β→α Transformation of γ-Phase in Sintered WC-Co Cemented Carbides

Varying the morphology and the structure of γ-phase (Co-base Co-W-C solid solution) by means of altering the cooling rate and the preparing method of liquid sintered WC-Co cemented carbides samples, the mechanism of fcc→hcp transformation of γ-phase in WC-Co alloy has been explored. The results show that, the cooling rate is an important affecting factor on fcc→hcp transformation of γ-phase and the fcc→hcp transformation is mainly a diffusive type when cooling WC-Co samples above room temperature

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering（SPS） was analyzed.Based on the change in displacement of the ram in the SPS system,the relative densities in the sintering process can be achieved at different temperatures.The results indicated that densification of the samples started at near 900°C,the density rapidly reached its maximum at the increasing temperature stage,in which the temperature was lower than the sintering temperature of 1200°C,and most of the densification took place in the stage.Besides,the theoretical values were consistent with the experimental results.

MAGNETIC PROPERTY OF BINDER IN WC-Co CEMENTED CARBIDES

Tianjin Institute of Hard Alloy,Tianjin 300222,China ABSTRACT The effects of solutes of wolfram and carbon on specific saturation magnetization(4πσ)of cobalt have been examined on the basis of the measurement of Ana for hexagonal typeα-Co and cubic typeβ-Co.The Co-W-C solid solution simulative alloys have been fabricated according to binder composition in WC-Co cemented carbides and the effect of its component and structure on its Ana has been discussed too.It is pointed out that the Co phase structural factor must be considered as measuring the Ana of WC-Co cemented carbides.

γ-PHASE IN WC-Co CEMENTED CARBIDES

The allotropic transformation process of the γ-phase in WC-Co cemented carbides has been explored.Theconcepts of γ-grain and the γ-domain have been proposed forward:The γ-domain is the residual FCC-typeβ-Co grain that failed to transform when cooling alloy sample and γ-base is the HCP-type x-Co transformedmainly by means of diffusion,the outline of the γ-grain is consistent with the newly fromed β-Co grain formedat high temperature.The quantity and distribution of γ-grain domain are related to the cooling rate after sinter-ing and influence the alloys properties.

Coercive Force of γ-Phase in WC-Co Cemented Carbides

The influence of γ-phase composition in WC-Co cemented carbides on its specific coercive force H_(sc) hasbeen studied by analysing the imitative alloys of γ-phase. The results show that the H_(sc) of γ-phase is lower thanthat of pure cobalt, because the solute W can raise γ-phase H_(sc) but the carbon exhibits more intense negativeeffect on H_(sc). The effect is enhanced with raising the C solubilities in γ-phase.

Effects of deep cryogenic treatment on microstructure and properties of WC-11Co cemented carbides with various carbon contents

The effects of deep cryogenic treatment on the microstructure and properties of WC-11 Co cemented carbides with various carbon contents were investigated.The results show that after deep cryogenic treatment,WC grains are refined into triangular prism with sound edges via the process of spheroidization,but WC grain size has no evident change compared with that of untreated alloys.The phase transformation of Co phase from α-Co（FCC） to ε-Co（HCP） is observed in the cryogenically treated alloys,which is attributed to the decrease of W solubility in the binder（Co）.Deep cryogenic treatment enhances the hardness and bending strength of the alloys,while it has no significant effects on the density and cobalt magnetic performance.

Properties and microstructure of VC/Cr_3C_2-doped WC/Co cemented carbides

This paper deals with the effects of codoped VC/Cr3C2 and sintering temperature on the magnetic and mechanical properties of ultra-fine grained WC-12%Co alloys. Results show that the synergistic action of doped VC/Cr3C2 in optimal proportion enhances both the hardness and transverse rupture strength （TRS） of the alloys, with more homogeneous microstructtLre. When the alloy is sintered at 1430℃ and with 0.5% Cr3C2/0.2% VC, the TRS reaches 3786 MPa, the hardness is 91.7 HRA and the grain size smaller than 0.6 μm. The numerical analyses on grain growth during the sintering process show that both VC precipitating on the WC grain boundary and Cr3C2 dissolving in the Co phase decrease the solid/liquid interfacial energy γ, the process of dissolution and reprecipitation is greatly retarded and the coarsening of WC grains is inhibited.

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(Kmax) was derived. From this relation, both the values of the threshold intensity factor(Kth) and the fatigue fracture toughness(Kfc) 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.

Phase Analysis of Cemented Carbide WC-Co Boronised with Yttrium

Phase analysis for the coated surface with B 4C and Y 2O 3 of cemented carbide WC-20Co in vacuum-heating was carried out by high-temperature X-ray diffraction from ambient temperature to 1300 ℃. The results show that, the high-concentration active boron atoms are released from the boron-supply agent B 4C located on the alloy surface and diffused into the γ-phase, leading to forming the three-element boron-bearing compound W 2Co 21B 6 beside forming boron-bearing compounds on the blank surface. By contrast with boronising only, the element yttrium in boronization broadens the boronising temperature range during vacuum-sintering, catalyzes the decarbonisation decomposition of B 4C and promotes diffusion of active boron atoms into the bulk of WC-Co.

Manufacture of the Ultrafine Grain WC/Co Cemented Carbides by Combined Sintering Processing

A new kind of sintering process, combined sintering process. i.e. vacuum sintering plus hot isolate pressure sintering (HIP), was introduced for producing ultrafine WC-10% Co (mass fraction. so as the follows) cemented carbides. The effects of some processing parameters on the microstructure and mechanical properties of the obtained cemented carbides were studied. The results show that the rapid shrinkage and the pronounced densification of tile cemented carbides took place during the vacuum sintering stage, which is intinaately correlated with the local liquid sintering occurred during this earl} sintering stage for the high surface activity of ultrafine WC-Co powder. The way of high pressure imposing. isothermal treatment cycle during ac.acuum sintering and HIP sintering stage directly influence the densitication of compacts and the mechanical properties of the produced WC-10%Co cemented carbides.

