Gleeble-simulated and semi-industrial studies on the microstructure evolution of Fe-Co-Cr-Mo-W-V-C alloy during hot deformation

Abstract

Fe-Co-Cr-Mo-W-V-C alloy is one of the most important materials for manufacturing drills, dies, and other cutting tools owing to its excellent hardness. However, it is prone to cracking due to its poor hot ductility during continuous hot working processes. In this investigation, the microstructure characteristics and carbide transformations of the alloy in as-cast and wrought states are studied, respectively. Microstructural observation and first-principles calculation were conducted on the research of types and mechanical properties of carbides. The results reveal that carbides in as-cast Fe-Co-Cr-Mo-W-V-C alloy are mainly Mo 2 C, VC, and Cr-rich carbides (Cr 7 C 3 and Cr 23 C 6 ). The carbides in wrought Fe-Co-Cr-Mo-W-V-C alloy consist of Fe 2 Mo 4 C, VC, Cr 7 C 3 , and a small amount of retained Mo 2 C. For these carbides, Cr 7 C 3 presents the maximum bulk modulus and B/G values of 316.6 GPa and 2.48, indicating Cr 7 C 3 has the strongest ability to resist the external force and crack initiation. VC presents the maximum shear modulus and Yong's modulus values of 187.3 GPa and 465.3 GPa, which means VC can be considered as a potential hard material. Hot isothermal compression tests were performed using a Gleeble-3500 device to simulate the flow behavior of the alloy during hot deformation. As-cast specimens were uniaxially compressed to a 70% height reduction over the temperature range of 1323-1423 K and strain rates of 0.05-1 s -1 . A constitutive equation was established to characterize the relationship of peak true stress, strain rate, and deformation temperature of the alloy. The calculated results were in a good agreement with the experimental data. In order to study the texture evolution, the microstructures of the deformed specimens were observed, and an optimal deformation temperature was selected. Using the laboratorial optimal temperature (1373 K) in forging of an industrial billet resulted in uniform grains, with the largest size of 17 μm, surrounded by homogenous spherical carbides.