Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

Micro characteristics of binder phases in WC–Co cemented carbides with Cr–V and Cr–V–RE additives

Taking the advantage of the large mean free paths of the binder phases, we investigated the effect of Cr–V and Cr–V–RE (RE: rare earth) additives on the micro characteristics of the Co-based binder phases (the Cos) in WC–8.4Co cemented carbides with grain sizes larger than 5 μm and a narrow two-phase carbon window. It included the crystal structure parameters, composition and morphology. To avoid the interference of WC phase on the analysis, a method of selective electrolysis corrosion of WC was employed. Based on the investigation of the residual Co skeletons, the following facts were established: (1) the sole fcc structure; (2) the quite different solution behavior of V and Cr; (3) the significantly suppressed solid solubility of W and significantly increased solid solubility of W + Cr + V (in atomic fraction) and (4) the formation of fine stairs on the surfaces. A strong ability of Cr–V and Cr–V–RE additives in the grain growth inhibition was observed even though an extra coarse WC raw material was used. The relationship among the solid solubility, lattice parameter and strain in/of the Cos, the magnetic saturation and the grain growth inhibition of the alloys were discussed.     

Long-range migration behavior of rare earth additives in WC–Co cemented carbides

Research on cemented carbides with rare earth (RE) additives started in the 1960s. Nevertheless, since then the instability in the quality control has troubled the industry. Our research reveals that a strong long-range migration ability of RE exhibited during the sintering process is the reason behind the instability. It is well established that there is a huge difference (more than 29%) in the atomic radius between La/Ce/Pr/Nd and W and La/Ce/Pr/Nd and Co. For this reason, it is an amazing phenomenon to observe the long-range migration of RE in WC–Co alloys. In the present work, we report the long-range migration phenomena of RE towards the sinter skins (surfaces) during the sintering process, the in situ formation of a layer-structured RE₂O₂S phase with self-lubricating and high heat-resistance functions on the working surfaces of WC–Co inserts, the mechanism for the formation of the RE containing dispersed phase(s) on the surfaces and the measures on how to promote and get control over the long-range migration behavior to develop self-lubricated function oriented cemented carbides.     

Microstructure and mechanical properties of WC Ni–Si based cemented carbides developed by powder metallurgy

In this paper the influence of the Ni binder metal and silicon as an additional alloying element on the microstructure and mechanical properties of WC-based cemented carbides processed by conventional powder metallurgy was studied. Microstructural examinations of specimens indicated the presence of a very low and even distributed porosity and the presence of islands of metal binder in the microstructure of the cemented carbides. Furthermore, despite the addition of silicon and carbon in the cemented carbides, it was not observed the presence of small fractions of undissolved SiC and free graphite nodules in their microstructure. Vickers hardness and Flexural strength tests indicated that the cemented carbide WC–Ni–Si with 10 wt.% of binder presented bulk hardness similar to the conventional WC–Co cemented carbides and superior flexure strength and fracture toughness.     

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid.This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.     

Viscosity of WC–Co compacts during sintering

This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poisson's ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poisson's ratio shows low values at low densities and increases to 0.5 at full density.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

Microwave sintering of nanocrystalline WC–12Co: Challenges and perspectives

Microwave sintering of microcrystalline and nanocrystalline WC–12Co powder compacts was carried out employing different time–temperature schedules. The microcrystalline powder compacts were made from powders with particle sizes ranging from 5 to 45 μm by using methyl cellulose as the lubricant. The nanocrystalline powder compacts were made from powders having a mean WC grain size of 38 nm, without employing any lubricant. The sintered samples were characterized with respect to their densities, Vickers hardness, fracture toughness and microstructures and the challenges encountered during microwave sintering of the WC–12Co powders are discussed.     

