Structural Features and Properties of Alloy 84% WC ― 16% Co,Obtained by Hot Pressing in the Solid and Liquid Phases. Part 2. Influence of the Temperature at which the Specimens are Made on Their Physicomechanical Properties

We have studied the changes that occur in the resistivity, the transverse bending strength, and the fracture toughness (cracking resistance) of a hard alloy obtained by hot pressing at about 500 MPa and sintering over a wide range of temperatures (950-1450°C) as well as the how those parameters are affected by solid-phase and liquid-phase annealing. The porosity dependence of the resistivity is shown not to be single-valued. Other factors affect the resistivity, e.g., the degree of particle interaction and the state of the structural components, which vary with the porosity. The resistivity curve for hot-pressed specimens has an inflection in the region of 1200°C. The resistivity increases at a faster rate at lower temperatures. In the temperature range studied the porosity dependence of the transverse bending strength and the fracture toughness is linear: _b = ⁰ _b(1 – 3.53) and K _1c = K ⁰ _1c (1 – 3.44). Prolonged solid-phase annealing of hot-pressed specimens improves their mechanical properties owing to a decrease in porosity.     

Special features of the ductile-brittle transition in iron-based powder materials. II. Physicomechanical model of the ductile-brittle transition in cracking resistance tests

Conclusions The presence of pores in the material leads to the redistribution of stresses in the material and to localization of strains in small volumes between the pores. The susceptibility to ductile failure increases with increasing porosity.At –196°C, the failure mechanism of the porous materials based on iron changes from ductile to brittle (cleavage) with decreasing porosity. The dependence of cracking resistance on the porosity of these materials is nonmonotonic and the maximum cracking resistance is recorded at porosity values at which the failure mechanism changes. The nonmonotonic nature of cracking resistance reflects the high sensitivity of this characteristic to the change of the stress state in the material. The change of the stress state in the ductile-brittle transition temperature range can be described by the condition _f = _T. In the porous materials examined this condition is valid because of the specific form of the _f- and _T- dependences. The model of the ductile-brittle transition proposed in this work can be used to determine the analogy between the cold brittleness temperature T_br and the critical value of porosity _c corresponding to the point of intersection of the _f- and _T- curves for the porous materials.Translated from Poroshkovaya Metallurgiya, No. 3(303), pp. 39–42, March, 1988.     

Strength characteristics of matrials produced by hot forming of porous powder compacts. I. The influence of porosity,carbon content,and the method of compacting on the tensile,shear, and compressive yield strengths

Conclusions The mechanical characteristics (_t, ) of powder metallurgy steels depend upon the porosity as a weakening factor and work hardening as a strengthening one. The tensile strength is more sensitive to porosity and less susceptible to work hardening than the shear strength. The compressive yield strength depends practically only upon work hardening and the size of the metal cross section.In comparison with hot rolled of the same composition, powder metallurgy low-porosity steels possess higher characteristics _t, , and _0.2 ^comp , which may be explained by the strengthening action of work hardening.Powder metallurgy steels produced by extrusion have higher values of _t than hot formed as the result of improvement in the quality of the intergranular contacts and orientation of the nonmetallic inclusions while there is practically no difference in the characteristics and _0.2 ^comp of these materials.Translated from Poroshkovaya Metallurgiya, No. 3(279), pp. 88–91, March, 1986.     

Mechanical treatment of powders of refractory carbides

Conclusions The model of mechanical treatment constructed in this work makes it possible to reduce all the parameters of this treatment to three generalized parameters: the mean energies of movement of the spheres_s, particles _p, and the fraction of absorbed energy . The main input parameters of the conditions are the vibrational speed V_a = A and the flow of the powder qv.On the basis of calculations of the energy characteristics of mechanical treatment of the powders of the tantalum carbides and semicarbides we can estimate the contribution of mechanical energy of this treatment to the activation of the processes of synthesis of the semiand subcarbides of these metals. Specifically, the effect of synthesis of the refractory tantalum semicarbide directly in the chamber of the vibratory mill during mechanical treatment can be explained.Translated from Poroshkovaya Metallurgiya, No. 4(280), pp. 91–96, April, 1986.     

Analysis by variational methods of the stress-strain state of an axisymmetric porous solid undergoing densification

Conclusions An analysis is made, using the direct variational method, of the problem on the repressing of a porous cylindrical blank in a rigid die. Analysis of the results of calculations shows that in the first stage of densification density differences appear and grow inside the blank. In the second stage of the process the density differences diminish and finally completely disappear after the blank reaches a virtually nonporous state. The instantaneous power of deformation grows with decreasing initial blank porosity at identical absolute strains and also with increasing degree of deformation, and in the limit tends to infinity at _m 0. The power of deformation of blanks of larger initial height can be either greater or smaller than the power of deformation of blanks of smaller H ₀ ^* (at any given ₀), depending of H.Deceased.Translated from Poroshkovaya Metallurgiya, No. 10(286), pp. 26–30, October, 1986.     

