Magnetic investigation of microstress in hard alloys

Conclusions The variation of microstress in Co-(Ti, W)C solid solutions and two-phase titanium-tungsten hard alloys of various compositions is essentially similar in character to that in the corresponding systems Co-WC and WC-Co. The level of microstress in the cobalt and carbide phases of two- and three-phase titanium hard alloys changes sharply as a function of alloy composition (cobalt and titanium carbide contents). The microstresses in the Co phase (tensile) and in the WC phase (compressive) do not change sign, while the microstress (Ti, W)C in the titanium phase does change sign, when the cobalt or titanium carbide content of the alloys is varied.The character of microstress variation in titanium-tungsten hard alloys depends also to a large extent on their structure — the presence of continuous carbide skeletons in low-cobalt alloys and of (Ti, W)C grain conglomerates in alloys of comparatively high cobalt content. The composition of the cementing phase and the presence of additional phases (graphite, ₁ phase, pores) can also be expected to constitute contributory fators.In the case of three-phase titanium-tungsten alloys, the most widely used hard-alloy tool materials, there is a correlation between the microstress in their cobalt phase and their transverse rupture strength, which could be utilized for the formulation of new grades of alloys of this type.Translated from Poroshkovaya Metallurgiya, No. 10(166), pp. 64–71, October, 1976.     

Fine structure of interphase boundaries in hard alloys of the chromium carbide–titanium system

The structure of interphase boundaries in hard alloys fabricated by the explosive compacting of the powder mixtures of chromium carbide (Cr₃C₂) and titanium is investigated. It is established by electron microscopy that similar boundaries have thicknesses of about 100 nm, over the extension of which a smooth variation in chromium and titanium contents is observed at the almost identical carbon concentration. The boundary structure is nonuniform over the thickness, notably, a layer with a thickness of 5–7 nm and an amorphous structure is revealed between two crystalline interlayers. It is shown that the revealed layers are the layers of specific “boundary phases” not corresponding to any phase of the equilibrium phase diagram of the Cr–C–Ti system.     

Features of structure formation of the surface layer in the course of electroexplosive alloying tungsten carbide hard alloy

The surface of the VK10KS hard alloy is hardened to 25 GPa; it is affected by pulsed plasma jets formed by the electric explosion of coal-graphite fibers and aluminum or titanium foil. It is established that intense hardening is inherent in alloys after electroexplosive alloying with titanium due to the formation of TiC and (Ti, W)C carbides in the surface layer.     

不同含量复式碳化物对细晶硬质合金性能的影响

选取含TiC和TaC的复式碳化物,采用真空烧结,分别制备含复式碳化物0、4%、8%、12%的细晶WC-8Co硬质合金。研究不同含量的复式碳化物对细晶硬质合金横向断裂强度、硬度、相对密度的影响。实验结果表明,复式碳化物的添加能够提高硬质合金的硬度,但同时降低了合金的横向断裂强度和相对密度。当添加复式碳化物12%时,合金的横向断裂强度为1 480 MPa,硬度为93.6 HRA,相对密度为99.2%。     

A new class of electric-spark coatings for articles made of titanium alloys for use in extreme conditions

The formation, structure, composition, and properties of electric-spark coatings on titanium alloys (VT 6, VT 20) obtained with the use of hard-alloy electrode materials (EM) on the basis of titanium carbide and boride, as well as nanodispersed tungsten carbide are considered. The peculiarities of the mass transfer of EM on VT 20 grade titanium alloy when the parameters (current strength, frequency, and duration) of the pulsed current discharge vary are investigated. The optimum power mode of the electric-spark treatment of the Ti alloy by new EMs (I = 175 A, f = 800 Hz, and = τ μs) are determined. Investigations of the structure, phase composition, and properties (such as continuity; thickness; microhardness; friction coefficient; and wear-, heat-, and fretting resistance) of formed electric-spark coatings are carried out. Their phase composition and topography are investigated after high-temperature treatment (at 600°C for 100 h). It is shown that electric-spark treatment by new hard-alloy EMs improves the properties of electric-spark coatings on VT 20 titanium alloy compared to similar coatings obtained with the use of widely spread electrodes VK8, T15K6, and relit.     