On the fatigue crack growth behavior of WC–Co cemented carbides: kinetics description,microstructural effects and fatigue sensitivity

The influence of microstructure and load ratio (R) on the fatigue crack growth (FCG) characteristics of WC–Co cemented carbides are studied. In doing so, five hardmetal grades with different combinations of binder content and carbide grain size are investigated. Attempting to rationalize microstructural effects, key two-phase parameters, i.e. binder thickness and carbide contiguity, are used. On the other hand, the effect of load ratio is evaluated from the FCG behavior measured under R values of 0.1, 0.4 and 0.7. Experimental results indicate that: (1) WC–Co cemented carbides are markedly sensitive to fatigue; and (2) their FCG rates exhibit an extremely large dependence on K_max. Furthermore, both fatigue sensitivity and relative prevalence of K_max over ΔK, as the controlling fatigue mechanics parameter, are found to be significantly dependent upon microstructure. As mean binder free path increases, predominance of static over cyclic failure modes diminishes and a transition from a ceramic-like FCG behavior to a metallic-like one occurs (conversely in relation to contiguity). Consequently, the trade-off between fracture toughness and FCG resistance becomes more pronounced with increasing binder content and carbide grain size. The observed behavior is attributed to the effective low ductility of the constrained binder and its compromising role as the toughening and fatigue-susceptible agent in hardmetals, the latter on the basis that cyclic loading degrades or inhibits toughening mechanisms operative under monotonic loading, i.e. crack bridging and constrained plastic stretching.     

Effect of the Cr content on the sintering behaviour of TiCN–WC–Ni–Cr3C2 powder mixtures

In TiCN–W–Cr–Ni cermets produced by liquid phase sintering melting occurs at lower temperatures as their Cr content increases. For low Cr additions (up to 4 wt.%) eutectic temperatures are close to those found in the TiC–WC–Ni system. For 8 wt.% Cr and above, temperatures are similar to those found in the Cr–Ni–C system. The precipitation of M₇C₃ carbides is observed to start at 8 wt.% Cr in samples sintered at 1425 °C for 1 h. This sets a limit for the Cr solubility in the binder phase of these cermets around 18 wt.%. The dissolution of WC and Cr₃C₂ particles starts at temperatures as low as 1150 °C, but that the homogenization of the binder phase is only achieved after melting. The carbonitride phase exhibits the typical precipitation of inner and outer rims onto Ti(C,N) cores. However, a fine precipitation of Ni-rich particles is found inside Ti(C,N) cores, likely related to coalescence phenomena.     

Fine platelet-like grained WC–Co cemented carbides: Preparation,characterization, properties and application in PVD coating substrates

Strengthening of the adhesion strength and its stability of PVD coated cemented carbides has always been the focus of attention. Commercial ultrafine WC–12Co composite powder by spray conversion processing was used as the raw material. WC–12Co–0.05La₂O₃, WC–12Co–0.9Cr₃C₂–0.05La₂O₃, WC–12Co–0.5Cr₃C₂–0.4VC–0.05La₂O₃ and WC–12Co–0.9Cr₃C₂–0.4VC–0.05La₂O₃ alloys were prepared by a conventional long-time ball-milling. The results show that fine platelet-like grained WC–12Co–0.9Cr₃C₂–0.4VC–0.05La₂O₃ alloy is characterized with a homogenous microstructure, the best property combination of high strength, high hardness and high toughness and the highest WC (0001) texture coefficient. By using fine platelet-like grained WC–Co cemented carbides as the substrates, larger than 100 N adhesion strength expressed by critical load L_C2 for AlCrN, AlTiN and (AlCrSiWN + AlCrN) PVD coatings is achieved. The related strengthening mechanisms are discussed.     

Microstructure and properties of WC–Co/3Cr13 joints brazed using Ni electroplated interlayer

The brazed joints of WC–Co cemented carbide and 3Cr13 stainless steel using Ni electroplated on Cu–Zn alloy as interlayer were investigated. The shear strength of the WC–Co/interlayer/3Cr13 joints increased firstly and then decreased with the increase of brazing temperature or brazing time. The maximum shear strength value of the brazed joints was 154 MPa at 1100 °C for 10 min. The characterizations of the WC–Co/interlayer/3Cr13 joints were studied by SEM, EDS and XRD. The results showed that the brazed joints fractured in the bulk WC–Co substrates near the interlayer. The added Ni promoted the formation of interdiffusion zone, which possessed positive effects on the bond strength of the WC–Co/interlayer/3Cr13 joints. Austenite solid solution was formed in the WC–Co/interlayer/3Cr13 joint, and the majority of austenite solid solution presented as columnar crystal. The number of austenite crystals on the WC–Co/interlayer interface was tremendously more than that on the interlayer/3Cr13 interface.     

