Question

Minimum percent of nickel needed in stainless steel to stabilize the austenitic FCC phase is:

• A. 2
• B. 4
• C. 9
• D. 16

Hint:

At least \textbf{9% Ni} is required to retain the \textbf{austenitic FCC phase} in stainless steel such as AISI 304.

Answer 1

The Correct Option is C

Step 1: Austenitic Stainless Steel
- Austenitic stainless steels have a face-centered cubic (FCC) crystal structure at room temperature. - Nickel is a strong austenite stabilizer, meaning it retains the FCC phase even after cooling. Step 2: Nickel Requirement
- For a steel to be classified as fully austenitic, at least 8–10% nickel is typically needed. - 9% is widely accepted as the minimum content to ensure stabilization of the FCC phase, particularly in grades like AISI 304. Step 3: Role of Nickel
- Nickel lowers the martensite start temperature, - Prevents ferrite and martensite formation during cooling, - Enhances corrosion resistance and ductility. Conclusion: To stabilize the austenitic (FCC) phase in stainless steel, a minimum of 9% nickel is required.