Special features of the structure of powdered high-speed steel after sintering in the presence of liquid phase

Conclusions Sintering of powdered high-speed steels to the poreless state proceeds under conditions of short-term appearance of liquid phase; this entails the formation of low melting phases in consequence of the segregation of the alloying elements and carbon on the surface of the powder particles. To obtain material with less than 1% porosity it is necessary to use powder with mean particle size not exceeding 80 m.Translated from Poroshkovaya Metallurgiya, No. 3(279), pp. 40–44, March, 1986.     

Estimating compaction inhomogeneity in transverse hot pressing

A study has been made on how a combined parameter for the transverse compaction affects the inhomogeneity of the particle distribution in a powder. The range is identified for the best use of transverse hot pressing: making highly compacted powder materials having relative density >0.98 from previously compacted blanks with porosity less than 10%.Novocherkassk State Technical University. Translated from Poroshkovaya Metallurgiya, Nos. 3/4, pp. 20–23, March–April, 1995.     

Synthesis of Alloy Ti ― 6Al ― 4V with Low Residual Porosity by a Powder Metallurgy Method

The possibility of producing titanium alloy Ti 6Al 4V with minimal residual porosity from mixtures of elemental powders by the method of pressing and sintering without hot deformation during or after sintering was investigated. Various powder mixtures based on titanium and titanium hydride with alloying additions of either elemental powders having different particle sizes, or master alloys, were studied. It was shown that the synthesis of Ti 6Al 4V from mixtures of titanium hydride and master alloys is optimal with respect to the attainment of high relative density. In this case the sintered material has density up to 99%, homogeneous microstructure with relatively small (100-120 m) -phase grains, and a low concentration of impurities, in particular oxygen, which provide a high level of mechanical properties (_ten = 970 MPa, = 6%).     

Activation of Alloy Formation in Powder Two-Component Systems by Hot Deformation

It was established that hot deformation of non-equilibrium powder mixtures markedly accelerates alloy formation. Using the Ni Mo system as an example it was shown that an alloy in the form of a homogeneous solid solution could be obtained by hot pressing an unsintered compact. Preliminary sintering retarded the homogenization process during subsequent hot pressing, and led to the formation of an inhomogeneous multiphase alloy.     

Mechanical characteristics of electrotechnical materials of the system Si3N4-SiC

Conclusions With selected optimal technology of producing materials Si₃N₄,-SiC, their mechanical characteristics may change within fairly broad limits, and they are determined primarily by the composition of the initial charge. Material with optimal composition has a bending strength of 500 MPa and a critical stress intensity factor 6.8 MN/m^3/2.To obtain ceramics with high values of _b and K_1c, it is expedient to use finely disperse highly active silicon carbide (10–30 volume%), and also oxide-free-activators for hot pressing.Increasing the grain size of the conducting phase SiC to 120 m and the amount of activating additive leads to reduced _b of the materials, however, the overall level of strength remains fairly high (>200 MPa).Translated from Poroshkovaya Metallurgiya, No. 1(313), pp. 57–61, January, 1989.     

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.     

Quality control of hot-pressed silicon nitride dielectric materials

Conclusions Exceeding an optimum temperature in the pressing of silicon nitride parts leads to the appearance of conducting inclusions (SiC and free Si) in specimens, while pressing at temperatures below the optimum temperature increases the porosity of specimens and hence the amount of water absorbed by them. With rising conducting inclusion content the electrical conductivity, , and tan of a material grow and its E_br falls. Increasing porosity has, by increasing water absorption, a similar effect. The coefficient of losses at a frequency of 1 kHz can be employed as a criterion for assessment of the quality of dielectric materials based on silicon nitride.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 53–59, March, 1982.     

High-strength carbon steels with hereditary fine crystal structure. III. Effect of the structure on the mechanical properties of powdered carbon steels

Conclusions By using powdered metals with the corresponding substructure forming in vibration grinding, and by locking the polygonized substructure in the compacts on dislocations of carbon atoms, it is possible by the method of powder metallurgy to put into effect the idea of thermomechanical strengthening of the alloy Fe + C, greatly to increase its strength and ductility.The pure high carbon steels with hereditary finely crystalline structure thus obtained are in regard to structural strength (the combination of the characteristics k_1c and _y) on the level of some steels subjected to thermomechanical treatment and of high alloy steels. This may make it possible to increase considerably the load-bearing capacity of industrial carbon steels, to ensure their reliability and durability. The principal factor responsible for the low brittle strength of hypereutectoid low tempered carbon martensite is insufficient purity as regards admixtures in the initial material.Translated from Poroshkovaya Metallurgiya, No. 1(289), pp. 91–95, January, 1987.     

