Question

Nickel and manganese elements in steel 0.4pt the critical temperature.

• A. Raises
• B. Lowers
• C. Do not change
• D. Raises and lowers

Hint:

To easily remember the effect of alloying elements, categorize them: Austenite formers (e.g., Ni, Mn, C, N) lower critical temperatures, while Ferrite formers (e.g., Cr, Si, Mo, V) raise them.

Answer 1

The Correct Option is B

Step 1: Recall how alloying elements split into two camps.
Every alloying element in steel either prefers to dissolve more easily in the FCC austenite lattice, called an austenite stabilizer, or in the BCC ferrite lattice, called a ferrite stabilizer, and this preference directly shifts the transformation temperatures on the phase diagram.
Step 2: Identify which camp nickel and manganese belong to.
Both nickel and manganese are well known austenite stabilizers, they expand the temperature and composition range over which the FCC gamma phase is stable, essentially making it easier for the steel to stay austenitic even as temperature drops.
Step 3: Translate that into the effect on the critical temperature.
If austenite is being made stable over a wider, lower temperature range, then the temperature at which austenite must transform into ferrite plus cementite, the critical or eutectoid temperature, has to drop as well. In fact, nickel and manganese are exactly the elements used industrially to push austenite stability all the way down to room temperature in austenitic stainless steels.
Step 4: Conclusion.
Since both elements act to stabilize austenite, they pull the critical transformation temperature ($A_1$ / $A_3$) downward rather than upward.
\[ \boxed{\text{Lowers}} \]