Mechanical Metallurgy

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.

Step 1: Understanding ductile fracture.
Ductile fracture is characterized by significant plastic deformation before failure. It usually involves the formation and growth of microvoids.
Step 2: Role of shear stress.
In ductile materials, failure typically occurs along planes of maximum shear stress. This leads to cup-and-cone type fracture surfaces.
Step 3: Analysis of other options.
Direct, compressive, and bending stresses may contribute to loading, but the actual fracture in ductile materials is governed by shear stress.
Step 4: Conclusion.
Therefore, ductile fracture occurs primarily due to shear stress.

Step 1: Understanding Griffith’s theory.
Griffith’s theory of fracture explains crack propagation based on energy balance. It assumes that materials fracture without plastic deformation.
Step 2: Assumptions of the theory.
The theory assumes elastic behavior up to fracture and neglects plastic deformation at the crack tip. These assumptions hold true only for brittle materials.
Step 3: Elimination of other options.
Ductile materials undergo significant plastic deformation before fracture, which violates the assumptions of Griffith’s theory. Hence, it is not applicable to ductile or all materials.
Step 4: Conclusion.
Griffith’s theory is applicable only to perfect brittle materials.

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 C removes residual stresses without significantly altering its mechanical properties.
Step 4: Conclusion.
Cold working followed by stress relieving at around 300 C effectively prevents season cracking in brass.

Step 1: Understanding the application of connecting rods and gear shafts.
Connecting rods and gear shafts are machine components that are subjected to repeated cyclic loads, torsion, and bending stresses during operation. Therefore, the material used must possess good strength, toughness, and wear resistance.
Step 2: Properties of medium carbon steels.
Medium carbon steels contain approximately 0.3%–0.6% carbon. This composition provides a good balance between strength and ductility. They can also be heat-treated to improve hardness and fatigue strength, making them suitable for dynamically loaded components.
Step 3: Eliminating other options.
(A) Low carbon steels: These have high ductility but low strength, making them unsuitable for high-stress components.
(C) High carbon steels: These are very hard and brittle, increasing the risk of fracture under cyclic loads.
(D) Stainless steels: These are mainly used for corrosion resistance and are not commonly selected due to higher cost.
Step 4: Final conclusion.
Medium carbon steels best satisfy the mechanical requirements of connecting rods and gear shafts.