Kinetics of the Phase Formation in the Fe - C System

	Below the important part of the Fe - C phase diagram is shown. Three lines indicate hypoeutectic, eutectic and hyereutectic compositions.
		If we look at what we will find at the indicated points, this is what we would expect:
	In all three cases we start with some polycrystalline g- structure: It is drawn identically in each cases to point out that the basic phenomena are the same (and to save work).
	For the blue hypoeutectic case, we start to grow some a phase as soon as we cross the boundary between the pure g region and the a + g mixed phase.
		It is pretty safe to assume that nucleation of the a grains starts at the grain boundaries, in particular at the nodes.
		At the eutectic temperature, the a grains have their maximal size. As soon as we cross over to the a + Fe3C eutectic, the pure g grains must now transform to a + Fe3C, i.e. to perlite.
	For the red hypereutectic case, pretty much the same thing happens
		We start to grow some Fe3C, i.e. cementite, as soon as we cross the boundary between the pure g region and the a + Fe3C mixed phase.
		It is pretty safe to assume that nucleation of the Fe3C grains starts at the grain boundaries, but it also sage to assume that it will be somehow different from the way the a grains did it. This is indicate schematically; we simply have growth along the boundaries
		How nucleation and growth takes place in reality, however, cannot be told from the phase diagram alone; we need some additional information
		At the eutectic temperature, the cementite grains have their maximal size. As soon as we cross over to the a + Fe3C eutectic, the pure g grains must now transform to a + Fe3C, i.e. to perlite.
	It only remains to discuss what happens if we have the precise eutectoid composition; this is shown in green.
		It is simple enough. below the eutectoid temperature, the the pure g grains transform to a + Fe3C, i.e. to (pure) perlite.
		However, this does not happen everywhere all at once, again, some nucleation and growth takes place as indicates.
	One important point becomes clear, no matter exactly what happens: The original grain structure of the g phase e very much what kind of grain structure we will have in the room temperature structure.

Zum Index

8.4.1 Plain Carbon Steels

© H. Föll