Normalizing

Normalizing is a heat treatment that improves the toughness of steel. After heating the steel components to 30~50℃ above the Ac3 temperature, they are kept for some time and then released for air cooling. The main feature is that the cooling rate is faster than annealing but lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling, which can not only obtain satisfactory strength but also significantly improve toughness (AKV value), and reduce The cracking tendency of the member. After some low-alloy hot-rolled steel sheets, low-alloy steel forgings, and castings are normalized, the comprehensive mechanical properties of the material can be greatly improved, and the cutting performance is also improved.

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Basic Information

Normalizing, also known as normalizing, is to heat the workpiece to Ac3 (Ac refers to the final temperature at which free ferrite is completely transformed into austenite during heating, generally from 727 ° C to 912 ° C) or Acm (Acm is In actual heating, the critical temperature line for complete austenitization of hypereutectoid steel is above 30~50℃. After holding for a while, the metal heat treatment process of cooling in air or water spray, spray, or blowing is taken out of the furnace. The purpose is to refine the grains and homogenize the carbide distribution. The difference between normalizing and annealing is that the cooling rate of normalizing is slightly faster than that of annealing, 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 occupy equipment, and the productivity is high. Therefore, normalizing is used as much as possible to replace annealing in production. For important forgings with complex shapes, high-temperature tempering (550-650°C) is required after normalizing. The purpose of high-temperature tempering is to eliminate the stress generated during normalizing cooling and improve toughness and plasticity.

The main application areas of normalizing are:

①For low carbon steel, the hardness after normalizing is slightly higher than that of annealing, and the toughness is also better, which can be used as a pretreatment for cutting.

②For medium carbon steel, it can replace the quenching and tempering treatment (quenching + high-temperature tempering) as the final heat treatment, and can also be used as a preparatory treatment before surface quenching by induction heating.

③Used for tool steel, bearing steel, carburized steel, etc., it can reduce or inhibit the formation of network carbide, to obtain a good structure required for spheroidizing annealing.

④ For steel castings, it can refine the as-cast structure and improve the machining performance.

⑤ For large forgings, it can be used as the final heat treatment to avoid a large cracking tendency during quenching.

⑥Used for ductile iron to improve hardness, strength, and wear resistance, such as for manufacturing important parts such as crankshafts and connecting rods of automobiles, tractors, and diesel engines.

⑦ One normalizing before spheroidizing annealing of hypereutectoid steel can eliminate the reticulated cementite to ensure that all cementite is spheroidized during spheroidizing annealing.

Normalizing is mainly used for steel workpieces. Generally, normalizing steel is similar to annealing, but the cooling rate is slightly larger and the structure is finer. Some steels with a small critical cooling rate can be cooled in the air to transform austenite into martensite. This treatment is not a normalizing property but is called air quenching. In contrast, some large-section workpieces made of steel with a large critical cooling rate cannot obtain martensite even if they are quenched in water, and the quenching effect is close to normalizing. The hardness of steel after normalizing is higher than that after annealing. When normalizing, it is not necessary to cool the workpiece with the furnace-like annealing, which takes up a short time in the furnace and has high production efficiency. Therefore, normalizing is generally used instead of annealing in production. For low carbon steel with carbon content less than 0.25%, the hardness achieved after normalizing is moderate, which is more convenient for cutting than annealing. Generally, normalizing is used to prepare for cutting. For medium carbon steel with a carbon content of 0.25 to 0.5%, it can also meet the requirements of cutting after normalizing. Normalizing can also be used as the final heat treatment for lightly loaded parts made from this type of steel. The normalizing of high carbon tool steel and bearing steel is to eliminate the network carbide in the organization and prepare the organization for spheroidizing annealing.

The final heat treatment of ordinary structural parts, because the workpiece after normalizing has better comprehensive mechanical properties than the annealed state, for some ordinary structural parts with low stress and low-performance requirements, normalizing can be used as the final heat treatment to reduce the process, Save energy and improve production efficiency. In addition, for some large or complex parts, when quenching is in danger of cracking, normalizing can often replace quenching and tempering as the final heat treatment.

Purpose

(1) Remove the internal stress of the material

(2) Adjust the hardness of the material (generally increase), and reduce the plasticity slightly

This is to prepare for the next processing. Similar to annealing, it is only to improve efficiency and reduce costs.