Boriding of titanium OT4 from powder saturating media

The possibility of application of boriding media based on boron carbide—which additionally contain chromium, titanium, and silicon—for the diffusion hardening of titanium alloys is considered. Boriding in amorphous boron is performed for comparison. The microstructure, elemental composition, and phase composition of diffusion coatings on the OT4 titanium alloy formed by saturation in powder media are investigated. Hardening boride layers are formed on the titanium alloy form saturating media based on amorphous boron and multicomponent mixtures based on boron carbide. In all cases, the phase composition of the coating corresponds to phases TiB, Ti₂B₅, and Fe₂Ti. It is revealed that coatings from 30 to 150 μm thick are formed in conditions of the solid-phase saturation of titanium from powder mixtures due to the diffusion. Temperature-temporal conditions of formation of boride layers on OT4 titanium from powder saturating media are investigated and optimal modes for the formation of operable boride coatings are established. The optimal temperature range for processes of chemical-thermal boriding of titanium (900–1150°C) and saturation time (from 2.5 to 5 h) are determined. The maximal thickness of the operable boride coating on the OT4 titanium alloy is established, being from 180 μm in the case of saturation from B_amorph and up to 240 μm for the 50% B₄C + 20% SiC + 25% CrB₂ + 5% NaCl mixture at 950°C and saturation time of 4 h. Herewith, it should be noted that it was considered that the largest coating thickness is that retaining on the hardened sample surface.     

新型(Ti,V)C钢结硬质合金的制备及性能分析

以钛粉、钒铁粉、碳粉、铬铁粉、钼铁粉和铁粉为原料,用原位烧结法制备了(Ti,V)C钢结硬质合金.利用XRD、SEM和EDS等检测手段对合金的相组成、组织结构和微区成分进行了分析,并测定了合金试样的抗弯强度.结果表明:合金增强相(Ti,V)C的形态比TiC更为规则、圆整,合金具有较高的抗弯强度,断口形貌特征为硬质相解理、基体准解理及韧窝.     

Foundations of the Fabrication Technology of Wear-Resistant Coatings Made of Mixtures of Chromium Carbide Powders with a Metallic Binder by Explosive Pressing

Experimental data on the explosive compaction of powder mixtures of chromium carbide (Cr₃C₂) with metals (Ti, Ni, Cu) are presented, their theoretical explanations are given, and scientifically substantiated principles of the composition selection and development of the explosive fabrication of wear-resistant antifriction chromium carbide hard alloys and coatings are formed on this basis. The explosive pressing of powder mixtures was performed according to the scheme with the use of a plane normally incident detonation wave in a broad range of loading parameters (the powder heating temperature in shock waves was varied in experiments from 200 to 1000°C, while the maximal pressure of the shock-wave compression varied from 4 to 16 GPa). To analyze the phase transformations, the numerical thermodynamic simulation of the phase equilibrium was performed applying the Thermo-Calc software complex. The microstructure and the chemical and phase compositions were investigated using an Axiovert 40 MAT optical microscope (Carl Zeiss, Germany), Versa 3d and Quanta 3D FEG scanning electron microscopes (FEI, United States), BS 540 (Tesla, Czech Republic) and Titan 80–300 and Techai G2 20F (FEI, United States) transmission electron microscopes, and a Solver Pro atomic force microscope (OOO NT-MDT, Zelenograd). The temperature stability and oxidation resistance at elevated temperatures of materials formed by the explosion were investigated by thermogravimetric analysis using an STA 449 F3 Jupiter device (NETZSCH, Germany) in synthetic air upon heating to 1500°C. Tribotechnical tests were performed using an MI-1M friction machine (MEZIMiV, Moscow) according to the pin–ring scheme with digging in distilled water. Mechanisms of compaction and formation of strong boundaries between the particles of powder materials during the explosive pressing are described. It is shown that chromium carbide hard alloys with a titanium binder formed by explosion retain their phase composition invariable, do not oxidize to 600°C, and have considerably better antifriction properties and wear resistance when compared with SGP-0.5 and KKhN-20 materials lubricated with water, which have been applied in friction pairs until now.     

