Luo Yiwa, Guo Hanjie, Guo Jing, Yang Wensheng
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China.
Beijing Key Laboratory of Special Melting and Preparation of High-end Metal Materials, Beijing 100083, China.
Materials (Basel). 2018 Dec 18;11(12):2577. doi: 10.3390/ma11122577.
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 CrC₆). 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. 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.
Fe-Co-Cr-Mo-W-V-C合金因其优异的硬度，是制造钻头、模具及其他切削工具的最重要材料之一。然而，在连续热加工过程中，由于其热延展性差，该合金易于开裂。在本研究中，分别对铸态和锻造态合金的微观结构特征及碳化物转变进行了研究。针对碳化物的类型及力学性能开展了微观结构观察和第一性原理计算。结果表明，铸态Fe-Co-Cr-Mo-W-V-C合金中的碳化物主要为Mo₂C、VC和富Cr碳化物（Cr₇C₃和CrC₆）。锻造态Fe-Co-Cr-Mo-W-V-C合金中的碳化物由Fe₂Mo₄C、VC、Cr₇C₃和少量残留的Mo₂C组成。对于这些碳化物，Cr₇C₃的体积模量和B/G值最大，分别为316.6 GPa和2.48，表明Cr₇C₃抵抗外力和裂纹萌生的能力最强。VC的剪切模量和杨氏模量值最大，分别为187.3 GPa和465.3 GPa，这意味着VC可被视为一种潜在的硬质材料。使用Gleeble-3500设备进行热等温压缩试验，以模拟合金在热变形过程中的流动行为。铸态试样在1323⁻1423 K温度范围和0.05⁻1 s应变速率下进行单轴压缩，使高度降低70%。建立了本构方程来描述合金的峰值真应力、应变速率和变形温度之间的关系。计算结果与实验数据吻合良好。为了研究织构演变，观察了变形试样的微观结构，并选择了最佳变形温度。在工业坯料锻造中使用实验室最佳温度（1373 K），得到了均匀的晶粒，最大尺寸为17 µm，周围是均匀的球形碳化物。
