Why do steel pipe welds need to be annealed? How to check the quality of annealing?

The steel pipe weld adopts Induction heating furnace , and there are two types of steel pipe longitudinal straight weld annealing and butt circular weld annealing.

(1) Annealing of longitudinal straight welds of steel pipes The main purpose of annealing of longitudinal straight welds of steel pipes is to refine the grains. Because the welding seam is heated to the welding temperature and welded in the induction heating furnace, the grains of the heating zone become coarse, and the subsequent rapid cooling will form low-carbon martensite in this area, although the material made of low-carbon steel is used [w (C) 0. 05%~0.15%] Try to avoid low-carbon martensite in the weld, but due to segregation during the welding process, the local carbon content can reach 2~3 times the normal carbon content, such as trash (C) For the steel pipe of 0.06%, the segregation section can reach m (C) 0.2%, plus the factor of the ingot content in the steel, and the local hardness of the weld can reach about 40HRC. This hardness zone is the most prone to cracks and make the steel pipe work ineffective. Weld annealing is precisely aimed at this problem. It can refine grains, eliminate stress and reduce hardness. The Induction heating furnace is used for heating, and only the weld section is heated. For steel pipes with a wall thickness of 8-15mm, the weld width is about 20mm.

The welding seam annealing induction heating furnace adopts the transverse magnetic field heating of the plane heating. At this time, even if the steel pipe wall thickness/d800℃≧3.5, its electrical efficiency is still very high. Therefore, a lower frequency power supply, such as 1-3kHz, can be used. The annealing temperature of the weld seam directly affects the quality of the normalized structure. If the temperature is too high, the grains in the weld area will be larger than the base metal.

(2) Induction heating furnace is used for butt-joining steel pipe welds. Induction heating furnace is used for butt-joining steel pipe welds. Generally, an annular Induction heating furnace  is used. In terms of the heating efficiency of the induction heating furnace, a higher frequency should be used. Wall thickness/d800℃≧3.5

(3) Inspection of steel pipe weld hardness and microstructure. The microstructure of the normalizing zone must be similar to that of the base metal. The grains of the heat-affected zone are coarser than those of the base material, indicating that the normalizing temperature is too high; The hardness is higher than that of the base material, indicating that the normalizing temperature is too low; if a small amount of coarse grains are mixed in many small grains in the microstructure, it indicates that the normalizing temperature is slightly higher and should be slightly lowered.