Different from the seamless steel pipe, which is directly perforated by round steel, the welded pipe is a steel pipe made of steel plate or strip steel after crimping and welding.It is easily affected during the welding process, resulting in cracks, pores and other defects in the steel pipe (especially the weld part)
Hazards of non-standard welds
1. During the forming process of the weld, the protection of the molten metal has a direct impact on the welding quality,If the weld is too large and poorly protected, oxygen, nitrogen and hydrogen in the air will dissolve into the liquid metal, causing pores, slag inclusions, and cracks, which will damage the performance of the steel pipe.
2. The thermal cycle in the welding process has an impact on the welding quality. Due to the high temperature of the molten pool metal, the alloying elements may evaporate or oxidize, and the heat-affected zone may cause coarse grains due to overheating, reducing the performance.Uneven temperature distribution will also cause welding deformation and welding stress, reducing welding quality.
3. During the welding process of the welded pipe, if the weld is too wide, not only the welded joint will be severely heated, but the grain size of the weld will be enlarged, and the plasticity and toughness will decrease.Moreover, the welding heat-affected zone is large, which is easy to generate welding stress and deformation; in addition, the waste of materials increases the cost.
4. If the welding seam is too narrow, the base metal and the welding seam may be poorly fused, causing stress concentration, and at the same time, the welding seam will produce welding defects such as undercuts and cracks, which will affect the strength of the joint. Therefore, the correct determination of the weld size is the key to ensuring the welding quality.
Weld control
1. Welding temperature control
The welding temperature is mainly affected by the high-frequency eddy current thermal power. As long as the capacitance, inductance or voltage and current in the loop are changed, the excitation frequency can be changed, so as to achieve the purpose of controlling the welding temperature. For low carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the penetration requirement of 3~5mm tube wall thickness. In addition, the welding temperature can also be achieved by adjusting the welding speed.
2. Control of extrusion force
After the two edges of the broken pipe are heated to the welding temperature, under the extrusion of the extrusion roller, the common metal particles are formed to penetrate and crystallize each other, and finally form a firm weld. If the extrusion force is too small, the number of common crystals formed will be small, the strength of the weld metal will decrease, and cracks will occur after being stressed;If the extrusion force is too large, the molten metal will be squeezed out of the weld, which not only reduces the strength of the weld, but also produces a large number of internal and external burrs, and even causes defects such as welding laps.
3. Control of the position of the high-frequency induction coil
The high-frequency induction coil should be as close as possible to the position of the squeeze roller. If the induction coil is far away from the extrusion roller, the effective heating time will be longer, the heat affected zone will be wider, and the strength of the weld will decrease.
4. The impedance device is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impedance device should usually not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe.Its function is to form an electromagnetic induction loop between the induction coil, the edge of the welded seam of the tube and the magnetic rod, resulting in a proximity effect.
Hazards of non-standard welds
1. During the forming process of the weld, the protection of the molten metal has a direct impact on the welding quality,If the weld is too large and poorly protected, oxygen, nitrogen and hydrogen in the air will dissolve into the liquid metal, causing pores, slag inclusions, and cracks, which will damage the performance of the steel pipe.
2. The thermal cycle in the welding process has an impact on the welding quality. Due to the high temperature of the molten pool metal, the alloying elements may evaporate or oxidize, and the heat-affected zone may cause coarse grains due to overheating, reducing the performance.Uneven temperature distribution will also cause welding deformation and welding stress, reducing welding quality.
3. During the welding process of the welded pipe, if the weld is too wide, not only the welded joint will be severely heated, but the grain size of the weld will be enlarged, and the plasticity and toughness will decrease.Moreover, the welding heat-affected zone is large, which is easy to generate welding stress and deformation; in addition, the waste of materials increases the cost.
4. If the welding seam is too narrow, the base metal and the welding seam may be poorly fused, causing stress concentration, and at the same time, the welding seam will produce welding defects such as undercuts and cracks, which will affect the strength of the joint. Therefore, the correct determination of the weld size is the key to ensuring the welding quality.
Weld control
1. Welding temperature control
The welding temperature is mainly affected by the high-frequency eddy current thermal power. As long as the capacitance, inductance or voltage and current in the loop are changed, the excitation frequency can be changed, so as to achieve the purpose of controlling the welding temperature. For low carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the penetration requirement of 3~5mm tube wall thickness. In addition, the welding temperature can also be achieved by adjusting the welding speed.
2. Control of extrusion force
After the two edges of the broken pipe are heated to the welding temperature, under the extrusion of the extrusion roller, the common metal particles are formed to penetrate and crystallize each other, and finally form a firm weld. If the extrusion force is too small, the number of common crystals formed will be small, the strength of the weld metal will decrease, and cracks will occur after being stressed;If the extrusion force is too large, the molten metal will be squeezed out of the weld, which not only reduces the strength of the weld, but also produces a large number of internal and external burrs, and even causes defects such as welding laps.
3. Control of the position of the high-frequency induction coil
The high-frequency induction coil should be as close as possible to the position of the squeeze roller. If the induction coil is far away from the extrusion roller, the effective heating time will be longer, the heat affected zone will be wider, and the strength of the weld will decrease.
4. The impedance device is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impedance device should usually not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe.Its function is to form an electromagnetic induction loop between the induction coil, the edge of the welded seam of the tube and the magnetic rod, resulting in a proximity effect.
