Hardness of steel increases with the increase of _______________.

Answer

**Step-by-Step Solution:**

1. **Understanding Steel Composition**: Steel is an alloy primarily made of iron and carbon. The properties of steel, including its hardness, can be significantly affected by the amount of carbon present in it.

2. **Effect of Carbon on Hardness**: As the carbon content in steel increases, the hardness of the steel also increases. This is because carbon atoms fit into the iron lattice and create a stronger bond, making it more difficult for the layers of atoms to slide over each other.

3. **Carbon Content Range**: For hard steel, the carbon content typically ranges from 0.5% to 1.5%. Within this range, the hardness of steel is optimized, making it suitable for various applications where strength and durability are required.

4. **Consequences of Increased Hardness**: While increased hardness is beneficial for many applications, it can also make the steel more brittle, especially if it is subjected to heat treatment. This brittleness can lead to fractures under stress.

5. **Conclusion**: Therefore, the answer to the question is that the hardness of steel increases with the increase of **carbon content**.

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