1. 1.
FIRST PHASE: Ferrite (alpha – Fe)
From 0°C up to 910°C iron exists as Ferrite, which is pure iron, and has a body-centred cubic crystal
structure (BCC). All atoms composing Ferrite are located at the corners of a cube, the model of a structural
unit of Ferrite, and one is at the centre of its diagonals. There are millions of these cubic blocks in the BCC
crystal structure of Ferrite.
SECOND PHASE: Austenite (gamma – Fe)
Above 910°C iron mixed with carbon exists as Austenite with face-centred cubic crystal structure (FCC). One
atom is present at each corner of the cube adopted as a graphic representation for its structural unit.
THIRD PHASE: Cementite (Fe3C)
Three atoms of iron mixed with an atom of carbon and by virtue of their metallic bonding they form a
compound known as iron carbide or Cementite.
FORTH PHASE: Pearlite (alpha – Fe and Fe3C)
Ferrite and Cementite bound together constitute Pearlite, so-called because the layered structure of the
material is similar to that of the mother of pearl. Inhomogeneous layers of Pearlite cover iron and these
casting structures imply the segregation of the elements in steel. A structure composed of alternating dark
layers of Cementite with the white ones of Ferrite can be identified with a microscope.
2.
Pure iron is so soft to sustain structures. Carbon has to be dissolved in iron in order to make it stronger. At
room temperature up to 0.01 weight percentage of carbon (wt% c) can be dissolved in iron. Rapidly heating
iron from 0° to 727°C and mixing it with 0.022 wt% C in solution with iron, we obtain the first phase of the
iron-carbon phase diagram, defined as Ferrite, pure iron, which exists until 0.4 wt% C is dissolved in iron.
Decreasing the wt% C dissolved in iron but heating iron from 727°C to 910°, Austenite forms in the iron
with Ferrite. Slowly cooling it down, this solution reaches 727°C again and, reaching the value of 0.8 wt% C
dissolved in iron, Austenite forms. This exponentially decreasing curve that describes the process of cooling
down and the addition of wt% C in iron till 0.8% is defined by the curve A3. Until we reach 2.1wt% C
dissolved in iron, we can increase the temperature of the solution in a directly proportional way to the rate
of the increasing wt% C in iron. When the temperature is higher than 1147°C, Austenite becomes slightly
liquid. The straight and diagonal line representing the direct proportionality linking the increasing
temperature to the augmenting wt% C, is denominated Acm. We can diminish the wt% of carbon in iron,
but raise the temperature until it reaches 1490°C and then do the reversal. We can raise the wt% C
dissolved in iron and reduce the temperature until 1147°C again. Quickly cooling the solution down, so that
it becomes solid again, Cementite or iron carbide forms from 1.2 until 6.7 wt% C dissolved in iron. By virtue
of the cooling, a bigger quantity of Pearlite is constituted. While carbon is in solution, the excess of carbon
comes out of the solution and each atom of carbon mixes with three atoms of iron, forming Cementite. The
Ferrite and the Cementite constitute the Pearlite. The more carbon is added to iron, the more pearlite
forms, the tougher and the stronger the structure is. Whilst shaping any structure, the first process
happening is casting. Then, elements segregate in steel and the structure becomes inhomogeneous or
heterogeneous. Inhomogeneous layers of pearlite cover iron. Yet, casting structures cannot be sustained,

2. because they imply the segregation of elements in stell, which comes from the different cooling rates
within the components of the material. So, we have to get rid of casting structures as much as possible. In
order to obtain a homogenous structure we use the heat treatment. The success of the product is
guaranteed.
3. By virtue of the microstructure of the steels, the data analysed above reflect onto their macrostructures
modifying them. Ferrite, as pure iron, has a very weak structure. Adding more atoms of carbon in solution
the structure becomes all pearlited and is tougher than before. The aim of the heat treatment of iron,
dissolving some amounts of carbon in it, is improving the chemical and mechanical properties of the
material so that we can obtain a good and useful product. Pearlite is the strongest material and the
toughest one among the ones analysed, cementite is the hardest one and Ferrite is the most ductile and
the softest one.