Axles are made by heating rods of iron embedded in charcoal powder. The process is known as :

Options

A. tempering
B. annealing
C. nitriding
D. case hardening

Answer

To determine the process by which axles are made by heating rods of iron embedded in charcoal powder, we can analyze the options provided and the characteristics of each process.

### Step-by-Step Solution:

1. **Understanding the Process**: The question states that iron rods are heated while embedded in charcoal powder. This indicates a heat treatment process that alters the properties of the iron.

2. **Identifying the Options**: The options given are:
- A) Tempering
- B) Annealing
- C) Nitriding
- D) Case Hardening

3. **Evaluating Each Option**:
- **Tempering**: This process is used to improve the toughness of iron-based alloys and reduce brittleness. However, it does not involve embedding in charcoal powder, so this option is incorrect.

- **Annealing**: This is a heat treatment process that enhances ductility by allowing the metal to be worked into thin wires. Like tempering, it does not involve charcoal powder, making this option incorrect as well.

- **Nitriding**: This process involves the diffusion of nitrogen into the surface of the metal to increase hardness. It does not relate to heating iron rods in charcoal powder, so this option is also incorrect.

- **Case Hardening**: This is a heat treatment process where iron or steel is heated in the presence of carbon (from charcoal powder) to harden the surface while maintaining a softer core. This matches the description given in the question.

4. **Conclusion**: The correct answer is **Case Hardening**. This process is specifically used for manufacturing axles by heating iron rods embedded in charcoal powder.

### Final Answer:
The process is known as **Case Hardening**.