In nitriding process of steel

Options

A. Steel is heated in an atmosphere of ammonia
B. Steel is made red hot and then cooled
C. Steel is made red hot and then plunged into oil for cooling
D. none of these

Explanation

### Step-by-Step Solution:

1. **Understanding Nitriding**:
- Nitriding is a heat treatment process used to harden the surface of steel by introducing nitrogen into the surface layer. This process enhances the hardness and wear resistance of the steel.

2. **Atmosphere Used in Nitriding**:
- The nitriding process involves heating steel in an atmosphere rich in ammonia (NH₃). The ammonia gas dissociates and allows nitrogen to diffuse into the surface of the steel.

3. **Evaluating the Options**:
- **Option 1**: "Steel is heated in an atmosphere of ammonia."
- This is correct because nitriding specifically requires ammonia to introduce nitrogen into the steel.
- **Option 2**: "Steel is made red hot then cooled."
- This is incorrect as it does not specify the use of ammonia, which is essential in the nitriding process.
- **Option 3**: "Steel is made red hot then plunged into oil for cooling."
- This describes quenching, not nitriding, so it is incorrect.
- **Option 4**: "Steel is heated in an atmosphere of ammonia."
- This is a repetition of Option 1 and is correct.

4. **Conclusion**:
- The correct answer is **Option 1**: "Steel is heated in an atmosphere of ammonia."