in the content of M6C particles due to their liquid-phase dissolution enhances the wear resistance of the drills by a factor of 1.7. This is associated with the fixation of undissolved particles in the matrix, the formation of residual compressive stresses and of dispersed M3C carbide particles as well as the high (∼50%) content of the metastable γ-phase in the surface layer.