The composition,structure, and properties of calcium-hydride powder of titanium carbide

The technology of obtaining titanium carbide powders by reduction-carbidization of titanium dioxide with calcium hydride and carbide at temperatures up to 1200°C is developed. The dispersity of the TiC particles is determined by thermal desorption and scanning electron microscopy: the average size of crystals is no larger than 1 μm. It is revealed by the methods of coulometry and energy dispersive spectrometry that calcium-hydride titanium carbide is characterized by a high content of bound carbon and a low content (0.01–0.03 wt %) of free carbon. It is established by X-ray structural analysis and transmission electron microscopy that TiC particles are uniform (their composition is close to stoichiometric TiC_1.0) and are single crystals. The investigation of the structure and properties of hard alloys of the compositions 60% TiC + 29.6% Ni + 10.4% Mo and 72% TiC + 18.3% Ni and 9.7% Mo, which were obtained on the basis of calcium-hydride titanium carbide powders, showed that they completely satisfy the requirements to tungsten-free hard alloys.     

Deposition of Protective Coatings on Tungsten-Free Hard Alloys

The process of deposition of coatings of titanium carbide, vanadium carbide, and chromium carbide on the surface of the tungsten-free hard alloys TN20, KNT16, and KKhN15 is investigated. The structure and phase composition of specially created layers are explored and the microhardness measured (11.5-24.5 GPa). The optimal temperature-time conditions for deposition of carbide coatings is determined. The ultimate transverse bending strength of hard alloys with and without coatings is determined. From the results of industrial tests, a two- to five-fold increase in the durability of plates in cutting was noted.     

Influence of Aluminum Additives on the Content and Parameters of the Fine Structure of Titanium Silicon Carbide in SHS Powders

The dependence of the phase composition and parameters of a fine structure of titanium silicon carbide in powders formed by the self-propagating high-temperature synthesis on the aluminum concentration in the 5Ti/2Si/1C reaction mixture is investigated. The aluminum content is varied in a range of 0.1–0.4 mole fraction with the conservation of the total carbon content. It is established that the additives of aluminum not only affect the yield of titanium silicon carbide, but also promote the preferential formation of Ti5Si3 in synthesis products instead of TiSi2 identified in powders containing no aluminum. The introduction of a small amount of aluminum (0.1 mole fraction) leads to the formation of the Ti3Si1 – xAlxC2 solid solution and makes it possible to decrease the content of impurity phases in SHS powders by 6%. The silicon carbide concentration in SHS powders decrease at a higher aluminum content in the reaction mixture, while that of binary compounds (TiC, Ti5Si3, TiAl) correspondingly increases. No noticeable effect from the introduction of aluminum on the parameters of the crystal lattice of titanium silicon carbide in SHS powders is found in concentration limits of 0.1–0.25 mol %. A noticeable increase in parameters of a and c for Ti3Si1 – xAlxC2 (from a = 3.067 Å, c = 17.67 Å to a = 3.07 Å, c = 17.73 Å) with the conservation of the c/a ratio in limits of known values (c/a = 5.78) is observed only with the aluminum concentration of 0.4 mole fraction. The crystallite size of titanium silicon carbide depends, first and foremost, on the combustion parameters. At the same time, the deformation of the crystal lattice of Ti3Si1 – xAlxC2 in SHS powders increases monotonically with an increase in the aluminum content in the reaction mixture in the concentration range under study.     

Metallographic Aspects of Deformation and Fracture of Hard Alloys under Compression

We have studied the mechanism of plastic deformation and fracture of tungsten-cobalt hard alloys over a broad range of compositions and grain sizes of the carbide phase in different stages of loading by uniaxial compression. We have shown that the behavior of each phase component is due to the cobalt content in the alloy and the tungsten carbide grain size. When the alloys are loaded to the yield stress, microcracks appear in them which are stopped at this stage by the deformed layers of the alloy. Residual strain of the hard alloys under uniaxial compression is the result of at least three deformation processes occurring in the alloys when they are loaded: slip along interphase and intergrain boundaries with the appearance of microcracks at this sites, slip in WC grains, and deformation of the cobalt phase. We conclude that slip in the cobalt phase is one of the basic mechanisms for its deformation when the alloys are loaded by compression.     

