Explanation:

Hardness and tensile strength in austenitic stainless steel can be increased by:

- 4. Cold working:
- Cold working, also known as cold forming or cold rolling, is a process in which the stainless steel is deformed below its recrystallization temperature.
- This process increases the dislocation density and introduces defects in the crystal structure, resulting in an increase in the hardness and tensile strength of the material.
- Cold working involves processes such as cold rolling, cold forging, cold extrusion, and cold drawing.
- These processes impose plastic deformation on the material, causing it to undergo strain hardening, which increases its strength and hardness.

Reason for other options being incorrect:

- 1. Normalizing:
- Normalizing is a heat treatment process in which the stainless steel is heated to a temperature above its critical transformation temperature and then cooled in still air.
- This process is used to refine the grain structure and relieve internal stresses in the material, but it does not significantly increase the hardness or tensile strength of austenitic stainless steel.

- 2. Hardening:
- Hardening is a heat treatment process in which the stainless steel is heated to a temperature above its critical transformation temperature and then rapidly cooled, usually by quenching in oil or water.
- This process produces a martensitic structure in the material, which is harder and stronger than the original austenitic structure. However, austenitic stainless steel is not easily hardenable by conventional hardening processes.

- 3. Martempering:
- Martempering is a heat treatment process in which the stainless steel is heated to a temperature above its critical transformation temperature and then quenched in a bath at a temperature slightly above its martensite start temperature.
- This process produces a structure consisting of tempered martensite, which is harder and stronger than the original austenitic structure. However, austenitic stainless steel is not easily martempered and does not exhibit significant hardness or tensile strength improvement through this process.

Therefore, the correct answer is option 'D' - 4 alone. Cold working is the most effective method for increasing the hardness and tensile strength of austenitic stainless steel.