This transformation arises in steel having carbon upto 0.55 percent. This transformation occurs at point P and is characterized by transformation of solid and liquid phase into a single solid phase. For instance, the transformation occurring at P is correspond to by the following equation,
Liquid (0.5% C) + d iron (0.08% C) → iron (0.18% C)
Explanation of Reading Iron-Carbon Phase illustration
Suppose that in liquid state mixture of iron and carbon contains 0.4 percent carbon and is illustrated by point 1. The line xx shows the path of cooling.
As cooling begin the freezing start in intersection point 2 of xx and liquid ABC at 1510oC. At a temperature of 1470oC at point 3 the liquid is solidified totally in solid austenite. Such phase gradually cools till point 4 on line IK is reached. The proeutectoid ferrite precipitation commences and the carbon content of austenite varies along line A3. The composition of ferrite varies beside the line IL. In the region enclosed between IK and IL the percentage of ferrite and austenite can be found by utilizing lever rule. The remainig austenite transforms into peralite 88 percent ferrite and 12 percent cementite at eutectoid temperature of 723oC. The percent weight of as ferrite +cementite can be calculated by lever rule along line LKM. Additionally cooling will affect no modify in microstructure as carbon percentage in ferrite practically keeps constant of 0.008. The microstructure is built via pearlite matrix embedded along with ferrite crystals.
Taking example of alloys along with 3 percent carbon and knowing its cooling from a point above liquids along yy. The composition naturally is in the region of cast iron. The austenite divides from liquid and its content will rise along the solidus PG and the liquid amount reduces along with composition varying along liquidus BC.
While the temperature of 1130oC is reached, the mixture will comprises of austenite containing 2 percent carbon and liquid of eutectic composition.
The liquid will solidify at stable temperature into ledeburite - a combination of austenite and cementite. On additionally cooling eutectic austenite molders to cementite beside the line GK. On temperature of 723oC as eutectoid. The kept austenite will convert to pearlite. Below the temperature of 723oC all ledeburite will be changed into a mixture of pearlite and cementite.