In slowly cooled hypoeutectoid steels, one normally finds a microstructure in which the pearlite colonies are surrounded by ferrite grains. Explain.

Question

What will be an ideal response?

Answer 1

Solution:
In the previous example, a hypoeutectoid steel containing 0.31 percent carbon was considered. This can serve as an example. At 850°C, its microstructure was composed of only austenite
crystals containing 0.31 percent carbon in solid solution. On cooling from this temperature, the alloy will enter a two phase y plus ? field. In other words, ? alpha crystals will begin to precipitate out of the y crystals. This precipitation occurs along the original austenitic grain boundaries. There are several features to this process. As the temperature continues to drop, the ferrite crystals grow and from a continuous band along the original austenite boundaries with the result that the remaining austenite crystal are surrounded by ferrite crystals. Furthermore, because the ferrite is very low in its carbon concentration, the carbon concentration in the austenite increases. When the temperature reaches that of the eutectoid, the austenite will contain 0.77 percent carbon, and transforms into pearlite if the steel is lowered through the eutectoid temperature. The final result is a microstructure consisting of isolated pearlite colonies surrounded by ferrite crystals.