The difference

(1) The cooling rate of normalizing is faster than that of annealing, the lamellar spacing of the obtained pearlite structure is smaller, and the pearlite is thinner. The purpose is to normalize the structure of the steel, so it is also called the normalization treatment. For example, when the carbon content is less than 0.4%, normalizing can be used instead of full annealing.

(2) Compared with full annealing, normalizing can obtain higher strength and hardness.

(3) The normalizing production cycle is short, the equipment utilization rate is high, the energy is saved, and the cost is low, so it has been widely used.

Choose

Annealing and normalizing belong to the same type of heat treatment. In actual production, the choice of annealing and normalizing is mainly considered from the following three aspects.

• Considering machinability
  Machining properties of metals, including hardness, chip brittleness, surface roughness, and tool wear. Generally speaking, the hardness of the metal is in the range of 170-230HBW, and the cutting performance is good. If the hardness is too high, it is not only difficult to process but also causes the tool to wear quickly; if the hardness is too low, it is easy to cause sticking and entanglement of chips during cutting, reducing tool life and rough cutting surface. In general production, normalizing is suitable for low and medium carbon structural steels as preparatory heat treatment, and high carbon structural steels (such as bearing steel) and tool steels are preferably annealed (spheroidizing annealing). For alloy steel, the hardness of the steel is increased due to the inclusion of alloying elements, so in most cases, alloy steel above medium carbon is often annealed.

• Considering the use of performance
  If the performance requirements of the steel parts are not too high, normalizing can be used as the final heat treatment. However, if the size of the part is large or the shape is complex, normalizing may cause large residual force or deformation and crack of the part. At this time, annealing should be selected for higher mechanical properties, and quenching + tempering must be performed frequently for the final heat treatment of the parts. , In terms of reducing the tendency of deformation and cracking, annealing should be used for preparatory heat treatment.

• Economical considerations
  Normalizing has a shorter production cycle than annealing and is easy to operate. Where possible, normalizing should be prioritized instead of annealing, especially in mass production.

Heat treatment process

The heat treatment of steel is divided into two categories: integral heat treatment and surface heat treatment. Commonly used overall heat treatment includes annealing, normalizing, quenching, and tempering; surface heat treatment can be divided into surface quenching and chemical heat treatment.

Normalizing is a heat treatment process in which the steel is heated to 30-50 °C above the critical temperature, and cooled in still air after holding for an appropriate time. The main purpose of normalizing is to refine the structure, improve the properties of the steel, and obtain a structure close to the equilibrium state.

Compared with the annealing process, the main difference between normalizing and annealing is that the cooling rate of normalizing is slightly faster, so the production cycle of normalizing heat treatment is short. Therefore, when annealing and normalizing can also meet the performance requirements of parts, normalizing should be used as much as possible. Most of the medium and low carbon steel billets are generally normalized. Generally, alloy steel billets are often annealed. If normalizing is used, due to the fast cooling rate, the hardness after normalizing is high, which is not conducive to cutting.

Application

(1) As the final heat treatment:

• It can refine the austenite grains and make the structure uniform.
• Reduce the content of ferrite in hypoeutectoid steel, and increase and refine the content of pearlite, thereby improving the strength, hardness, and toughness of steel.
• For ordinary structural steel parts, such as when the carbon content is 0.4% to 0.7%, and the mechanical properties are not required very high, normalizing can be used as the final heat treatment.
• To improve the mechanical properties of some steel plates, pipes, strips, and sections, normalizing can be used as the final heat treatment.

(2) As a pre-heat treatment:

• Alloy structural steel parts with large cross-sections are often normalized before quenching or quenching and tempering (quenching and high-temperature tempering) to eliminate Widmanderin and banded structures and obtain fine and uniform structures.
• For hypereutectoid steel, the amount of secondary cementite can be reduced, so that it does not form a continuous network, and it is prepared for spheroidizing annealing.
• For large forgings and steels with larger sections, normalizing can be done first to prepare the structure 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 machinability.

(4) Improve and refine the as-cast structure of steel castings.

(5) For some large and heavy steel parts or steel parts with complex shapes and sharp changes in cross-section, if the rapid cooling of quenching is used, serious deformation or cracking will occur. On the premise of ensuring performance, normalizing can be used instead of quenching.

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