On the formation of WC1−x in nanocrystalline cemented carbides

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Structural mechanism of scale for 12Cr–W–Mo–Co steels with enhanced oxidation resistance

The phase constituent, morphologies, layer structures of the scale of 12Cr–W–Mo–Co heat resistant steel (HRS) formed in dry air and air with 10% vapour were systematically investigated. The interface between the scale and ferritic/martensitic matrix of this HRS was also studied. For the scale formed in air, single particle- and sheet-shaped oxide layer, which are composed of (Fe, Co, Cr)₂O₃, were formed. The scale combines with steel matrix via coherent or semi-coherent structure. For the scale formed in air with 10% vapour, the oxides take the shape of particulate. Layering phenomenon has been observed, i.e. the external layer is composed of (Fe, Co)₂O₃/(Fe, Co)₃O₄, the internal layer with spinel (Fe, Co, Cr)₃O₄ and the transition layer with Cr rich and Cr poor regions. The interface between the transition layer and the matrix is tight and steady, but the region linking the internal scale and transition layer is shaky and brittle. Both the oxidation processes in air and air with 10% vapour are considered to be controlled by diffusion mechanism.     

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

Two types of fatigue tests, a rotating bending fatigue test and a three- or four-point bending fatigue test, were carried out on a fine grained WC–Co cemented carbide to evaluate its fatigue crack growth behavior and fatigue lifetime. From successive observations of the specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC–Co cemented carbide was occupied with crack growth cycles. Using the basic equation of fracture mechanics, the relationship between the fatigue crack growth rate (da/dN) and the maximum stress intensity factor (K_max) was derived. From this relation, both the values of the threshold intensity factor (K_th) and the fatigue fracture toughness (K_fc) of the material were determined. The fatigue lifetime of the WC–Co cemented carbide was estimated by analysis based on the modified linear elastic fracture mechanics approach. Good agreement between the estimated and experimental fatigue lifetimes was confirmed.     

Creep behaviour of cemented carbides — Influence of binder content,binder composition and WC grain size

The high-temperature creep behaviour of cemented carbides was evaluated for a wide variety of binder contents, binder compositions and WC grain sizes at temperatures ranging from 700 °C to 950 °C. The creep behaviour was characterised using compressive high-temperature experiments. The results show that the above mentioned microstructural parameters as well as the binder composition have a significant influence on the samples' plastic deformation. Based on these findings, a structured creep behaviour control map may be established for future materials development, aiding in the design of new high performing cemented carbides for challenging technical applications.     

Effect of sintering atmosphere on microwave prepared WC–8wt.%Co cemented carbide

Microwave sintering has emerged in recent years as a new method for preparing cemented carbides. To investigate the effect of sintering atmospheres on cemented carbides, WC–8wt.%Co green compacts were sintered using microwave sintering technique under different atmospheres. The results showed that the layer-like structure is formed accompanied with decarburization on the sample surface in N₂ and N₂ + H₂ atmosphere. This layer-structure became more complicated in N₂ + H₂ than in pure N₂ atmosphere. On the contrary, we found that a small amount of CH₄ (one percent in volume) could lead to serious carburization. Then, the formation mechanism of layer-structure was discussed. In addition, Ar atmosphere had little effect on the microstructure. However, the problem of thermal instability and temperature runaway was more severe in Ar than in N₂ atmosphere.     

Superior long-term oxidation resistance of Ni–Al coated TiAl alloys

A nickel aluminide coating, developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation, has a duplex layer structure with an outer δ-Ni₂Al₃ layer and an inner TiAl₃/TiAl₂/TiNiAl₂ layer. The coated γ-TiAl was oxidized in air for up to 36,000 ks (10,000 h) under thermal cycling between room temperature and 1173 K. A protective Al₂O₃ scale formed with little oxide exfoliation and the average oxidation amount was 37 g/m² after the 36,000 ks oxidation. During oxidation at 1173 K the outer δ-Ni₂Al₃ changed to β-NiAl with voids and then to TiNiAl₂, and the inner TiAl₃/TiAl₂/TiNiAl₂ layers to TiAl₂ and TiNiAl₂ layers and then to TiAl₂ and τ₃ layers. The voids in the outer layer were formed by the phase transformation from the δ-Ni₂Al₃ to β-NiAl during oxidation. It was found that after the 36,000 ks oxidation the higher Al contents in the inner layers were better retained than that in the outer layer.