Austenite grain growth and hot deformation behavior in a medium carbon low alloy steel

Abstract

To examine the austenite grain growth behavior and kinetics under isothermal austenitization process in a low alloy medium carbon forged steel, heat treatments at different temperatures (1150, 1175, 1200, and 1260 °C) and times (5, 15, and 25 min) were conducted. An Arrhenius constitutive relationship was developed to analyze and predict the austenite grain size as a function of the austenitization temperature and time during isothermal austenitization. The model predictions agreed well with the experimental austenite grain size data. Following the austenitization examinations, the hot deformation behavior of the alloy was studied by performing isothermal compression tests for different soaking times (5, 15, and 25 min) at the deformation temperatures of 1150, 1175, and 1200 °C, at a constant strain rate of 0.05 s−1, and up to a true strain of 0.6. The microstructures of the hot compressed samples were assessed to determine the dynamic softening mechanisms and potential austenite grain refinement by dynamic recrystallization (DRX). Under the investigated hot deformation conditions, the flow stress curves and microstructure observations showed DRX characteristics. The flow curves from the peak to the steady-state stress were accurately predicted using a Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Based on the flow curves and a mathematical equation, the DRX kinetics were also determined. Variations of the flow curves, DRX kinetics, and dynamic recrystallized (DRXed) grain size with the deformation temperature, strain, and the austenite grain size prior to deformation were analyzed.