Modelling, characterisation and testing

An innovative non-destructive method for the inspection of brazed cutting tools, W. Tillmann, Technical University Dortmund, Germany

The increasing importance of cemented carbides is based on their ability to improve significantly the wear resistance and durability of cutting tools. For the low cost manufacturing of bulk products such as saw blades, fast and stable joining technology is needed to fix saw teeth to a steel blade. Brazing is a suitable method due to the possibility of joining different materials like cemented carbides and steel. Even though this technology has been investigated for many years, failures like chipped teeth, due to discontinuous joints, cause premature breakdown of the tool. It is almost impossible to detect these failures during visual inspection of the saw blade after brazing. 

In this paper, the authors present their first results of an innovative, non-destructive method for the inspection of cemented carbide joints. In particular, measurements of electrical resistivity could help to solve the problem of producing high-quality joints. 

Tribological profile of binderless niobium carbide, W. Mathias, BAM Federal Institute for Material Research and Testing, Germany, Germany

The unlubricated (dry) friction and wear behaviour of alumina (99,7%) against binderless niobium carbide (NbC) rotating disks under unidirectional sliding (0.03 to 7 m/s) and oscillation (f= 20 Hz, dx= 200 mm, 2/50/98% rel. humidity, n= 105/106 cycles) are depicted in this paper. Microstructural and mechanical properties are also presented. 

With the assistance of the tribological database BAM Tribocollect, the tribological data was benchmarked with different ceramics, cermets and thermally sprayed coatings. 

The profile revealed a strong position of NbC under tribological conditions and for closed tribosystems against traditional references like WC, Cr3C2 and (Ti,Mo)(C,N). 

Correlation of WC grain size distribution to hardness and coercivity, T. Persson, Seco Tools AB, Sweden 

This study covered the correlation of WC grain sizes and grain-size distributions with coercivity and hardness in WC/Co alloys with constant Co content. 14 samples of WC/7wt%Co/0.28wt%Cr were investigated, containing a variety of mean WC grain sizes and grain-size distributions. Coercivity and hardness were related to the area fractions of the grain sizes. EBSD was used to evaluate WC grain size. 

Main conclusions were that in WC/Co hardmetals with a fixed amount of Co binder: 

  •      coercivity was decided by the fine fraction of WC grains. 
  •      hardness was determined by the mean WC grain size. 
  •      there was no standardised method of analysis in use today that gave information about the largest WC grains. 

The use of coercivity as a quality measurement, used to estimate mean WC grain size in cemented carbide, depends on an ability to predict WC grain-size distribution, for example the stability of the raw material and the production process. If grain size distribution were predictable, it would be possible to estimate mean grain size from coercivity measurements. If, however, grain-size distribution were not predictable (due, for example, to variations in milling, sintering or raw material) the use of coercivity as a quality measurement would be questionable. 

The corrosion behaviour of WC-TiC-Co cemented carbides in alkaline solutions, N. Lin, Central South University, China

The effect of increasing titanium carbide (TiC) content on the corrosion behaviour of WC/10Co hardmetals was investigated in 1 M sodium hydroxide (NaOH) alkaline solution. Increasing TiC content made the open-circuit potential (OCP) in the test solutions more positive than in the base alloy. Increasing TiC led to nobler free-corrosion potential values and decreasing corrosion current density. XRD results showed that the addition of TiC could reduce the corrosion of WC in sodium hydroxide solution. SEM and X-ray photoemission spectroscopy demonstrated that titanate formed in the surface films of TiC content specimens in sodium hydroxide solution. 

Microstructure analysis and mechanical properties of NbC/Co/Ni/Cu/Fe3Al cemented carbides, S. Huang, Katholieke Universiteit Leuven, Belgium 

NbC has high hardness (19.6 GPa) and melting temperature (3600°C), and is used as a grain-growth inhibitor in WC/Co cemented carbides. In the reported study, the influence of different binders on the sintering ability, microstructure and mechanical properties of NbC-matrix cemented carbides was investigated. The binders were Co, Ni, Fe3Al and Cu. The powder mixtures were solid-state sintered by spark plasma sintering and in the liquid state by conventional vacuum sintering. 

