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Definition and Function of Tempering After Normalizing

The purpose of tempering after normalizing is generally mainly for large cross-section parts. It has the following functions, which are very important.

For parts such as motor shafts and press containers (the material is generally carbon steel or low-alloy structural steel), normalizing + high temperature tempering is the final heat treatment, so the timely tempering treatment after normalizing can effectively stabilize The organization and size can further eliminate the internal stress of the workpiece and have a significant effect on improving the comprehensive mechanical properties.

For large forgings, high temperature tempering in time after normalizing (generally adding a subcooling in the middle) can not only achieve the above-mentioned purpose, but also has an important role of hydrogen expansion, especially for high hardenability steels. , (Hydrogen is the factor that produces white spots), and large forgings generally have to be scrapped if they have white spots inside. So important parts (or special materials) normalizing + high temperature tempering is very necessary.

Normalizing generally does not perform tempering. When materials with good hardenability or mist-cooled normalizing are used, high-temperature tempering is required, because materials with good hardenability are prone to appear mixed crystal structure during normalization, and mist-cooling will cause certain Therefore, tempering is necessary, which not only eliminates the stress, but also improves the overall performance of the material.

Normalizing is mainly to refine grains and eliminate network carbides, and tempering is to eliminate stress.

Normalizing is a metal heat treatment process that heats the material to a temperature below Ac3 and cools it in the air after heat preservation. The difference between normalizing and annealing is that the normalizing cooling rate is slightly faster than the annealing cooling rate, so the normalizing structure is finer than the annealing structure, and its mechanical properties are also improved. In addition, the external cooling of the normalizing furnace does not take up equipment, and the productivity is high. Therefore, normalizing is used as much as possible to replace annealing in the production.

The Effect of Tempering

Improve the stability of the structure, so that the structure of the workpiece no longer changes during use, so that the geometric size and performance of the workpiece remain stable.

Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometric size of the workpiece.

Adjust the mechanical properties of steel to meet the requirements of use. Divided into high temperature, low temperature and medium temperature tempering, among which high temperature tempering is also called quenching and tempering treatment.

The purpose of tempering after normalizing is to remove stress and temper. The reason is very simple: the cooling of the normalizing process is not balanced cooling, and the obtained structure is not a balanced structure, and a certain internal stress naturally exists in the non-balanced structure.

This can be traced back to the original definition of tempering: the literature of tempering in the past defined tempering as-heating the quenched or normalized steel to a certain temperature (below the lower critical point temperature), Keep sufficient time to eliminate the residual stress caused by quenching or normalizing, and obtain a more stable microstructure and heat treatment process with comprehensive mechanical properties required by the design. Most of the current literature defines tempering as: the use of heating means to transform the unstable quenched structure into a relatively stable tempered structure, and reduce the quenching stress, so as to increase the strength of the steel as much as possible while maintaining high strength and hardness. Toughness and plasticity, good mechanical properties are obtained, so that the shape and size of the workpiece will not be out of tolerance during use.

Due to the problems in the heating, rolling and cooling of steel during hot working, it is easy to cause uneven grain size, proeutectoid structure network precipitation, abnormal structure, etc. in the steel, such as direct tempering treatment cannot improving the above-mentioned defects directly affects the heat treatment effect. Therefore, for higher-grade steels, such as high-pressure boiler tubes, normalizing is generally carried out first. The steel is heated to the critical temperature of Ac3 or 30～50℃ above the Accm line, then kept for a period of time austenitized, and then air-cooled. Larger, it can be cooled by forced cooling means such as air blowing, water spray, etc., to achieve the purpose of uniform material grain size, reduction of network, reduction of abnormal structure, and then tempering to eliminate internal stress of steel , Improve plasticity and obtain suitable strength and plasticity.