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 between 727°C and 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 period of time, 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 to 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 be used as a final heat treatment instead of quenching and tempering treatment (quenching + high temperature tempering), and it 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, so as 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 in 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.
The structure after normalizing: hypoeutectoid steel is ferrite + pearlite, eutectoid steel is pearlite, hypereutectoid steel is pearlite + secondary cementite, and it is discontinuous.
Normalizing is mainly used in the production of steel workpieces, such as
seamless pipes (SMLS). 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 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 a 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.
For the final heat treatment of ordinary structural parts, since the workpiece after normalizing has better comprehensive mechanical properties than the annealed state, normalizing can be used as the final heat treatment for some ordinary structural parts with low stress and low performance requirements to reduce the number of processes, 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.
Go here to learn more about "Differences between Annealing, Normalizing, Tempering and Quenching of Seamless Pipes"