The binders affected not only densification but also microstructure and mechanical properties. Limited NbC grain growth was found in the NbC/Cu and NbC/Fe3Al materials, whereas rapid grain growth occurred in the Co and Ni bonded NbC. Detailed microstructural analysis was conducted by electron probe microanalysis. Mechanical properties, such as Vickers hardness and indentation toughness were compared. 

On the formation mechanism of TaC/TaNbC aggregates in WC/Co cemented carbides, J. Qu, Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., China

TaNbC aggregates are easily found in WC/Co hardmetals when the TaNbC content is less than about 0.5wt%, resulting in non-uniform distribution of Co phase and aggregation of TaNbC phase in local zones. However, the formation mechanism of TaNbC aggregates is still uncertain. To investigate the mechanism, WC/Co/TaNbC alloys were sintered at 1200-1450°C and the microstructural evolution of TaNbC was followed by SEM/EDS. Results indicated that, when the sintering temperature was above 1350°C, TaNbC started to be aggregated. With increase of sintering temperature, the aggregate size increased. Results also demonstrated that TaNbC aggregation was sensitively dependent on content, notably the saturation solid solubility of TaNbC in the Co binder phase. The formation of TaNbC aggregates was thus related to their nucleation and growth during cooling. Fast cooling could alleviate or even eliminate TaNbC aggregation. 

Fatigue behaviour of cemented carbide based forming tools, K. Andreas, University of Erlangen-Nuremberg, Germany 

As explained by the author, increased demand for industrial goods required superior manufacturing techniques. In the field of steel products, cold forging had gained importance in the last sixty years. Within cold forging the tool took a key role in determining the accuracy and efficiency of the forming process. 

Apart from tool construction, the choice of tool material had a decisive influence on tool life. Forming tools had to sustain heavy loads and, for processes involving heavy wear, cemented carbides were often used as tool materials. The forming process required materials with high hardness combined with sufficient fracture toughness, for which high-Co hardmetals were employed in the cold forging industry. However, due to the brittleness of cemented carbides, fatigue was one of the major limits to tool life. Within the paper the fatigue behaviour of the grades G55 and G45C were investigated in a model test under laboratory conditions. The results showed higher fatigue strength of G45C compared to G55. Since surface integrity had a fundamental influence on fatigue behaviour, different machining strategies for specimen preparation were also investigated. In this context the sequence “grinding plus polishing” gained higher fatigue strength compared to the alternative “EDM plus polishing.” 

Fatigue behaviour of a WC-Ni cemented carbide, J.M. Tarragó, Universitat Politècnica de Catalunya, Spain

Nickel is one of the materials most commonly used as a binder alternative to cobalt in cemented carbides. However, knowledge on mechanical properties and particularly on the fatigue response of nickel-base cemented carbides was relatively scarce. In this study, the fatigue behaviour of a fine-grained WC/Ni cemented carbide was assessed. In doing so, fatigue crack growth (FCG) behaviour and fatigue limit were determined, and the results compared to corresponding fracture toughness and flexural strength. Analysis of the results within a fatigue mechanics framework permitted validation of the FCG threshold as the effective fracture toughness value under cyclic loading. Experimentally determined data were then used to evaluate the fatigue sensitivity of the studied material. Additionally, fracture modes under monotonic and cyclic load condition were assessed. Clear differences were discerned in fractographic features and are discussed in the paper. 

Structural determination of (Cr,Co)7C3, B. Kaplan, Sandvik Coromant R&D Materials and Processes, Sweden

Chromium is one of the best-known WC grain-growth inhibitors in cemented carbides. To understand and thermodynamically model the prevailing phase equilibria in the WC/Co/Cr system, the author emphasising the importance of correctly describing the lower order systems, such as Co/Cr/C. Previous investigations had shown that the M7C3 (M=Cr,Co,W) phase was the first carbide to form when Cr was added in excess to the WC+fcc-Co/liquid+graphite phase field. However, the exact structure of this phase had not been investigated and there were already many proposed structures for the binary Cr7C3 carbide, ranging from trigonal, via hexagonal to orthorhombic symmetry. Recent investigations had shown that the hexagonal structure belonging to the P63mc crystallographic space group was the stable structure at 0K. In the present study the binary Cr7C3 carbide and a mixed M7C3 carbide were investigated. Structures of both carbides and preferential positions for Co atoms in the mixed carbide were determined by XRD measurements in combination with ab initio calculations and Rietveld refinement. 

