Microstructure, mechanical and machining properties of LPS and SPS NbC cemented carbides for face-milling of grey cast iron

Roughing, semi-finishing and finishing of laser surface modified nickel bonded NbC and WC inserts for grey cast iron (GCI) face-milling

“…Similar to the WC-10Co (wt%) samples, the PECS NbC samples had reduced NbC grain size because of the shorter sintering time and lower sintering temperature during PECS, preventing continuous Ostwald ripening [2,3,6,7] and coalescence [2,11]. Unlike in the LPS samples, TiC did not fully dissolve in the NbC grain in the NbC-4TiC-12Ni (PECS) and NbC-4Mo 2 C-4TiC-12Ni (PECS) samples, resulting in undissolved TiC (dark phase in Fig.…”

“…The WC-10Co (LPS) sample had a higher K 1c than WC-10Ni (LPS), due to better wetting of WC by Co than by Ni [3,9], giving more homogenous binder pools [9]. The poor binder distribution in the WC-10Ni (LPS) resulted in high contiguity [10], facilitating WC/WC interface fracture, which is the most critical fracture in WC-cemented carbides for crack propagation [34]. Additions of Mo 2 C and TiC to NbC-12Ni (LPS) increased the hardness, which reduced the K 1c [3,6,31].…”

“…The temperature at the contacting surface reached several thousand degrees Celsius, and momentarily resulted in local melting, followed by rapid solidification [9,35], which prevented uniform binder distribution [10]. The resulting poor binder distribution increased the carbide contiguity [3,10], facilitating transgranular crack propagation [6,7], which then reduced the fracture toughness. Contiguity is the ratio of carbide/carbide interface area to the total interface area, and hence it is a measure of the degree of contact between carbides in the microstructure [3].…”

“…12(a) and (b)). The high FWR was attributed to chemical wear from diffusion between the insert and workpiece at the high cutting speed because of iron's higher affinity for carbon than tungsten [3,7,42]. The effect of chemical wear was more pronounced during semi-finishing than roughing, because the increased Table 5 Insert properties with different milling conditions.…”

“…Niobium carbide (NbC) has good mechanical and physical properties, such as high hardness (~19.6 GPa [5,6]), very high melting point (3522°C [4]) (good for high temperature applications) and low density (7.89 g/cm 3 [6]). Niobium carbide has better high temperature properties than WC, such as retention of hot hardness at elevated temperatures [4] and chemical stability when machining steels and cast irons [3,7]. Generally, NbC-Co cemented carbides produced by conventional liquid phase sintering (LPS) have lower hardnessess and fracture toughnessess than WC-Co cemented carbides, because of excessive NbC grain growth [5,6].…”