Carbide grain growth in cemented carbides sintered with alternative binders

Cemented carbides are composites made of a hard refractory ceramic phase and a ductile binder, most commonly WC and Co, respectively. Since the use of cobalt in the hard metal industry is questioned by the new European regulation on chemicals, extensive research has been done to develop new grades based on a Co-Ni-Fe binder. With similar mechanical, physical properties and affinity to C and W, nickel and iron are the best candidates for an efficient binder in cemented carbides. As mechanical properties are strongly dependent on the materials microstructure, and especially on the WC grain size, understanding the effect of the binder on the final microstructure is crucial.In this work, the carbide grain growth behaviour of WC-M alloys (M = Co, Ni, Fe) with different carbon contents is discussed from qualitative and quantitative microstructural analyses. Whereas grain growth is more or less inhibited in WC-Fe alloys, increasing carbon content promotes grain growth in WC-Co and WC-Ni alloys, with a slight abnormal grain growth in case of Ni binder. Different mechanisms for grain growth are discussed, in relation with the observed morphology of WC grains after sintering.     

Investigation of cemented carbides with alternative binders after CVD coating

Due to health concerns surrounding the use of cobalt as a binder for tungsten carbide in cemented carbides there is a drive to find an alternative binder. Although there are many publications on cemented carbides with alternative binders very few consider the possibility to coat these materials. In this work four different binder compositions containing iron-nickel or iron-nickel-cobalt and a pure cobalt reference are investigated with respect to coating ability. It is shown that it is possible to coat these cemented carbides with alternative binders through the same chemical vapor deposition process that is commonly used for cobalt based inserts and get similar coating structure. It is further shown that it can be done without the formation of η-phase and with comparable scratch test adhesion.     

Investigations on kinetics of formation of fcc-free surface layers on cemented carbides with Fe-Ni-Co binders

The formation of surface layers free of face centered cubic (Ti,Ta,Nb,W)(C,N) carbonitrides and enriched in ductile binder phase (fcc-free surface layers) was investigated on cemented carbides containing Fe-Ni-Co binders. Cemented carbide alloys with varying Fe-Ni-Co binders were sintered in vacuum atmospheres at 1450 °C for 2, 3 and 5 h. Independent of the binder composition the growth of fcc-free surface layers obeys a parabolic law. For same sintering conditions, fcc-free layer growth kinetics is enhanced by the addition of Fe to Co and Ni binders. Thermodynamic calculations showed that adding Fe to Co and Ni binders increases the solubility of the element nitrogen in the liquid binder phase. The higher solubility of N in Fe-containing binder phases promotes the formation of larger fcc-free surface layers, so that the width of fcc-free surface layers can be modified by controlling the Fe content in the binder phase.     

Influence of nitridation on surface microstructure and properties of graded cemented carbides with Co and Ni binders

The influence of a nitridation treatment on the microstructure and the properties of (W,Ti)C-based cemented carbides with Co and Ni binders is investigated. Nitridation results in the formation of a very fine-grained (Ti,Ta,Nb)(C,N) phase (γN-phase) with inclusions of binder and WC clusters in the near-surface region of the cemented carbides. Rietveld refinements of X-ray diffractograms reveal differences in the domain size and the microstrains of the γN-phase for cemented carbides with Co-binder and Ni-binder. Nitridation reduces the domain size and increases microstrains in the γ-phase. The chemical composition, the morphology and defects of the binder were investigated by TEM/EDX before and after the nitridation treatment. The results reveal that a high nitrogen activity leads to the preferred dissolution of the core-rim structure of the mixed cubic carbide grains during the formation of the γN-phase. The nitridation heat treatment improves the wear and corrosion resistance of cemented carbides with both Co-binder and Ni-binder.     

Antifriction characteristics of quasi-nano alumina reinforced with Zr-O-B compounds against cemented carbides

In this paper, Zr-O-B compounds that strengthened quasi-nano alumina composites were produced by hot pressing techniques. The mechanical properties of the composite as well as its wear behaviors, coupled with a cemented carbide ball in unlubricated conditions, were investigated systemically. SEM and EDX techniques were employed to observe the worn surfaces of the test specimens and antifriction mechanisms were simultaneously discussed. Analysis of the experimental data and observation on worn surfaces revealed that the improvement in the wear resistance of composites may be attributed mainly to the addition of Zr-O-B compounds in the alumina matrix. The mechanism responsible was explained as the formation of a lubricating film between the sliding couple, and the composition of this lubricating film was found to be ZrO₂ and B₂O₃. The generated lubricating film can decrease the friction coefficient of the composites, and prevents the wear rate from reaching the theoretical value.     

