Introduction and application of normalizing heat treatment process
Normalizing is a heat treatment process in which steel parts are heated to 30-50 ℃ above the critical temperature and cooled in static air after holding for appropriate time. The main purpose of normalizing is to refine the microstructure, improve the properties of steel and obtain the structure close to equilibrium state.
(1) As final heat treatment:
1) It can refine austenite grain and homogenize the structure.
2) By reducing the content of ferrite in hypoeutectoid steel, the content of pearlite is increased and refined, so as to improve the strength, hardness and toughness of steel.
3) For ordinary structural steel parts, normalizing can be used as the final heat treatment when the carbon content is 0.4% ~ 0.7% and the mechanical properties are not very high.
4) Normalizing can be used as the final heat treatment to improve the mechanical properties of plate, tube, strip and section steel.
(2) As a pre heat treatment:
1) In order to eliminate widmanstatten and banded structure and obtain fine and uniform microstructure, normalizing is often carried out before quenching or quenching and tempering treatment (quenching and high temperature tempering) for large section alloy structural steel parts.
2) For hypereutectoid steel, the amount of secondary carburizing can be reduced and the continuous network can not be formed, which can be used for spheroidizing annealing.
3) For heavy forgings and steel with large cross-section, normalizing can be carried out to prepare the microstructure for quenching.
(3)Improve cutting performance: the hardness of low carbon steel or low carbon alloy steel after annealing is too low, which is not convenient for cutting. Normalizing can increase its hardness and improve its cutting performance.
(4)The as cast microstructure of steel castings is improved and refined.
(5) For some large and heavy steel parts or steel parts with complex shape and sharp change in cross section, serious deformation or cracking will occur if quenching is adopted. Normalizing can be used instead of quenching on the premise of ensuring performance.
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