The high-frequency steel pipe welding unit is the core equipment of the entire HFW production line and also requires the most technical expertise. Improper adjustment will directly affect the quality of the welded steel pipe. This article uses five-roll extrusion welding as an example to detail common problems with high-frequency welding and their causes.
First, External Burr Shape
External burrs are formed by the high-frequency heating and extrusion of the plate edge. The shape of the external burr is affected by factors such as the welding extrusion volume, welding power, and plate edge shape. Therefore, the shape of the external burr is the most intuitive indicator of weld quality and can be used to judge weld quality.
1. Uniform External Burr: A good external burr shape is achieved when the extruded metal burr forms a mushroom-shaped pattern, overlapping the deformed metal burr and partially merging with it. This results in a weld with a moderately wide heat-affected zone.
2. Flat External Burrs: Extruded metal burrs appear as flakes overlying deformed metal burrs, forming a flat, arc-shaped burr that lies flat on the surface of the welded steel pipe. This condition can easily lead to an excessively wide weld heat-affected zone. Deformed metal burrs are relatively abundant, while extruded metal burrs are relatively few or absent. This prevents impurities from being removed from the weld and can lead to cracking during the flattening test. Both conditions can easily lead to an excessively narrow weld heat-affected zone.
Cause: Generally speaking, for a constant extrusion rate, higher welding power contributes to a higher proportion of extruded metal burrs to the total external burr volume, and the burr shape becomes flatter. For a constant welding power, higher extrusion rates contribute to a higher proportion of deformed metal burrs to the total external burr volume, and the burrs become more upright. Low extrusion rates require a lower welding power to ensure weld quality; conversely, high extrusion rates require a higher welding power to ensure weld quality. However, too low an extrusion rate hinders the removal of weld inclusions, while too high an extrusion rate can lead to excessive molten metal extrusion, resulting in a cold weld.
3. Inclined external burrs. Cause: Misalignment between the two plate edges during butt welding results in different stress states, causing the molten metal to be squeezed out and deviate from its original position.
Second, the impact of strip edge morphology on welding quality.
1. Strip edge morphology. Method for determining strip edge morphology:
Semi-welded test: Take a semi-welded specimen at high frequency. The so-called semi-welded specimen is the section of the butt weld edge where the weld transitions from butt welding to weld fusion. Using a flattening tester, the semi-welded specimen is fractured along the weld seam, revealing two distinct zones: the welded fusion zone and the unfused zone. The fracture of the fusion zone appears silvery-white, while the unfused zone appears bluish-black. The boundary between the two zones forms a C-shaped curve.
2. Observe the degree of the upper and lower streamline angles in high-frequency weld metallography. Generally, a positive V-shape is observed when the lower streamline angle is greater than the upper streamline angle; an I-shape is observed when the upper and lower streamline angles are equal or nearly equal; and an inverted V-shape is observed when the upper streamline angle is greater than the lower streamline angle.
3. Misaligned edge shape and adjustment. Misalignment of plate edges (also called lap welding) is an important factor affecting welding quality. If misalignment occurs due to improper adjustment, the effective wall thickness of the weld is reduced, which easily leads to stress concentration.
The reasons for misalignment during steel pipe welding are:
(1) In most cases, it is caused by the different heights of the two extrusion upper rollers. Therefore, it can be corrected by adjusting the height of one of the extrusion upper rollers.
(2) Sometimes it is caused by the different heights of the welding extrusion edge rollers. It can be corrected by adding or removing metal gaskets at the bottom or side of the extrusion edge roller frame.
Misalignment caused by different angles of the welded plate edges. This misalignment can easily cause the weld burr to fall to one side. The high-frequency welding metallographic test shows a bent fusion line or asymmetric streamline angles on the left and right sides of the fusion line. The reasons for this misalignment are:
(1) The position of the welding extrusion upper roller is asymmetric along the rolling center of the welded steel pipe, causing the two extrusion upper rollers to exert unequal forces on the two plate edges. The extrusion upper roller frame can be adjusted horizontally so that the extrusion upper roller moves to the left or right at the same time, and finally the weld is at the center of the extrusion upper roller (Figure 12). (2) The heights of the two extrusion side rollers are different or the extrusion roller frame is tilted.
(3) Plate edge quality reasons. The strip has defects such as sickle bend and wave bend, or the strip edge is worn on one side.
