explain the Eutectic and Eutectoid reaction in cast irons
Eutectic and eutectoid reactions are important concepts in understanding the behavior of cast irons. These reactions describe the phase transformations that occur in the microstructure of cast iron alloys during cooling or heating.
1. Eutectic Reaction:
The eutectic reaction occurs when a liquid phase solidifies into two or more different solid phases simultaneously. In the case of cast irons, the eutectic reaction involves the transformation of the liquid phase into both ferrite and cementite.
To understand the eutectic reaction in cast irons, you can refer to a phase diagram specific to the composition of the alloy. The phase diagram will show the temperature-composition relationship for the iron-carbon system. Locate the eutectic point on the phase diagram, which represents the specific composition at which the eutectic reaction occurs.
During the cooling of a molten cast iron alloy, when the temperature reaches the eutectic point, the liquid phase transforms into two solid phases, ferrite and cementite. The microstructure consists of alternating lamellar layers of these two phases. This lamellar arrangement contributes to the unique mechanical properties of cast irons, such as their high strength and wear resistance.
2. Eutectoid Reaction:
The eutectoid reaction is a type of reaction that occurs when a solid phase transforms into two different solid phases at a specific temperature and composition. In cast irons, the eutectoid reaction involves the transformation of austenite into pearlite.
To understand the eutectoid reaction in cast irons, again, refer to the phase diagram. Locate the eutectoid point on the phase diagram, which represents the specific temperature and composition at which the eutectoid reaction occurs.
When a cast iron alloy is heated to the eutectoid temperature, the austenite phase, which is a high-temperature solid solution of carbon in iron, undergoes a eutectoid reaction to form pearlite. Pearlite is a lamellar microstructure consisting of alternating layers of ferrite and cementite. The formation of pearlite significantly affects the mechanical properties of cast irons, such as their hardness and strength.
To conclude, the eutectic and eutectoid reactions are fundamental to understanding the microstructure and properties of cast irons. By consulting the specific phase diagrams of cast iron alloys, you can determine the composition, temperature, and phases involved in these reactions.