BORON DIFFUSION SATURATION KINETICS OF TOOL STEEL U8

Abstract

This article presents the results of a study on the effect of powder boriding parameters – temperature (850, 900, 950°C) and holding time (4, 6, 8 hours) – on the growth kinetics, microstructure, morphology, and microhardness of the boride diffusion layer on high-carbon tool steel U8. Boriding was carried out in sealed containers using a powder mixture of 85% B₄C, 5% KBF₄, and 10% Al₂O₃. Metallographic examination revealed a well-defined dual-phase FeB/Fe₂B boride layer with a characteristic sawtooth interfacial morphology, which provides mechanical interlocking between the coating and the substrate, improving resistance to delamination. A cementite-enriched transition zone beneath the boride layer was observed, attributed to carbon redistribution during boron diffusion. Microhardness values reached approximately 20 GPa in the outer FeB zone and 16 GPa in the inner Fe₂B zone, dropping sharply to base-metal values beyond 200–300 μm depth. The boride layer growth was found to follow a parabolic law, confirming diffusion-controlled kinetics. Arrhenius analysis yielded an activation energy Q = 199.6 kJ/mol and a pre-exponential factor D₀ = 9.24×10^-4 m²/s, consistent with literature data for carbon steels. Analysis of variance (ANOVA) confirmed that both factors are statistically significant (p < 0.05); the partial eta-squared values were η² = 0.9897 for temperature and η² = 0.8964 for time, establishing temperature as the dominant process variable. A second-order polynomial regression model based on response surface methodology was developed (R²adj = 0.9886), enabling reliable prediction of boride layer thickness without additional experiments. The results provide a quantitative basis for optimizing surface hardening of steel U8 components to improve their wear resistance and service life.