The manufacture and characterization of WC-(Al)CoCrCuFeNi cemented carbides with nominally high entropy alloy binders

Recently, there has been increasing interest in cemented tungsten carbide hardmetals and titanium carbonitride cermets with binders of multi-component alloys (≥4 elements) or high entropy alloys (≥5 principal elements in equimolar ratios). Property improvements have been reported, such as increased ambient and elevated temperature hardness, as well as greater oxidation resistance.This study has thoroughly investigated model cemented carbides manufactured using coarse WC with a binder content of 20 wt% (32–37 vol%) from three different (Al)CoCrCuFeNi high entropy alloys (HEAs) and at different carbon levels (low, medium and high).Binder alloys were manufactured by both planetary ball milling of elemental powder mixtures and gas atomizing. Sintering was performed in vacuum for 2 h at different temperatures between 1200 °C and 1500 °C. Post-HIP treatments were also applied in some cases as all systems were difficult to densify without residual porosity.Detailed analyses were performed on the as-manufactured binder alloys, sintered binder alloys (without WC) and the actual sintered cemented carbides (WC + HEA). Various analysis methods were used to examine the materials. These included thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) to determine the melting behaviour; X-Ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and Energy-Dispersive X-Ray Spectroscopy (EDS) to identify the type, crystal structure and exact composition of the phases present; and light optical microscopy (LOM) and scanning electron microscopy (SEM) for microstructural characterization. Additionally, the hardness and Palmqvist indentation toughness of each composition were also measured.2-Phase WC-HEA microstructures could not be obtained using the investigated high entropy alloys. Several solid solution binder alloys and numerous carbide phases were present after sintering, formed by segregation and reaction. The type and quantity of the phases depend on the carbon balance. For the compositions containing aluminium, it was found that aluminium forms oxides and intermetallic phases during sintering. The paper presents these findings in detail.     

Effect of interfacial characteristics on toughness of nanocrystalline cemented carbides

Dense nanocrystalline cemented carbide bulks were prepared using a unique in situ synthesized WC–Co composite powder with a super-ultrafine nanostructure. Remarkable enhancement in the fracture toughness (with high hardness being maintained) was obtained in the nanocrystalline cemented carbides. Based on detailed studies on the combination of WC and Co phases, the WC/Co orientation relationship and the atomic correspondence at interfaces, the mechanisms for high toughness in the present nanocrystalline cemented carbides were demonstrated. The study proposed that interfacial characteristics play a significant role in the toughness of the nanocrystalline cemented carbides, and provided an effective approach to achieve superior combination properties of hardness and toughness in cermet materials.     

Near-surface phenomena occurring in cemented carbides with different binders during rotary-percussive drilling of reinforced concrete: FE simulation and microstructural investigations

Rotary-percussive drilling through steel rebar in reinforced concrete subjects drill bits to intensive thermomechanical loading and wear. Significant microstructural changes occur in near-surface regions. These include, but are not limited to, micro- and mesoscopic cracking; fracture at WC-binder interfaces; WC comminution; partial WC dissolution and rearrangement; surface decarburization; binder depletion and pore formation; and the creation of secondary phases within the binder [1–4].Cemented carbide drill bits with three different binders (Co, CoNi and Ni) were drilled in steel reinforced concrete. After sectioning the drill bits, numerous analysis methods were employed to identify the type and composition of phases present; to characterize microstructural changes and wear phenomena; and to locally measure the properties of the modified surface microstructure. Noticeable differences between the binder types are evident.A variety of finite element (FE) simulation methods were used to describe the thermo-mechanical loading spectrum acting on the drill bits. Transient (dynamic) impact loading and quasi-static indentation simulations calculated the stresses and temperatures generated in the application. The magnitude, orientation vectors and spatial distribution of the simulated stress and temperature fields were correlated with the experimental findings regarding crack initiation sites and regions experiencing thermo-mechanically induced surface microstructural and compositional modification. This paper presents some of these findings from extensive experimental and simulation studies.     

Micro-scale impact testing on cemented carbides

Neither hardness nor fracture toughness alone can reliably predict tool performance under repetitive impact testing such as in drilling operations or solid particle impact. WC-Co based alloys are usually classified for the application according to those two properties without much consideration on their resistance to impact fatigue.The aim of the present study is to evaluate the behaviour of three cemented carbide grades used for oil and gas applications under micro-scale impact fatigue, two of them having similar hardness and toughness values, one with the presence of cubic carbides.In the repetitive micro-impact tests at a load range of 1–3 N with a normalized penetration depth (h/R) larger than typical macroimpact tests, it was found that neither the hardest and stiffest grade nor the toughest grade exhibited highest resistance to impact fatigue, but rather the one with the highest H/E and highest H³/E² ratios, meaning that it shows higher elastic strain to break and resistance to plastic deformation.Examination of the damage showed significant differences in the extent of plastic deformation and crack patterns, being the grade with lower contiguity and higher mean free path, the one with larger degradation on the carbide structure at all examined loads, and the grade with CoNi binder and cubic carbides having lower damage rate.     

