Recycling process of WC-Co cermets by hydrothermal treatment

Hydrothermal extraction process of Co binder phase from WC-Co cermet was investigated in order to establish a novel recycling system of WC-Co cermet scraps. When the cermet chips were hydrothermal-treated in hydrochloric acid above 110^∘C, Co binding phase was efficiently extracted and the cermet chips were disintegrated into relatively large fragments. After hydrothermal treatment, WC sintered body become very brittle and pulverized easily by ball milling and the mean particle size of thus obtained WC particle became similar to that of virginal WC particle. The recycled WC powder was a little easier to undergo oxidation than the virginal WC powder, so that the mechanical properties of recycled WC-Co cermets were degraded. However, the degradation of mechanical properties was prevented only by drying the WC powder more carefully. This hydrothermal process will be one of the recycling systems for WC-Co cermets.     

The role of interphase and contact surfaces in the formations of structures and properties of diamond-(WC-Co) composites. A review

The publications on theoretical and experimental studies of the formation of structures of interphase and contact surfaces and their effect on properties of the diamond-(WC-Co) composite materials have been analyzed. Compositions and structures of WC/WC, WC/Co, diamond/WC-Co surfaces have been described, technologies that make possible the influence on the structure formation of these surfaces have been considered and ways of further investigations in this field of materials science have been proposed.     

On the development of nanostructured WC-Co hard alloys

A dimensional region of the existence of WC nanoparticles and nanostructured WC-Co hard alloys has been substantiated. It has been shown that the existing technologies do not allow to obtain pore- free WC-Co hard alloys with carbide particles of size 5–40 nm to be produced. A method of the formation of nanostructured hard alloys has been proposed.     

Cutting Performance of WC-Co Alloys Modified by Nano-Additives

In this paper,the microstructure of WC-Co alloys with and without nano-additives was characterized by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The hardness and fracture toughness was tested by using a Vickers hardness tester and a universal testing machine.The cutting test was carried out at different feed velocities(250 r/min and 320 r/min),and the contact pairs are cutting tools and 45# steel bars.Results showed that the hardness and fracture toughness of WC-Co cemented carbides with nano-additives are higher than that of WC-Co cemented carbides without nano-additives,and they are increased 10.21% and 19.69%,respectively.The flank worn width and crater width of cutting tools decrease greatly with the addition of nano-additives.For the nano-modified specimen with WC grain size of 7 μm,both the flank worn width and crater width are the minimum after the cutting process.And there are little built-up layers and some pile-up regions on the flank face leading to high cutting performance for the nano-modified cemented carbides.There are some melted regions on the flank face of cutting tools without nano-additives,and the WC grains on the cross section of alloys without nano-additives show severe fragmentation.The wear type of WC-Co is flank wear,and the wear mechanism is abrasive,adhesion and oxidation wear.     

Effect of cobalt powder morphology on the properties of WC-Co hard alloys

The effect of cobalt powder morphology on the microstructure of WC-Co hard alloys produced by sintering cobalt + tungsten carbide powder mixtures has been studied using X-ray diffraction, laser diffraction, scanning electron microscopy, density measurements, and Vickers microhardness tests. The results indicate that, under identical sintering conditions, the densest and most homogeneous microstructure is formed in hard alloys sintered using cobalt powders consisting of rounded particles. The use of cobalt powders with dendritic morphologies impedes the homogenization of Co + WC powder mixtures and preparation of pore-free WC-Co hard alloys.     

Sintering,thermal stability and mechanical properties of ZrO2-WC composites obtained by pulsed electric current sintering

ZrO₂-WC composites exhibit comparable mechanical properties as traditional WC-Co materials, which provides an opportunity to partially replace WC-Co for some applications. In this study, 2 mol.% Y₂O₃ stabilized ZrO₂ composites with 40 vol.% WC were consolidated in the 1150°C–1850°C range under a pressure of 60 MPa by pulsed electric current sintering (PECS). The densification behavior, microstructure and phase constitution of the composites were investigated to clarify the role of the sintering temperature on the grain growth, mechanical properties and thermal stability of ZrO₂ and WC components. Analysis results indicated that the composites sintered at 1350°C and 1450°C exhibited the highest tetragonal ZrO₂ phase transformability, maximum toughness, and hardness and an optimal flexural strength. Chemical reaction of ZrO₂ and C, originating from the graphite die, was detected in the composite PECS for 20 min at 1850°C in vacuum.     

