Carbon profile

In some cases, the carbon profile may not provide the necessary hardness or other properties. For example, if the carbon content is too high, quenching to room temperature may not produce all martensite at the surface because the high carbon content places the martensite finish temperature, Mj below room temperature. This results in the presence of soft retained austenite, and a low surface hardness. Conversion to martensite by subzero cooling to below the temperature can increase the hardness (Fig. 6) (12). 

An important advancement in carburizing has been the development of diffusion models to calculate the carbon gradient as a function of time as the gas composition and temperature change (13). Such models can be coupled with computer control of the gas composition and temperature to produce desired carbon profiles. 

Absorption in the p-layer can be reduced by using an a-SiC H alloy with a bandgap of about 2 eV [584]. Carbon profiling within the p-layer further improves the window properties [585]. An intentionally graded p-i interface (buffer layer) 10 nm in thickness enhances the spectral response in the blue [125, 494, 586], which can be attributed to a reduced interface recombination. 

Carbon nitride, 27 214—215 conductivity of, 2 7 204-206 field emission properties of, 27 221 theoretical band structure calculations for, 27 203-204 Carbon nitride films, 2 7 205 Carbon nitride solids, 2 7 203 Carbonochloridic esters, 6 290 Carbon paper, 4 738-739 Carbon profile, in gas carburizing, 26 204 Carbon Raschig-ring tower packing, 22 745 Carbon reduction, of ferrovanadium, 25 518... 

Organic carbon profiles (Figure 4F) in all three subenvironments show the same decreasing profile with depth (13). The evaporative panne (core 1) contain the highest amounts of organic carbon which corresponds to an abundance of Spartina roots and filamentous algae visible in hand specimen. 

The calcium ion (Ca2+) and total carbonate concentrations are shown in Figures 7 and 8, respectively. The concentration levels and the trends of Ca2+ and total carbonate profiles obtained at the three Mg2+ concentrations are very similar. 

Temperature and fixed carbon profiles In the central core... 

H. Surm, O. KeBler, E. Hoffmann, and P. Mayr, Carburized, CVD Coated and Gas Quenched Steels Carbon Profile, Microstructure and Hardness, EUROMAT, (Proceedings of Conference), Munich, Germany, 1999, to be published. 

Table 11.2 Soil carbon content for the 0-100 cm layer of the main soil type of the western Amazon, calculated using an exponential modelisation of the vertical carbon profile.

Bennema, J. 1974. Oiganic carbon profiles in Oxisols. Pedologie XXIV, 2 119-146. 

A number of questions on the composition of the carbon mounds remain unanswered. It is not clear whether the carbon cap contains iron with the concentration increasing with depth from the surface, or whether the iron present (Figure 11) has been deposited by the sputtering process. The outer surface of the unetched deposit usually consists of 98-100 At% carbon. The composition of the material under the cap is also uncertain the validity of the observed carbon profile requires confirmation. 

Figure 17.3.12 AES depth profiles for carbon and fluorine in 2000-A-thick MgPc films. Carbon profiles of separate samples were normalized to a common curve, (a) MgPc film immersed in aqueous 0.1 M KPF6. (b) MgPc film oxidized in 0.1 M KPF solution.

SIMS was also used to study naphtha reforming catalysts It was found that the carbon profile along the pellet had wide irregularities. Two different regions could be distinguished, areas quite free of coke where the carbon signal was very low, and areas where the peak signal was close to the maximum. The size of these areas varied between 20 and 100 pm. 

If we apply the same reasoning to the carbon profiles shown in Figure 4.33, we would anticipate a progressive narrowing of the zone of activity progressing backwards from the exit, which is preferentially deactivated in this case, to the inlet. There the reaction zone is literally squeezed out of the front of the bed at a sufficiently long time of reaction. 

Figure I Measurement of carbon profiles, (a) Experimental data. (6) Calculated profile. Parallel fouling mechanism (from Richardson [36]).

Figure 5.45 (a) Iron-carbon phase diagram with different heating conditions A, B, and C indicated a = ferrite, y = austenite, (b) Carbon profile and model for the decarburization of plain carbon steel above 910 °C, corresponding to condition C in (a). 

In the model shown for the carbon profile in Figure 5.45(b), Co is the original carbon content, Cs is the carbon content at the scale-metal interface, x is distance measured from the original metal surface, and X is the position of the scale-metal interface. 

Combining Equations (5.47), (5.48), and (5.49), carbon profiles can be calculated and, as shown in Figure 5.47, these give excellent agreement with measured profiles. 

The actual value of the depth of decarburization is more difficult to establish since the carbon profile is smooth and it is difficult to define an inner Unfit. Although this could, and perhaps should, he set by defining a carbon content below which the mechanical properties of the steel are below specification, current practice relies on the choice of this position by eye by an experienced metallographer. By comparing reported depths of decarburization with the carhon profiles, the inner Umit was established as the position where the carbon content is 92% of the original uniform carbon content of the steel. This is also indicated in Figure 5.47. 

To illustrate this, calculated carbon profiles are plotted in Figure 5.48 for an 0.85 wt% C steel heated for 90 min at 1050 °C for various assumed values of kc, the corrosion constant.The profiles are plotted relative to the original metal surface and refer to the following values of kc 4.1 x 10 mm s , which is a realistic value for a furnace atmosphere a hypothetical value of 4.1 x 10 mm s to represent a very high scaling rate and 0 mm s representing the case where the atmosphere just fails to form a scale but still reduces the surface carbon content to very low values. 

Figure 5.37 Radial carbon profile in ring-shaped reformer catalyst [381].

Measured radial carbon profiles in the decoked particles are depicted in Figure 6.9.13. The experimental times to reach 50% burn-off are compared wiffi the numerically calculated data. [Coked catalyst particles were regenerated at different temperatures up to a burn-off degree of 50% (Kern, 2003).] The calculated and... 

The carbon profile investigated on 4C-4N specimen using EDS FE-SEM is depicted in Fig. 9. It is found that higher carbon at the deeper layer which indicates that carbon pnished-ahead by the incoming nitrogen atom and the dissolved carbon is accumulated at the front of the nitride layer which has also been reported in the literature (Lewis et al, 1993). 

Fig. 9. Carbon profiles (a) and nitrogen profiles (b) along the depth.

In the case of an ascending carbon profile, which is obtained when the carbonaceous deposit results from a consecutive mechanism, the rate is continuously decreasing with time in all points of the reactor, except at z = 0, where coke is not deposited yet. A gradually decreasing part of the reactor will then be effective in the conversion to the main product. 

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