Induction annealing can effectively eliminate the structural defects of steel pipe welds, improve the structure, homogenize the composition and refine the grains, improve the mechanical properties of the steel, and reduce the residual stress. After induction annealing, the hardness of the weld is reduced and the toughness is improved, which is beneficial to the subsequent processing.
When welding, the temperature of the welded area is very high, and the temperature of the place without welding is relatively low. Temperature differences cause thermal expansion and contraction. Due to thermal expansion and contraction, welding stress is generated internally. Therefore, it is necessary to use induction annealing equipment to carry out the weld annealing process to eliminate welding stress and improve toughness.
Benefits of Induction Annealing: Two Methods of Induction Annealing for Steel Pipe Welds
According to the difference of the steel pipe weld, the annealing of the steel pipe weld is divided into two types: the annealing of the longitudinal straight weld of the steel pipe and the annealing of the butt circular weld.
1. Annealing of longitudinal straight welds
The main purpose of the annealing of the longitudinal straight weld of the steel pipe is to refine the grains of the metal structure. Because the weld is induction heated to the welding temperature and welded, the grains in the heated area become coarser, and the subsequent rapid cooling will cause the formation of low carbon martensite in this area. Although low carbon steel materials are used to try to avoid low carbon martensite in the weld, the local carbon content can reach 2-3 times the normal carbon content due to the formation of segregation during the welding process. For example, for a steel pipe with ω(C) of 0.06%, the segregation section can reach ω(C) 0.2%. In addition, due to the manganese content in the steel, the local hardness of the weld can reach 40HRC. This hardness section is most prone to cracks, making the steel pipe work failure.
Induction annealing of the weld can effectively solve the above hardness problem. Induction annealing can refine grains, relieve stress and reduce hardness. With induction heating, only the weld section is heated. For steel pipes with a wall thickness of 8-15mm and a weld width of about 20mm, induction heating is a very effective and fast annealing method, and it is easy to automate the annealing process.
The induction coil adopts the transverse magnetic field heating method of plane heating. At this time, the electrical efficiency is very high, so a lower frequency power supply, such as 1-3kHz, can be used. The annealing temperature of the weld directly affects the quality of the normalized structure. If the temperature is too high, the grain size of the weld area will be larger than that of the base metal. If the temperature is too low, the weld will not be heated through, and the structure will not be fully refined.
2. Annealing of butt steel pipe welds
The annealing of the butt weld generally uses a circular induction coil. In terms of induction heating efficiency, a higher frequency should be used. Steel pipe welds are usually annealed using high frequency induction heating machines. It has a fast annealing speed and high efficiency.
Inspection of Hardness and Microstructure of Steel Pipe Welds
The microstructure of the normalized zone must be similar to that of the base metal. The grains in the heat-affected zone are thicker than those of 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 there is a small amount of coarse grains mixed in many small grains in the microstructure, it means that the normalizing temperature is slightly higher and should be slightly lowered.
Induction annealing treatment is an efficient and energy-saving heat treatment process, which can effectively improve the toughness and plasticity of metal workpieces and improve the processing performance.
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