What are the factors affecting the quality of high-frequency welded steel pipes

In the production process of high-frequency welded steel pipes, to ensure that the product quality meets the requirements of technical standards and customer needs, it is necessary to analyze the factors affecting product quality during the steel pipe production process. The factors affecting the quality of high-frequency welded steel pipes during production include seven aspects: raw materials, welding process, roll adjustment, roll material, equipment failure, production environment, and other reasons. Among these, the main influencing factors are raw materials, welding process, and roll adjustment. The other four factors—roll material, equipment failure, production environment, and other reasons—are relatively minor. Therefore, in the steel pipe production process, the three key aspects of raw materials, welding process, and roll adjustment should be carefully controlled.

First, how much influence do raw materials have on the welding quality of steel pipes?

(1) The influence of the mechanical properties of steel strip on the quality of steel pipes.

The commonly used steel for welded steel pipes is carbon structural steel, with main grades including Q195, Q215, Q235, SPCC, SS400, SPHC, etc. Excessively high yield point and tensile strength of the steel strip will cause difficulties in forming the steel strip, especially when the pipe wall is thick, the material has a large springback force, and the steel pipe has a large deformation stress during welding, making the weld seam prone to cracking. When the tensile strength of the steel strip exceeds 635 MPa, and the elongation is less than 10%, the weld seam is prone to spalling during the welding process. When the tensile strength is less than 300 MPa, the steel strip is prone to wrinkling on the surface during the forming process due to the soft material. It can be seen that the mechanical properties of the material have a great influence on the quality of the steel pipe, and the quality of the steel pipe should be effectively controlled from the perspective of material strength.

(2) The influence of steel strip surface defects on steel pipe quality.

Common steel strip surface defects include camber, wavy shape, and longitudinal shearing. Camber and wavy shapes generally occur during the cold rolling process of steel strip and are caused by improper control of the reduction amount. During the forming process of steel pipe, camber and wavy shapes will cause the strip to deviate or overturn, which can easily cause lap welding of the steel pipe weld seam and affect the quality of the steel pipe. Edge chipping of the steel strip (i.e., the serrated, uneven edge of the steel strip) generally occurs on the slitting strip, caused by dull or dull blades of the slitting machine disc. Due to edge chipping, localized missing sections frequently occur, making the steel strip prone to cracks and fissures during welding, thus affecting the stability of the weld quality.

(3) Influence of steel strip geometry on steel pipe quality.

When the width of the steel strip is less than the allowable deviation, the extrusion pressure during welding the steel pipe decreases, resulting in weak welds at the steel pipe weld, leading to cracks or open pipes. When the width of the steel strip is greater than the allowable deviation, the extrusion pressure during welding the steel pipe increases, resulting in welding defects such as pointed tips, lap welds, or burrs at the steel pipe weld. Therefore, fluctuations in the width of the steel strip not only affect the accuracy of the outer diameter of the steel pipe but also seriously affect the surface quality of the steel pipe. For steel pipes requiring a wall thickness variation within the same cross-section not exceeding a specified value (i.e., pipes demanding high wall thickness uniformity), fluctuations in the steel strip thickness will transfer any excess thickness variation within the same coil of steel strip to the finished steel pipe, causing a large number of pipes to exceed the allowable tolerance and be rejected. Thickness fluctuations not only affect the thickness accuracy of the finished steel pipe, but also, due to the inconsistent thickness of the steel strip, lead to unstable extrusion pressure and welding temperature during welding, resulting in inconsistent weld quality. Furthermore, internal material defects such as inclusions, impurities, and pinholes within the steel are also significant factors affecting pipe quality. Therefore, before welding the steel strip, the surface quality and geometric dimensions of each coil must be inspected. Steel strips that do not meet the standard requirements should not be produced to avoid unnecessary losses.

Second, the impact of the welding process on the quality of high-frequency welded steel pipes.

(1) Control of weld gap.

After the steel strip enters the steel pipe welding unit, it is formed by the forming roller and oriented by the guide roller, forming a circular steel pipe blank with an open gap. The extrusion amount of the extrusion roller is adjusted to control the weld gap within 1-3 mm, and to keep the two ends of the weld flush. If the weld gap is too large, it will result in poor welding, incomplete fusion, or cracking; if the weld gap is too small, the excessive heat will cause the weld to burn out, molten metal to splash, and affect the welding quality.

(2) Adjustment of the position of the high-frequency induction coil.