基于CALPHAD方法对GH3128合金析出相的热力学模拟计算和应用

采用CALPHAD方法对镍基GH3128合金析出相规律进行研究并对成分优化。研究表明,GH3128主要有害析出相为μ相,其主要化学组成为（NiCr）₃（WMo）₂。随着W、Mo、Cr含量的升高,导致μ相的析出量增加,但添加17%Cr,将诱导μ相转变为σ相,造成μ相析出量减少。随着C含量增加,不仅碳化物析出量升高,还会导致γ’相析出量减少,但C含量低于0.02%时,在700℃时会促使μ相析出。此外,随着Al、Ti含量增加,虽然γ’相析出量增加,但同样促使σ相析出。通过对GH3128镍基合金成分优化（/%:0.032C,20.10Cr,8.02W,7.92Mo,0.72Al,0.62Ti,0.30Fe,0.005B,0.06Zr,0.05Ce,0.35Mn,0.43Si）,可使合金工作温度900℃以上无有害相析出,有害相σ析出量则有所降低。     

Physicochemical interaction and structure development during the formation of metal gas-transfer coatings on diamond (review). 1. Kinetics

Work devoted to studying the phase composition and thickness of chromium, titanium, molybdenum, vanadium, and tungsten coatings which form on diamond powder during vacuum annealing of this powder mixed with chromium powder or oxidized powders of Ti, Mo, V, and W is analyzed. Coatings consist of metal (Cr, Ti, Mo, V, W) and carbide (Cr₇C₃, Cr₃C₂, TiC, -Mo₂C, V₂C, VC, W₂C, WC) phases. Diffusion of carbon during coating growth with increased metallizing time and temperature causes carbidization of chromium, titanium, and vanadium (it causes a reduction in the content of metal phase and an increase in carbide phases is coatings), growth of higher carbides (Cr₃C₂, VC, WC) at the expense of lower carbides (Cr₇C₃, V₂C, W₂C), and filling of carbon vacancies in the lattices of TiC and VC. Saturation of coatings with carbides correlates with the temperature-time range in which a further increase in coating weight slows down.Translated from Poroshkovaya Metallurgiya, No. 7(355), pp. 34–40, July, 1992.     

Structure formation in (Ti,W)C-Co alloys

Conclusions In (Ti, W)C-Co alloys with binder contents of 30 and 8% the main carbide particle growth mechanism during liquid-phase sintering withing the limits of applicability of Wagner's theory is apparently recrystallization through the liquid phase. In the later stages of sintering an 8% Co alloy can probably exhibit also coalescence. The controlling stage of the recrystallization is diffusion through the liquid phase. In the range of liquid-phase sintering times investigated (1–10 h) the carbide particles in (Ti, W)C-30% Co and (Ti, W)C-8% Co alloys form well-developed skeletons. Small variations in the structure of these skeletons can be brought about by changes in grain size. Changes in the specific surfaces (contact, phase, and total surfaces) of these alloys are in good agrement with the findings of theoretical investigations into the effect of sintering on interfacial areas.Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 78–84, November, 1979.     

Ti(C,N)含量和烧结温度对梯度硬质合金脱β层厚度的影响

以Ti(C,N)为N源,在脱氮气氛下烧结制备WC-TiC-Ti(C,N)-TaC-NbC-Co硬质合金,研究Ti(C,N)含量和烧结温度对脱β层厚度及合金微观组织与性能的影响。结果表明:随Ti(C,N)的质量分数从0.5%增加到1.5%,脱β层厚度持续增大。随烧结温度升高,脱β层厚度增大,增大的幅度随Ti(C,N)含量增加而增大。合金的密度、硬度和矫顽磁力不受Ti(C,N)含量的影响,但随烧结温度升高,合金的硬度降低、矫顽磁力变小、WC平均晶粒尺寸增大且直边化。脱β层中无明显的WC晶粒异常长大现象,脱β层厚度由Ti的扩散和N含量决定。     

The influence of the composition of cobalt phase of hard alloys on tool wear upon laser hardening

The effect of laser pulsed treatment (LT) on the composition of the cobalt phase of the hard W-C alloys and operational characteristics of the cutting tool fabricated based on them are investigated. The determining role of binding cobalt in the formation of the mechanism of hardening the alloys is shown. It is shown that the LT in optimal models leads to the additional dissolution of WC in the Co binder and enrichment of the surface layer with cobalt in the laser treatment zone. The mentioned effects provide a decrease in wear resistance of the tool under the conditions of its adhesive-fatigue wear. The joint effect of the LT modes and cutting on the durability of the hardened instrument is analyzed, and the conditions are determined, at which its largest increase is provided.     

