A correlation was made of the microstructure, wear resistance, and fracture toughness of hardfacing alloys reinforced with complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel substrate by a submerged arc welding (SAW) method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside hardfacing electrodes were employed. Microstructural analysis of the hardfaced layer showed that cuboidal carbides, in which a TiC carbide core was encircled by a WC carbide, and rod-type carbides, in which W and Ti were mixed, were homogeneously distributed in the bainitic matrix. In the surface layer hardfaced with FeWTiC powders, more complex carbides were formed, because of the efficient melting and solidification during hardfacing, than in the case of hardfacing with WTiC powders. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, the hardness and wear resistance increased. In-situ observation of the fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. The hardness, wear resistance, and fracture toughness of the hardfacing alloys reinforced with complex carbides were improved in comparison with high-chromium white-iron hardfacing alloys, because of the homogeneous distribution of hard and fine complex carbides in the bainitic matrix.
This study investigates the effects of high-energy electron-beam (1.4 MeV) irradiation on surface hardening and microstructural modification in a gray cast iron currently used for a diesel engine cylinder block. The gray cast-iron samples were irradiated in air using an electron accelerator. Afterward, their microstructure, hardness, and wear properties were examined. The original microstructure, which contained graphite flakes in a pearlitic matrix, was changed to martensite, ledeburite, and retained austenite, along with complete or partial dissolution of the graphite. This microstructural modification occurred only when the surface was irradiated with an input-energy density over 1.1 kJ/cm 2 , and it greatly improved the surface hardness and wear resistance. In order to investigate the complex microstructures, thermal analysis and simulation testing were also carried out. The results indicated that the irradiated surface was heated to the austenite-temperature region and then quenched to room temperature, which was enough to obtain surface hardening through martensitic transformation. The thermal analysis results matched well with the microstructures of the thermally simulated samples.
Surface composites reinforced with TiC particulates were fabricated by high-energy electron-beam irradiation. In order to investigate the effects of flux addition on the TiC dispersion in surface composite layers, four kinds of powder mixtures were made by mixing TiC with 5, 10, 20, and 40 wt pct of the flux components (MgO-CaO). To fabricate TiC-reinforced surface composites, the TiC-flux mixtures were deposited evenly on a plain carbon steel substrate, which was subjected to electronbeam irradiation. Microstructural analysis was conducted using X-ray diffraction and Mössbauer spectroscopy as well as optical and scanning electron microscopy. The microstructure of the surface composites was composed of a melted region, an interfacial region, a coarse-grained heat-affected zone (HAZ), a fine-grained HAZ, and an unaltered original substrate region. TiC agglomerates and residual pores were found in the melted region of materials processed without flux, but the number of agglomerates and pores was significantly decreased in materials processed with a considerable amount of flux. As a result of irradiation, TiC particles were homogeneously distributed throughout the melted region of 2.5 mm in thickness, whose hardness was greatly increased. The optimum flux amount, which resulted in surface composites containing homogeneously dispersed TiC particles, was found to be in the range of 10 to 20 pct to obtain excellent surface composites.
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