The paper is devoted to examination of the effect of boron modification and temperature conditions for metal cooling in a mold on phase composition, morphology and chemical composition of structural components of heatand wear-resistant white cast iron of Fe – C– Cr– Mn – Ni – Ti–Al – Nb system. The phase composition of the metallic base changed from the dualphase (α- and γ-phases) to the completely single-phase (γ-phase). Boron modification influenced on the type of secondary carbides, while secondary hardening in a mold occurs through extraction of dispersed niobium carbides (without boron, but with chromium carbides). The structure of modified cast iron is presented by the primary complex carbides (Ti, Nb, Cr, Fe)C, as well as by solid solution dendrites, eutectics and secondary carbides MeC. Boron addition changes the chemical composition of primary carbides with decrease of niobium content from 44 to 2 % and increase of titanium content from 24 to 65 %; content of eutectic carbides rises as well. As for hypereutectic carbides, they are characterized by increase of ferrum content and lowering of chrome content. Parameters of the primary phases (MeC carbides and solid solution dendrites) were investigated using the methods of quantitative metallography. The special technique of Thixomet PRO image analyzer was used for evaluation of the F form factor which is the criterion of compatibility of the primary phases. The following parameters were used in this work as the parameters of dendrite structure: dispersity of the dendrite structure (δ), volumetric part of dendrites (V), distance between the axes of second order dendrites (λ2 ), form factor (F), average dimensions of dendrites – square (S), length (l) and width (β). All the suggested characteristics (parameters) allowed not only to provide quantitative evaluation of the dendrite structure, but also to determine modification degree as relative variation (in %) of each criterion in modified cast iron in comparison with non-modified iron. Quantitative relation between modification degree and crystallization conditions were established as well.
Special features of the formation of the structure and properties of plasma-powder coatings of 250Kh15G20S are investigated. The effect of additional alloying of the coating with nitrogen by adding nitrided ferrochrome to the filler powder is considered. The effect of the deposition conditions and of the chemical composition of deposited metal on its hardness and impact-abrasive wear resistance is described.In most cases, tools working in the conditions of impact loading and simultaneous abrasive wear are produced from alloys with the austenitic structure of the metallic base 1 -3 . To ensure that the particles of the hardening phase remain in the structure, austenite should undergo martensitic or polydeformation transformation during wear 4 . The majority of commercial surfacing materials, used for hardening of components, do not satisfy the conditions of impactabrasive wear because the metallic base on the deposited metal is not capable of adapting to the conditions of abrasive wear under the effect of impact loading. Therefore, the aim of this work is the investigation of the effect of nitrogen alloying of the deposited metal of the Fe -C -CrMn -Si system on impact-abrasive wear resistance and examination of the structure and properties in plasmapowder surfacing.Investigations were carried out on 100 £ 30 £ 20 mm specimens of St3 steel. The beads were deposited in a single layer with a thickness of not less than 4 mm. The surfacing conditions were selected using the diagram published in Ref. 5 and resulted in the minimum penetration of the metal of the base and mixing of the metal in the deposited metal.Surfacing was carried out using a combined action plasma torch with a current of 120 and 180 A with cooling of the deposited specimens in still air or with spraying with water.The deposited specimens were tested for impactabrasive wear in accordance with the GOST 23.207-79.The design of the plasma torch, used for the deposition of coatings, and the connection diagram are shown in Figure 1.The surfacing mixture was produced by mixing the basic surfacing powder 250Kh15G20S with the powder of nitrided ferrochrome at a ratio of 19:1. The chemical composition of the produced powder compositions is shown in Table 1.In depositing the powder 250Kh15G20S at a current of 80 A, the high viscosity of the melt prevents efficient melting of the substrate and the formation of metallurgical bonds with the deposited metal -the coating separates and breaks up.The increase in current to 120 A leads to the formation of high-quality beads of deposited metal but the coating is characterized by scattered porosity caused by the high viscosity of molten metal and high tensile stresses formed during solidification of the weld pool. Metallographic analysis of the deposited metal shows that the shape of the pores is irregular and the edges are uneven (Figure 2(a)), and the pores are distributed at the boundaries of the growing eutectic crystals. They are natural stress concentrators and ready microcracks resulting in brittle failure of th...
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