It is rapid cooling at various rates. This is cheved by immersing the heated metal from above critical temperature into cold water or any type of other cooling medium. The quenching results in high stiffness or hardness. Steels enhance high hardness by retaining iron carbide that forms at critical temperature and does not determine time to decompose as the cooling happens very fast. Cooling media such as like ice water, oil, cool water, hot oil, molten lead and molten salts are used depending upon desired cooling rate. Such result of quenching is increased strength and hardness and reduced ductility while not proper quenching may result in surface cracks.