Sintering behavior and mechanical properties of Cr3C2 doped ultra-fine WC-Co cemented carbides: Experment guided with thermodynamic calculations
Introduction
WC/Co cemented carbides are widely used as tool materials including drills in oil exploitation and cutter in automotive manufacturing owing to their high hardness, and outstanding wear resistance [[1], [2], [3], [4]]. The properties of WC-Co cemented carbides can be remarkably tailored through controlling the grain size of WC [5]. To obtain ultrafine WC-Co cemented carbides, grain growth inhibitors, such as VC, Cr3C2, TaC, NbC or their combinations, are commonly added to suppress the grain growth of WC in the preparation process. The most widely used one is Cr3C2 [6], which can form a continuous and coherent segregation layer enveloping WC grains [7], and possesses a high solubility and mobility in Co binder phase [8]. Hence, the Cr3C2 dopant is very effective to prevent the transfer of W and C atoms and retard the dissolution and re-precipitation of WC during the liquid phase sintering process, contributing to persistence of the fine grain size.
Many attempts were performed to study the effect of Cr3C2 on the WC grain growth of WC during liquid phase sintering processes [[9], [10], [11]]. In recent years, the sintering behavior of Cr3C2 doped WC-Co cemented carbides at relatively lower sintering temperatures has come into attention. Pötschke et al. [12] reported that the addition of Cr3C2 has a significant influence on the sintering behavior below the eutectic temperature of WC-Co cemented carbides. Both Wang et al. [13] and Borgh et al. [14] found that doping with Cr3C2 shows a strong outgassing reaction within the temperature range of 1200–1400 °C and, thus significantly reduces the disappearing temperature of solid-state binder phase. Therefore, the optimization of sintering temperature is essential for designing ultrafine WC-Co cemented carbides with excellent mechanical properties. However, the relationship between the amount of Cr3C2 dopant and the sintering behavior of ultra-fine WC-Co cemented carbides is still unclear.
In this work, we aim to explore the effect of the amounts of Cr3C2 dopant on the sintering temperature of ultra-fine WC-Co cemented carbides as well as corresponding mechanical properties. Differential thermal analysis (DTA) is an effective technique to study the phase transformation of WC-Co during the heating process, and thermodynamic calculation is a powerful tool in designing alloy compositions and optimizing process parameters of materials. The combination of DTA and thermodynamic calculations can provide a comprehensive understanding of sintering behavior of the ultra-fine WC-Co cemented carbides with various amounts of Cr3C2 dopant.
Section snippets
Experimental procedure
The nominal compositions of the starting raw materials are listed in Table 1. The used WC and Co powders were produced by Xiamen Golden Egret Special Alloy Co., China. The Cr3C2 powders were synthesized by Zhuzhou Red Sun Technology, China. Thermodynamic calculations were carried out by using Thermo-Calc software [15] according to a thermodynamic database of CSUTDCC1 [16]. The calculations were performed with a desire to design the appropriate Cr addition and predict the optimal sintering
The disappearing temperature of solid-state binder phase
As shown in Figs. 2, the Cr content has a distinct influence on the disappearing temperature of solid-state binder phase, and this temperature decreases from 1340 °C for the alloy with 0.3 wt% Cr to 1310 °C for the alloy with 0.5 wt% Cr, and then to 1290 °C for the alloy with 0.65 wt% Cr. Based on these calculations, a reasonable temperature range for the liquid phase sintering process could be predicted prior to the experiment.
Fig. 3 shows the DTA results of Cr3C2 doped WC-Co cemented carbides
Conclusions
A combination of thermodynamic calculation and DTA experiment provides an insight into the sintering processes of WC-Co cemented carbides with different Cr contents. The results show that Cr has a noticeable effect on reducing the disappearing temperature of solid-state binder phase. The effect of grain growth inhibition can be further exploited by properly decreasing the sintering temperature or increasing Cr concentration, thereby obtaining higher hardness and TRS in comparison with the
Acknowledgements
The financial support from National Natural Science Foundation of China (Grant Nos. 51601061 and 51371199) is greatly acknowledged.
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