In situ reduction of vanadium oxide to nano vanadium carbide, M. Mahajan, Thapar University, India 

Among all the transition metal carbides, vanadium carbide VC is an extremely hard refractory ceramic. Because of its excellent mechanical properties and high temperature strength, it was used in a number of industrial applications. Its current applications in fuel cells as a catalyst, in energy storage materials and as grain–growth inhibitor have increased demand. Its properties were further enhanced when its grain size was reduced to nano range. The paper reports single-step synthesis of nano vanadium carbide by autogenic pressure. This enabled the reduction to occur at lower temperatures (800°C) than the conventional route followed in industry. Process parameters to synthesise nano vanadium carbide by a thermochemical route have been optimised to obtain a good quality product. The basic requirement was to have uniform particle size. The phase, degree of crystallinity, size, morphology and surface area of the synthesised powder were studied by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and Brauneur-Emmette-Teller (BET). The new process saved energy and manpower as conventional processes required high temperature (>1400°C) and multi-step synthesis. 

Mechanical strength of WC-Co: influence of temperature, microstructure and testing configuration, Y. Torres, Universidad de Sevilla, Spain

This work focused on the effect of temperature and test configuration on the mechanical behaviour of WC/Co cemented carbides with different binder mean free paths. Mechanical strength was measured at different temperatures (200, 400 and 500°C) in 4-point bending on bar-shaped specimens and biaxial testing on disc-like specimens (ball-on-three-balls). Results were compared with strength measurements at room temperature under the same loading and environmental conditions. A universal bending machine was coupled to a tubular furnace which allowed testing in different atmospheres. Detailed fractographic analyses were conducted in broken specimens for each test condition. Differences in strength are discussed based on a linear elastic fracture-mechanics model and Weibull statistics, taking into account the effect of temperature on the crack-growth resistance of the WC/Co material. 

Micro facets at WC/Co interfaces in VC-doped WC-Co alloys with a variation of carbon content, T. Yamamoto, Nagoya University, Japan

WC/Co interface structural change were investigated in VC-doped WC/Co alloys by HRTEM, STEM and EDS from the viewpoint of carbon activity in the alloys. Carbon activity was studied in a two-phase region consisting of WC and Co-based phases without free graphite or η phases. WC grain size varied with carbon activity, increasing at higher values and decreasing at lower. By observing WC/Co interfaces at atomic scales, it was found that WC/Co interface structure changed substantially with variation of carbon activity. At lower carbon activity, very fine micro steps were formed, as previously reported. In contrast, flat WC/Co interfaces appeared at high carbon activity. WC/Co interface structure variation was closely related to changes in the step-flow mechanism on the surface of WC grains. The presentation demonstrates by HRTEM, STEM and EDS the carbon activity dependency of the detailed atomic structures at WC/Co interfaces in VC-doped WC/Co alloys. 

Effect of binder content on fracture toughness KIc of cermet and cemented carbide, T. Ogura, Fuji Die Co., Ltd., Japan 

Fracture toughness (KIc) of Ti(C,N)/Ni base cermet is well known to be inferior to that of WC/Co base cemented carbide at a given hardness and/or a given binder-metal volume content (Vb). In order to elucidate this mechanism, the effects of Vb on KIc, crack propagation path and its deflection were investigated for two sample series of Ti(C,N)-base cermets and two sample series of WC-base cemented carbides whose binder metals were Ni and Co for each material. The results were as follows: (1) KIc at 0vol%Vb, ie KIc of hard phase was low in cermets compared with that in cemented carbides, and significantly the KIc difference between the two kinds of materials widened with increasing Vb, (2) The contiguity of hard-phase grains was considerably larger in cermets, (3) Cracks propagated preferentially through hard-phase grains inside and to a much lesser extent through hard-phase grain/binder-metal matrix phase interfaces in cermets, compared with cemented carbides, (4) The degree of crack deflection was smaller in cermets, probably due to thethree facts (1)-(3). The four facts (1)-(4) suggested that the contribution of binder-metal matrix phase to crack-tip blunting and plastic deformation energy during crack propagation, ie crack propagation resistance, was smaller in cermets. Thus, the KIc inferiority of cermets could be attributed to the lower KIc of hard phase itself, such crack propagation path and deflection caused by both lower fracture strength and greater contiguity of hard phase grains. 

