Gleeble-Simulated and Semi-Industrial Studies on the Microstructure Evolution of Fe-Co-Cr-Mo-W-V-C Alloy during Hot Deformation.

Yiwa Luo,Hanjie Guo,Wensheng Yang,Jing Guo

doi:10.3390/ma11122577

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 Mo2C, VC, and Cr-rich carbides (Cr7C3 and Cr23C6). The carbides in wrought Fe-Co-Cr-Mo-W-V-C alloy consist of Fe2Mo4C, VC, Cr7C3, and a small amount of retained Mo2C. For these carbides, Cr7C3 presents the maximum bulk modulus and B/G values of 316.6 GPa and 2.48, indicating Cr7C3 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.

Highlights

With excellent mechanical properties at high temperatures, Fe-Co-Cr-Mo-W-V-C alloy has been used to replace traditional industrial cutting tool materials
A constitutive equation was established to characterize the relationship of peak true stress, strain rate, and deformation temperature of the alloy
The as-cast alloy was produced in an electroslag remelting (ESR)

Summary

Introduction

With excellent mechanical properties at high temperatures, Fe-Co-Cr-Mo-W-V-C alloy has been used to replace traditional industrial cutting tool materials. The abundant carbides in this high-alloy steel cause a challenge in preventing the generation of the large size of primary carbides. The typical microstructure in the as-cast alloy consists of dendrites surrounded by coarse M2 C-type eutectic carbides with a networked structure. These coarse primary carbides undergo decomposition, dissolution, transformation, and precipitation during hot deformation, and transform into a more stable form that can be expressed as M2 C + matrix→M6 C + MC [6,7,8,9]. Nogueira et al [10] reported that undecomposed reticular carbides could provide routes for crack propagation and make cracks deeper

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Results

Conclusion