The induction coil should be placed on the same center line as the steel pipe. The distance between the front end of the induction coil and the center line of the extrusion roller should be as close as possible to the steel pipe specifications, without burning the extrusion roller. If the induction coil is too far from the extrusion roller, the effective heating time is longer, the heat-affected zone is wider, and the strength of the steel pipe weld decreases, or it is not fully penetrated; conversely, the induction coil is prone to burning out the extrusion roller.

(3) Adjustment of Impedance Device Position

An impedance device is one or a group of special magnetic rods for welded steel pipes. The cross-sectional area of the impedance device should generally 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 with the induction coil, the edge of the weld seam of the pipe blank, and the magnetic rod, generating a proximity effect. Eddy current heat is concentrated near the edge of the weld seam of the pipe blank, so that the edge of the pipe blank is heated to the welding temperature. The impedance device should be placed in the V-shaped heating section, and the front end should be at the center of the extrusion roller, so that its center line is consistent with the center line of the pipe. If the impedance device is not placed properly, it will affect the welding speed and welding quality of the welded steel pipe, causing cracks in the steel pipe.

Third, how much influence do high-frequency welding process parameters have on the quality of high-frequency welded steel pipes?

The heat input to the weld seam of the steel pipe by the high-frequency power supply is called the input heat. When electrical energy is converted into thermal energy, the formula for the input heat is: Q=KI2 Rt (1) where Q—the heat input to the pipe blank; K—energy conversion efficiency; I—welding current; R—loop impedance; t—heating time. Heating time: t = Lv (2), where L is the center distance from the induction coil or electrode head to the extrusion roller; v is the welding speed.

When the high-frequency input heat is insufficient, and the welding speed is too fast, the edge of the heated pipe body cannot reach the welding temperature, and the steel remains in its solid structure and cannot be welded, forming cracks that are not fused or penetrated. When the high-frequency input heat is too large, and the welding speed is too slow, the edge of the heated pipe body exceeds the welding temperature, which easily leads to overheating or even burning, causing the weld to break through and resulting in metal spatter and shrinkage cavities. From formulas (1) and (2), it can be seen that the amount of high-frequency input heat can be controlled by adjusting the high-frequency welding current (voltage) or the welding speed, so that the weld of the steel pipe can be both penetrated and not broken through, resulting in a steel pipe with excellent welding quality.

Fourth, how much does the adjustment of the rolls affect the quality of high-frequency welded steel pipes?

During the production process, when the rolls are damaged or severely worn, some rolls need to be replaced on the unit, or a certain variety has been continuously produced in sufficient quantities, and the entire set of rolls needs to be replaced. At this point, the rolls must be adjusted to obtain good steel pipe quality. Improper roll adjustment can easily cause defects such as pipe seam twisting, lap welding, edge wavy lines, bulges, indentations, or scratches on the pipe surface, and excessive ovality. Therefore, roll adjustment techniques must be mastered when changing rolls. Changing steel pipe specifications generally requires replacing the entire set of rolls. The roll adjustment method is as follows: use a steel wire to draw a center line from the unit inlet to the outlet, and adjust it to ensure that the pass patterns of each stand are on the same center line and that the forming bottom line meets the technical requirements. After changing the roll specifications, first perform a comprehensive adjustment of the forming roll, guide roll, extrusion roll, and sizing roll, and then focus on adjusting the closed pass pattern of the forming roll, the guide roll, and the extrusion roll. The guide roll's function is to control the direction of the pipe seam and the bottom line height of the billet, alleviate edge extension, control the edge springback of the billet, and ensure that the pipe seam is straight and does not twist before entering the extrusion roll. If the guide roll is not properly adjusted during the welding process, welding defects such as pipe seam twisting, lap welding, and edge wavy lines can easily occur. The extrusion roller is a key piece of equipment in the steel pipe welding unit. Its function is to pressure weld the pipe body, whose edges have been heated to the welding temperature, under the extrusion pressure of the roller. During production, the opening angle of the extrusion roller must be controlled. If the extrusion pressure is too low, the weld metal strength decreases, and cracking will occur under stress; if the extrusion pressure is too high, the weld strength decreases, and the amount of external burrs increases, easily causing welding defects such as lap welds. During the slow start-up of the steel pipe welding unit, close attention should be paid to the rotation of the rollers in each part, and the rollers should be adjusted as needed to ensure that the welding quality and process dimensions of the welded steel pipe meet the specified requirements.