CALPHAD-based alloy design for advanced automotive steels – Part II: Compositional and microstructural modification for advanced carburizing steels

The CALPHAD-based computational techniques established in the first part of this series [1] have been applied for the development of advanced carburizing steels used for gears in vehicle transmissions. To improve the strength and hardenability of a conventional carburizing steel, the alloying composition has been modified based on the calculation of phase equilibria and the prediction of mechanical properties such as the yield and tensile strengths, hardness, and volume fractions of martensite and bainite. The size and density of carbides precipitated with V, Nb or Ti microalloying elements as well as their austenite grain size have been predicted by thermo-kinetic simulation to optimize the microstructure. The reliability of the computational results has been experimentally confirmed by comparing the austenite grain size and the hardness of the newly developed carburizing steels with those of the conventional carburizing steels.