Steel is heated to below red heat and then, cooled slowly. This process refers to

Options

A. Hardening
B. Annealing
C. Tempering
D. Nitriding

Explanation

To solve the question regarding the process that occurs when steel is heated to below red heat and then cooled slowly, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Process**: The question describes a scenario where steel is heated to a temperature below red heat and then cooled slowly. It is important to identify what this heating and cooling process aims to achieve in terms of the properties of steel.

2. **Identify the Purpose of Heating and Cooling**: The main goal of heating steel to below red heat and then cooling it slowly is to alter its physical properties. This process typically aims to increase ductility and reduce hardness.

3. **Define the Process**: The process of heating steel to a specific temperature and then allowing it to cool slowly is known as **annealing**. This is a heat treatment process used to relieve internal stresses, soften the material, and improve its ductility.

4. **Differentiate from Other Processes**:
- **Hardening** involves heating steel to a high temperature and then quenching it rapidly, which increases hardness but reduces ductility.
- **Tempering** is done after hardening to improve toughness by reheating the material to a lower temperature.
- **Nitriding** involves heating the metal and diffusing nitrogen into its surface to enhance hardness.

5. **Conclusion**: Based on the definitions and purposes of the processes discussed, the correct answer to the question is that the process referred to is **annealing**.

### Final Answer:
The process referred to when steel is heated to below red heat and then cooled slowly is called **annealing**.

---