Kinetics carburization

As a result of fliese temperature - programmed runs, additiond experiments were conducted isothermally at temperatures between 723 - 873 K. Based on tbe second phase peak (peak 2), the conversion - time profiles at different temperatures exhibited a characteristic Sigmoid shape and may therefore be used to interpret the solid carburization kinetics. As detailed in Brown [5], the transient convrasion data may be d cribed by... 

DEVELOPMENT OF NOVEL SUPPORTED MO2C CATALYSTS CARBURIZATION KINETICS AND OPTIMAL CONDITIONS... 

Carburization Kinetics. The trends in the mass increase during carburization of reduced B-2 and B-6 catalysts (Figure 1) are qualitatively similar to those reported (1,6,13). The rapid initial mass gain, presumably attributable to carbide formation, is followed by a lower, constant rate of increase that is attributed to free carbon (6). The formation of the free carbon, which has been associated with catalyst swelling, occurred only on exposure to H2/CO mixtures but not CO alone and when the Fe2C (Hagg) phase predominated. 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

Mossbauer spectroscopy provides phase identification, determination of oxidation states, and incidentally structure information and particle size. A little used application is to follow in real time the kinetics of phase transitions (carburization, reduction) in catalysts by monitoring the intensities of a few selected peaks in a single velocity experiment. Examples of applications on catalysts have recently been reviewed [43]. 

Fluid-solid reactions include thermal decomposition of minerals, roasting (oxidation) of sulfide ores, reduction of metal oxides with hydrogen, nitridation of metals, and carburization of metals. Each t3 e of reaction will be discussed finm the thermodynamic point of view. Then reaction kinetics for all of the various rate determining steps in fluid-sohd reactions will be discussed for two general models shrinking core and shrinking particle. 

There is a relationship between surface segregation and surface reaction kinetics. Thus for the carburization and decarburization of Fe, CH4 C(dis-solved) + 2H2, the rate is given by... 

Although carburization can enhance the performance of certain components, in cases such as reformer tubes in the treatment of oil and hydrocarbons, carburization of stainless steels is deleterious and life limiting. In this case internal carbides form with kinetics analogous to those for internal oxidation." Consequently, alloys that are resistant to carburization are developed, primarily by alloying with nickel to reduce the diffusion coefficient of carbon, and with silicon and aluminium, which are thought to impart some protection by the formation of impervious silica and alumina surface hlms in the low-oxygen-potential atmospheres. ... 

Bun] Bungardt, K., Preisendanz, H., Mersmann, T., Carburization of Plain Carbon and Chrome-Alloy Steels in Hydrogen-Benzene Vapour Mixtures (in German), Arch. Eisenhuettenwes., 36(10), 709-724 (1965) (Interfaee Phenomena, Kinetics, Experimental, 60)... 

H2-CH4 gaseous mixtures may be used to measure carbon activity in steels [1969Bun] by equilibrating the carbon potential in gaseous and solid phases. The kinetics of carburization and decarburization of steels with methane according to the reaction CH4 C (dissolved in Fe) + 2 H2 was measured at 928°C by [1965Gra] on austenite and at 786°C by [1973Gra] on the ferrite. The kinetics of the carburization at... 

Gra] Kinetics of carburization - decarburization of iron under CH4 928°C, < 0.1 MPa... 

Y. Sun and T. Bell Effect of layer thickness on the rolling-sliding wear behavior of law- temperature plasma-carburized austenitic stainless steel. Tribology Letters (2002) 13,1, 29-34 Y. Sxm, Kinetics of low temperature plasma carburizing of austenitic stainless steels, J. Mater. Proc. Tech. 168 (2005) 189-194. 

In carburizing environments, if the relative partial pressures of oxygen and carbon favor AI2O3 stability and the temperature is such that the kinetics of AI2O3 formation enable rapid coverage of the alloy surface, iron aluminides would be expected to have very... 

An insidious aspect of carburization is its nonuniform nature. Just as for other forms of localized corrosion, it is extremely difficult to predict and model localized carburization damage. As a rule of thumb, carburization problems only occur at temperatures above 815°C, because of unfavorable kinetics at lower temperatures. Carburization is therefore not a common occurrence in most refining operations because of the relatively low tube temperatures of most refinery-fired heaters. 

Ohno, K Hino, M Kinetic analysis of iron carburization during smelting reduction, Science and technology of innovative ironmaking for aiming at energy half consumption, Japan, November, 2003,17-20. 

As already mentioned in previous sections, corrosion by liquid metals can proceed by various ways dissolution, oxidation, carburization, formation of intermetallic compounds, etc. The nature of the corrosion process will firstly depend on the thermodynamic data of the liquid metal/sohd material system considered. These thermodynamic data give the equilibrium state of the system and thus the products susceptible to form. Of course, kinetics data are also essential to determine whether the products predicted by thermodynamics will indeed form and in that case at what rate they are going to form. These two aspects will be treated in the following paragraphs. 

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