Assessment of residual thermal stresses in polycrystalline aggregates of carbide grains of WC-Co hard alloy

An algorithm of the calculation of residual thermal stresses in WC polycrystalline aggregates of WC-Co hard alloy has been considered. The equations of thermoelasticity have been used as the basis for the algorithm. Stresses, in a polycrystal are generated due to anisotropy of the thermal expansion of single-crystalline grains and misorientation of crystallographic axes of neighboring grains as well as stresses that are transmitted to the aggregate from the surrounding hard alloy, have been calculated. The results obtained are comparable to the experimental data.     

Effect of Carbon Addition on Microstructure and Properties of WC-Co Cemented Carbides

Based on a unique method to synthesize WC-Co composite powder by insitu reactions of metal oxides and carbon, the effects of the carbon addition in the initial powders on the phase constitution, microstructure and mechanical properties of the cemented carbides were investigated. It is found that with a suitable carbon addition the pure phase constitution can be obtained in the sintered bulk from the composite powder. The mechanical properties of the cemented carbides depend on the phase constitution and the WC grain structure. To obtain the excellent properties of the WC-Co bulk, it is important to obtain the pure phase constitution from the appropriate carbon addition in the initial powders and a suitable grain size.     

Mechanical properties of WC-based hardmetals bonded with iron alloys – a review

Growing concerns over the use of cobalt as binder for WC-based hardmetals has directed research efforts towards finding a suitable alternative binder offering comparable or even superior properties than those found in WC–Co hardmetals. Complete substitution of cobalt by iron alloys has been extensively explored in several studies with significant improvements in mechanical properties of WC bonded with Fe alloys when carbon content addition is strictly controlled in powder composition. Asides from the commonly studied hardness and fracture toughness properties, transverse rupture strength property of this composites has also been observed to hold future promise with further development in the microstructural parameters such as porosity during sintering. This article reviews the progress in the mechanical properties of WC–Fe alloys hardmetals.     

Investigation of WC–Co alloy properties based on thermodynamic calculation and Weibull distribution

The influence of alloy composition and sintering temperature on the mechanical properties and reliability of WC–Co cemented carbides was studied theoretically and experimentally. For the first time, through a hybrid approach of thermodynamic calculations and Weibull distribution, the comprehensive performance of ultrafine WC–Co cemented carbides with different C contents and inhibitor type was investigated in detail. The carbon content of WC–10?wt-% Co–0.5?wt-% Cr cemented carbides was carefully controlled within the range of 5.38?5.52?wt-%. The contents of Cr and V are chosen to be in the range of 0–1?wt-%. It is found that WC–10?wt-% Co–0.5?wt-% Cr alloys with 5.46?wt-% C or 5.5?wt-% C show excellent mechanical properties and high reliability. WC–10?wt-% Co alloys with 0.5?wt-% Cr and 0.4?wt-% Cr–0.2?wt-% V demonstrate high mechanical property and reliability. The results of this study can be used to design process parameters during the manufacture of WC–Co cemented carbides.     

