Hardening of Steel
Plain carbon steel is hardened by heating it above the critical temperature and cooling it rapidly by plunging it into water, iced brine, or other liquid. When heating through the critical temperature range, iron undergoes a transformation and changes from a form with low carbon solubility to one with high carbon solubility. Upon cooling, a reverse transformation occurs. Since these changes are progressive and require time for completion, they may be stopped if the cooling period is shortened.
If the cooling is very rapid, as in water quenching, the transformation takes place much below the critical temperature range. The carbon is fixed tied in a highly stressed, finely divided state, and the steel becomes hard, brittle, and much stronger than steel that is slowly cooled.
The presence of alloying elements alters the rate of transformation on cooling. Each alloy element shows individuality in its effect; therefore, alloy steels are manufactured and heat treated to meet specific performance requirements.
Tempering is usually associated with the ***hardening of steel ***because hardening leaves the steel very brittle and susceptible to breaking during use. Tempering reduces the hardness of the forged steel very slightly, but improves the overall product, as it results in steel that is much less brittle. Once the hardening of steel has completed, it is ready for the second phase, the tempering, which involves re-heating the steel which received the most intense heat in the hardening process. The metal is re-heated until it turns the indicative blue color, which means tempering has occurred and the heat source is cut off. The newly hardened and tempered steel to is left to slowly cool on its own