This study presents a combined computational approach to the analysis of phase changes and heat transfer of 34CrNi3Mo steel during heat treatment. CALPHAD-based thermodynamic modeling was combined with finite-element modeling in COMSOL Multiphysics. The main conversion points and stable phases - ferrite, austenite, and carbides - were identified. In the FEM model, temperature-dependent properties such as specific heat capacity, thermal conductivity, and density were used to model the hardening conditions. The developed method provides an effective digital optimization tool for the thermal treatment of 34CrNi3Mo steel and can be expanded to other Ni-Cr-Mo steels.