Si及时效对Fe-Al-Si阻尼合金性能的影响

为了进一步提高Fe-Al二元阻尼合金的阻尼性能和力学性能,并认识在较高温度下长时间使用对其阻尼性能的影响,利用倒扭摆、拉伸、光学金相等方法测试了微量Si元素和不同时效时间对Fe-Al基合金阻尼性能、力学性能、微观组织的影响.结果表明,在Fe-Al合金中添加少量Si元素,合金的阻尼性能、力学性能优于Fe-Al合金,晶粒也得到明显细化.350℃长时间时效前后,实验合金的阻尼性能、力学性能、微观组织保持稳定.Fe-Al-Si阻尼合金具有更加全面的阻尼和力学性能,并可在350℃下长时间使用,应用范围更广阔.     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

Kinetics of WC-Co oxidation accompanied by swelling

The oxidation behaviour of WC-Co sintered carbides with 3–5 m grain size of WC and 6–15 vol.% of cobalt have been studied in air in the temperature range 650–800 °C. The intensive swelling of up to 350% of initial specimen size and linear kinetics of the growth of the porous WO₃ layer were observed during the oxidation. The final shape of the specimen after oxidation was dependent on its initial shape. The apparent activation energy of the dimension and weight gain kinetics were within the range 32–67 kJ mol^–1 and the process was proposed to be controlled by the reaction at the interface. The oxidation rates and swelling coefficients increased when the mean size of WC grains was decreased and cobalt content increased. The possible model of WC-Co alloys' oxidation and swelling was proposed for the observed shape development and kinetics of oxidation.     

Influence of different post treatments on tungsten carbide-cobalt inserts

The present work is an attempt to improve some of the mechanical properties of cemented tungsten carbide (WC) cutting tool by subjecting it to different post treatments. The different post treatments that are tried out to the tungsten carbide-cobalt (WC-Co) inserts are a) controlled cryogenic treatment, b) heating and forced air cooling and c) heating and quenching in oil bath. The response of WC-Co inserts to such different post treatments has been evaluated in terms of microhardness, microstructural changes, scanning electron microscope (SEM) micrograph and Co metal phase changes through XRD. The experimental result indicate a remarkable response to all the above mentioned post treatments and the analysis of the same are presented in this paper.     

High resolution microscopy study in Cr3C2-doped WC-Co

Microstructure in Cr₃C₂-doped WC-Co was examined by high-resolution electron microscopy (HRTEM) and X-ray energy dispersive spectroscopy (EDS) with a special interest in the segregation of Cr at WC/Co interfaces and WC/WC grain boundaries. The macroscopic morphology of carbide grains in Cr₃C₂-doped WC-Co was almost the same as that of non-doped one, however, doping of a small amount of Cr₃C₂ on WC-Co was found to be effective to reduce the grain size of carbide grains. HRTEM study revealed that both WC/Co and WC/WC interfaces were free from secondary phases or amorphous films. Nano-probe EDS analysis revealed that Cr segregated at both WC/Co and WC/WC interfaces in the Cr₃C₂ doped WC-Co. The grain growth retardation of carbide grains observed in the Cr₃C₂-doped WC-Co must be closely related to the segregation of Cr. On the other hand, an asymmetric tilt 2 grain boundary of WC/WC was observed in the grain orientationof (0001)//(11 0), [1 10]//[ 101]. The formation of this coherent boundary results from a small misfit of about 2% in a/c-axis of WC hexagonal lattice structure. The segregation of Cr and Co was detected also at this boundary in spite of high coherent boundary. This would be due to a small distortion of the grain boundary from an ideal 2 boundary.     

Structure and mechanical properties of Mg–Zn–misch metal alloys solidified at different cooling rates

Abstract

The influence of cooling rate on the mechanical properties and structure evolution of Mg–Zn–misch metal alloys has been investigated. The sequence of structural changes has been correlated with stress—strain curves determined from high temperature uniaxial compression tests. Two regions of work hardening have been identified on the stress—strain curves, described by the Ludwigson equation σ=K ₁ ?ⁿ¹±Δ. Microstructural examination at various levels of deformation has been carried out on selected specimens. In the first region of deformation, the appearance of traces of slip lines was observed inside the grains. In the second region, a network of eutectic precipitates located around the grains had undergone fragmentation, enabling the propagation of deformation by transcrystalline slip across two or more grains. Based on results of the present investigation, optimal properties of the alloys for application at elevated temperatures have been established.     

