It is not uncommon for those people not familiar with heat treating processes to talk about the process of hardening and the process of tempering as if they are the same thing. They are very different methods to bring about changes in the physical nature of the steel that makes it better suited for specific types of uses and applications.
In terms of the processes, both hardening and tempering of steel make it stronger, but with different final results as to the other properties of the piece. Understanding why this is the best option requires first having a clear picture of the desired end characteristics for the part or component.
Hardening
A big difference in hardening and tempering of steel is the amount of heat and the following cooling process. With the hardening of steel, the goal is to make the steel more rigid and strong. The increased rigidity in the component will increase the fatigue life of the part, with a corresponding increase in brittleness.
Hardening is completed by heating the steel to a specific temperature, which is above the austenitizing temperature, and then quenching with oil, water, marquencing salt, or high-pressure gas to cool the part. The fast cooling converts the material into a martensite form.
Tempering
After hardening, and to reduce the brittleness of the part, it can then be tempered. The part must be at room temperature to start the process, which involves heating the metal to a lower level as determined by the specific steel. It is then allowed to cool slowly under very controlled conditions.
The result of the hardening and tempering of steel is the strength and rigidity required for the metal without the brittleness of simply hardening the steel. Tempering relieves the inner stress in the component, with makes it more ductile and stronger.