A Detailed Discussion On The Normalizing Process Of Square Tubes

The normalizing process is a heat treatment process that heats the square rectangular tube to above Ac3, holds it for an appropriate time, and then cools it in still air. Normalizing that is heated to 100 to 150°C above Ac3 is called high-temperature normalizing. The purpose of normalizing square and rectangular tubes is to refine the grains and uniformize the distribution of carbides. 

Because the hardness of low carbon steel square rectangular tubes is too low after annealing, the phenomenon of tool sticking occurs during cutting, and the cutting performance is poor. Increasing the hardness by normalizing can improve the cutting performance. Some medium carbon structural steel parts can be normalized instead of quenched and tempered. , simplifying the heat treatment process. Hypereutectoid steel normalizes and heats the knife above Acm, so that all the original network-shaped cementite is dissolved into the austenite, and then cooled at a faster speed to inhibit the precipitation of cementite at the austenite grain boundary, thereby eliminating network carbides and improve the structure of hypereutectoid steel. Normalizing can improve the weld structure and ensure the weld strength for welded square and rectangular pipes that require weld strength. During the heat treatment process, the reworked workpiece must be normalized. Structural parts that require mechanical performance indicators must be normalized and then quenched and tempered to meet the mechanical performance requirements. Medium and high alloy steel square tubes and large forgings must be tempered at high temperatures after normalizing to eliminate the internal stress generated during normalizing. Some alloy steels undergo a partial martensitic transformation during forging to form a hard structure. To eliminate this bad structure, when normalizing is performed, the normalizing temperature is heated and kept at a temperature about 20°C higher than the normal normalizing temperature. 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 square and rectangular tubes that are not subject to large forces and have low-performance requirements to reduce processes, save energy, and improve production efficiency. 

As another heat treatment process commonly used in the production of square and rectangular tubes, annealing is also widely used. So what are the differences between normalizing and annealing in production? We have made eight points on this: (1) Normalizing is a variant or special case of complete annealing. Usually, annealing is done with the furnace cooling while normalizing is cooled in the air. Normalizing applies to both hypereutectoid steel and hypereutectoid steel. For super eutectoid steel and eutectoid steel, normalizing is generally used to eliminate network carbides;

(2) The normalizing cooling rate is slightly faster than the annealing cooling rate, so the normalized structure is finer than the annealed structure, and its mechanical properties are also improved;

(3) Generally, alloy steel blanks are often annealed. If normalized, the cooling rate is faster, making the hardness higher after normalizing, which is not conducive to cutting processing;

(4) In the production practice of square and rectangular pipes, if the steel contains reticular cementite, it must be eliminated by normalizing before other processes can be used;

(5) Since cooling outside the normalizing furnace does not occupy equipment and has high productivity, normalizing should be used instead of annealing as much as possible in production;

(6) For hypereutectoid steel, the purpose of normalizing is the same as that of annealing, which is mainly to refine the grains and eliminate defects in the structure. However, the pearlite sheets in the normalized structure are thinner than those in the annealed one, and in the hypereutectoid steel The amount of pearlite is large and the amount of ferrite is small, so the hardness and strength of the steel after normalizing are higher than those after annealing;

(7) High-temperature tempering is required after normalizing workpieces with complex shapes. The purpose is to eliminate the stress generated during normalizing and cooling and improve toughness and plasticity;

(8) Different processes should be selected according to the requirements for square and rectangular tubes that have performance requirements after heat treatment. For example: if a certain strength and hardness are required after treatment, the normalizing process can be selected; if a certain plasticity is required, the strength and hardness should be reduced as much as possible. For hardness, the annealing process should be selected.