ANS:C - brittle & hard.

Sudden immersion or dipping of a red-hot steel bar in water results in it becoming brittle and hard. Explanation:

1. Quenching Process:
   • The process of quenching involves rapidly cooling a heated metal object, such as a steel bar, by immersing it in a quenching medium, typically water, oil, or brine.
   • Quenching is commonly used in metallurgy to alter the mechanical properties of metals, particularly steel.
2. Effect on Steel:
   • When a red-hot steel bar is immersed in water during quenching, the rapid cooling rate causes the formation of a specific microstructure known as martensite.
   • Martensite is a hard and brittle phase that forms in steel when it is quenched from high temperatures.
   • The sudden cooling prevents the normal transformation of austenite (the high-temperature phase of steel) into softer, more ductile phases such as pearlite or bainite.
3. Resulting Properties:
   • The steel bar becomes hard due to the formation of martensite, which increases its hardness significantly.
   • However, martensite is also brittle, meaning the steel becomes prone to fracture or breaking under stress.
   • This brittleness makes the steel less malleable and less tough compared to its pre-quenched state.
4. Industrial Applications:
   • Quenching is commonly used in the heat treatment of steel to achieve specific mechanical properties, such as increased hardness and wear resistance.
   • However, the resulting brittleness may need to be tempered by subsequent heat treatment processes to improve toughness and reduce the risk of fracture.