Heat treatment

Heat treatment introduction
normalizing: heat the steel to above the AC3 or ACCM 30-50 ° C, holding for some time, put out from the furnace , and cool in air or by water, spray,wind. The Intent of this Technology is to make the grain in steel refine and carbide distribute uniformly. hardenning / quenching heats up the steel to critical temperature Ac3 (hypo-eutectoid steel) or Ac1 (hyper-eutectoid steel) above some temperature, keeps warm for some period of time. causes it completely or the partial austenite 1,than cool the steel quickly at the speed which is faster than the speed of critical cooling.

The purpose of quenching is to transform the underheating austenite to martensite or bainite, obtains their organization, then coordinate with the different temperature's tempering,to largely enhances the steel's intensity, hardness, the wear resistance, fatigue as well as tough strength and so on. Tempering Heat the steel which was quenched at the temperature below AC1, than cool down after holding for period of time.
The tempering may divide into the tempering, the mid temperature tempering and the high-temperature tempering.
1.low-temperature tempering (250℃ below) The goal is to maintain quenched steel's high degree of hardness and the resistance to wear, reduces residual stress and brittleness caused by quenching. 
2.mid temperature tempering (250～500 ℃) the purpose of it is to obtains the high elasticity and the yield point, suitable toughness. Mechanical properties: 35~50HRC, 
3.high-temperature tempering. (above 500 degree) purpose of high-temperature tempering is to get the good comprehensive mechanical (200～350HBS) like ,intensity, plasticity and toughness. Annealing Heats up the metal slowly to certain temperature, hold warm for enough time, cooling down slowly. is suitable speed cooling. Annealing can reduce hardness degree, improve cutting workability; Eliminate residual stress, stay size, reduces possibility of distortion and crack ; refine grain, adjust organization, eliminates organization flaw. 
There are recrystallization annealing, isothermal annealing, homogenizing annealing, spheroidizing annealing. Normalizing & Tempering The cooling speed of normalizing maybe too quick, material possibly creates too high hardness degree and forms the stress, the tempering can reduces degree of hardness, increases the plasticity, the stress-relieving. quenching and high temperature tempering Adjusts the steel comprehensive mechanical properties