Causes of weld torsion:
(1) The heights of the two welding side rollers are different, which can be corrected by adding or removing metal gaskets at the bottom or side of the horizontal roller frame.
(2) The positions of the two extrusion side rollers are asymmetric along the rolling center of the welded steel pipe, resulting in unequal forces applied by the two extrusion side rollers to the tube billet, causing the tube billet to torsion.
(3) The bearings or guide plates of the finishing rollers are damaged, resulting in the inability to control the torsion of the weld.
(4) The guide roller position is adjusted too high in the vertical direction, and only part of the guide roller hole contacts the steel billet, resulting in too little force applied to the tube billet, and the torsion of the roller cannot be controlled, or the guide roller or bearing is damaged. Third, steel pipe indentations and pits There are two types of indentations. One is a linear indentation, which is caused by improper roller position adjustment; the other is a crescent-shaped indentation, which is caused by the bulges produced by roller wear. The pits on the steel pipe are generally caused by the bulges on the roller and the oxide scale adhered to the roller.
1. Linear indentation
(1) Continuous linear indentation: It is caused by the poor matching of the roller positions between the rollers. It often appears between the roller gaps. The indentation between the upper roller and the side roller is the most common indentation. In addition, the side roller and the bottom roller may also produce indentations.
Cause: It is generally caused by improper matching of the roller positions of the side roller and the upper roller.
Adjustment method: It can be adjusted by raising the position of the upper extrusion roller or reducing the distance between the two side rollers.
Cause: It is generally caused by improper matching of the roller positions of the side roller and the lower roller.
Adjustment method: It can be adjusted by lowering the position of the lower extrusion roller or reducing the distance between the two side rollers.
Cause and adjustment method:
a. The height of one of the upper extrusion rollers is too low. Adjust the height of the single upper extrusion roller.
b. The position of the extrusion side roller is asymmetrical. Due to the different forces, the side roller position on the side where the indentation occurs can be tightened, or the side roller position on the side where the indentation does not occur can be loosened.
c. The side rollers are caused by different heights.
d. The distance between the two extrusion upper rollers and the weld is unequal due to the misalignment of the extrusion upper rollers. Adjust the extrusion upper roller frame horizontally.
(2) Discontinuous linear indentation Common causes:
a. Generally, the roller bearing is damaged.
b. On the split roller, the roller component deviates from its original position due to the partial breakage of the roller screw.
2. Crescent-shaped indentation Generally, the roller surface is worn, and there is an uneven contact surface, which causes a crescent-shaped indentation on the steel pipe. The crescent-shaped indentation usually appears at the position where the roller radius is the largest. The reason is that the linear speed of the roller at this position is the largest and the friction is the largest. The crescent-shaped indentation can be removed by online grinding.
3. Pits
Cause 1: A protrusion on the roll causes a pit in the steel pipe. The cause is the same as for a crescent-shaped indentation.
Cause 2: An indentation caused by the sizing roller. Due to the medium-frequency heat treatment at the weld seam, a large amount of oxide scale forms. After passing through the sizing roller, this scale adheres to the surface of the sizing roller. If the emulsion spray is improperly positioned or the emulsion flow is too low, the scale is not washed away. Over time, the scale accumulates, eventually causing pits near the weld seam of the welded steel pipe.
Fourth: High-frequency sparking
High-frequency sparking is very common in high-frequency welding, seriously affecting welding quality and potentially causing the welding unit to shut down.
Common causes:
1. Due to metal oxide impurities in the emulsion, some of the metal oxides can cause electrical contact between insulated components during the cooling of the rolls.
2. Burrs on the edges of the plate after slitting. Burrs on the plate edge can easily overlap before welding the V-angle, causing sparks.
3. Long-term use of the FP rollers causes severe wear of the sealing blades, resulting in grooves at the bottom of the blades. This step between the grooves and the unworn area creates burrs on the plate edge due to shearing. These burrs overlap prematurely before the V-angle is welded, causing sparks.
4. Dirt accumulated near the induction coils during long-term operation can easily cause electrical contact and sparks.
5. Improper installation of the induction device components can lead to loose connections or damage to the insulation components due to long-term operation.
Many factors contribute to the quality of welded steel pipe forming, including the strip edge shape, misalignment, internal and external burr configuration, weld extrusion, welding temperature, impedance, cooling water, and high-frequency weld metallographic testing. Only with a comprehensive understanding of all factors influencing forming welding can the weld condition be quickly and accurately assessed.
Post time: Oct-21-2025