**Heat treatment of steel involves heating steel to a temperature above the melting point and then cooling.**

Heat treatment is a process used to alter the physical and mechanical properties of a material, such as steel, by subjecting it to heating and cooling cycles. This treatment can be applied to steel to achieve various desired properties, such as improved hardness, toughness, strength, and ductility.

There are several methods of heat treatment, including annealing, quenching, tempering, and normalizing. Each method involves different temperature ranges and cooling rates, resulting in different material properties.

**Explanation:**

**1. Annealing:**
- Annealing is a heat treatment process used to soften the steel and improve its machinability.
- It involves heating the steel to a temperature above its critical temperature (the temperature at which the steel becomes fully austenitic) and then cooling it slowly.
- This process helps reduce internal stresses and strains in the steel, making it more ductile and less brittle.
- Annealing can be used to change the carbon percentage of steel by allowing carbon atoms to diffuse and redistribute within the material.

**2. Quenching:**
- Quenching is a heat treatment process that involves heating the steel to a temperature above its critical temperature and then rapidly cooling it by immersing it in a quenching medium, such as oil or water.
- This rapid cooling rate results in the formation of a hard and brittle microstructure known as martensite.
- Quenching is often used to increase the hardness and strength of steel.
- However, it does not directly affect the carbon percentage of the steel.

**3. Tempering:**
- Tempering is a heat treatment process that follows quenching.
- It involves reheating the quenched steel to a temperature below its critical temperature and then cooling it slowly.
- This process helps relieve internal stresses and strains induced by quenching and improves the toughness and ductility of the steel.
- Tempering can be used to change the carbon percentage of steel by allowing some carbon atoms to diffuse out of the material.

**4. Normalizing:**
- Normalizing is a heat treatment process similar to annealing, but with a faster cooling rate.
- It involves heating the steel to a temperature above its critical temperature and then cooling it in still air.
- Normalizing helps refine the grain structure of the steel, making it more uniform and improving its mechanical properties.
- Normalizing does not directly affect the carbon percentage of the steel.

Based on the explanations above, it is clear that heat treatment of steel involves heating the steel to a temperature above the melting point (the critical temperature) and then cooling it. This process can be used to achieve various desired properties in the steel, but it does not directly change the carbon percentage. Therefore, option B is the correct answer.