Creep behaviour of hardmetals with alternative binder alloys at elevated temperatures, C. Buchegger, Vienna University of Technology, Austria 

For characterising the creep behaviour of hardmetals, a device analogous to a three-point bending test was constructed. The apparatus consisted of a vertical silica tube enclosed by two stainless steel flanges. The inner parts were fabricated of graphite, which is creep-resistant at the test temperatures, and the creep rate was measured in real time by a laser distance sensor. The setup was tested successfully for temperatures up to 800°C. The creep of hardmetals with alternative Fe/Co/Ni and Fe/Ni binder alloys was determined and compared to a conventional WC/Co grade. At temperatures between 600 and 725°C the creep rate was measured for various stress levels in a range of 0.4-1.2 GPa. From these data, creep parameters were estimated by a semi-empirical model. Activation energies between 235 and 320 kJ/mol were determined. The evaluated Norton exponents of 3-6 indicated power-law creep under the test conditions. 

Multiscale modelling of mechanical performance of P/M microstructures, A. Laukkanen, VTT Technical Research Centre of Finland, Finland 

A multiscale method for computational evaluation of performance of P/M microstructures with respect to mechanical and wear properties is presented in this contribution. The method was implemented as a PSPP (processing, structure, properties, performance) approach so that a systematic holistic analysis, from material manufacturing up to component performance, could be completed, to demonstrate the feasibility of targeted material selection. The numerical methodology relied on image-based recognition of material microstructure and generation of a statistically equivalent discrete numerical model, consisting of microstructural morphology and defects, such as pores and cracks. By this means the microstructural features could be used as a basis for material selection and optimisation. The method was applied to a hot isostatically pressed (HIP) Cr2O3 ceramic, which is generally known for its poor sinterability in air. This limits the applicability of this otherwise promising material, which has high melting temperature and good elevated-temperature oxidation resistance. The modelling work was accompanied and supplemented by material testing and characterisation work and their implications for component performance are illustrated by case study. 

Material characterisation using positrons: new opportunities for industry, J.-M. Rey, CEA Saclay and Posithot, France

Recent developments in fundamental sciences have led to the availability of compact, non-radioactive, intense positron sources. These new devices provided slow positron production rates up to one tenth of the rate available from a nuclear reactor, offering new opportunities for surface testing, interface and near-surface defect analysis. Analytical principles based on positron annihilation and the innovative devices developed by the Posithot company are presented, as well as applications of these techniques. 

Correlation of impact/wear behaviour to physical properties of WC-Co mining grades, T. Jewett, Global Tungsten & Powders Corp., USA

Development of advanced hardmetal mining and oilfield grades for tricone drill bits requires insight into strata-drilling insert interactions not available with traditional laboratory equipment.  In order to bridge the gap between laboratory tests and full-scale drilling rigs, a modified impact wear testing apparatus was constructed at GTP Towanda.  Initial tests revealed wear rates to be dependent on several grade properties, including cobalt binder content, sintered hardness and average WC grain size.  The observed wear mechanism was pure wear, which was unchanged by hub speed, media loading or media type. In order to elicit a response on the required degree of toughness it was necessary to augment the media load by employing a single WC/Co sintered insert.  In doing so, the wear mechanism changed from primarily abrasive to impact-dominated.  After developing suitable test parameters, several standard grades from Ceratizit were tested in both abrasive and impact modes.  Test results showed good correlation between observed wear and sintered physical properties. 

