The reaction occurs at 910 C in pure iron but takes place between 910 C and 723

The reaction occurs at 910 c in pure iron but takes

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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 Ae 1 , 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 hyper-eutectoid 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. The Austenite-Cementite Transformation The Dube classification applies equally well to the various morphologies of cementite formed at progressively lower transformation temperatures. The initial development of grain boundary allotriomorphs is very similar to that of ferrite, and the growth of side plates or Widmanstaten cementite follows the same pattern. The cementite plates are more rigorously crystallographic in form, despite the fact that the orientation relationship with austenite is a more complex one. As in the case of ferrite, most of the side plates originate from grain boundary allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries in austenite. The Austenite-Pearlite Reaction Pearlite is probably the most familiar micro structural feature in the whole science of metallography. It was discovered by Sorby over 100 years ago, who correctly assumed it to be a lamellar mixture of iron and iron carbide. Pearlite is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength. Lamellar eutectoid structures of this type are widespread in metallurgy, and frequently pearlite is used as a generic term to describe them. These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth, and are, therefore, diffusion controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.
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~ Page 22 of 73 ~ Plain Iron Carbon Steels Introduction The discussion hereunder includes notes on iron and what happens when iron /carbon mixtures are cooled from liquid to solid. The notes are based on the Iron Phase Diagram (equilibrium diagram). A "Phase" is a form of material having characteristic structure and properties. It is a form of the material which has identifiable composition, structure and boundaries separating it from other phases in the material volume.
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