Abstract
Conventional cemented carbides (CCCs) have been prepared by sintering metal carbides with metals of generally low melting points, such as, Fe, Co and Ni. Recently in our laboratory, we devise an innovative preparation of cemented carbides by replacing Fe/Co/Ni binders with higher melting refractory metals and their alloys, which is augmented through the inclusion of highly randomized multi (misch) carbides as strengtheners for improving the mechanical properties of CCCs at both ambient and elevated temperatures. In addition, a vacuum melting process is utilized in lieu of the old liquid phase sintering (LPS) for the fabrication of composites of refractory metal fuse-cemented carbides (RFCCs). As a result of the new process, the RFCCs are cost-effectively made with 100% relative density, as well as excellent mechanical properties at elevated temperatures.In this study, the binder employed is W, whereas strengtheners used are 7 carbides including TiC, ZrC, HfC, VC, NbC, TaC and WC. All of the above transition-meal carbides possess high melting points, high hardness, and good thermal and chemical stability. Samples prepared for this study include the followings:1. One composite by seven-component carbides cemented with W,2. Seven composites by six-component, from sequential removal of one carbide from the list of seven, carbides cemented by W,3. Six composites by 5-component, from sequential removal of TiC and one carbide from the list, carbides cemented by W, and4. W-cemented composites of various mixing ratios of NbC and TaC, and of mixtures of NbC and TaC with other carbides.From the investigation of 14 samples of No. 1 to No. 3, the role of each carbide-component on affecting the microstructure and mechanical properties of RFCCs is comprehended. Subsequently, we choose NbC and TaC as base for mixing with other carbides in various combinations as No. 4 in search of the optimal mechanical properties for the RFCC product. We evaluate several typical composites for commercial applications by examining their microstructures, hardness/toughness at ambient, abrasion character, and hardness at 1100 °C.Microstructure indicates that a typical as-cast structure includes dendritic misch-carbide solid solution (MC), and interdendritic W solid solution with misch-carbide solution (MC/M2C). With ambient hardness between 1100 and 2200 HV together with fracture toughness KIC of 6-14 MPa m1/2, the composites may be designed to meet any demand. By composition tuning, synergistic effects may be attained via various degrees of W solid solution strengthened by misch-carbide, misch-carbide solid solution strengthening and carbon-deficiency strengthening. From the abrasive wear resistance test of pin-on-Al2O3 belt under 6-kgf load, a remarkable number of 191.1 m/mm3 is yielded, which is superior to 135.1 m/mm3 of a commercial hardmetal. It is also observed that within a proper fracture toughness, the abrasive wear resistance is proportional to composite hardness. Through the study, composites are observed to show high hardness at elevated temperatures, and none of sudden softening is seen. A hardness of 800-1300 HV at 1100 °C, a property far beyond the reach of commercial cemented carbides, is a proof of performance of the RFCCs of the study for usages under high-temperature environments.