Assertion : Nitriding is the process of heating steel in presence of `N_(2)` to form iron nitrides.
Reason : The surface of steel becomes hard after nitriding process.

Options

A. If both assetion and reason are CORRECT, and reason is the CORRECT explanation of the assertion
B. If both assertion and reason are CORRECT, but reason is NOT the CORRECT explanation of the assertion
C. If assertion is CORRECT but reason is INCORRECT
D. If assertion is INCORRECT but reason is CORRECT

Answer

To solve the given question, we will analyze both the assertion and the reason provided, and determine their correctness.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states: "Nitriding is the process of heating steel in presence of N2 to form iron nitrides."
- Nitriding is indeed a process that involves the diffusion of nitrogen into the surface of steel. However, the primary purpose of nitriding is to harden the surface of the steel, not necessarily to form iron nitrides. While iron nitrides can form during this process, the assertion is misleading as it implies that the formation of iron nitrides is the main objective.

2. **Understanding the Reason**:
- The reason states: "The surface of steel becomes hard after nitriding process."
- This statement is correct. The nitriding process does lead to the hardening of the steel surface due to the diffusion of nitrogen, which increases the hardness and wear resistance of the steel.

3. **Evaluating the Relationship**:
- The assertion is incorrect because it inaccurately describes the primary outcome of nitriding.
- The reason is correct as it accurately describes the effect of the nitriding process on the steel surface.
- Since the reason does not correctly explain the assertion (because the assertion itself is incorrect), we conclude that the assertion and reason are not in agreement.

4. **Final Conclusion**:
- Based on the analysis, the correct answer is that the assertion is incorrect, but the reason is correct. Therefore, the answer to the question is Option 4: "Assertion is incorrect, but Reason is correct."