The Iron-Carbon System

Of all binary alloy systems, the one that is possibly the most important is that for iron and carbon. Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially iron-carbon alloys. This section is devoted to a study of the phase diagram for this system and the development of several of the possible microstructures. The relationships among heat treatment, microstructure, and mechanical properties are explored in Chapters 10 and 11. 

A portion of the iron-carbon phase diagram is presented in Figme 9.24. Pure iron, upon heating, experiences two changes in crystal structure before it melts. At room temperature, the stable form, called ferrite, or a-iron, has a BCC crystal structure. Ferrite experiences a polymorphic transformation to FCC austenite, or y-iron, at 912°C (1674°F). This austenite persists to 1394°C (2541°F), at which temperature the FCC austenite reverts back to a BCC phase known as 5-ferrite, which finally melts 

Tutorial Video Eutectic Reaction Vocabulary and Microstructures 

Eutectic reaction for the iron-iron carbide system 

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The iron—carbon system contains the orthorhombic iron carbide (3 1) [12011 -67-5] which melts congmendy and represents the cementite in... 

Figure 6.4 The time-temperature-transformation diagram of the iron-carbon system, beginning at the composition of austenite...

The iron-carbon system has a eutectic find it and mark it on the diagram (Fig. A1.37). At the eutectic point the phase reaction, on cooling, is... 

Binary and Ternary Interstitial Alloys. III. The Iron-Carbon System The... 

Table 5.49. Current names of iron alloys Phases and Phase complexes in the iron-carbon system.

The iron—carbon system contains the orthorhombic iron carbide (3 1) [12011-67-5], Fe3C, which melts congruently and represents the cementite in steel metallurgy. The existence of other carbides, eg, iron carbide (2 1) [12011 -664], Fe2C, iron carbide (5 2) [1212745-6], Fe5C2, and iron carbide (7 3)... 

Many two- and three-component systems have been studied and recorded in detail. The iron-carbon system is one that has attracted much attention and been of great value in iron metallurgy. 

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. 

In SELECTED OUTPUT FeHC03+, and FeC03 for the iron - carbon system as well as FeS04+, FeHS042+, Fe(S04)2, FeHS04+, and FeS04 for the iron - sulfur system must be defined for the output besides the iron species from chapter 4.1.3.1. For both systems numerical problems occur at pH 4 and pH 14. 

In the iron - carbon system (Fig. 54) the FcOH1 - field vanishes, the zero charged FeCO30 - field predominating instead under the same pE - pH conditions. In the iron - sulfur system (Fig. 55) the predominance field of the iron - sulfate species FeS04+ enlarges at the expense of Fe3+, while FeOH2+ disappears completely. 

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

Shterenberg L. E., Slesarev V. N., Korsunskaya I. A., and Kamenetskaya D. S. (1975) The experimental study of the interaction between the melt carbides and diamond in the iron-carbon system at high pressures. High Temp.-High Press. 7, 517-522. 

The interstitial structures of by far the greatest technical importance are those which occur in the iron-carbon system, and the application of X-ray analysis to this system has resulted in a great extension of our understanding of the properties of carbon steels, and in a considerable simplification in the description of their behaviour. We cannot give here a detailed account of all the work in this field but certain features are of general interest and may be briefly discussed. 

The crystal chemistry of the iron-carbon system is especially complex on account of the relatively small size of the iron atom, resulting in a carbon iron radius ratio of about o 6o, which is so close to the critical value 0 59 discussed above that both interstitial structures and structures of greater complexity may be expected. Added to this is the further complication that iron is dimorphous. Below about 910 °C, and from about 1400 °C to the melting point, the structure is cubic body centred, and is known as a iron. Between these two temperatures a cubic close-packed structure, termed y iron, is formed. The ferromagnetism of iron... 

Interstitial solutions are characterized by atoms of smaller atomic size fitting into the interstices between the larger atoms of the structure of another solid. The iron-carbon system is an example there the carbon atoms fill the spaces between the iron atoms. 

Iron always contains carbon. A part of the phase diagram of the iron/carbon system is shown in Figure 10.5. The carbon has different solubilities in the different iron modifications, which form mixed crystals (solid solutions). In a-iron the solubility is only 0.04% (ferrite) and in 5-iron the solubility is 0.36%. In the y-modification with its fee struemre, carbon and iron form an intercalation lattice as a solid solution called austenite with the maximum solubility of 2.06% carbon at 1147 °C. Iron with more carbon is called cast iron. Iron with less than 2.06% carbon is called steel. During slow cooling of a melt (above 1147 °C) iron solidifies either as austenite (carbon content of the melt <4.3%) or as cementite (FcjC, carbon content of the melt > 4.3%). At 1147 °C the melt solidifies in a eutectic mixture of both these phases called ledeburite. 

Figure 10.5 Phase-diagram of the iron carbon system at lower carbon concentration. ...

Eno] Enomoto, M., Aaronson H.I., Calculation of a+y Phase Boundaries in Fe-C-X Systems from the Central Atoms Model , Calphad, 9(1), 43-58 (1985) (Thermodyn., Calculation, 31) [1985Gus] Gustafson, R, A Thermodynamic Evaluation of the Iron-Carbon System , Scand. J. Metall,... 

Jae] Jaenecke, E., About half Two Phase Diagrams of the Iron-Carbon System and their Relationships in Ternary Alloys, in Partieular by Silicon (in German), Z. Metallkd., 32(5), 142-144 (1940) (Abstract, Phase Diagram, Phase Relations, 23)... 

Depending on the binding energy between the atoms and on the possibility of additional phases that may form, real phase diagrams can be much more complex than the simple examples shown here. One example is the phase diagram of the iron-carbon system in figure 6.50. 

The iron-carbon system has been investigated in our laboratory. The powders were subjected to thermal treatment and next they were investigated by the potentiometric method in powder electrodes, and by the metallographic method using grinds and corrosions of the powder in shellac. Both the carbon content and the structure of the system had an influence on the rates of change and the value of the powder electrode potential in aqueous solution of potassium sulphate. The potentiometric measurements enabled us to define the structure when the quantitative composition of an alloy was known. 

Physical metallurgy is a rather wide field of applications of Mossbauer spectroscopy and it is possible to enumerate only the main topics phase analysis, order-disorder alloys, surfaces, alloying, interstitial alloys, steel, ferromagnetic alloys, precipitation, diffusion, oxidation, lattice defects etc. Alloys are well represented by the iron-carbon system, the mechanism of martensite transformation, high-manganese and iron-aluminium alloys, iron-silicon and Fe-Ni-X alloys. 

The eutectic of the iron-carbon system, the constituents of which are austenite and cementite. The austenite decomposes into ferrite and cementite on coohng below the temperature at which transformation of austenite to ferrite or ferrite plus cementite is completed. 

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