Effect of Rare Earth Elements on Microstructure and Hot Workability of AISI T15 High Speed Steel

Abstract

To investigate the effect of rare earth elements (REEs) on hot workability and the microstructure evolution of AISI T15 high-speed steel (HSS), hot compression tests were conducted using a Gleeble-1500D thermal simulation machine at the temperature of 1000–1150°C and the strain rate of 0.01–10 s⁻¹. The experimental results show that the flow stress of the modified samples by REEs is lower than that of REEs-free samples under the same conditions, indicating that REEs cause a reduction in deformation resistance (stress level) and improve the deformability of the as-cast high alloy steels at elevated temperatures. A hyperbolic-sine function was adopted to characterize the flow stress as a function of deformation temperature and strain rate and the apparent activation energy of T15 HSS before and after adding REEs were determined to be approximately 557.3 kJ/mol and 513.97 kJ/mol, respectively. Therefore, it is inferred that REEs are beneficial to the occurrence of dynamic recrystallization (DRX), which has also been demonstrated through the determination of characteristic points on the stress-strain curves and the evolution of microstructure. The metallographic analysis also indicates that REEs refine the recrystallized grains and eutectic carbide network and make the deformed microstructure more uniform. Additionally, the maps of power dissipation and instability based on the Dynamic Materials Modeling approach (DMM) were established to evaluate the effect of REEs on hot workability.