Nanohardness of wear-resistant surfaces after electron-beam treatment

Abstract

The nanohardness, Young’s modulus, and defect substructure of the metal layer applied to Hardox 450 low-carbon martensitic steel by high-carbon powder wire (diameter 1.6 mm) of different chemical composition (containing elements such as vanadium, chromium, niobium, tungsten, manganese, silicon, nickel, and boron) and then twice irradiated by a pulsed electron beam are studied, so as to determine the correct choice of wear-resistant coatings for specific operating conditions and subsequent electron-beam treatment. The metal layer is applied to the steel surface in protective gas containing 98% Ar and 2% CO2, with a welding current of 250–300 A and an arc voltage of 30–35 V. The applied metal is modified by the application of an intense electron beam, which induces melting and rapid solidification. The load on the indenter is 50 mN. The nanohardness and Young’s modulus are determined at 30 arbitrarily selected points of the modified surface. The defect structure of the applied metal surface after electron-beam treatment is studied by means of a scanning electron microscope. The nanohardness and Young’s modulus of the applied metal after electron-beam treatment markedly exceed those of the base. The increase is greatest when using powder wire that contains 4.5% B. A system of microcracks is formed at the surface of the layer applied by means of powder wire that contains 4.5% B and then subjected to an intense pulsed electron beam. No microcracks are observed at the surface of layers applied by means of boron-free powder wire after intense pulsed electron-beam treatment. The boron present increases the brittleness. The increase in strength of the applied layer after electron-beam treatment is due to the formation of a structure in which the crystallites (in the size range from tenths of a micron to a few microns) contain inclusions of secondary phases (borides, carbides, carboborides). The considerable spread observed in the nanohardness and Young’s modulus is evidently due to the nonuniform distribution of strengthening phases.