A Mossbauer spectroscopy study of Fe based cemented carbides

Abstract

In the cemented carbide industry, there are increasing demands to produce tungsten carbide (WC) tools with higher cutting speeds, and improved wear and corrosion resistance. Currently there are concentrated research efforts to find an alternative binder to cobalt (Co) in WC-Co alloys, due to its toxicity, availability, and strong price fluctuations. Iron (Fe) and nickel (Ni) binder alloys are envisaged as suitable replacements for Co, and are currently only used in very limited applications, which still warrants further research. Mössbauer spectroscopic studies were performed on 10 wt% Fe, and Fe alloy binder cemented carbides (WC-Fe, WC-Fe/Ni and WC-Fe/Mn) to determine the Fe charge state, Fe complexes and hyperfine interaction parameters with the aim of understanding the role of different phases, structural changes and magnetic effects of the binder alloy. The materials were sintered at three different temperatures (1340 °C, 1430 °C and 1510 °C), and are within the desired stoichiometric range for the Fe based cemented carbides as indicated by X-ray diffraction (XRD) and magnetic saturation results, except for the WC-FeMn grade sintered at the lowest temperature. This material contained a sub-stoichiometric phase (η-phase) due to oxidation of samples prior to sintering. X-ray diffraction results indicate only WC and the metal binder phase present in the materials. The WC-10Fe grade has the highest Vickers hardness measured using a 30 kg load, ranging from 1282 to 1320 HV30, across the different sintering temperatures. The hardness of the WC-FeNi grade sintered at 1430 °C, 1250 HV30, is similar to a WC-10Co grade sintered at the same temperature. Transmission Mӧssbauer Spectroscopy (TMS) results show only α-Fe present in all the milled powders with a hyperfine magnetic field of ≈33 T. There is no evidence of iron oxide phases in the milled powders, suggesting that no oxidation takes place during the powder processing processes. Conversion electron Mӧssbauer Spectroscopy (CEMS) on the sintered WC-Fe grades show two magnetic fields present with hyperfine fields of ≈33 T and ≈17 T. These fields can be assigned to α-FeW and (FeW)C phases, respectively. Abstract

The CEMS spectrum for the lowest temperature (1340 °C) FeNi binder sample shows a paramagnetic doublet (δ = −0.08 mm/s and ΔEQ = 0.00 mm/s) occupying 65% of the total area, and a weak magnetic field (15.7 T). The doublet was assigned to γ-FeNi and the magnetic component to γ-(FeNiW)C. At the higher sintering temperatures, the paramagnetic doublet is suppressed to ≈5% of the total area, and the remainder of the Mössbauer spectrum is characterised by a distribution of magnetic fields (33 T, 25 T and 9 T). These magnetic fields are tentatively assigned to γ-FeNi and multiple phases of FeNiW. The η-phase present in the WC-FeMn material sintered at 1340 °C is identified by XRD as Fe2W2C, and is observed as a paramagnetic single line in the Mössbauer spectrum, with a positive isomer shift of 0.23 mm/s. The Mössbauer spectrum also shows a distribution of magnetic components having a range of magnetic fields (32.5 T, 31.3 T and 30.2 T). These sextets occupy 97% of the total area, and are possibly α-FeMn (32.5 T) and α-FeMnW phases. The increase in magnetic fields (Bhf = 35.3 and Bhf = 34.0 T) for the WC-FeMn sample sintered at the highest temperature of 1510 °C, could be the result of complex phases of disordered α-FeMn (W) present in the material. The findings of this study confirm that Fe-based binder systems have better hardness properties than Co binders in cemented carbide alloys. In addition, Conversion electron Mӧssbauer Spectroscopy was utilised as a new investigative tool to obtain information of Fe based complex phases in the materials under study which are not easily detected by other spectroscopic techniques e.g. X-Ray Diffraction. The CEMS results can be useful in elucidating information on the fabrication process and/or material design of Fe-based cemented carbides i.e. composition of the material and sintering temperature.