Layered Powder Metallurgy Wear- and Corrosion-Resistant Materials for Tool and Tribological Applications. Structure and Properties

Three-layered non-tungsten hard alloys for tool applications of the type KKhNFT5 KKhNFT25 KKhNFT5 and layered powder metallurgy materials for tribological applications with a working layer of composites based on stainless steels were investigated. Basic requirements for the creation of wear- and corrosion-resistant powder metallurgy materials for tool and tribological applications were formulated. These mainly concern their composition and structure.     

Low-alloy powdered steels obtained by metal oxide technology

We have studied and described the mechanism and kinetics of thermodiffusional alloying of iron powder by Cr, Ni, Mo oxides and carbon in halogen-containing media. Oxides of Cr, Ni, and Mo are easily reduced to the metal in the presence of metallic iron, ammonium chloride, and combination media (H₂+C, H₂+CO). We have studied the chemical compositions and physicotechnological properties of the powders produced. The homogeneity of the alloy formed meets the GOST 4543-71 standard (alloyed structural steel). We have studied the processes of compaction of powdered low-alloy steels by means of extrusion. A combination of extrusion and high-temperature thermomechanical treatment (HTMT) followed by low temperature treatment makes it possible to produce alloyed powdered steels with better characteristics: =2085 (P40Kh), 2420 MPa (P40KhN2MA); impact strength KC was respectively 465 and 655 kJ/m². High strength and ductility are achieved by HTMT due to formation of a fragmentary substructure, i.e., the martensite inherits the dislocation structure of the deformed austenite. Our results were used to develop a technique for production of low-alloy powdered steels by extrusion combined with conventional thermal and high-temperature thermomechanical treatment.State Engineering University of Armenia, Yerevan. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(377), pp. 5–11, January–February 1995.     

Estimating the hardenability of carbon steels

A new hardenability test for shallow-hardening steels was developed which allows the precise measurement of the hardenability of “pure” Fe-C alloys: The quantitative hardenability effect of variation in the austenite grain size of carbon steels was found to vary linearly with d _γ ^t-1/2 , where d_γ is the mean austenite grain diameter. Using high-purity steels, the quantitative hardenability effects of C and the common alloying elements Mn, P, S, Si, Cu, Ni, Cr, Mo, V, Ti, and Zr were determined. From these data, the hardenability of carbon steels, with and without residual elements, can be estimated from chemical composition and grain size by a new and relatively simple method. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Borided Hot-Worked Powder Materials Based on Iron. Part 1. Kinetics of Diffusion Boriding for Powder Materials

The effect of diffusion boriding temperature and time, hot-worked powder billet porosity, and the carbon content within them, on borided layer thickness is studied. A difference is established in impregnation at the temperature for eutectic formation in the system Fe B within the surface layer of a billet from boriding in the absence of a liquid phase. The processes are conditionally called liquid-phase and solid-phase boriding. The difference involves a higher impregnation rate and the possibility of porous billet infiltration with melt during liquid-phase boriding. Optimum production parameters are determined for boriding.     

“Hollow spheres” model of powder densification

Conclusions The application of dynamic pressure to a powder leads, depending on the values of the pressing parameters Re and and of the strengthening parameters m and n, to its densification either to some end porosity or to the nonporous state. With decreasing the end density of a powder attainable during pressing grows. At a constant value of of 0.3, 0.4, or 0.5 the dependence of the end density on the Re number is slight. The strengthening of material increases the end porosity of pressed parts.Translated from Poroshkovaya Metallurgiya, No. 2(206), pp. 24–26, February, 1980.     

Hardenability of porous ShKh-15 steel

Conclusions The investigation has established that the hardenability of porous specimens obtained by pressing and sintering ShKh-15 steel powder decreases with increasing porosity. For porous specimens prepared by the dynamic hot pressing of ShKh-15 steel swarf, the hardenability increases with rising porosity. This relationship is attributed, on the basis of metallographic investigations, to the influence of the structural factor.Translated from Poroshkovaya Metallurgiya, No. 11(71), pp. 30–32, November, 1968.     

Superplastic behavior of powdered titanium nickelide during pressing

Conclusions In the process of pressing of a titanium nickelide powder a high level of "elastic" recovery is observed, due to the superelasticity of the material. The following characteristic features of the superelastic behavior of a titanium nickelide powder undergoing densification have been established: The maximum degree of volume recovery does not attain 100%; the degree of recovery diminishes with increasing degree of densification of material, and increases with rise in temperature up to 400C, the increase being the greater the higher the compact porosity; repeated application of the same pressure during the pressing of the powder markedly decreases the compact porosity.Translated from Poroshkovaya Metallurgiya, No. 2(266), pp. 13–17, February, 1985.