Q1: Fatigue limit of a material should not be increased by

ANS:B - surface decarburisation

Surface decarburization is a phenomenon that occurs when the carbon content of the surface layer of a metal is reduced due to exposure to elevated temperatures in oxidizing environments. Here's an explanation of surface decarburization:

1. Process: Surface decarburization typically occurs during heating processes where metal components are exposed to high temperatures in the presence of oxygen. Common processes that can lead to surface decarburization include:
   • Annealing: Heating metal components to high temperatures for the purpose of softening or stress relief.
   • Forging: Heating metal components to facilitate shaping or forming.
   • Welding: Heating metal components during welding operations.
   • Heat Treatment: Heating metal components for processes such as hardening, tempering, or annealing.
2. Mechanism: During exposure to elevated temperatures in oxidizing environments, such as air or furnace atmospheres, the carbon atoms at the surface of the metal react with oxygen to form carbon monoxide (CO) or carbon dioxide (CO2) gas. This reaction causes the carbon atoms to diffuse out of the metal surface, leading to a reduction in the carbon content of the surface layer.
3. Effects:
   • Reduction in Carbon Content: Surface decarburization results in a decrease in the carbon content of the affected metal surface layer. This reduction in carbon content can lead to a corresponding decrease in hardness, strength, and wear resistance of the material.
   • Changes in Microstructure: Surface decarburization can alter the microstructure of the metal surface layer, affecting properties such as grain size, phase composition, and distribution of alloying elements.
   • Surface Quality: Decarburized surfaces may exhibit discoloration, roughness, or surface irregularities, depending on the severity of the decarburization process.
4. Prevention:
   • Protective Atmospheres: Performing heat treatment or processing in controlled atmospheres, such as vacuum or inert gas environments, can prevent surface decarburization by minimizing the availability of oxygen.
   • Surface Coatings: Applying protective coatings or surface treatments to metal components can help prevent exposure to oxidizing environments and inhibit surface decarburization.
   • Optimized Processing Conditions: Adjusting processing parameters, such as temperature, heating rate, and duration, can help minimize the occurrence of surface decarburization during heat treatment or processing operations.

In summary, surface decarburization is a surface degradation phenomenon that occurs when the carbon content of the metal surface layer is reduced due to exposure to elevated temperatures in oxidizing environments. It can have detrimental effects on the mechanical properties and surface quality of metal components, highlighting the importance of prevention and control measures during heat treatment and processing operations.