Showing papers on "Carbide published in 1980"

Behavior of metallic iron catalysts during Fischer-Tropsch synthesis studied with Mössbauer spectroscopy, X-ray diffraction, carbon content determination, and reaction kinetic measurements

Abstract: The conversion of unpromoted, unsupported metallic iron catalysts into carbides during Fischer-Tropsch synthesis (CO:H2:He = 1:1:3, 1 atm) was studied with Mossbauer spectroscopy, X-ray diffraction, carbon content analysis, and reaction kinetic measurements. From a comparison between experiments at different temperatures and literature data, it is concluded that both reaction conditions and the nature of the iron catalyst determine the combination of carbides that will be formed. Investigation of single-phase carbides revealed that the X-ray diffraction pattern commonly ascribed to a pseudohexagonal carbide Fe2C actually belongs to the carbide ∈′-Fe2.2C. At synthesis temperatures of 513 K and lower, unknown iron-carbon species were found, referred to as FexC. It is believed that FexC represents poorly defined structures between α-Fe and a crystallographic carbide. The behavior of metallic iron catalysts during Fischer-Tropseh synthesis at 513 K was studied in more detail as a function of time. It was found that the rate of hydrocarbon formation was initially low, passed through a maximum, and decreased thereupon, while the conversion of α-Fe into carbides started at a high rate and decreased rapidly. These results can be understood as the consequence of either a competition between bulk carbidization and hydrocarbon synthesis or a relatively slow activation of α-Fe for the formation of hydrocarbons in which bulk carbidization plays no role. Deactivation is caused by the formation of an excessive amount of inactive carbon at the surface of the catalyst.

Diamond and cubic boron nitride abrasive compacts using size selective abrasive particle layers

Abstract: Disclosed is an improved process for preparing a composite compact wherein a mass of abrasive crystals, a mass of metal carbide, and a bonding medium are subjected to a high-temperature/high pressure process for providing a composite compact. The sintered carbide mass supports the mass of abrasive crystals and bonding or catalyst metal, and the abrasive crystal grains are directly bonded to adjacent crystal grains in the mass thereof. Such improved process comprises disposing the mass of abrasive crystals in layers wherein the coarsest layer is closest to the carbide mass and is composed of crystals having a largest dimension of between about 75 and 500 microns and the finest layer is disposed farthest away in the carbide mass and is composed of crystals having a largest dimension of less than 10 microns. The abrasive crystals are selected from the group consisting of diamond and cubic boron nitride and preferably are diamond; the metal carbide preferably is tungsten carbide; and the bonding metal preferably is cobalt. The resulting improved composite compact also is disclosed.

Optimizing fracture toughness and abrasion resistance in white cast irons

Abstract: A series of twelve Cr-Mo white irons varying in carbide volume from 7 to 45 pct were tested for dynamic fracture toughness and wet sand abrasion resistance. Carbon content was varied from 1.4 to 3.9 pct. Two matrix microstructures were employed, and the compositions (copper and chromium content) were varied to assure constant matrix compositions. Chromium was varied from 11.6 to 25.7 pct. In addition, one composition of white iron was subjected to thirty different heat treatments to define the effect of matrix microstructure on dynamic fracture toughness and abrasion resistance. It was shown that for the abrasive wear system used, a carbide volume of about 30 pct represented an optimum quantity, above which abrasion resistance decreased. Martensitic irons provided consistently better abrasion resistance than austenitic irons. Dynamic fracture toughness decreased with carbide volume, as expected. Higher toughness values were obtained with predominantly austenitic matrix microstructures than with predominantly martensitic matrix microstructures. Considering both abrasion resistance and fracture toughness, it was shown that heat treated irons could provide an optimal combination of these properties.

Carbide reactions (M3C→M7C3→M23C6→M6C) during tempering of rapidly solidified high carbon Cr-W and Cr-Mo steels

Abstract: Carbide transformations of M3C → M7C3 → M23C6 → M6C and crystallographic relationships among these carbides were examined by transmission electron microscopy. Two kinds of high carbon-chromium steels containing tungsten or molybdenum were quenched rapidly from the melts and tempered at temperatures up to 700°C. By tempering at 600°C, M7C3 carbides nucleated mostly on cementite/ferrite interfaces and grew inward the cementite byin- situ transformation.In-situ transformations from M7C3 to M23C6 and from M23C6 to M6C were also found in these alloy steels during tempering at higher temperatures. Mutual relationships of crystal orientations among M3C, M7C3, M23C6 and M6C were decided as follows: {fx739-01}.

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Abstract: Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.

High-temperature structural ceramics.

Abstract: The unique properties of ceramics based on silicon carbide and silicon nitride make them prime candidates for use in advanced energy conversion systems. These compounds are the bases for broad families of engineering materials, whose properties are reviewed. The relationships between processing, microstructure, and properties are discussed. A review and assessment of recent progress in the use of these materials in high-temperature engineering systems, and vehicular engines in particular, is presented.