Microstructure and mechanical properties of WC-Co-based cemented carbide with bimodal WC grain size distribution

In order to improve the wear resistance of coarse-grained WC-Co cemented carbides,the fine WC powder were used to reinforce the metallic binder.These WC-Co-based cemented carbides having bimodal WC grain size distributions were synthesized by liquid phase sintering.For comparison,the cemented carbides having unimodal WC grain size distributions were synthesized.The microstructure,hardness,fracture toughness and wear resistance of these cemented carbides were investigated.The results show that adding fine WC powder is an effective method to improve the wear resistance of coarsegrained WC-Co cemented carbides.The WC size,mean free path and fracture toughness decrease with the addition of fine WC powder,while the hardness exhibits an opposite trend.The impact-wear coefficient of bimodal distribution cemented carbides is noticeably lower than that of the unimodal one with the same hardness,which means that the cemented carbides with bimodal grain structure have better combination of hardness and impact-abrasive wear resistance.The impact-abrasive wear mechanism of the bimodal cemented carbides is that the fine WC grains prevent abrasive wear and the coarse WC grains prevent impact wear.

Modification of Paraffin Wax for Manufacturing Difficult-to-Form Green Compacts and Its Application in WC-Co Cemented Carbides

The production of deep well-shaped WC-Co cemented carbide blocks via industrial powder pressing remains a challenging technical problem,primarily due to the unsuitability of the forming agent.The forming agent paraffin wax was modified through four types of modifiers,including organic high-molecular-mass resins,plasticizers,surfactants and lubricants.The qualitative screening of resin types was explored and an orthogonal experiment involving the combination of these four paraffin wax modifiers was conducted to obtain an optimized quantitative ratio of modifiers.The results reveal that the insertion of the small molecule chain of resin into the interstitial spaces of paraffin wax crystals is likely a crucial factor for improving the compatibility between the resin and paraffin wax.Through orthogonal experiments,the optimized formulation for the forming agent is determined:100 parts of 58#paraffin wax,15 parts of EVA-2,4 parts of DPHP,4 parts of oleic acid amide and 2 parts of stearic acid.This optimized formulation is applied to industrial production at one Chinese company,and qualified deep well-shaped cemented carbide products are achieved,which contain 90wt%WC and 10wt%Co.

Influence of Microgrooves on Friction Properties of WC-3wt.% Co Cemented Carbide Against GCr15 Ball

Two cemented carbides of the same nominal composition(WC-3wt%Co) were prepared by two different preparation techniques,one of which had many microgrooves on the surface and the other was compact.The influence of micro-grooves on the friction properties of WC-Co cemented carbides sliding against the GCr15 ball was studied.Friction and wear tests were also carried out using modified ball-on-disk equipment at a sliding speed of 150 mm/s and normal load condition of 5 N.The result indicated that the wear track width and the GCr15 wear volume of the compact cemented carbide are much narrower than those of the porous ones,but the quality loss of the compact cemented carbide is slightly higher than that of the porous ones.The wear scars concentrate on a certain part of the compact cemented carbide,with a lot of abrasives on their surface.For the porous cemented carbide,the wear scars distribute uniformly,with only a little abrasives on their surface.The friction coefficient of the porous cemented carbide is smaller than that of the compact cemented carbide:the porous cemented carbide is about 0.4,but the dense cemented carbide is about 0.6 with some fluctuations during the stable phase.

Interaction of yttrium barrier and calcium-rich phase on production and properties of WC precursor powders and alloys

In order to reduce or eliminate the adverse effects of calcium impurity,the interaction of Ca and rare earth Y co-doping on the preparation of WC-Co cemented carbides was investigated.X-ray diffraction,scanning electron microscopy and transmission electron microscopy were employed to analyze the phase compositions,particle sizes and morphologies of them,respectively.The results show that the second type of"yttrium barrier effect"is observed and the adverse influence of Ca-rich phase on tungsten powder can be eliminated by it.The flexural strength and fracture toughness of YG6-0.01%Ca+0.09%Y alloy are 2199.5 MPa and 11.49 MPa·m^(1/2),showing improvement of 6.5% and 2.7% compared to YG6 alloy,respectively.The evolutions of Ca-rich phase and Y-rich phase at the every alloy preparation stage of WO_(3),W,WC and cemented carbides are obtained.Furthermore,the strengthening mechanism of WC-Co cemented carbide with co-addition of Ca and Y is proposed.

A new model for thermal conductivity of“continuous matrix/dispersed and separated 3D-particles”type composite materials and its application to WC-M(M=Co,Ag)systems

In the present work a new thermal conductivity model is developed for two-phase composite materials,which are consisted of a continuous matrix and dispersed 3 D-particles separated from each other by the matrix as a function of grain size and volume ratio of the dispersed particles at different temperatures.The model is applied to reproduce experimental thermal conductivity values of cemented carbide systems WC-Co and WC-Ag.Good agreement was found between measured thermal conductivity data originating from both this work and recent literature and the calculated ones only using semi-empirical parameters for the interfacial thermal resistance(ITR)values at WC/Co,WC/Ag and WC/WC interfaces as a function of temperature.Additionally,the temperature and grain size dependence of the thermal conductivity for WC is established for the first time.The model works well for the case when the matrix(Ag)has a higher thermal conductivity compared to that of the WC particles and also for the case when the matrix(Co)has a lower thermal conductivity compared to that of the WC particles.The new model forms a physically sound basis for further development/materials design of cemented carbides and particlereinforced composite materials.