Phase iron-carbon alloys

As you can see, the process by which the iron-carbon alloy is processed and solidified is jnst as important as the overall stoichiometry. Although a discussion regarding phase transformations is more the realm of kinetic processes, it is nonetheless pertinent to snmmarize here the types of important ferrous alloys, particularly those in the cast iron categories. This is done in Fignre 2.13. 

Metallurgy was one of the first fields where material scientists worked toward developing new alloys for different applications. During the first years, a large number of studies were carried out on the austenite-martensite-cementite phases achieved during the phase transformations of the iron-carbon alloy, which is the foundation for steel production, later the development of stainless steel, and other important alloys for industry, construction, and other fields was produced. 

Before proceeding to discuss the application of the Phase Rule to the study of the iron-carbon alloys, however, the main facts with which we have to deal may be stated very briefly,... 

Present as second phase in iron-carbon alloys. 

Neu] Neumann, F., Schenck, H., Patterson, W., Iron-Carbon Alloys in Thermodynamic Consideration (in German), Giesserei Tech.-Wiss. Beih., 23, 1217-1246 (1959) (Phase Diagram, Phase Relations, Thermodyn., Theory, 72)... 

Part V. Compendium of Phase Diagram Data , Tech. Rep. AFML-TR-65-2, Part V, Air Force Materials Laboratory, Wright-Patterson AFB, OH, 1969 (Phase Diagram, Experimental, 1) [1969Ruh] Ruhl, R.C., Cohen, M., Splat Quenching of Iron-Carbon Alloys , Trans. AIME, 245, 241-251 (1969) (Crys. Structure, Experimental, Phase Relations, 50)... 

Nauk Ukr. RSR, A, (6), 566-569 (1978) (Experimental, Crys. Structure, Phase Diagram, 10) [1978Sha2] Shapovalov, V.I., Influence of Hydrogen on the Eutectoid Transformation in Iron-Carbon Alloys , Phys. Met. Metallogr, 46(3), 183-185 (1978), translated fiomEVz. Metal Metalloved, 46(3), 664-666 (1978) (ExperimentaL Phase Relations, Morphology, 8)... 

Wel] Wells, C., Walters, F.M., Alloys of Iron, Manganese and Carbon-Part XIV. Iron-Carbon Alloys Containing 7% Manganese , Trans. Amer. Soc. Met, 23, 751-761 (1935) (Phase Relations, Experimental, 6)... 

Gom] Gomersall, D.W., MeLean, A., Ward, R.G., The SolubiUty of Nitrogen in Liquid Iron and Liquid Iron-Carbon Alloys , Trans. Met. Soc. AIME, 242(7), 1309-1315 (1968) (Experimental, Phase Relations, 24)... 

Svy] Svyazhin, A.G., Kindop, V.E., El Gammal, T, Solubility of Nitrogen in Liquid Iron-Carbon Alloys , Mater. Sci. Forum, 318-320, 81-88 (1999) (Experimental, Review, Phase Diagram, 21)... 

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)... 

Kei] Keil, O., Kotyza, F., The Influence of Silicon and Manganese on the Solidification of Iron-Carbon Alloys (in German), Arch. Eisenhuettenwes., 4(6), 295-297 (1930) (Experimental, Morphology, Phase Diagram, 20)... 

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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)... 

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. 

Ferrous alloys are those in which iron is the prime component, but carbon as well as other alloying elements may be present. In the elassification scheme of ferrous alloys based on carbon content, there are three types iron, steel, and cast iron. Commercially pure iron contains less than 0.008 wt% C and, from the phase diagram, is composed almost exclusively of the ferrite phase at room temperature. The iron-carbon alloys... 

Figure 9.29 Schematic representations of the microstructures for an iron-carbon alloy of hypoeutectoid composition Cq (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature.

In this discussion of the microstructural development of iron-carbon alloys, it has been assumed that, upon cooling, conditions of metastable equilibrium have been continuously maintained that is, sufficient time has been allowed at each new temperature for any necessary adjustment in phase compositions and relative amounts as predicted from the Fe-FejC phase diagram. In most situations these cooling rates are impracti-cally slow and unnecessary in fact, on many occasions nonequilibrium conditions are desirable. Two nonequilibrium effects of practical importance are (1) the occurrence of phase changes or transformations at temperatures other than those predicted by phase boundary lines on the phase diagram, and (2) the existence at room temperature of nonequilibrium phases that do not appear on the phase diagram. Both are discussed in Chapter 10. 

The development of microstrnctnre for many iron-carbon alloys and steels depends on a eutectoid reaction in which the austenite phase of composition 0.76 wt% C transforms isothermally (at 727°C) into a-ferrite (0.022 wt% C) and cementite (i.e., a + FejC). 

The microstructural product of an iron-carbon alloy of eutectoid composition is pearlite, a microconstituent consisting of alternating layers of ferrite and cementite. The microstructures of alloys having carbon contents less than the eutectoid (i.e., hy-poeutectoid alloys) are composed of a proeutectoid ferrite phase in addition to pearlite. Pearlite and proeutectoid cementite constitute the microconstituents for hypereutec-toid alloys—those with carbon contents in excess of the eutectoid composition. 

For iron-carbon alloys (i.e., steels), an understanding of microstructures that develop during relatively slow rates of cooling (i.e., pearlite and a proeutectoid phase) is facilitated by the iron-iron carbide phase diagram. Other concepts in this chapter were presented as a prelude to the introduction of this diagram—the concepts of a phase, phase equilibrium, metastability, and the eutectoid reaction. In Chapter 10, we explore other microstructures that form when iron-carbon alloys are cooled from elevated temperatures at more rapid rates. These concepts are summarized in the following concept map ... 

The Iron-Iron Carbide (Fe-FesC) Phase Diagram Development of Microstructure in Iron-Carbon Alloys... 

For an iron-carbon alloy of composition 3 wt% C-97 wt% Fe, make schematic sketches of the microstructure that would be observed for conditions of very slow cooling at the following temperatures 1250°C (2280°F), 1145°C (2095°F), and 700°C (1290°F). Label the phases and indicate their compositions (approximate). 

The development of microstructure in both single- and two-phase alloys typically involves some type of phase transformation—an alteration in the number and/or character of the phases. The first portion of this chapter is devoted to a brief discussion of some of the basic principles relating to transformations involving solid phases. Because most phase transformations do not occur instantaneously, consideration is given to the dependence of reaction progress on time, or the transformation rate. This is followed by a discussion of the development of two-phase microstructures for iron-carbon alloys. Modified phase diagrams are introduced that permit determination of the microstructure that results from a specific heat treatment. Finally, other microconstituents in addition to pearlite are presented and, for each, the mechanical properties are discussed. 

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