Seamless steel pipe heat treatment defects and their prevention: Regardless of whether normalizing, annealing, tempering, quenching, or other heat treatment processes are used, seamless steel pipes need to go through basic processes such as heating, insulation, and cooling during heat treatment, and these processes may cause defects in seamless steel pipes. Seamless steel pipe heat treatment defects mainly include unqualified structure and performance of seamless steel pipes, oversized, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, and surface oxidation of seamless steel pipes during protective gas heat treatment.
Seamless steel pipe structure and performance are unqualified: During heat treatment, the performance of seamless steel pipes often does not meet the requirements due to incorrect heating temperature of seamless steel pipes, unreasonable insulation time, too fast or too slow cooling speed, etc.
1. When formulating the heating process, the influence of alloy elements in steel, the heating temperature of seamless steel pipes, the original structure, and the size of the austenite change of steel should be fully considered.
2. Formulate the heating temperature for heat treatment of seamless steel pipes according to the iron-carbon balance diagram.
3. Specify the heat treatment method, heating temperature, tempering temperature, and cooling rate. After the process plan is formulated, it must be verified by small batch production before mass production can be carried out.
① The size of the seamless steel pipe is unqualified: After the seamless steel pipe is heat treated, the size will change significantly in some cases, including changes in outer diameter, ovality, and curvature. The change in outer diameter often occurs in the quenching process. After the seamless steel pipe is quenched, the main structure becomes martensite and bainite. The change in volume due to the change in structure increases the outer diameter of the seamless steel pipe. To reduce the change in outer diameter, a sizing process is often added after the tempering process. The change in ovality usually occurs at the end of the seamless steel pipe, which is mainly caused by the long-term high-temperature heating of large-diameter thin-walled seamless steel pipe. To prevent the change in 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 seamless steel pipe will “burn down”, and the end will appear “out of round”. In this case, as long as the seamless 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 seamless steel pipes, especially inconsistent cooling rates in the longitudinal or transverse parts of seamless steel pipes during quenching. Generally speaking, the bending of seamless steel pipes can be eliminated by straightening them with a straightening machine.
② Surface cracks of seamless steel pipes: During the heat treatment of seamless steel pipes, excessive temperature stress will cause surface cracks in seamless steel pipes. The main reason is that the heating rate or cooling rate is too fast. When the alloy thick-walled seamless steel pipe is heated, if the temperature in the furnace is too high, the seamless steel pipe will encounter high-temperature rapid heating after entering the furnace. At this time, it is easy to cause a large temperature difference between the surface of the seamless steel pipe and the internal metal, resulting in temperature stress. When the stress reaches the tensile strength limit of the material, cracks appear on the surface of the seamless steel pipe. Due to the quenching process, the probability of surface cracks in seamless steel pipes during metallographic quenching treatment is relatively high. When there are non-metallic inclusions, composition and organizational segregation in seamless steel pipes, the possibility of quenching cracks in seamless steel pipes will increase. To reduce the heat treatment cracks of seamless steel pipes, on the one hand, the heating and cooling systems of seamless steel pipes should be formulated according to the steel type, and the appropriate quenching medium should be selected; on the other hand, the quenched seamless steel pipes should be tempered or annealed as soon as possible to eliminate their internal stress.
③ Scratches and bruises on the surface of seamless steel pipes: mainly defects formed on the surface of seamless steel pipes due to the failure or collision between the tools and workpieces in contact with the seamless steel pipes when or after heating in the heating furnace, in the quenching device or during the roller conveying process. To prevent the occurrence of this defect, while ensuring the normal operation of the heating equipment, the relative sliding speed between the seamless steel pipe and the workpiece, tools, and rollers should be reduced as much as possible to reduce the chance of collision between them.
In short, whether it is the heating of the hot-rolled seamless steel pipe before perforation, the heating of the rough pipe before sizing (reducing) after rolling, or the intermediate annealing of the cold-rolled (drawn) seamless steel pipe, as long as the heating process parameters are not properly designed and controlled, the pipe billet (seamless 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 seamless steel pipe. Therefore, it is necessary to strengthen the quality control of each link of the heating of the pipe billet (seamless steel pipe).
Post time: Jan-17-2025