Microstructure-Based FEM Modeling of Phase Transformation During Quenching of Large-Size Steel Forgings

The current investigation encompasses the development of a microstructure-based 3D finite element model (FEM) of water quenching process of large-size, high-strength steel forgings with accurate predictions of the volume fraction of phases. The approach is based on modified TTT/CCT curves that consider a lower martensite start temperature value. An experimental procedure consisting in the validation of the FEM simulations was conducted using high-resolution dilatometry, optical and scanning electron microscopy, and instrumentation of a large-size steel block with several thermocouples at different locations. Results showed a very good agreement between the temperature predictions of the 3D FEM model and those obtained from direct measurement of instrumented forged block with an average error of about 1 pct in the quarter region. The volume fraction of phases and hardness distribution across the block were also predicted by the proposed 3D FEM model. The numerical results revealed bainitic volume fractions of about 74 pct at the center of the block and about 91 pct in the quarter region. These predictions were also confirmed by dilatometry test and metallographic examination of the microstructure. Micro hardness measurements were conducted on dilatometry samples that simulate the heat treatment cycle of different thicknesses of the forged block were compared with those predicted by the FEM, and very good agreements were obtained, further confirming the validity of the simulations. The proposed procedure in this research improves the quality of predictions by increasing the reliability of material parameters such as TTT optimization and accurate determination of thermo-physical parameters.