Question

Which alloying element is typically added to tool steels to achieve secondary hardening during tempering?

• A. Silicon
• B. Nickel
• C. Copper
• D. Molybdenum

Hint:

Secondary hardening is essential for high-speed tool steels (like the M-series containing Molybdenum) because it allows the tools to maintain their cutting edge and hardness even when they heat up to red-hot temperatures during high-speed machining.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the alloying element that is added to tool steels to promote the phenomenon of secondary hardening during the tempering process.

Step 2: Detailed Explanation:

• Mechanism of Secondary Hardening: When standard carbon steels are tempered, they continuously soften as temperature increases due to the coarsening of iron carbides (\( \text{Fe}_3\text{C} \)) and recovery of the dislocation structure.
However, in tool steels containing strong carbide-forming elements (like Molybdenum, Tungsten, Vanadium, and Chromium), a different behavior is observed at high tempering temperatures (\( 500^\circ\text{C} \text{ to } 600^\circ\text{C} \)).
- At these elevated temperatures, alloy elements like Molybdenum become mobile enough to diffuse through the lattice.
- They react with carbon to form extremely fine, highly stable, and coherent alloy carbides (such as \( \text{Mo}_2\text{C} \) or MC).
- These fine alloy carbides replace the coarser iron carbides, pinning dislocations and causing an increase in hardness. This phenomenon is known as secondary hardening (or red hardness).

• Role of Other Elements:
-

Silicon (Option A) retards tempering and prevents softening at low temperatures but does not form secondary carbides.
-

Nickel (Option B) and

Copper (Option C) are non-carbide-forming elements in steel and do not contribute to secondary hardening through carbide precipitation.


Step 3: Final Answer:
Hence, Molybdenum is typically added to tool steels to achieve secondary hardening, corresponding to Option (D).