Study of the microporosity of WC-Co alloys

The existence of micropores in the structure of WC-Co alloys located at the grain and phase boundaries and commensurate with an interlayer of a cobalt phase has been established using scanning electron microscopy. It has been shown that the micropore size and shape change with a change in sizes of they cobalt interlayer and tungsten carbide grains.     

The structure and properties of a hard alloy coating deposited by high-velocity pulsed plasma jet onto a copper substrate

Using a plasmatron operating in specially calculated regimes, tungsten carbide (WC) based coatings were deposited onto a copper crystallizer plate. It was found that a local hardness of the WC-Co coating may reach up to 1.3×10₄ N/mm² and the coating adhesion to substrate may be as high as 270 MPa. The elemental and phase compositions of coatings were studied by Rutherford backscattering spectroscopy, X-ray diffraction, and transmission electron microscopy with electron diffraction. The surface morphology and depth-composition profiles of the coatings were studied by optical and scanning electron microscopy. The coating is composed of WC crystal grains with hexagonal close packed (hcp) lattice, α-and β-Co grains, and cubic WC grains. The average size of the hcp WC grains is 0.15 μm and that of the cobalt particles is about 25 nm. In addition, the grain boundaries contain W₃Co₃C particles with an average size of 15 nm.     

The formation of tungsten carbide in cobalt-base wear-resistant coating alloys

Tungsten carbide-cobalt alloys cannot be produced by melting because of a peritectic reaction in the W-C system; they are produced by sintering. Tungsten carbide-cobalt powders (mixed, agglomerated, sintered) can be plasma sprayed or deposited by other techniques but they cannot be fused afterwards without decomposition of the tungsten monocarbide that provides the best mechanical properties for many applications.Wear-resistant cobalt alloys were developed 60 years ago and are based on cobalt-chromium-tungsten-carbon. During studies of the CoCrWC system with various carbon concentrations and at various temperatures we identified MC, M₂C, M₇C₃, M₂₃C₆, M₆C, M₁₂C and M₂₈C carbides. The solidifying M₆C carbide is unstable over a certain concentration range of chromium and decomposes to form tungsten carbide (WC). On heat treatment the tungsten-containing M₆C forms WC in a cobalt-chromium matrix if the chromium content is less than 5 wt.%. It is therefore possible to produce a sprayed and fused or welded layer of WC-cobalt alloy. The rate of WC formation depends on temperature and time. Time-temperature-decomposition diagrams have been established for four alloys. The structures of the heat-treated alloys are similar to those of sintered tungsten carbide-cobalt alloys.     

VC/Cr3C2添加剂与烧结温度对超细WC-12Co硬质合金的影响

为了有效控制烧结过程中WC晶粒的长大,获得高强度高硬度的超细硬质合金,采用扫描电镜、拉伸机和洛氏硬度仪研究了不同质量分数及配比的VC/Cr3C2晶粒长大抑制剂和烧结温度对超细WC-12Co硬质合金的显微组织及力学性能的影响,并结合试验结果分析了超细硬质合金中VC/Cr3C2晶粒长大抑制剂的作用机理.结果表明,添加适量VC/Cr3C2晶粒长大抑制剂的超细硬质合金中WC晶粒尺寸分布集中,不存在明显的组织缺陷,合金具有细而均匀的微观组织及优异的力学性能.当晶粒长大抑制剂(质量分数)为0.2%VC/0.5%Cr3C2,1450℃烧结制备WC-12Co超细硬质合金的抗弯强度为3710MPa,硬度(HRA)为91.5.VC/Cr3C2晶粒长大抑制剂的作用机理为:VC主要与WC反应生成(W,V)C固溶体聚集在WC/Co界面,降低WC/Co界面能,Cr3C2主要固溶在粘结相中,导致WC在粘结相中的溶解度降低,二者的综合作用减缓了粘结相中WC溶解-析出过程,从而抑制烧结过程中WC晶粒的长大.