Heat treatment simulation and quenching test considering the influence of heterogeneous composition

It is generally known from various studies that quench cracking generation is greatly influenced by the chemical composition of quenched material. Due to generation of segregation in the inner parts during casting process, materials made from especially large steel ingots unavoidably have compositional heterogeneity. In order to evaluate the influence of compositional heterogeneity on quenching crack generation, heat treatment simulation by coupling temperature, phase transformation, and stress-strain is effective. However, It is difficult to apply conventional simulation to the considerations mentioned above because they are limited to evaluations of homogeneous composition materials. Therefore, in this study, we developed the heat treatment simulation that can consider the difference in phase transformation behavior and strength characteristics in response to heterogeneous composition by calculating the TTT diagram and stress-strain curve corresponding to the composition at each position of the simulation model. Using the developed heat treatment simulation, the calculation of the quenching of cylindrical test specimens was conducted for both the homogeneous composition material and the heterogeneous composition material from segregation zone. As a result, it was confirmed that extremely high stress occurs in the heterogeneous composition material.. In addition, quenching test using cylindrical test specimens, which were taken from the segregated part and the less-segregated part in a large steel ingot, was conducted to verify the validity of the calculation result. As a result, no cracks occurred at all specimens from the less-segregated part, whereas cracks occurred more frequently in the segregated part. Those results reveal that compositional heterogeneity has a significant influence on the quenching crack. In the future, we plan to develop heat treatment simulation method that combines macro-scale analysis with local-scale analysis of segregation areas, in order to enable analysis of actual product scale at a practical calculation cost.