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 μms 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.