) The external surface of the part made of mild steel can be hardened by

The external surface of a part made of mild steel can be hardened by a process called case hardening or surface hardening. Here are the steps:

1. Cleaning: The first step is to clean the steel part thoroughly to remove any dirt, oil, or oxide layer. This can be done using a wire brush or a cleaning solution.

2. Heating: The next step is to heat the steel part. This is usually done in a furnace at a temperature of around 900 to 950 degrees Celsius. The part is heated until it is red hot.

3. Carburizing: The heated steel part is then subjected to a carburizing environment. This is usually a carbon-rich material like charcoal or carbon monoxide gas. The part is kept in this environment for a certain period of time. During this time, carbon atoms diffuse into the surface of the steel, increasing its carbon content.

4. Quenching: After carburizing, the steel part is quickly cooled or quenched in a liquid like water or oil. This rapid cooling causes the carbon atoms in the surface to lock into place, forming a hard, wear-resistant layer.

5. Tempering: The final step is tempering, which involves reheating the steel part to a lower temperature and then allowing it to cool slowly. This reduces the brittleness of the hardened surface and improves its toughness.

This process results in a part with a hard, wear-resistant surface (the case) and a tough, ductile core.