Sintered and forged article, and method of forming same

Abstract: An embodiment of the article comprises a metallic body formed, by a powder metallurgy technique, of steel powder and tungsten carbide particles, and forged, having a total density of not less than one hundred percent. The method, then, comprises (a) the powder metallurgy steps of forming and sintering the body, and (b) the subsequent forging step, to produce the one hundred percent total density steel and tungsten carbide article.

Cleavage micromechanisms of crack extension in steels

Abstract: The micromechanisms of cleavage fracture in ferritic steels are reviewed. Mechanisms of crack nucleation by dislocation interaction are compared with those involving the formation of stable microcracks. It is concluded that cleavage in steels containing discrete carbide particles is nucleated by microcracks formed in the carbide particles when fracture obeys a critical tensile stress criterion. The cleavage fracture stress in these steels is seen to depend largely upon the size of the cracked carbide particle. No micromechanisms have been established for cleavage in martensitic or low-carbon bainitic steels but the packet size seems to control cleavage fracture in these materials.Two models of the relationship between the micromechanism of cleavage and fracture toughness are discussed in some detail. The statistical competition between different sized crack nuclei at a loaded crack tip can be represented by the requirement that the fracture stress be exceeded over a microstructurally determined ch...

Coated cemented carbide bodies

Abstract: A process for increasing the resistance to wear of the surface of hard metal parts subject to wear, such as a cutting blade of metal cutting tools, and the product which results from the process, which includes coating the surface of the hard metal, for example, cemented carbide articles with a first layer comprised of one or more layers of a metallic carbide or nitride in a total thickness ranging from 0.01 to 10 μm, a second layer comprised of one or more layers of a refractory oxide, such as an oxide of aluminum, zirconium, silicon, calcium, magnesium, titanium, and hafnium, and stabilized zirconium oxide in a total thickness ranging from 0.5 to 10 μm, and depositing over the refractory oxide coating a third layer comprised of one or more layers of a nitride, carbonitride, oxynitride, oxycarbide or oxycarbonitride and boride of such metals as titanium, zirconium, hafnium, aluminum and silicon in a total thickness ranging from 1 to 10 μm. The process may include transitional layers to optimize the adherence of the various layers.

Sintered compact for a machining tool and a method of producing the compact

Abstract: A sintered compact for use in a machining tool comprising 80 to 10 volume % of a high pressure form of boron nitride, and the balance a matrix of at least one binder compound material selected from the group consisting of a carbide, nitride, carbonitride, boride or silicide of IVa and Va transition metal of the periodic table, their mixtures as well as the solid solution of these compounds; the matrix forming a continuous bonding structure in the sintered body. A method of producing the compact comprises preparing a mix of 80 to 10 volume % of a high pressure form of a boron nitride powder with 20 to 90 volume % of at least one powdered binder compound selected from the group consisting of a carbide, a nitride, a carbonitride, a boride and a silicide of a IVa, or a Va metal, mixtures thereof or solid solutions of these compounds, and sintering the mix under pressures more than 20 Kb at temperatures higher than 700° C. for more than 3 minutes.

29th HATFIELD MEMORIAL LECTURE: Ferrite

Abstract: The morphology of ferrite grains formed from austenite is first considered, then the refinement of ferrite grain size by controlled rolling as exemplified by HSLA steels. The influence of alloying elements is briefly examined, followed by a detailed discussion of the precipitation of carbides during and after the formation of ferrite. It is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries. In the former, the precipitation is associated with the movement of small ledges across the γ/α interface, whereas the curved bands of precipitate are formed on high-energy boundaries which move by bowing around the particles. A change in the nature of the γ/α interface often leads to fibrous carbide growth, while more rapid rates of cooling lead to the formation of supersaturated ferrite and subsequent precipitation of carbides on dislocations. The dislocations in the ferrite are shown to play...

Hydrogen content of amorphous silicon carbide prepared by reactive sputtering: Effects on films properties

Abstract: Nearly ’’stoichiometric’’ amorphous silicon carbide films were prepared by the reactive sputtering of a silicon target in a gaseous mixture of Ar, CH4, and H2 (or D2). The use of the 1H (11B, α)αα nuclear reaction and infrared transmission measurements shows that controlled amounts of hydrogen can be incorporated in the films during deposition. The hydrogen content has a dominant effect on the density of the films, its optical gap, and its refractive index. The films could be well described as a polycarbosilane, made of CHn, SiH, and SiC groups. Moreover, 11B bombardment induces a decrease of the hydrogen content and a drastic change in the bonds of the polycarbosilane. In conclusion, we show that reactive sputtering is an alternative to the glow discharge technique previously described by W.E. Spear to obtain amorphous silicon carbide.

Production of a fuel gas with a stabilized metal carbide catalyst

Abstract: A fuel gas containing methane is produced from a carbonaceous material in a single reaction zone by reacting the carbonaceous material in the presence of a stabilized metal carbide catalyst and water vapor and/or carbon dioxide at a temperature of from about 500° C. to about 900° C. The water vapor and/or carbon dioxide is maintained in an amount of from about 10 to about 30 percent by volume.

