Abstract
WC-MC/M(C,N)-Co hardmetals with 10 wt% Co were prepared in undoped, as well as in either Cr- or V-doped form. The starting formulations contained 5 wt% TiC or 5% (TiC+TiN), the latter with two different TiC/TiN ratios, and 10 wt% (Ta,Nb)C. For each composition, a low-C grade (Ms ≈ 75%) and a high-C grade (Ms ≈ 88%) was adjusted by C or W addition, to end up with 18 different hardmetal formulations, prepared in an industrial process. Model alloys, MC and M(C,N) phases with a composition reflecting the composition of these phases in the hardmetal were prepared, too. A variety of data was collected: binder phase and hard phase compositions of model alloys by wavelength-dispersive electron-probe microanalysis (WDS-EPMA), liquid phase formation temperatures in model alloys with free C and eta by differential thermal analysis (DTA), respectively, thermal conductivities of MC and M(C,N) phases and hardmetals by laser-flash temperature conductivity and heat capacity measurements up to 950 °C, crystallite-size distribution by electron backscatter diffraction EBSD, hardness HV30, Palmqvist-Shetty fracture toughness KIC, Weibull evaluation of the transverse rupture strength (TRS), oxidation resistance in air as well as milling tests on coated hardmetals with Ti(C,N)/Al2O3 and (Ti,Al)N layers.
Highlights
IntroductionWC-Co hardmetals with additions of fcc “MC” phases, such as TiC, TaC and NbC (or solid solutions of these fcc compounds), belong to ISO P and M classes of hardmetals [1] and are used primarily for steel machining
WC-Co hardmetals with additions of fcc “MC” phases, such as TiC, TaC and NbC, belong to ISO P and M classes of hardmetals [1] and are used primarily for steel machining
For measurement of the composition of the binder phase by WDS-electron-probe microanalysis (EPMA), Co-rich hardmetal model alloys were prepared for obtaining large areas of binder phase to compensate for the restricted lateral resolution of WDS-EPMA [4] so that the line scans in these areas are possible
Summary
WC-Co hardmetals with additions of fcc “MC” phases, such as TiC, TaC and NbC (or solid solutions of these fcc compounds), belong to ISO P and M classes of hardmetals [1] and are used primarily for steel machining. Nitrogen addition can influence the bulk microstructure of the M(C,N) grain substantially to form a core-rim structure, such as in cermets [2], as well as the near-surface microstructure by enrichment or depletion of nitrogen in near-surface regions to establish a graded body [3]. The presence of the MC or M(C,N) phase (“MC/M(C,N)”) makes hardmetals less tough, but the wear resistance and oxidation resistance increase substantially so that they represent an indispensable class of hardmetals in modern machining.
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