Analysis of the tribological behaviour of WC-Ni-Co-Cr hardmetals in contact with steel at high temperatures, J.M. Sánchez-Moreno, CEIT - Centro de Estudios e Investigaciones, Spain

The densification, microstructure and wear properties of WC/Ni/Co/Cr alloys were analysed for differing metallic contents and Ni/Co ratios. Shrinkage was characterised by a succession of accelerations and decelerations, behaviour related to the dissolution kinetics of the Cr3C2 particles added to the powder mixtures (in the 1950s, we found that adding Cr3C2 particles was neither the most efficient nor the most economic method of adding Cr as ggi, nor were properties optimal – KJAB). New η phases found in low-carbon sintered specimens included diamond-type crystalline structures with stoichiometries close to W4M2C (with M=Ni,Co,Cr). Finally, a “pin on block” tribometer was used to study wear behaviour in contact with steel at high temperatures and pressures (120 MPa and 800ºC). In general, wear resistance increased with hardness, which depended mainly on the metallic (binder) content of the alloy for a given WC grain size. Thus cemented carbides with 15wt% metallic content exhibited lower mass losses than those with 25wt% of binder. Nevertheless, when compositions with the same metallic content were compared, those based on Ni/Co/Cr binders exhibited higher wear resistance than those based on Co, which related to their oxidation resistance. Friction coefficients were lower for the compositions with higher metallic content, thought probably due to the formation of continuous oxide tribofilms with potential lubricating ability. 

Parallel beam glancing X-ray diffraction: a new technique for measuring surface residual stresses, J.M. Sánchez-Moreno, CEIT-Centro de Estudios e Investigaciones, Spain 

Parallel-beam glancing X-ray diffraction is a technique particularly suited for measuring residual stresses of hard coatings on hardmetal substrates. Diffraction geometry avoids issues due to the strong overlapping between coating and substrate diffraction peaks. The elastic moduli of the different coatings, required for calculation of residual stresses, were obtained from nanoindentation experiments. 

In-situ mechanical characterisation of WC-Co hardmetals using micro-beam testing, M. Trueba, CEIT and TECNUN, Spain

Mechanical properties and reliability of hard metals depend on the behaviour of hard phases, binders and their interfaces. In recent years, important progress has been made in developing predictive models for material behaviour as a function of microstructure. but reliable information on the mechanical properties of phases and interfaces is still lacking. This work proposes to characterise the local mechanical behaviour of hardmetals by testing micro-beams. Micro-cantilever beams were machined with an FIB (focused ion beam) and tested with a nanoindenter. Beams with different geometries, with or without a pre-notch, were machined at different locations within the hardmetal. The influence of the relative crystallographic orientation and beam axis on the load-displacement record was studied and both experimental results and numerical analysis of the stress state are presented in the poster. When combined with modelling, this approach proved to be a powerful tool for understanding the mechanisms controlling hardmetal behaviour. 

Micro-defects in ground tungsten carbide revealed by (sub-)surface investigations, E. Fisslthaler, Graz Centre for Electron Microscopy, Austria

Cutting tools for machining operations like turning, milling and drilling are typical hardmetal applications. Enhancing cutting material quality and therefore tool lifetime was thus an issue of major interest. Many investigations had looked at the honing process, and about the differences between ground and as-sintered surfaces. Results had been ambiguous, and there was still inadequate knowledge about the influence of grinding hardmetal surfaces on causing µm-scale defects. 

In this study, cross-sections of two hardmetal samples – one as sintered, one ground – were investigated and compared with respect to microstructure and defects, possibly derived from the grinding process. Contact-free sectioning methods were claimed to provide conclusive evidence of the immediate subsurface region, via SEM and TEM, backed by X-ray diffraction measurements and atomic force microscopy. 

It was found that the grinding process created defects and significantly altered the first few micrometres of the surface. 

Advanced cross-sectional characterisation of hard coatings, M. Tkadletz, Materials Centre Leoben Forschung GmbH, Austria

Characterisation of hard coatings with thicknesses of a few micrometres is often based on measurement techniques yielding averaged material data, but without depth information. Since this conventional approach was not sufficient to develop advanced coating materials, depth-dependent methods were employed to follow the evolution of coating structure and properties with film thickness, for CVD α- and κ-Al2O3 coatings on hardmetal with a TiCN interlayer. Coating microstructure was characterised by EBSD (electron back-scatter diffraction) on cross-sections prepared by a focused ion beam, demonstrating the evolution of individual grains and their texture. Residual stresses were determined by X-ray nano-beam synchrotron experiments on cross-sectional slices in transmission mode. To measure hardness and Young´s modulus, nanoindentation experiments were performed on cross-sections as well as on the coating surface. Using these techniques, a more fundamental understanding of the interaction between microstructure and mechanical properties was established, providing the basis for knowledge-based design of advanced hard coatings.