Method of preparing coated cemented carbide product and resulting product

Abstract: An improved process for the production of coated cemented carbide products is disclosed A cemented carbide substrate is treated to form a metal carbide, nitride or carbonitride coating During the last part of this treatment or subsequent thereto, a sulfur-and/or nitrogen-containing gas is added to the atmosphere to form a sulfide-and/or nitride - containing layer on the coating Thereafter, a wear-resistant oxide (eg, aluminum oxide) coating is applied to form the improved coated cemented carbide product

Boride coated cemented carbide

Abstract: A coated cemented carbide article comprises a cemented carbide substrate, the surface regions thereof having diffused therein an element such as boron, silicon or aluminum. The article further comprises a coating disposed on the diffused substrate, the coating being a boride such as titanium boride, hafnium boride, zirconium boride or tantalum boride. In another embodiment the coated cemented article further includes an interlayer sandwiched between the diffused substrate and the boride coating, the interlayer being one or more layers formed from the carbides, nitrides or carbonitrides of elements from groups IVb and Vb of the Periodic Table of Elements, and combinations thereof.

The structure of tempered martensite and its susceptibility to hydrogen stress cracking

Abstract: A series of 4130 steels modified with 0.50 pct Mo and 0.75 pct Mo were tempered at temperatures between 300 and 700 °C for one hour. The changes in the carbide dispersion and matrix substructure produced by tempering were measured by transmission electron microscopy. These measurements were correlated with resistance to hydrogen stress cracking produced by cathodic charging of specimens in three-point bending. Scanning electron microscopy showed that specimens tempered between 300 and 500 °C failed by intergranular cracking while those tempered at higher temperatures failed by a transgranular fracture mode. Auger electron spectroscopy showed that the intergranular fracture was associated with hydrogen interaction with P segregation and carbide formation at prior austenite grain boundaries. Transgranular cracking was initiated at inclusion particles from which cracks propagated to produce flat fracture zones extending over several prior austenite grains. The 4130 steels modified with higher Mo content resisted tempering and showed better hydrogen stress cracking resistance than did the unmodified 4130 steel. The transition in fracture mode is attributed to a decohesion mechanism in the low temperature tempered samples and a pressure mechanism in the highly tempered samples.

STEM Analysis of Grain Boundaries in Cemented Carbides

Abstract: There is controversy over whether the cobalt binder of liquid-phase sintered WC forms a continuous skin over the WC particles or whether the carbide grains form a continuous skeleton. It is shown that the atomic Co/W ratio in the 20-A grain boundaries is >3 times larger than that in the grains, supporting the former model.

The Elastic Modulus and Fracture of Boron Carbide

Abstract: Young's modulus at room temperature and the fracture surface energy, up to 1200°C, were determined for boron carbide specimens with 0 to 15% porosity. Sonic modulus measurements indicated that the Young's modulus decreased as porosity increased in a manner characteristic of the cylindrical pore model. The fracture surface energy appeared to be independent of porosity but was inversely related to temperature. Fractography supported the low values of fracture surface energy that were obtained.

A model to predict carburization profiles in high temperature alloys

Abstract: A mathematical model has been developed which, by means of finite difference computation techniques, permits the prediction of carbon concentration profiles in carburized high temperature alloys. It is assumed that a proportion of the carbon which diffuses into the alloy reacts with elements such as chromium to form carbide precipitates. The amount of carbon remaining in solution is determined from the solubility product of the carbide. Only this carbon in solution is able to diffuse through the alloy matrix, and thus the carbide precipitation process reduces the rate of carburization. Applying the model, the diffusion coefficient of carbon in Alloy 800 H at 900 °C has been determined as (3.3 ± 0.5) × 10−8 cm2/s. The model can also treat the carburization of an alloy containing two carbide-forming elements, but application to alloys containing both chromium and niobium (columbium) was successful only to a limited extent, probably as a result of the slow, complex kinetics of carbide precipitation. The model can be used to adapt carbon concentration profiles from one geometrical configuration to another. On the basis of profiles determined experimentally on small, cylindrical test specimens, carbon concenration profiles have been predicted for thick section tubes of Alloy 800 H exposed to a carburizing environment for up to 100,000 h.

Method for the production of copper-boron carbide composite

Abstract: A process for manufacturing nuclear radiation shields consisting of neutron-absorbing boron carbide particles embedded in a heat-dissipating copper matrix. Copper is electroplated through a layer of loose, electrically nonconductive boron carbide particles on a metal substrate. The carbide particles may be deposited on the substrate while electroplating, and heat exchanger ductwork may be incorporated. To make cylindrical shields, a cylindrical metal substrate is rotated about its axis giving rise to centrifugal forces which hold the carbide particles on the inner surface and aid electrodeposition. A thermo-mechanical process is described in which boron carbide particles pre-encapsulated with copper are consolidated into a unitary mass on the inner surface of a heated cylindrical substrate with or without the aid of a roller within the cylinder.

Method for preparing silicon carbide

Abstract: What is disclosed is an improved method of preparing silicon carbide which is characterized by the preparation of a specific pre-silicon carbide polymer. The method allows the preparation of silicon carbide from specific polysilane polymers without the cumbersome extractions and purifications found in the newer methods of silicon carbide preparation.