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The eutectoid of carbon in iron, above lower critical temperature, when cooled,results in:
- a)Ferrite and austenite
- b)Ferrite and cementite
- c)Cementite and austenite
- d)Ferrite, cementite and austenite
Correct answer is option 'B'. Can you explain this answer?
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The eutectoid of carbon in iron refers to the phase transformation that occurs in iron when it is cooled below a certain temperature known as the eutectoid temperature. At this temperature, the carbon in the iron forms a new phase called cementite (Fe3C).
Explanation:
When iron is heated above its lower critical temperature, it exists in a phase called austenite. Austenite is a solid solution of carbon in iron, and it has a face-centered cubic crystal structure. As the iron is cooled, the carbon atoms start to come out of solid solution and form a new phase known as cementite.
Cementite is a compound that consists of iron and carbon in a specific ratio of 6.7% carbon by weight. It has a complex crystal structure and is extremely hard and brittle.
During the phase transformation from austenite to cementite, the carbon atoms diffuse through the iron matrix and combine to form cementite particles. These particles grow and coalesce, and eventually, the entire austenite phase is transformed into a mixture of ferrite and cementite.
Ferrite is another phase of iron that has a body-centered cubic crystal structure. It is a relatively soft and ductile phase, and it contains very little carbon. The presence of cementite particles in the ferrite matrix gives rise to a composite microstructure that is commonly known as pearlite.
Pearlite is a lamellar structure consisting of alternating layers of ferrite and cementite. It is a relatively soft and ductile phase, and it is commonly found in steels with a eutectoid composition.
Therefore, the correct answer is option B: Ferrite and cementite.
Explanation:
When iron is heated above its lower critical temperature, it exists in a phase called austenite. Austenite is a solid solution of carbon in iron, and it has a face-centered cubic crystal structure. As the iron is cooled, the carbon atoms start to come out of solid solution and form a new phase known as cementite.
Cementite is a compound that consists of iron and carbon in a specific ratio of 6.7% carbon by weight. It has a complex crystal structure and is extremely hard and brittle.
During the phase transformation from austenite to cementite, the carbon atoms diffuse through the iron matrix and combine to form cementite particles. These particles grow and coalesce, and eventually, the entire austenite phase is transformed into a mixture of ferrite and cementite.
Ferrite is another phase of iron that has a body-centered cubic crystal structure. It is a relatively soft and ductile phase, and it contains very little carbon. The presence of cementite particles in the ferrite matrix gives rise to a composite microstructure that is commonly known as pearlite.
Pearlite is a lamellar structure consisting of alternating layers of ferrite and cementite. It is a relatively soft and ductile phase, and it is commonly found in steels with a eutectoid composition.
Therefore, the correct answer is option B: Ferrite and cementite.
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