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

Step 1: Understanding intergranular failure.
Intergranular failure in steels occurs when fracture propagates along grain boundaries rather than through the grains. This type of failure is often caused by the presence of impurities such as sulfur at grain boundaries, which weaken them.
Step 2: Role of sulfur in steels.
Sulfur is a harmful impurity in steel. It forms iron sulfide (FeS), which segregates at grain boundaries and leads to hot shortness and intergranular cracking.
Step 3: Effect of manganese addition.
Manganese reacts preferentially with sulfur to form manganese sulfide (MnS), which is less harmful and remains distributed within the grains instead of accumulating at grain boundaries.
Step 4: Conclusion.
Thus, the addition of manganese prevents intergranular failure by neutralizing the adverse effects of sulfur.

Answer 2

Intergranular failure in steel is driven by harmful impurities segregating at grain boundaries. The fix must work by neutralizing that specific harmful segregation. Let's check the role each element actually plays in the steel.

1. Sulfur: Sulfur is the impurity that causes the problem in the first place. It combines with iron to form iron sulfide (FeS), which has a low melting point and segregates as a thin film at grain boundaries, causing hot shortness and cracking along those boundaries during hot working.
2. Manganese: Manganese has a stronger affinity for sulfur than iron does. It reacts preferentially to form manganese sulfide (MnS), which has a much higher melting point than FeS and forms as small, rounded, well-dispersed inclusions within the grains rather than as a continuous film at the boundaries, removing the source of intergranular weakness.
3. Silicon: Silicon is added mainly as a deoxidizer during steelmaking, to remove dissolved oxygen and prevent gas porosity. It plays essentially no role in tying up sulfur or protecting grain boundaries.
4. Phosphorous: Phosphorous is itself a harmful impurity, contributing to cold shortness and temper embrittlement by segregating at grain boundaries, similar in effect to sulfur. It does not counteract intergranular failure, it can worsen it.

Because manganese is the element that actively converts the harmful grain-boundary sulfur film into harmless dispersed inclusions, it is the correct choice.

Therefore, the correct answer is Manganese.