Why should the steel pipe weld seam be annealed in medium frequency induction furnace, and how to check its annealing quality?

The main purpose of annealing is to reduce the hardness of the workpiece, improve the plasticity of the workpiece, and eliminate residual stress. Workpieces generally require stress-relief annealing after welding, and many manufacturers use intermediate frequency induction furnaces for annealing heat treatment on workpieces. Today, we will take the annealing of steel pipe welds as an example to tell you why an intermediate frequency induction furnace is used for annealing and how to check its annealing quality.

There are two types of steel pipe weld annealing: steel pipe longitudinal straight weld annealing and butt round weld annealing.

(1) Steel pipe weld hardness and microstructure inspection

The microstructure of the normalizing zone must be similar to that of the base metal, and the grains in the heat-affected zone are thicker than the base metal, indicating that the normalizing temperature is too high: the hardness of the heat-affected zone is higher than that of the base metal, indicating that the normalizing temperature is too low ; If a small amount of coarse grains are mixed with many small grains in the microstructure, it means that the normalizing temperature is slightly higher and should be slightly lowered.

(2) Annealing of longitudinal straight weld seam of steel pipe

The main purpose of annealing the longitudinal straight weld of the steel pipe is to refine the grain. Because when the weld is heated to the welding temperature and welded by induction, the grains in the heating zone become coarser, and the subsequent rapid cooling will form low-carbon martensite in this zone, although low-carbon steel materials are used to try to avoid welding Low-carbon martensite is produced in the seam, but due to the formation of segregation during the welding process, the local carbon content can reach 2-3 times the normal carbon content, and the local hardness of the weld can reach about 40HRC. Cracks are most likely to occur in this hardness zone, which makes the steel pipe work invalid. Weld annealing is aimed at this problem. It can refine grains, eliminate stress and reduce hardness. Use intermediate frequency induction furnace for induction heating, only heat the weld section, for steel pipe with wall thickness 8-15mm, the weld width is about 20mm. The seam annealing sensor is heated by a transverse magnetic field with planar heating. We can choose a lower frequency power supply, such as a 1-3kHz medium frequency induction furnace. The weld annealing temperature directly affects the quality of the normalizing structure. If the temperature is too high, the grains in the weld area will be larger than the base metal. If the temperature is too low, the weld will not be heated through and the structure will not be fully refined.

(3) Annealing of butt welded steel pipe welds

For the annealing of the butt steel pipe weld seam, the circular inductor is generally used (about the circular inductor, you can see how the inductor of the intermediate frequency induction heating equipment should be designed). From the perspective of induction heating electrical efficiency, cunning should be used high frequency.

Zhengzhou Gou's has been engaged in the induction heating industry over fifteen years and has a good understanding of the heat treatment of workpieces. The article briefly describes why steel pipe welds should be annealed and how to inspect the annealing quality. Regarding this, if you still don’t understand, welcome to inquire.