Question

Season-cracking of brass can be prevented by cold-working at

• A. 15$^\circ$C
• B. 100$^\circ$C
• C. 300$^\circ$C
• D. 550$^\circ$C

Hint:

Season cracking in brass is mainly prevented by removing residual stresses through stress-relief heating at about 300$^\circ$C.

Answer 1

The Correct Option is C

Step 1: Understanding season cracking.
Season cracking is a type of stress corrosion cracking observed mainly in brass alloys. It occurs due to the combined effect of residual tensile stresses and a corrosive environment such as ammonia.
Step 2: Role of residual stresses.
Residual stresses are introduced during cold working processes like rolling or drawing. These stresses make brass susceptible to cracking over time.
Step 3: Prevention method.
Season cracking can be prevented by stress-relief treatment. Heating brass to about 300$^\circ$C removes residual stresses without significantly altering its mechanical properties.
Step 4: Conclusion.
Cold working followed by stress relieving at around 300$^\circ$C effectively prevents season cracking in brass.

Answer 2

Season cracking (stress-corrosion cracking) in brass happens when residual tensile stresses from cold working combine with a corrosive atmosphere, typically traces of ammonia. Preventing it means removing those residual stresses without undoing the cold work's shape or ruining the alloy's properties. Let's check each temperature option against that requirement.

1. \( 15^\circ\text{C} \): This is close to room temperature. At such a low temperature, atoms in the brass lattice have essentially no extra thermal energy to rearrange and relax internal stresses, so residual stress remains fully locked in and season cracking is not prevented.
2. \( 100^\circ\text{C} \): This is still far too low for meaningful stress relief in brass. Some very minor stress relaxation can occur, but nowhere near enough to eliminate the residual tensile stress responsible for cracking.
3. \( 300^\circ\text{C} \): This falls within the stress-relief annealing range for brass. At this temperature, enough thermal energy is available for dislocations and grain boundaries to rearrange and reduce the internal residual stress, while the temperature stays low enough that grain growth and softening (which would spoil the cold-worked properties) are avoided.
4. \( 550^\circ\text{C} \): This is high enough to cause recrystallization and grain growth in brass, which removes the beneficial effects of cold working altogether, so it is not the correct low-temperature stress-relief choice.

Only the intermediate temperature of \( 300^\circ\text{C} \) relieves the residual stress without destroying the cold-worked structure.

Therefore, the correct answer is \( 300^\circ\text{C} \).