Iron carbon equilibrium

Figure 8.1 The iron-carbon equilibrium diagram up to 6.67 weight% C. (Reprinted from ref [8.1].)...

FIGURE 3.3 Extract from the iron carbon equilibrium diagram, including the temperature ranges for certain heat treatment processes... 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Figure 6.3 The iron-carbon phase diagram showing the alternative production of iron and cementite from the liquid alloy, which occurs in practice, to the equilibrium production of graphite...

EQUILIBRIUM DIAGRAM. A diagram showing the phase fields of an alloy system under the conditions of complete equilibrium using as coordinates the temperature, the compositions in terms of the components, and the pressure. The most frequently used equilibrium diagrams in metallurgy are drawn with the pressure considered constant. See iron-carbon diagram under iron Metals, Alloys, and Steels. See also Distillation. 

The iron-carbon diagram showing the true equilibrium with graphite (dotted lines) as well as the metastable equilibrium with cementite. 

In most systems the martensitic reaction is geometrically reversible. On heating, the martensite will start to form the higher temperature phase at the As temperature and the reaction will be complete at an Af temperature, as illustrated in Figure 11.19. Martensite in the iron-carbon system is an exception. On heating, the iron-carbon martensite decomposes into iron carbide and ferrite before the As temperature is reached. Martensite can be induced to form at temperatures somewhat above the Ms by deformation. The highest temperature at which this can occur is called the Md temperature. Likewise, the reverse transformation can be induced by deformation at the Ad temperature somewhat below the As. The temperature at which the two phases are thermodynamically in equilibrium must lie between the Ad and Md temperatures. 

Figure 3.10. The equilibrium phase diagram for the iron-carbon system.

Carbonate iron sediments are formed when iron is precipitated in the presence of dissolved carbonic acid or as a result of interaction of the primary sediments with organic matter in the course of diagenesis. The determination of the stability of primary iron carbonates with respect to iron oxides and hydroxides was made on the basis of using the functional dependence of and on pH, calculated by the method of Garrels and Christ (1968) for the system carbonate-water. According to this dependence, in conditions of FeCOj in equilibrium with water the value of partial pressure of carbon dioxide decreases in proportion to increasing pH. 

In all the experiments and thermodynamic calculations, it was assumed that carbon monoxide was stable at the time of decomposition of iron carbonate and was removed from the rocks being metamorphosed. Nor were possible secondary reactions in the case of oxidation of FeC03 by water taken into account—the result of the reactions would be the formation of some CO, CH4 and more complex hydrocarbons. Such assumptions are valid in examining the results of short-term experiments in which metastable carbon monoxide arises and does not undergo subsequent decomposition. However, in long-term natural processes the establishment of complete equilibrium between all the components of the gas phase, with the decom-... 

The most important equilibrium relations met with in the case of the iron-carbon alloys are represented graphically in Figs. 60 and 61. 

Microbial reactions also change the chemical composition of the groundwater e.g. due to production of carbonate and sulfide. If the inorganic composition of groundwater is to be used to estimate microbial degradation, chemical reactions have to be considered, too. In this example a simplified geochemistry is simulated, consisting of a calcium-carbonate equilibrium system and precipitation of iron sulfide. 

Figure 4.16 The iron-rich region of the iron-carbon existence diagram. The phase cementite (FesC not shown) occurs at 6.70 wt% carbon. This phase is a nonequilibrium phase and does not occur on the equilibrium phase diagram, which is between iron and graphite. The diagram is not to scale, and the a-ferrite and 5-ferrite phase fields have been expanded for...

Following equilibrium constant and interaction parameters (ej, are recommend for solubility of nitrogen in iron-carbon melts [1999Svy] ... 

Kasl] Kase, T., Ternary System of Iron-Carbon-Nickel , Sci. Rep. Tohoku Imp. Univ., 1(14), 193-217 (1925) (Phase Diagram, Phase Relations, Morphology, Experimental, 5) [1925Kas2] Kase, T., On the Equilibrium Diagram of the Iron-Carbon-Nickel System , Sci. Rep. 

Sch] Schwartz, H.A., Payne, H.R., Gorton, A.F., Austin, M.M., Conditions of Stable Equilibrium in Iron-Carbon Alloys , Trans. Amer. Inst. Min. Met Eng., 68, 916-929 (1922) (Experimental, Phase Diagram, Meehan. Prop., 9)... 

Zhu] Zhukov, A.A., Improving die Metastable Equilibrium Diagram of the Iron-Carbon-Silicon... 

Mal] Malinochka, Y.N., Dolinskaya, V.Z., A New Metastable Equilibrium Diagram and Strac-ture of Iron-Carbon-Sihcon Alloys (in Russian), Liteinoe Proizvod., 7, 26-27 (1970) quoted in [1992Rag]... 

Vog] Vogel, R., About the Influence of the Titanium on the Perlit Creation in Carbon Steel , Ferrum, 14(11-12), 177-195 (1917) (Experimental, Phase Diagram, Phase Relations, 9) [1925Tam] Tamaru, K., On the Equilibrium Diagram of the System Iron-Carbon-Titanium , Sci. Rep. 

Equilibrium Between Titanium Carbide (TiC ) of Various Stoichiometries and Iron-Carbon Alloys , Scr. Mater., 35(7), 791-797 (1996) (Thermodyn., Calculation, Phase Diagrams, Phase Relations, 14)... 

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