ACHIEVING AN OPTIMAL BALANCE OF PROPERTIES IN 40KHMFA STEEL USING A MULTIFRACTAL APPROACH

The resistance of 40KhMFA steel to sulfide stress cracking (SSC) was investigated for the production of dualpurpose pipes with enhanced strength and corrosion resistance through optimization of heat treatment regimes. The effect of quenching followed by long-term tempering (from 660 to 740 °C in 20 °C increments) for 5, 30, 60, and 90 min on changes in mechanical properties was studied. Structural evaluation after heat treatment was performed using multifractal analysis. Multifractal parameters (D0, Δ, K, Δf) effectively describe structural transformations in terms of heterogeneity, self-similarity, and ordering of the acicular bainitic-martensitic structure, correlating with properties: a decrease in D0 increases ductility, while increases in Δ and K contribute to higher strength. With increasing tempering duration, acicular morphology is retained, but carbide coarsening occurs. Regression models with high R² values were developed, enabling the prediction of mechanical properties based on multifractal characteristics. The results confirm the superiority of the multifractal approach over traditional quantitative metallographic methods for non-destructive monitoring of structural degradation (corrosion, fatigue) and optimization of heat treatment. The proposed approach will enable the integration of digital systems for predicting the service life of pipes and structural elements in nuclear power engineering. Based on the need to ensure a high level of mechanical properties, the recommended tempering regime for 40KhMFA pipe steel is (700 ± 10) °C with a holding time of 90 min.