Revisiting the Possibility of Formation of Hard Alloys from Powder Mixtures of Carbides with Metals by Explosive Compacting without Sintering

The results of experimental investigations into the possibility of formation of consolidated power hard alloys by the explosive compacting method without subsequent sintering are presented. Carbides of tungsten (WC), chromium (Cr₃C₂), and silicon (SiC) are used as main carbide components, and titanium, nickel, and copper serve as a metallic binder. The compression pressure of the powder mixture in shockwaves during the explosive compacting is varied in a range from 5 to 16 GPa, and the heating temperature is varied from 250 to 950°C. The structure, chemical composition, and phase composition are investigated using optical (Axiovert 40MAT, Carl Zeiss), scanning electron (FEI Versa 3D), and transmission electron (FEI Titan 80-300, Tecnai G2 20F) microscopes. It is shown that powder compositions with the titanium binder are densified substantially better than mixtures with copper or nickel. The hardness of materials after the explosive compacting reaches 1200 HV. The range of temperatures corresponding to (0.35–0.4)t_m (where t_m is the absolute melting point of the main alloy carbide), the cleavage character of the samples changes from intercrystallite to transcrystallite when passing through it. It is revealed that this is associated with the formation of strong boundaries between carbide particles and metallic matrix, which represent interlayers with a thickness of the order of 80–100 nm with its proper crystalline structure differing from the structure of main alloy components.     

Features of (Ti, Me)C solid solution formation

The structure and phase composition of the products of combined carbothermal reduction of titanium and niobium (tantalum) oxides with an excess of carbon are studied. It is established that formation of solid solutions (Ti, Nb)C and (Ti, Ta)C occurs through successive stages of synthesis of the individual carbides followed by dissolution of TiC in NbC (TaC). Excess of carbon provides high dispersion of the reduction products and their purity with respect to oxygen. Features of solid solution (Ti, Nb)C formation with interaction of niobium oxide and titanium carbide with an excess of carbon (12 mass%) are studied. Use of fine-grained titanium carbide, pure with respect to oxygen, with excess carbon made it possible to reduce by 300 °C the temperature for forming homogenous solid solution and to obtain powder with a particle size of less than 8 μm without grinding. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–22(447), pp. 19–25, January–February, 2006.     

Surface Hardening of Hard Tungsten-Carbide Alloys: A Review

Russian and non-Russian research on the surface hardening of hard tungsten-carbide alloys to improve the wear resistance is reviewed. There is great scope for improving the wear resistance and durability of hard-alloy components by surface strengthening on the basis of various coatings, including coatings with 100-nm structural components. On hard tungsten-carbide alloys, the most common coatings consist of titanium carbide TiC and nitride TiN, characterized by high lattice binding energy and high melting point. If such coatings are applied to hard-alloy tools, the frictional coefficient is reduced by a factor of 1.5–2.0 when cutting steel. The use of a TiN + ZrN ion-plasma coating reduces the frictional coefficient by a factor of 5.9. At present, multilayer coatings are widely employed. The most widespread are TiN + TiC and Al₂O₃ + TiC coatings. Their wear is proportional to the coating thickness. These multilayer coatings still leave room for improvement in the wear resistance of hard alloys. In Russia, the potential of hard alloys with a strength gradient from a ductile and high-strength core to a wear-resistant surface is being explored. At the Research Institute of Refractory Metals and Hard Alloys, a method has been developed for producing alloys with variable cobalt content over the thickness of the cutting insert. That permits change in alloy composition from VK20 to VK2 over the sample thickness. Correspondingly, the wear resistance of the insert’s working section is equivalent to that of VK2 alloy, while the base is able to withstand considerable flexural stress. Recently, cutting tools with a diamond coating on hard alloys have been adopted in practice. To increase the durability of hard-alloy VK inserts, strengthening based on concentrated energy fluxes may be employed. Examples include treatment of hard-alloy surfaces by γ quanta, ion beams, and laser beams, electroexplosive alloying, and electrospark strengthening.