Influence of boron modification and cooling conditions during solidification on structural and phase state of heat- and wear-resistant white cast iron

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.

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