Gradient Structure of the Layer Applied to Hardox 450 Steel by Fe–C–Cr–Nb–W Powder Wire after Electron-Beam Treatment

Surfacing with composite coatings strengthened by carbide, boride, and other particles is currently of great interest in materials physics. The performance of the applied layer is primarily determined by the phase composition of the coating. To permit the selection of coatings capable of withstanding extremal operating conditions, including high loads and abrasive wear, their properties and structure must be investigated in detail. In the present work, state-of-the-art techniques in materials physics are used to study the structure, phase composition, and tribological properties of coatings applied to Hardox 450 low-carbon martensitic steel by Fe–C–Cr–Nb–W powder wire and then subjected to electron-beam treatment. The electron-beam parameters are as follows: in the first stage, energy density per pulse E_S = 30 J/cm²; pulse length τ = 200 μs; and number of pulses N = 20; in the second stage, E_S = 30 J/cm²; τ = 50 μs; and N = 1. These conditions are selected on the basis of calculations of the temperature field formed in the surface layer of the material by a single pulse. It is found that electron-beam treatment of an applied layer of thickness about 5 mm leads to modification of a thin surface layer (about 20 μm), consisting largely of α iron and the carbide NbC; small quantities of the carbides Fe₃C and Me₆C (Fe₃W₃C) are also present. This modified surface layer differs from the unmodified coating mainly in terms of the morphology and dimensions of the secondary-phase inclusions. In the modified surface layer, the inclusions are smaller and take the form of thin layers along the grain boundaries. In the unmodified coating, the inclusions are mainly rounded particles, chaotically distributed within the grain. After electron-beam treatment, the wear resistance of the applied layer increases by a factor greater than 70 with respect to Hardox 450 steel, while the frictional coefficient is significantly less (about a third as much).