A modified iron-carbon diagram is shown in fig. The point A (1539oC) on the diagram is the melting point of pure iron. The point E shows the solubility limit of carbon in γ-iron at 1130o C (1.7 %). The iron-carbon alloys containing up to 1.7 % carbon are called steels and those containing over 1.7 % carbon are called cast irons. The iron-carbon alloys containing 4.3 % carbon are called eutectic cast irons, above 4.3 % carbon are termed as hyper-eutectic cast iron and those in the range of 1.7 to 4.3 % are called hypo-eutectic cast irons.
We have already discussed that the temperature point at which the change starts on heating is called the lower critical point and the temperature point where this change ends in heating is called the upper critical point. The range between these two critical points is knowns critical range. This temperature at which the change starts (i.e. lower critical point) is the same for all steels (i.e. 723oC), but the ending point of transformation (i.e. an upper critical point) varies according to the carbon content in the steel. It will be seen that for steel containing 0.8 % carbon (wholly pearlite), there is only one critical point.
- The steel which contains less than 0.8 % carbon is known as hypo-eutectoid steels which consist of ferrite and pearlite.
- The steel which contains 0.8 % carbon is known as eutectoid steels which consist entirely of pearlite.
- The steel which contains above 0.8 % carbon is known as hyper-eutectoid steels which consist of cementite and pearlite.
- Cementite consists of 93.33 % iron and 6.67 % carbon.
- pearlite consists of 87 % ferrite and 13 % cementite.