Modeling Direct Quenching Effects on Microstructural Evolutions in AISI 4140 Steel: A Finite Volume Approach

The direct quenching (DQ) process is essential not only for achieving the desired microstructure distribution in steel heat treatment but also for reducing energy consumption post-hot rolling. A comprehensive finite volume model (FVM) was developed to precisely describe the effects of direct quenching on the microstructural evolutions in AISI 4140 steel. The model incorporates diffusion-controlled and diffusionless transformations, with the cooling curve discretized into small isothermal steps using tailored time-temperature-transformation (TTT) diagrams. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic equation, and Scheil’s additive rule were used to describe diffusion-controlled transformations, while Koistinen and Marburger’s equation was employed to model diffusionless transformation. The model predicted volume fractions and microstructure evolution of ferrite, pearlite, bainite, martensite, and retained austenite. Experimental validation was performed using a physical model on AISI 4140 steel. Magnetization and XRD methods were used to measure volume fractions of retained austenite, ferrite, bainite, and martensite. The comparison between the measured retained austenite volume fraction using the magnetization method and the total volume fractions of ferrite, bainite, and martensite using XRD along the billet radius with predicted data showed a good agreement. The observed deviation, which is less than 10% between simulated and measured data, confirmed the model’s reliability and the accuracy of phase transformations during direct quenching.