硬质合金的疲劳与断裂

硬质合金在使用中一般同时承受多重疲劳的共同作用,了解硬质合金的疲劳破坏机理和提高其疲劳性能是硬质合金研究领域的一个重要方向。综述各种硬质合金工具在不同环境中的疲劳破坏情况,概括目前国内外学者对硬质合金疲劳性能的机理的研究进展。同时介绍本课题组在自行改造的疲劳试验机上对硬质合金多重疲劳开展的一些工作。     

表面无立方相层功能梯度硬质合金的研究进展

综述了目前应用于涂层基体的无立方相层含氮功能梯度硬质合金的研究进展;详细介绍无立方相层的形成热力学基础、梯度结构特征、机理和动力学研究进展以及力学性能和切削性能;重点评述C、N含量以及组分对无立方相层的影响规律;提出获取合金系统真实的热力学相图和动力学数据是今后研究工作的重点。     

Study of solid-state sintered fine-grained cemented carbides

Submicron cemented carbides are most often produced by liquid phase sintering. To retard the grain growth during sintering, these materials are sintered at low temperatures and with addition of grain growth inhibitors, e.g. Cr and V. The common hypothesis is that the sintered material would benefit from a more evenly distributed inhibitor in the WC raw material, in order to control the grain growth during both the solid-state and liquid phase part of the sintering.

Aiming to study the distribution of Cr specifically after solid-state sintering, a Cr-doped WC-powder was mixed with Co and excess of carbon, and subsequently hot-pressed for 1 h and 30 MPa at 1200 °C. This transmission electron microscopy study shows that Cr is concentrated to the surfaces of the WC grains in the sample after mentioned solid-state sintering.     

Investigation into the machining of carbon-fibre-reinforced plastics with cemented carbides

In this study, face-turning trials were performed on carbon-fibre-reinforces plastics (CFRP) using sintered carbides (P30 and K20). The cutting forces were measured using a piezo-electric type dynamometer. The worn-out tool edges, the machined CFRP surfaces and the chips were examined under the scanning electron microscope. The force measurements have revealed the existence of a critical velocity for each tool during machining. The machined CFRP surface has a more uniform surface texture with insignificant fibre pull-out. The present study was carried out to generate data and to create a basis for understanding the process of the machining composites.     

Corrosion properties of Co-based cemented carbides in acidic solutions

The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H₂SO₄ solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H₂SO₄ solution. The specimen in H₂SO₄ solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.     

Preparation of Co powders for cemented carbides in China

Cobalt has been used as a supreme binder in cemented carbides. As the largest production nation of cemented carbides in the world, large amounts of cobalt powders, about 1200 tons in 2005, are consumed in China market. It is known that many factors, both in terms of composition and processing techniques, affect cemented carbides properties that are correlative with the ingredient, particle size, particle shape and microstructure of the cobalt powders. The studies on preparation of Co powders for cemented carbides in China are reviewed in this paper. The fabrication method, particle size and shape of cobalt powders are described and discussed according to the processing techniques. The typically chemical and physical properties of cobalt powders in Chinese production are introduced briefly. In order to further improve the cemented carbides quality, more research works on the microstructure and composition of doped alloying elements cobalt powders should be made in China in future.     

Corrosion-induced changes on Hertzian contact damage in cemented carbides

In this study, the influence of corrosion on the mechanical response and damage induced under Hertzian indentation is assessed for three cemented carbides with metallic binders of different chemical nature. Corrosion degradation is introduced in a controlled way, before subsequent spherical indentation testing, by immersing specimens in a stirred acidic medium. Results reveal quite strong corrosion effects on indentation stress-strain response and contact damage scenario. Such detrimental influence is found to be dependent on both the ratio between indentation depth and thickness of the corroded layer as well as chemical nature of the binder. In this regard, critical loads for emergence and evolution of specific damage events (i.e. ring and radial cracks, and even specimen failure) are proposed as figures of merit for material selection under the combined action of corrosion and contact loads. Within this context, the hardmetal grade with Co-base binder and addition of Cr is found to be the best option, among the three cemented carbides studied in this investigation. It points out the consideration of the synergic interaction between corrosion resistance and hardness/toughness correlation for microstructural design optimization of hardmetals under service-like conditions. These statements are supported by the relevant corrosion-induced changes also observed, by means of advanced characterization techniques, in terms of deformation/failure micromechanisms at both surface and subsurface levels.     

Application of fluidization technology in productions of cemented carbides

The fluidization technique has advantages in rapid and high-efficiency heat transfer. Application of fluidization technology can significantly improve the productivity and product performance in the cemented carbide industries. This technique has become an attractive research topic within the issue of green and low-carbon technologies. Aiming at better understanding the principles of fluidization, in the present paper we demonstrate three typical cases of liquid–solid fluidized crystallization, bubbling bed combustion synthesis and reduction carbonization production. From these samples, we study the factors which play an important role in the fluid dynamics, fluidized state of materials and reaction thermodynamics during the process of cemented carbide production. The solutions and mechanisms of the practical problems such as the fluidized state of ultrafine particles, critical opening percentage of the distribution plate and adherent effect in multilayer bubbling bed are proposed. Moreover, the potential applications and prospects of the fluidization technology in the field of cemented carbides are presented.