Question

Solidification cracking occurs due to

• A. cold working
• B. low temperature
• C. grain growth
• D. segregation and tensile stress

Hint:

Remember:
- Hot cracking (Solidification cracking) occurs at high temperatures, is intergranular, and is caused by segregation + tensile stresses.
- Cold cracking occurs at low temperatures (typically below 150\(^{\circ}\)C), is transgranular, and is caused by hydrogen embrittlement + martensitic microstructure + tensile stresses.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks about the primary metallurgical and mechanical causes of solidification cracking (also known as hot cracking or hot tearing) during the solidification of weldments or castings.

Step 2: Key Formula or Approach:
Solidification cracking is a high-temperature defect occurring near the solidus temperature of an alloy.
It requires the simultaneous presence of two critical factors:
1. A metallurgical factor: the presence of low-melting-point liquid films along grain boundaries due to solute segregation.
2. A mechanical factor: tensile stress or strain acting on the solidifying structure due to thermal contraction and shrinkage.

Step 3: Detailed Explanation:

• Metallurgical Segregation: As an alloy solidifies, impurities and solute elements with low partition coefficients (like sulfur and phosphorus in steel) are rejected by the solidifying dendrites into the remaining liquid phase.
- This results in highly segregated, low-melting-point eutectic liquid films at the grain boundaries at temperatures well below the nominal bulk solidus.

• Mechanical Contraction: Concurrently, the solidifying metal undergoes thermal contraction and solidification shrinkage, generating high tensile stresses across the joint or mold.
- Since liquid has zero shear strength, the boundary films cannot support these tensile forces.
- The tensile strain pulls the weakly bonded grains apart, leaving open intergranular cracks.

• Prevention Methods:
- Control impurity elements (e.g., maintain low sulfur and phosphorus levels).
- Use alloying additions (like Manganese in steel to form high-melting-point MnS instead of low-melting-point FeS).
- Minimize joint restraint to reduce tensile stress during cooling.


Step 4: Final Answer:
Solidification cracking is driven by the synergistic action of liquid film segregation at grain boundaries and mechanical tensile stresses during cooling.
Therefore, the correct choice is option (D).