Carburization changes

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. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

The very striking change in XPS spectra for samples Ai and A2 with only a 100 K increase in the temperature of carburization has been also seen by Carver et al.24 They report an increase in W/Al ratio by low energy ion... 

The above changes in the microstructure upon annealing do not only apply to the bulk material, but also for the surface. If an iron material is placed at high temperature in the presence of carbon vapor, a procedure known as carburization occurs. 

Carburizing + CVD offers the possibilities of strengthening the substrate surface and inducing compressive residual stresses in the substrate surface. Further, it can be done sequentially in one chamber just by changing the atmosphere from low pressure carburizing to CVD. Figure 27 shows an example of a... 

Figure 3.21 shows the change of the product composition with carburization time in hydrogen for a tungsten-carbon black mixture at 1119°C. After only 10 minutes at this temperature most of the metal was transformed to W2C, which was ffien only slowly converted to WC. This result is characteristic for the temperature range of 1050 to 1850 °C and W particle sizes of 1.3 to 20 pm [3.71]. It is remarkable that W2C forms during carburization even at 900 °C, which is well below its eutectoid decomposition teniperature of 1250°C. 

Although the above characterization in regard to physical and chemical properties seems to be quite rigid and complete, in some cases imexpected and unsatisfactory results may occur diuing the further hardmetal production. The reason for this behavior is based on still insufficient specification, because the carbon black quality, carburization temperature, as well as type and diuation of milling after carburization are not clearly defined, and can be changed by the supplier or may differ between diverse suppliers. 

In the case of cobalt, unstable cubic cobalt was identified as the product of the reduction of standard cobalt catalysts, while hexagonal cobalt was found as a product of the hydrogenation of cobalt carbide. Used cobalt catalysts show no carbide by x-ray examination. Bulk phase carbide decreases the activity of cobalt catalysts. Surface area measurements show no appreciable change when the cobalt of cobalt catalysts was converted to cobalt carbide. Carburization at conditions where free carbon is formed increases the area considerably. 

Fig. 2. Mass changes of four alloys in exposures at 600°C and ac = 41.5 for one day and a second day, black columns removable coke and hatched columns remaining mass change by metal wastage (loss), carburization and oxidation (gain).

Iron catalysts present special problems for the determination of the adsorbed species. The iron may be carburized during the reaction and this leads to a change in the bulk composition of iron catalyst with time on stream. Bianchi et al. determined that iron was partly converted into a mixture of e -Fe2 2C and x Fe2 5C during the reaction using Mossbauer spectroscopy. The adsorbed carbon surface species were determined to be present in three forms small amounts of reactive CH species which produced the bulk of the hydrocarbon products during reaction, a carbidic species with some associated hydrogen, and inactive graphitic carbon species. 

Table I. Principal Carburization Reactions of Iron and Phase Changes...

The changes in the ferromagnetic phases during the 72-hour carburization of catalyst B-6 (Figure 5) were measured by TMA at the times indicated in Figure 1. As the a-Fe phase decreased, the Fe2C (Hagg) phase increased, and the FesC (cementite) phase exhibited a transient existence, reached a maximum at about five hours. 

He/CO = 1/1 (by volume). Carburized in tubular reactor. Mass change not measured catalyst did not swell. ... 

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