TABLE OF CONTENTS Under equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys containing up to 0.8 wt% carbon. The reaction occurs at 910 C in pure iron, but takes place between 910 C and 723 C in iron-carbon alloys. However, by quenching from the austenitic state to temperatures below the eutectoid temperature, ferrite can be formed down to temperatures as low as 600 C. There are pronounced morphological changes as the transformation temperature is lowered, which it should be emphasized apply in general to hypo- and hypereutectoid phases, although in each case there will be variations due to the precise crystallography of the phases involved. For example, the same principles apply to the formation of cementite from austenite, but it is not difficult to distinguish ferrite from cementite morphologically.
As a result of a survey of the behaviour of plain carbon steels, Dub proposed a classification of morphologies of ferrite which occur as the ?/ ? transformation temperature is lowered. Dub recognized four well-defined morphologies based on optical microscopy, later extended by Aaronson:
Grain boundary allotriomorphs: An allotriomorph has a shape which does not reflect its internal crystalline symmetry. This is because it tends to nucleate at the austenite grain surfaces, forming layers which follow the grain boundary contours (Fig. 3.3a). The allotriomorph is in contact with at least two of the austenite grains and will have a random orientation with one of them, but an orientation which is more coherent with the other. It may, therefore, be crystallographically...
