Hardness of Cementite

Cementite is a compound that is a major component of steel and cast iron. It is a hard and brittle material with a high level of hardness. The hardness of cementite is measured on the Brinell hardness scale, which measures the resistance of a material to indentation.

The correct answer to the given question is option D, which states that the hardness of cementite is of the order of 550 BHNa.

Explanation:

Brinell hardness is measured by pressing a hardened steel ball or a carbide ball into the surface of a material and measuring the diameter of the indentation left by the ball. The applied load and the diameter of the ball are used to calculate the Brinell hardness number (BHNa).

Cementite, also known as iron carbide (Fe3C), has a unique crystal structure that contributes to its high hardness. It is composed of iron and carbon atoms arranged in a specific lattice pattern. The strong bonding between the iron and carbon atoms makes cementite a hard and brittle material.

Cementite has a high hardness due to the following factors:

1. Carbon Content: Cementite has a high carbon content, typically around 6.7%. The presence of carbon atoms in the lattice structure increases the hardness of the material.

2. Crystal Structure: Cementite has a complex crystal structure known as orthorhombic. This crystal structure provides a strong and rigid arrangement of atoms, contributing to the hardness of the material.

3. Interstitial Carbon: Cementite contains interstitial carbon atoms, which are carbon atoms located between the iron atoms in the crystal lattice. These interstitial carbon atoms further strengthen the material, increasing its hardness.

The hardness of cementite is significantly higher than that of pure iron. It is one of the hardest constituents in steel and cast iron. The hardness of cementite is typically around 550 BHNa, indicating its high resistance to indentation.

In summary, the hardness of cementite is of the order of 550 BHNa. This is due to its high carbon content, complex crystal structure, and the presence of interstitial carbon atoms. Cementite plays a crucial role in determining the physical and mechanical properties of steel and cast iron, making it an important component in various industrial applications.