Why should steel pipe welds be annealed with high-frequency induction heating power? What is the purpose?

Induction annealing refers to a process in which the workpiece is heated to the annealing temperature by a high-frequency induction heating power supply, and then cooled in the air. This situation can also be called induction normalizing treatment. Induction annealing is widely used in steel pipes. Weld annealing, so that the coarse and uneven grains of the weld on the surface of the steel pipe can be turned into fine grains after normalizing. Or the recrystallization annealing after cooling of the workpiece is to ensure that the elongated grains inside the workpiece can be restored to a small and uniform shape after cooling, so as to avoid damage to this part of the workpiece due to reduced mechanical properties during use. Or some workpieces are subjected to local annealing after carburizing in order to reduce part of the hardness.

We will briefly introduce two cases of annealing of steel pipe welds. One is the annealing of longitudinal straight welds of steel pipes. Its main purpose is to refine grains. When the temperature is heated and welded, the grains of the heated part will become coarser, and then when rapidly cooled, low-carbon martensite will be formed in the heated area, the toughness will be reduced, and cracks will occur. Weld annealing can refine the grains, eliminate the stress of the steel pipe, and reduce the hardness. Moreover, when heating, only the weld section needs to be heated, and the flat inductor transverse magnetic field heating method is used to reasonably adjust the annealing temperature of the weld. If it is too high, the grain size of the weld area will become larger, and if it is too low, it will not be butt welded. The seam is heated thoroughly, and the tissue cannot be fully refined.

The second is the annealing of the welds of butt-jointed steel pipes. For the annealing of butt-joint steel pipe welds, circular inductors are generally used. From the perspective of heating effect, equipment with higher frequency should be used. If the annealing temperature is too high during heating, it will affect the coarsening of the grains in the heating zone. If the temperature is too low, the hardness of the heating zone will increase. We can judge the temperature of the heating equipment during annealing at any time according to the thickness of the grains in the heating zone.