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Defects in heat treatment of steel pipes and their prevention
Date:2024-06-11      View(s):81      Tag:steel pipe heat treatment, steel pipe prevention, steel pipe defects
Whether normalizing, annealing, tempering, quenching, or other heat treatment processes are used, steel pipes need to go through basic processes such as heating, insulation, and cooling during heat treatment, and defects may occur in steel pipes during these processes. Defects in heat treatment of steel pipes mainly include unqualified organizational properties of steel pipes, oversized, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, and surface oxidation of steel pipes during protective gas heat treatment.

Unqualified organizational properties of steel pipes: During heat treatment, the performance of steel pipes often does not meet the requirements due to factors such as incorrect heating temperature of steel pipes, unreasonable insulation time, too fast or too slow cooling rate, etc.
First, when formulating the heating process, the influence of alloy elements in steel, the heating temperature of steel pipes, the original organization, and the size of the austenite change of steel should be fully considered.
Second, the heating temperature of steel pipe heat treatment should be formulated according to the iron-carbon balance diagram.
Third, the heat treatment method, heating temperature, tempering temperature, and cooling rate should be clarified. After the process plan is formulated, it must be verified by small batch production before mass production.

Unqualified steel pipe size: After heat treatment of steel pipe, the size will change significantly in some cases, including changes in outer diameter, ovality, and curvature. The change of outer diameter often occurs in the quenching process. After quenching, the main structure of the steel pipe becomes martensite and bainite. The change of volume due to the change of structure increases the outer diameter of the steel pipe. To reduce the change of outer diameter, a sizing process is often added after the tempering process. The change of ovality usually occurs at the end of the steel pipe, mainly caused by long-term high-temperature heating of large-diameter thin-walled steel pipes. To prevent the change of ovality, the main thing is to ensure the rationality of the heating system. Sometimes even if the heating system is reasonable, once the D/S value is too large, the steel pipe will "burn down" and the end will appear "out of round". In this case, as long as the steel pipe can be heated and rotated at the same time, it can be prevented.

Many factors affect bending, mainly including uneven heating and cooling of steel pipes, especially the inconsistent cooling speed of each part along the longitudinal or transverse direction when the steel pipe is quenched. Generally speaking, bent steel pipes can be straightened by a straightening machine to eliminate them.

Surface cracks on steel pipes: During the heat treatment of steel pipes, excessive temperature stress will cause surface cracks on steel pipes. The main reason is that the heating speed or cooling speed is too fast. When the alloy thick-walled steel pipe is heated, if the temperature in the furnace is too high, the steel pipe will be heated rapidly at high temperatures after entering the furnace. At this time, it is easy to cause a large temperature difference between the surface of the steel pipe and the internal metal to produce temperature stress. When the stress reaches the tensile strength limit of the material, cracks will appear on the surface of the steel pipe. Due to the quenching process, the probability of surface cracks during metallographic quenching of steel pipes is relatively high. When non-metallic inclusions, composition, and organizational segregation exist in the steel pipe, the possibility of quenching cracks in the steel pipe will increase. To reduce the heat treatment cracks of the steel pipe, on the one hand, the heating system and cooling system of the steel pipe should be formulated according to the steel type, and the appropriate quenching medium should be selected; on the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate its internal stress.

Scratches and bumps on the steel pipe surface: mainly caused by the failure or collision between the tools and workpieces in contact with the steel pipe when it is heated in the heating furnace or after heating, in the quenching device, or during the roller conveying process. To prevent the occurrence of this defect, the relative sliding speed between the steel pipe and the workpiece, tool, and roller should be reduced as much as possible to reduce the chance of collision between them while ensuring the normal operation of the heating equipment.

In short, whether it is the heating of the billet before perforation, the heating of the rough pipe before sizing (reducing) the diameter after rolling, or the intermediate annealing of the cold-rolled (drawn) steel pipe, as long as the heating process parameters are not properly designed and controlled, the billet (steel pipe) will produce quality defects such as uneven heating, oxidation, decarburization, heating cracks, overheating or overburning, which will eventually affect the quality of the steel pipe. Therefore, it is necessary to strengthen the quality control of each link of the billet (steel pipe) heating.
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