Question

The intergranular failure of steels can be prevented by addition of

• A. Sulfur
• B. Manganese
• C. Silicon
• D. Phosphorous

Hint:

Manganese is added to steels mainly to counteract the harmful effects of sulfur by forming MnS.

Answer 1

The Correct Option is B

The key reaction that fixes this problem is manganese displacing iron from iron sulfide: \[ \text{FeS} + \text{Mn} \rightarrow \text{MnS} + \text{Fe} \] Manganese sulfide forms as small rounded inclusions inside the grains instead of a continuous film along the grain boundaries.

1. Sulfur: the cause of the problem, not the fix.
2. Manganese: removes the harmful FeS film by the reaction above, preventing hot shortness and intergranular cracking.
3. Silicon: only acts as a deoxidizer, unrelated to sulfur control.
4. Phosphorous: itself embrittles grain boundaries, it does not help.

So intergranular failure is prevented by adding manganese.

Answer 2

Comparing the melting points and physical form of the sulfide compounds involved explains why one element solves the problem and the others do not.

1. Sulfur: when uncombined with manganese, sulfur forms iron sulfide, which has a melting point well below the hot-working temperature of steel. This low-melting film liquefies during hot rolling or forging and tears apart along the grain boundaries, a phenomenon called hot shortness.
2. Manganese: manganese sulfide has a much higher melting point than iron sulfide and does not liquefy during hot working. Because it also forms as discrete rounded globules rather than a continuous grain-boundary film, it does not provide an easy crack path, directly solving the intergranular failure problem.
3. Silicon: silicon forms oxide compounds during deoxidation, not sulfides, so it does not affect the melting-point issue driving hot shortness at all.
4. Phosphorous: phosphorous segregates at grain boundaries in its own right and lowers the boundary cohesive strength directly, adding to embrittlement rather than removing it.

Because manganese sulfide's higher melting point and globular form eliminate the low-melting grain-boundary film responsible for cracking, manganese is the element that prevents intergranular failure.

Therefore, the correct answer is Manganese.