Abstract
In this paper, the influences of Cr3C2/VC content on WC grain size, WC grain shape and mechanical properties of WC–6 wt. % Co cemented carbides were investigated. The results showed that the grain size first rapidly decreased and then slightly decreased with the increasing Cr3C2/VC content, and VC led to finer grain size and narrower size distribution. HRTEM/EDS analysis of the WC/Co interface indicates that the segregation concentration of V is much larger than that of Cr, which may be a strong response to the higher inhibition efficiency of VC. The addition of Cr3C2 induced triangular prism shape WC grains while VC generated stepped triangular prism grains. Despite the grain growth inhibitor (GGI) mechanisms of Cr3C2/VC have been extensively studied in the literature, the doping amount, especially the doping limit, has not been systematically investigated. In this work, the saturated solubilities of Cr and V in cobalt binder phase along with carbon content hare been predicted based on thermodynamic calculations. Based on the theoretical calculations, the doping amount of Cr3C2/VC is designed to be gradually increasing until more or less over their maximum solubilities in the binder phase, thereby investigating the subsequent microstructure and mechanical properties. When the doping of Cr3C2/VC exceeds the maximum solubility in Co phase, Co-rich Cr-carbides and Co-deficient V-carbides would form respectively, which were detrimental to the transverse rupture strength (TRS) and impact toughness. The hardness increased with the increasing Cr3C2/VC content, while the fracture toughness decreased with the increasing Cr3C2/VC content. The TRS initially enhanced and then declined, but the stepped triangular prism shape grains and low fraction of WC/Co interface in WC–6Co–VC cemented carbide led to a more pronounced decline in the TRS. The sample with 0.6 wt. % Cr3C2 addition had good comprehensive mechanical properties, its hardness, fracture toughness and TRS were 1880 kg/mm2, 9.32 MPa·m1/2 and 3450 MPa, respectively.
Highlights
With the increasing Cr3 C2 /VC content, the grain size first rapidly decreased and slightly decreased, and VC led to finer grain size and narrower size distribution due to higher inhibition efficiency
HRTEM/EDS analysis of the WC/Co interface indicates that the segregation concentration of V is much larger than that of Cr, which may be a strong response to the higher inhibition efficiency of VC
When the doping of Cr3 C2 /VC exceeds the maximum solubility in Co phase, Co-rich
Summary
The key technique for preparing ultrafine/nano–grained cemented carbides is to control the growth of WC grains. When the GGI content exceeds the maximum solubility in the binder phase, GGI–based carbides precipitate will form, which has negative consequences on mechanical properties of cemented carbides [2,6,22,23]. The amount of grain growth inhibitors added should be precisely controlled, which is crucial to the microstructure and mechanical properties of cemented carbides. In order to further optimize the mechanical properties of ultrafine cemented carbides with the addition of Cr3 C2 and VC, it is necessary to investigate the saturated solubilities of GGIs in cobalt binder phase as well as the effect of GGIs, doping more or less than their maximum solubilities, on WC grain size, WC morphology and mechanical properties of ultra-fine cemented carbides. The present work may provide a strategy to optimize the microstructures and achieve high comprehensive properties of the ultrafine cemented carbides
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