Alloys of the Iron carbon system are of the most vital importance to the modern industry due to their extensive, versatile applications. The Iron carbon system provides the most prominent example of heat treatment and property alteration based on the transformation from one form to other on application of heat and eutectoid decomposition. Because of its outstanding importance, the heat treatment of the iron-carbon system has been studied in more details than most alloy system. Types of steels and cast iron depend upon the percentage of carbon present in the steel or cast iron. As per the carbon percentage, two types of steels are there.
Hypo-Eutectoid Steel. Steels that contain less than 0.87% carbon are known as hypo-eutectoid steels.
Hyper-Eutectoid Steel. Steels that contain more than 0.87% to 1.8% of carbon are known as hyper-eutectoid steels.
Iron is allotropic metal. It exits in more than one type of lattice structure depending upon temperature. It may be face Centered Cubic (FCC) or Body Centered Cubic (BCC).
At room temperature, iron known as Alpha iron (α-iron) is BCC in lattice arrangement and it is magnetic. At 768º C, α-iron BCC magnetic changes to non –magnetic i.e. there is no change in lattice arrangement but it becomes non-magnetic.
Above 910º C α-iron BCC non-magnetic changes to Gamma iron (γ iron) FCC lattice arrangement. Non-magnetic α-iron is stable only up to 910º C.
Upon heating to 1394 º C, the γ iron FCC changes to Delta iron (δ-iron) BCC. The lattice arrangement changes back from FCC to BCC. It is stable up to melting point 1538º C of pure iron.
Gamma iron (γ iron) is also known as Austenite. Most of the heat treatments of steel begin with the single-phase austenite structure.
Alpha iron (α-iron) is also known as ferrite. Alpha ferrite is the stable form of iron at temperatures below 910°C. This body centered cubic structure can hold only 0.02 % of carbon in solid solution.
As the metals are heated to their melting temperature, a thermometer will show a constant rise in temperature with a constant heat input. A point where the temperature remains constant (Also called Arrest point) for a period of time to change the internal structure (BCC, FCC and BCC) is known as critical point. There are three stages of critical points.
(a) Lower critical point
(b) Intermediate critical point
(c) Upper critical point
Lower Critical Point. It is the first arrest point while heating and the last arrest point while cooling.
Intermediate Critical Point. It is a point between the upper and lower critical points, sometimes referred to as the second critical point (for steels with less than 0.4% carbon). At this point steel becomes non-magnetic.
Upper Critical Point. It is the point at which the iron and the carbon merge completely into one another by dissolving the solute carbon in the solvent iron to form a solid solution (Austenite).
The various structures of iron and phase changes of iron in different temperatures can be seen in the image below. In this diagram, the carbon percentage in steel or iron is shown on X- axis,and the increase of temperature while heating is shown on Y-axis. As the temperature increases or decreases, the structure of the metal keeps changing with respect to the carbon percentage. The temperatures at which phase transformations take place are important for the heat treatment of the alloys. The important critical temperatures are A1, A3 and Acm. A1 is the temperature, at which pearlite changes to austenite. This transformation occurs at a fixed temperature of 723°C irrespective of the composition of the alloy. Line PK of the phase diagram is called the A1 line. This temperature is also known as lower critical temperature.
At 768º C, α-iron BCC magnetic changes to nonmagnetic. There is no change in lattice arrangement but it becomes nonmagnetic. This point is known as intermediate critical point.
A3 is the temperature, at which last traces of ferrite changes into austenite and the alloy becomes fully austenite. This temperature is shown by the line GS in the phase diagram. A3 temperature varies from 723°C to 910°C depending upon the carbon content of the alloy. This temperature is also known as upper critical temperature.
Acm (cm – a French word chauffage, which means heating per minute) is the temperature, at which last traces of cementite changes into austenite and the alloys become completely austenite. This temperature is shown by the line SE in the phase diagram and varies from 723°C to 1148°C depending upon the carbon content in the alloy. It is also known as upper critical temperature.