Carbide phase diagram

Fig. 16. Iron—iron carbide phase diagram (1). See text.

The iron-iron carbide phase diagram is probably the most important of all binary phase diagrams. Why is the diagram not a true equilibrium diagram Does it matter ... 

Properties Solid solutions, dislocations V Mechanical properties Y Dislocations, slip sterns, strengthening mechanisms Y Phase equilibria, the iron-iron carbide phase diagram Mechanical properties of Fe-C alloys Y ... 

Iron-iron carbide phase diagram (Chapter 9)... 

Important phases fonnd on the iron-iron carbide phase diagram (Figure 9.24) are a-ferrite (BCC), y-austenite (FCC), and the intermetallic compound iron carbide [or cementite (Fe3C)]. 

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 ... 

In terms of heat treatment and the development of microstructure, what are two major limitations of the iron-iron carbide phase diagram ... 

Genetically, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt% in practice, however, most cast irons contain between 3.0 and 4.5 wt% C and, in addition, other alloying elements. A reexamination of the iron-iron carbide phase diagram (Figure 9.24) reveals that alloys within this composition range become completely liqnid at temperatures between approximately 1150°C and 1300°C (2100°F and 2350°F), which is considerably lower than for steels. Thus, they are easily melted and amenable to casting. Furthermore, some cast irons are very brittle, and casting is the most convenient fabrication technique. 

Several different annealing procedures are employed to enhance the properties of steel alloys. However, before they are discussed, some comment relative to the labeling of phase boundaries is necessary. Figure 11.11 shows the portion of the iron-iron carbide phase diagram in the vicinity of the eutectoid. The horizontal line at the eutectoid lower critical temperature, conventionally labeled A, is termed the lower critical temperature, be-... 

Figure 11.11 The iron-iron carbide phase diagram in the vidnity of the entectoid, indicating heat-treating tem-peratnre ranges for plain carbon steels. (Adapted from G. Krauss, Steels Heat Treatment and Processing Principles, ASM International, 1990, page 108.)...

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