The Composition of Steel

Steel is an alloy of iron and carbon, with carbon being the primary alloying element. The amount of carbon in steel can greatly influence its properties, such as hardness, strength, and ductility. When the carbon content exceeds 0.8%, the steel is considered high carbon steel.

The Lower Critical Point

The lower critical point is a temperature at which a specific phase transformation occurs in steel during cooling. Below this temperature, the microstructure of steel changes, leading to different compositions and properties.

Slow Cooling of High Carbon Steel

When high carbon steel (containing more than 0.8% carbon) is cooled slowly below the lower critical point, it undergoes a transformation known as eutectoid transformation. During this transformation, the steel decomposes into two distinct phases: ferrite and cementite.

Ferrite
Ferrite is a solid solution of carbon in iron, where the carbon content is very low. It has a body-centered cubic crystal structure and is relatively soft and ductile. Ferrite is the purest form of iron and has a maximum carbon solubility of only 0.022%.

Cementite
Cementite, also known as iron carbide, is a compound of iron and carbon with a fixed composition of Fe3C. It has a very high carbon content (6.7%) and a distinct crystal structure. Cementite is extremely hard and brittle, making it a major contributor to the hardness and strength of high carbon steel.

Pearlite
Pearlite is a lamellar microstructure that consists of alternating layers of ferrite and cementite. It forms when high carbon steel is slowly cooled below the lower critical point. The transformation of austenite (a high-temperature phase of steel) into pearlite is a diffusion-controlled process. The presence of pearlite in steel imparts improved strength and hardness.

Conclusion
When high carbon steel with a carbon content greater than 0.8% is cooled slowly below the lower critical point, it undergoes the eutectoid transformation. This leads to the formation of a microstructure consisting of pearlite and cementite. Therefore, the correct answer to the question is option 'D' - high carbon steel cooled slowly below the lower critical point consists of pearlite and cementite.