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Defects Caused by Heat Treatment and Their Prevention

Posted: 03/10/2022 20:53:01 Hits: 36

Using normalizing, annealing, tempering, quenching and other heat treatment processes can cause defects of steel pipes when they undergo basic processes such as heating, heat preservation and cooling, which may cause defects in the steel pipe. Defects caused by heat treatment of steel pipes mainly include unqualified structure and properties of steel pipes, excessive sizes, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, and surface oxidation of steel pipes during protective gas heat treatment.

Unqualified structure and performance of steel pipes: the performance of steel pipe is often unsatisfactory due to factors such as incorrect heating temperatures, unreasonable holding time, and too fast or too slow cooling rates of steel pipes in the heat treatment process.

In this regard, first, when formulating the heating process, the influence of the alloying elements in the steel, the heating temperature of the steel pipe, the original structure and size on the austenite change of the steel should be fully considered. Second, formulate the heating temperature of the steel pipe's heat treatment according to the iron-carbon balance diagram. Third, determine the heat treatment method, heating temperature, tempering temperature and cooling rate. After the process plan is formulated, it needs to be verified by small batch production before mass production.

Unqualified sizes of steel pipes: After the steel pipe is heat treated, the size will change significantly in some cases, including changes in outer diameters, ovality and curvature. The change of the outer diameter often occurs in the quenching process, because the main structure of the steel pipe becomes martensitic and bainite after quenching. The change in volume due to that in structure makes the outer diameter of the steel pipe increase. In order to reduce the change in the outer diameter, the sizing process is often provided after the tempering process. The change of ovality usually occurs at the end of the steel pipe, which mainly happens in large-diameter thin-walled steel pipes due to high-temperature heating for a long time. The main thing to prevent the change of ovality is to ensure the rationality of the heating system. Sometimes, even if the heating system is reasonable, once the D/S value is too great, the steel pipe will burn and collapse, and the end will not be round. In this case, as long as the steel pipe can be rotated while heating, the problem can be prevented.

There are many factors affecting the bending, mainly including uneven heating and cooling of the steel pipe, especially when the steel pipe is quenched, the cooling rate of each part along the longitudinal or transverse direction is inconsistent. In general, bent pipes can be adjusted by straightening them with a straightening machine.

Cracks on surfaces of steel pipes: In the heat treatment of steel pipes, excessive temperature stress will cause cracks to occur on the surfaces of steel pipes. The main reason is that the heating rate or cooling rate 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 encounter high temperatures and rapid heating after entering the furnace. At this time, it is easy to cause a great temperature difference between the surface of the steel pipe and the internal metal and generate temperature stress. When the stress reaches the tensile strength limit of the material, cracks appear on the surface of the steel pipe. The probability of cracks on surfaces during the metallographic quenching of the steel pipe is relatively high. When there are non-metallic inclusions, composition and microstructure segregation in the steel pipe, the possibility of cracks caused by quenching in the steel pipe will increase. In order to reduce the crack caused by heat treatment 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 on the surface of the steel pipe: they are defects formed on the surface of the steel pipe due to friction or collision when it is in contact with the tools and workpieces in the heating furnace, after heating in the quenching device or the conveying process of the roller table. In order to prevent this kind of defect, under the condition of ensuring the normal operation of the heating equipment, reduce the relative sliding speed between the steel pipe and the workpiece, the tool and the roller table as much as possible, and the chance of collision with each other.

In a word, the hot-rolled seamless steel pipe is heated before the piercing of the tube blank; the diameter is determined, and the intermediate annealing of the cold rolled or cold drawn steel pipe, tube blanks will generate defects such as uneven heating, oxidation, decarburization, heating cracks, overheating and overburning as long as the heating process parameters are designed and controlled improperly, ultimately affecting the quality of steel pipes. Therefore, it is necessary to strengthen the quality control of the heating of the steel pipe.

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