What Does the Crystal Structure Look Like?
As the iron is heated up to above the lower transformation temperature of 1,350xF the crystal structure transforms to a face-centered cubic structure. Therefore, we can say that in order to make the phase of austenite, we need to apply sufficient heat to create the phase. In addition to this, there is a growth that will occur due to the atomic structural change. Therefore, the size of the mold will change as it is heated. This is called growth and should not be confused with distortion. So by definition, the lower transformation temperature (or the magnetic change line) is the temperature that the ferrite phase (a body-centered cubic structure) begins to change to austenite (a face-centered cubic structure). What Then Happens to Steel? Steel is simply an alloy of iron and carbon. If we look at Figure 2, it can be seen that the diagram only addresses iron. We are now considering steel, therefore a horizontal carbon line is now drawn and shown in Figure 3.
