Pearlite colonies

Figure 2.15. Surface replica of two adjacent pearlite colonies with a high-angle boundary. The microstructure is a lamellar mixture of ferrite (major phase) and carbide (minor phase). (Photo courtesy of B. L. Bramfitt and J. R. Gruver, Bethlehem Steel Corporation.)...

Figpre 9.33 Photomicrograph of a 1.4 wt% C steel having a microstructure consisting of a white proentectoid cementite network surronndrng the pearlite colonies. lOOOX. (Copyright 1971 by United States Steel Corporation.)... 

The microstructure at position (ii) consisted of grains of ferrite and colonies of pearlite. It was noticed that the pearlite had started to "spheroidise" (see Problem 5.2). The microstructure at position (i) consisted of grains of ferrite and grains of lower bainite in roughly equal proportions. Estimate the temperatures to which the tube been heated at positions (i) and (ii). Explain the reasoning behind your answers. 

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

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