Influence of Boron Modifications and Cooling Conditions during the Structural and Phase State Solidification of Heat- and Wear-Resistant White Cast Iron

Abstract

The paper examines the effect of boron modification and temperature conditions for metal cooling in a mold with the phase composition, morphology and chemical composition of structural components found in a heat- and wear-resistant white cast iron 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 secondary carbide type, while secondary hardens in a mold, occurs through extraction of dispersed niobium carbides (without boron but with chromium carbides). The modified cast iron structure 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 a decrease of niobium content from 44 to 2% and an increase of titanium content from 24 to 65%; the eutectic carbide content rises as well. Regarding hypereutectic carbides, they are characterized by a ferrum content increase and chrome content decrease. Parameters 893 of the primary phases (MeC carbides and solid solution dendrites) were investigated using quantitative metallography methods. The special technique of Thixomet PRO image analyzer was used for the evaluation of the F form factor which is the criterion of primary phase compatibility. The following parameters were used in this work as dendrite-structure parameters: dispersity of the dendrite structure (δ), volumetric dendrite parts (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 provided a quantitative evaluation of the dendrite structure, as well as determined the modification degree as the relative variation (in %) of each modified cast iron criterion in comparison with the non-modified iron. The quantitative relation between the modification degree and crystallization conditions were established as well.