Decarburization, iron

Stable oxides, such as those of clrromium, vanadium and titanium cannot be reduced to the metal by carbon and tire production of these metals, which have melting points above 2000 K, would lead to a refractoty solid containing carbon. The co-reduction of the oxides widr iron oxide leads to the formation of lower melting products, the feno-alloys, and tlris process is successfully used in industrial production. Since these metals form such stable oxides and carbides, tire process based on carbon reduction in a blast furnace would appear to be unsatisfactory, unless a product samrated with carbon is acceptable. This could not be decarburized by oxygen blowing without significairt re-oxidation of the refractory metal. 

Methane production seems to be governed by the equation r = k C(a) H(a), with C(a) > H(a) so that an increase in H(a) leads to a higher rate. The second peak in Fig. 20 represents methane generated by the decarburization of the bulk of the iron. 

AOD [Argon oxygen decarburization] A steelmaking process in which a mixture of oxygen and argon is injected into molten iron to reduce the carbon content. Developed by the Union Carbide Corporation in the mid 1970s. By 1989, 90 percent of the stainless steel made in the United States was made with this process. 

A plate of iron is exposed to a carburizing (carbon-rich) atmosphere on one side and a decarburizing (carbon-deficient) atmosphere on the other side at 700°C. If a condition of steady state is achieved, calculate the diffusion flux of carbon through the plate if the concentration of carbon at positions of 5 and 10 mm beneath the carburizing surface are 1.2 and 0.8 kg/m, respectively. Assume a diffusion coefficient of 3 x 10 m /s at this temperature. 

Decarburization is intentional in the processing of low-carbon sheet steels for electrical applications. In the production of malleable cast iron by annealing while cast iron deearburizalion is beneficial. 

Puddling A process for making wrought iron from pig iron, based on the partial decarburization of pig iron in a special furnace. Invented by H. Cort in Titchfield, Southampton, England, in 1784 and widely used in the United Kingdom and Europe until the end of the 19th century. In 1873, there were 8,000 puddling furnaces in the UK alone. 

A number of internal components, such as valves, valve seats, cylinder walls, pistons, and rings, will be exposed to hydrogen and water vapor. The potential effects are of two primary types (1) decarburization of steels and cast iron and (2) hydrogen embrittlement of aluminum pistons. Water vapor could cause excessive corrosion of exhaust systems, but this could be minimized by use of titanium. 

Decarburization occurs in steels and cast irons in hydrogen gas by the reaction of H with C in the steel. The decarburization rate is primarily dependent on the diffusion rate of C in the steel, but is also affected by the carbon content of the steel, alloying elements in the steel, such as chromium, impurities in the hydrogen, and of course time and temperature. Carburization of steels, the reverse of decarburization, is usually conducted at temperatures of about 900°C, but decarburization can occur at temperatures as low as 800°C. " ... 

The. principles and the difficulties of the analysis of empirical rate laws for heterogeneous reactions may be illustrated by a discussion of results obtained for the decarburization of iron by means of hydrogen and CO2-CO mixtures. 

The decarburization of thin foils of y-iron (austenite) in a stream of hydrogen at 928 and 1000°C has been investigated by Grabke (23). Since the iron foils were only 5-10 cm thick, diffusion equilibrium within the foil was virtually established while the decarburization proceeded. Thus the rate of the reaction. 

The foregoing example of the decarburization of iron is especially simple. In many cases, however, complications occur—first, because of partial coverage of the surface by one or more adsorbed species, and second, because of a heterogeneity of the surface involving different types of adsorption sites as indicated by a strong dependence of the heat of adsorption on surface coverage. 

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). 

The temperature below which a ferrite surface layer forms on decarburization depends on the nature and concentration of other alloying elements in the steel, but, for plain carbon steels, can be taken to be 910 °C as for pure iron. Prediction of the effect of alloying elements on this temperature is complicated by the fact that the alloying elements will undergo either denudation or concentration at the surface during scaling. 

Using mishmetal to modify high-chromium white iron results in a refinement of the alloy structure and increases the wear resistance by 13.6% [1994Pee]. Adding Cu to the C-Cr-Fe melt decreases the decarburization reaction rate [2002Shi]. 

Kan] Kaneko, K., Sano, N., Matsushita, Y., Decarburization and Denitrogenization of Iron And Iron-Chromium Alloys by Hydrogen Plasma (in Japanese), J. Fac. Eng., Univ. Tokyo, A-13, 60-61 (1975) (Phase Relations, Thermodyn., Experimental, 2)... 

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

Calculate the maximum drop size allowable so that the final drop after a 2.0-s fall contains on an average 0.1 wt % carbon. Assume that the mass transfer rate of gases at the surface is very great, so there is no outside resistance. Assume no internal circulation of the liquid. Hence, the decarburization rate is controlled by the rate of diffusion of carbon to the surface of the droplet. The diffusivity of carbon in iron is 7.5 x 10" m /s (S7). Hint Can Fig, 5.3-13 be used for this case )... 

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