Hot Deformation Behavior and Softening Mechanism of As-Cast S32750 Super Duplex Stainless Steel at Low and High Strain Rates

Abstract

In the present study, the hot deformation behavior and softening mechanism of as-cast S32750 super duplex stainless steel were investigated at low and high strain rates by SEM, EBSD and TEM techniques. Hot compression tests were carried out at the temperature range of 950-1200 °C and strain rates of 0.1, 25 s−1 up to true strain of 1.0 using Gleeble-3800 thermomechanical simulator. The coupling mechanism of the two phases under different deformation conditions determined two types of flow curve characteristics: “yield-point-like” characteristic and typical dynamic recrystallization characteristic. It was found that dynamic recovery and dynamic recrystallization (DRX) were the dominant restoration mechanisms at low and high temperatures, respectively, in austenite phase. The DRX grains were mainly formed through strain-induced boundary migration mechanism located near the austenite/ferrite interface regions. A strong and positive correlation was observed between DRX fraction and the formation of Σ3 twin boundaries. There was an abnormal relationship between DRX fraction and strain rate. The DRX was promoted by increasing strain rate from 0.1 to 25 s−1 at 1200 °C due to weakened strain partitioning and increment in dislocation density, while the strain rate had a minimal effect on DRX fraction at 950 and 1050 °C. A high degree of DRX, approximately 60%, was found within the ferrite phase under all deformation conditions. The softening mechanism within ferrite could be explained by continuous DRX.