Common Problems with High-Frequency Welded Steel Pipes

The high-frequency welded steel pipe 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 Formation of Steel Pipes

External burrs are formed by the high-frequency heating and extrusion of the plate edge. The shape of the burr is affected by factors such as the amount of welding extrusion, welding power, and plate edge shape. Therefore, the shape of the external burr is the most intuitive indicator of the quality of the welded steel pipe, and the weld quality can be judged by the shape of the external burr.

1. Uniform External Burr Formation of Steel Pipes

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 the deformed metal burr. This results in a weld with a heat-affected zone of moderate width.

2. Flat External Burrs on Steel Pipes

This is an overburning phenomenon caused by excessive welding power. Extruded metal burrs appear as flakes overlying deformed metal burrs, forming a flat, arc-shaped burr lying 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 flattening tests. Both conditions can easily lead to a narrow weld heat-affected zone. Generally speaking, for a constant extrusion rate and higher welding power, the proportion of extruded metal burrs in the total external burr volume increases, and the burr shape becomes flatter. For a constant welding power and higher extrusion rate, the proportion of deformed metal burrs in the total external burr volume increases, and the burrs become more upright. Low welding extrusion rates require a lower welding power to ensure weld quality; conversely, high welding extrusion rates require a higher welding power to ensure quality. However, too low an extrusion rate is not conducive to removing weld inclusions, while too high an extrusion rate can lead to excessive extrusion of molten metal, resulting in a cold weld.

3. Inclined External Burrs on Steel Pipes

Cause: Misalignment occurs when the two plate edges are butted together, resulting in different stress states on the two edges, causing the molten metal to be squeezed out and deviate from its original position.

Second, the Impact of Strip Edge Morphology on Steel Pipe Welding Quality

Strip Edge Formation

Methods for Determining Strip Edge Formation:

Method 1: Semi-Welded Test

A semi-welded specimen is taken at high frequency. A semi-welded specimen is taken from the transition between the butt-welded and welded fusion states.

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.

Method 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. 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. Misalignment caused by different plate edge heights is a common cause of welding misalignment:

(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 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 welding plate edges. This type of misalignment can easily cause the welding burr to fall to one side, and the high-frequency welding metallographic test will show 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.

(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 a sicle bend and a wave bend, or the strip edge is worn on one side.

Weld twist

Causes:

(1) The heights of the two welding side rollers are different. This 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 twist.

(3) The bearings or guide plates of the finishing rollers are damaged, resulting in the inability to control the twist 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 inability to control the twist of the roller, or the guide roller or bearing is damaged.

Third, steel pipe indentations and pits

Indentations are divided into two types. One is a linear indentation, which is caused by improper roller position adjustment; the other is a crescent-shaped indentation, which is caused by bulges caused by roller wear. Pit holes in steel pipes are generally caused by bulges on the rollers and oxide scale adhered to the rollers.

1. Linear indentations on steel pipes

(1) Continuous linear indentations

This is caused by poor roll position matching between the rollers. It often appears between the roll gaps. The indentation between the top roller and the side roller is the most common indentation. In addition, the side roller and the bottom roller may also produce indentations.

Cause: Generally caused by improper roll position matching between the side roller and the top roller.

Adjustment method: This can be done by raising the position of the extrusion top roller or reducing the distance between the two side rollers.

Causes and adjustment methods:

a. The height of one of the extrusion top rollers is too low. Adjust the height of the single extrusion top roller.

b. The extrusion side rollers are asymmetrical. Due to different forces, the side roller position on the side that produces the indentation can be tightened, or the side roller position on the side that does not produce the indentation can be loosened.

c. Caused by different side roller heights.

d. The distance between the two upper extrusion rollers and the weld seam is not equal due to the misalignment of the upper extrusion rollers. Adjust the upper extrusion roller frame horizontally.

(2) Discontinuous linear indentation. Common causes:

a. Generally, the roller bearing is damaged.

b. On the split roller, the roller screw is partially broken, causing the roller components to deviate from their original position and causing indentations.

2. Crescent-shaped indentation on steel pipes. Generally, the roller surface is worn, and there is an uneven contact surface, which causes crescent-shaped indentations 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 on steel pipes

Cause 1: There are protrusions on the roller, which cause pits on the steel pipe. The cause is the same as that of the crescent-shaped indentation.

Cause 2: Indentation caused by the sizing upper roller. Due to the large amount of oxide scale generated at the weld seam after the medium-frequency heat treatment, it adheres to the surface of the upper sizing roller after passing through the sizing rollers. If the emulsion spray is improperly positioned or the emulsion flow rate is too low, the oxide scale is not washed away. Over time, the oxide scale accumulates, eventually causing pitting near the weld seam of the welded steel pipe.

Fourth, High-Frequency Sparking

High-frequency sparking is very common during high-frequency steel pipe welding, seriously affecting the quality of high-frequency welding 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 rollers.

2. Burrs on the edges of the sheet after slitting. Burrs on the sheet edges 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 from 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 steel pipe forming welds, 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 the formation of welding can the weld condition be quickly and accurately assessed.