Steel pipe welding uses medium frequency diathermy furnace for induction heat treatment

During the welding process of metal, the weld pool metal condenses from liquid to room temperature, and the base metal near the weld is heated to close to the melting temperature. Therefore, there is a Widmanstatten structure with coarse grains in the weld and its adjacent base metal. . During the cooling process, great thermal stress and structural stress are also generated, so important welded components must be tempered or annealed in a medium frequency diathermy furnace to eliminate the above shortcomings.

The welds of thick-walled steel pipes are tempered or annealed using medium-frequency local induction heating, which can effectively eliminate welding stress and improve the weld structure. For small batch production of steel pipe welded parts, it is more suitable to use soft sensors without magnetic conductors. The coil conductor of the soft sensor can be a soft cable. Wrap two layers of glass ribbon around the surface of the cable or install the cable in a rubber tube. Place an aluminum silicate fiber mat between the coil and the steel tube. This kind of soft sensor is versatile. It is very flexible and can be changed according to the size of the steel pipe. This kind of soft inductor without magnetic conductors generally has fewer coil turns, a large magnetizing current, and a low terminal voltage of the inductor. Sometimes it needs to be powered by a step-down transformer. The secondary of the step-down transformer should have a large adjustment range. 

To eliminate the internal stress of the weld, the tempering temperature in the furnace is about 280°C, and the holding time is 9-10h. Medium frequency induction heating is used for tempering. The heating speed is fast, reaching 50-100°C/min, and there is no holding time. The impact value after tempering by medium frequency induction heating is higher than that of tempering in the furnace, indicating less residual stress. The key is that the tempering temperature must be increased to 450-550°C for tempering by medium frequency induction heating.

For mass-produced welded steel pipes, when the welds are treated with medium-frequency induction heating, the coil of the inductor is wound with pure copper tube and has a magnetic conductor, so that the overall inductor is very convenient to use. For example, the medium frequency inductor used for tempering the weld after butt jointing of oil drill pipes. The coil is wound with a rectangular pure copper tube of 12mmx9mmx2mm, arranged in 5 layers, with a total of 60 turns. The inner diameter of the coil is φ250mm, the outer diameter is φ370mm, and the length is 180mm. There are Magnetic conductor. When the diameter of the heated drill pipe is 141.3mm, the voltage is 380V, the current is 500-700A; when the diameter of the heated drill pipe is 114.3mm, the voltage is 380V, the current is 600-800A, and the heating rate is 120-300℃/min.