Direct carburization

There are no official specifications for obtaining a minimum level of engine cleanliness from a fuel. However, all additives in France are subject to approval by the Direction des Carburants (DHYCA), with the objective of having data that prove, first of all, the product to be harmless, and second, the product s effectiveness. Likewise, the automotive manufacturers, in establishing their specifications, set the minimum performance to be obtained by the fuel with regard to engine cleanliness. 

Governmental specifications. In France, they are published through inter-ministerial directives and are prepared at the Ministry of Industry by the DHYCA (Direction des Hydrocarbures et des Carburants). They govern the characteristics that products must adhere to in all French territories. 

DHYCA Direction des Hydrocarbures et des Carburants (French Ministry of Industry)... 

Carbides may also be prepared, either by direct carburizing, as in the case of steel, in which a surface carbide film dissolves into the substrate steel, or by refractory metal carbide formation as in the cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

What is the relationship between carbide composition and catalytic activity Figure 5.9 (right) compares the two properties. The catalytic activity starts low, increases rapidly to a maximum and decreases slowly thereafter. The increase in activity occurs simultaneously with the carburization, but, without any direct correlation, the maximum in rate occurs when there is still metallic iron present. The interpretation given to the results in Fig. 5.9 is the following [22,24],... 

The applications of IR spectroscopy in catalysis are many. For example, IR can be used to directly characterize the catalysts themselves. This is often done in the study of zeolites, metal oxides, and heteropolyacids, among other catalysts [77,78], To exemplify this type of application, Figure 1.11 displays transmission IR spectra for a number of Co Mo O (0 < x < 1) mixed metal oxides with various compositions [79]. In this study, a clear distinction could be made between pure Mo03, with its characteristic IR peaks at 993, 863, 820, and 563 cm-1, and the Mo04 tetrahedral units in the CoMo04 solid solutions formed upon Co304 incorporation, with its new bands at 946 and 662 cm-1. These properties could be correlated with the activity of the catalysts toward carburization and hy-drodenitrogenation reactions. 

The reduction of WO3 by carbon is used for the production of tungsten carbide (direct carburization). 

In the petroleum industry carbon monoxide and carbon dioxide reactions with steel are encountered which may also cause decarburization or carburization, depending upon the direction of the equilibrium. 

In Ref [315], HOD films were grown using the two-step process based on the BEN technique of Ref [289], and the interface structure was studied by TEM. As a consequence of the BEN process, (i) the carburization of Si created nanocrystalline, heteroepitaxially oriented fi-SiC, (ii) the [3-SiC layer formed nanometer size hillocks and ridges that were aligned parallel to the Si[110] direction, (hi) diamonds nucleated on the 3-SiC layer that was convex, and (iv) P-SiC existed only under the... 

Ac( rdingly, the direct carburization process starts with a pelletized mixture of WO3 and graphite passing step-by-step two rotary furnaces, the first operating at 950-1250 °C under nitrogen (reduction) and the second at 1400 °C under hydrogen (carburization). 

Direct Carburization [9.13-9.16]. An alternative for producing WC powders on an industrial scale is the direct carburization process, which was patented in Japan. It has been exclusively used there for several years to produce submicron and ultrafine WC powders of high quality. Other than the conventional process, where steering of the WC particle size is mainly conducted via the metal powder process, this is done through the quality of the oxide and carbon source as well as the process parameters. 

FIGURE 9.7. Schematic presentation of the direct carburization process [9.13-9.16]. By courtesy of Tokio Tungsten Co., Ltd., Japan. 

The Rapid Carbothermal Reduction (RCR) Process (The Dow Chemical Company) [9.19,9.20], In principle, the patented RCR process is another type of direct carburization. The tungsten carbide powder is continuously synthesized by an extremely rapid carbothermal reduction of WO3 in a graphite transport reactor. Calculated heating rates are in the range of 100 million K per second by thermal radiation. The WO3/C mixture during this process is converted to a tungsten carbide precursor (WCi c) within seconds. 

The powder properties are either directed by powder milling of coarser powders (high-temperatm-e carburized), or by steering the particle size through the oxide precursor and subsequent soft carburization (lower temperature, but longer carburization times). 

The lower members of the hydrocarbon chains have a tendency to evaporate, and column 2 of Table 5-1 gives the composition of vapor in the head space of a fuel tank. Starting in 1971, U.S. cars were equipped with evaporation control systems whereby the direct emission of fuel into the atmosphere from tanks and carburators was greatly reduced. Fuel evaporation remains important in the handling of fuel at filling stations. 

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