Heat treatment of steel is done mainly to change its

ANS:A - mechanical properties.

Mechanical properties refer to the characteristics of materials that describe their behavior under the action of external forces. These properties are crucial for understanding how materials will perform in various applications and environments. Some of the key mechanical properties include:

1. Strength: Strength is the ability of a material to withstand an applied force without failure or deformation. It is typically characterized by several parameters:
   • Tensile Strength: The maximum stress a material can withstand without fracturing under tensile loading.
   • Compressive Strength: The maximum stress a material can withstand without failure under compressive loading.
   • Yield Strength: The stress at which a material begins to deform plastically.
   • Ultimate Strength: The maximum stress a material can withstand before failure.
2. Hardness: Hardness is the resistance of a material to deformation, indentation, or scratching. It indicates the material's ability to withstand surface wear and abrasion.
3. Ductility: Ductility is the ability of a material to undergo plastic deformation before fracture. Ductile materials can be elongated or stretched under tensile stress.
4. Brittleness: Brittleness is the tendency of a material to fracture or break without significant plastic deformation. Brittle materials fail suddenly without warning under stress.
5. Toughness: Toughness is the ability of a material to absorb energy and deform plastically before fracture. It combines both strength and ductility and indicates the material's ability to resist fracture under impact loading.
6. Stiffness: Stiffness, also known as modulus of elasticity, describes a material's resistance to deformation in response to an applied force. It reflects the material's ability to return to its original shape after deformation.
7. Fatigue Resistance: Fatigue resistance is the ability of a material to resist failure under cyclic loading or repeated stress over time. It is crucial for components subjected to fluctuating loads.
8. Creep Resistance: Creep resistance is the ability of a material to withstand prolonged exposure to high temperatures and constant stress without excessive deformation.

These mechanical properties are determined through various testing methods, such as tensile testing, hardness testing, impact testing, and fatigue testing. Understanding the mechanical properties of materials is essential for material selection, design, and engineering applications across different industries.