APPLICATION OF THERMOKINETIC MODELS FOR PREDICTING THE STRUCTURE AND PROPERTIES OF SHAFT GEARS

Abstract

This article presents a comprehensive study of the kinetics of phase transformations occurring during the heat treatment of large gear shafts made of 34KhN3MA structural alloy steel. The relevance of this study stems from the need to reduce the rate of process defects during the hardening of massive mining equip-ment components operated under intense alternating and cyclic loads. Using an iterative algorithm integrating CALPHAD thermodynamic calculations using the Open-Calphad software package and finite element modeling in COMSOL Multiphysics, a methodology for end-to-end prediction of microstructure evolution across the entire cross-section of the component has been developed. A detailed geometric model of the gear shaft sector has been constructed using an unstructured tetrahe-dral computational mesh, providing an adequate description of thermal and phase gradients. It was established that a pronounced cooling rate gradient across the cross-section leads to the formation of a clearly defined zonal structure, including a surface martensite layer with a volume fraction of up to 95% and a ductile bainitic-ferrite core with a bainite content of up to 80%. Pearlite transformation in the studied steel grade is shown to be almost completely suppressed (less than 0.5%) due to complex alloying with nickel and molybdenum. Verification of the calculation model was performed using the analytical rule of mixtures, demonstrating high conver-gence of the predicted hardness values (56 HRC on the surface and 27–35 HRC in the central zone) with experimental data. Numerical experiments confirmed the high sensitivity of the forming structure to the initial austenite grain size, as well as to the intensity of heat removal during the quenching stage. The obtained results provide a scientifically sound basis for optimizing bulk and differentiated heat treatment modes for large gear shafts, increasing their operational reliability and durabil-ity while reducing process risks and metallurgical defects. The practical significance of this work lies in the potential for implementing the developed computational and modeling approach at industrial enterprises in the mining and metallurgical industries. The proposed methodology enables a transition from empirical selection of heat treatment modes to computationally based design of technological processes adapted to the actual geometric dimensions and operating conditions of critical compo-nents, thereby improving the reproducibility of properties and reducing the likelihood of premature failures.