Chemistry

Tempering :
The hardness and elasticity of steel can be controlled by heat treatment. The steel is heated to a temperature below redness. It is then cooled slowly. The process is called tempering of steel. It is used to bring the steel to a suitable state of hardness and elasticity.
Annealing of steel :
Hard steel can be softened by heating it to a high temperature and then allowing it to cool down slowly. This process is called annealing.
Quenching of steel :
Hard steel is heated to a high temperature. It is then suddenly cooled by plunging into oil or water. Steel becomes as hard and brittle as glass. Steel produced in this way is known as quenched steel and the process of making such steel is known as quenching or hardening of steel.
Properties of Iron
Physical properties :
Pure iron has a grey colour. It is malleable and ductile. It is a good conductor of heat and electricity. It melts at 1808 K and boils at 3023 K. It has a density of 7.9 × 10³ kg cm ^–3 होता है।
Chemical properties :
1. Valency : Iron shows variable valency: 2 and 3. It forms divalent ion (Fe²⁺) as well as trivalent ion (Fe₃₊) The compounds in which iron shows divalency are known as ferrous compounds, whereas the compounds in which iron shows trivalency are known as ferric compounds. For example, in FeCl₂ the valency of iron is 2. So, it is called ferrous chloride. In FeCl₃, the valency of iron is 3. Hence, it is called ferric chloride.
2. Action of air : In the presence of moist air and carbon dioxide, iron gets covered with a thin deposit of rust. The rust consists of hydrated ferric oxide (2Fe₂O₃ 3H₂O) होती हैं।
3. Action of water : Red hot iron decomposes steam, forming ferrosoferric oxide and evolving hydrogen gas.