Heat treatment is a process in which metal particles are exposed to hot or cold temperatures to attain required chemical and physical properties apt for the application.
Effect of Heat Treatment:
Heat treatment manipulates the physical and mechanical properties of steel, but not its shape. It is not only used for toughening, but also for changing its formability and machinability. Heat treatment process specifies the use of heating to high temperatures and then cooling for the desired result. It is a critical manufacturing process to upgrade a product, its properties, and its performance. Not all steels react extensively to heat treatment but for a low alloy spring steel principal, heat treatment processes are annealing, hardening, and tempering.
Annealing involves heating to a high temperature (over 800C) and then allowing it to cool very slowly, either by ramping down the temperature in a furnace or by packing in a coating material. The annealing process lessens work pressure and internal stresses and makes the steel more flexible and more plastic. Heavy cold working demands average annealing stages to prevent cracking.
Hardening: During this heat treatment the steel is heated to very high temperature and quickly cooled by quenching( cooling steel rapidly to obtain desired properties) e.g. in oil. This makes the steel tough but fragile with high ductile strength but low hardness and high surplus stresses.
Tempering: Tempering is done after hardening and allows fine control of the balance between hardness and toughness. The higher the tempering temperature, the softer and tougher the steel will result in.
So basically heat treatment allows you to manipulate the material properties of given steel for a particular application.
The reason steel springs are tempered is to gain a blend of high ductile strength toughness and hardness.