To ensure that the quality of high-frequency welded pipe products meets technical standards and customer needs during the production process, it is essential to analyze the factors affecting product quality in steel pipe production. By analyzing the statistics of non-conforming products from a certain Φ76mm high-frequency welded steel pipe unit for a specific month, it is believed that there are seven factors affecting the quality of steel pipe products during the production process: raw materials, welding process, roll adjustment, roll material, equipment failure, production environment, and other reasons. Among them, raw materials account for 32.44%, welding process for 24.85%, and roll adjustment for 22.72%, totaling 80.01%, which are the main factors. The factors such as roll material, equipment failure, production environment, and other reasons account for 19.99%, which are relatively minor. Therefore, in the steel pipe production process, emphasis should be placed on controlling three aspects: raw materials, welding process, and roll adjustment.

Impact of Raw Materials on Welded Pipe Quality

Factors affecting the quality of raw materials mainly include unstable mechanical properties of steel strips, surface defects of steel strips, and large geometric dimensional deviations. Therefore, emphasis should be placed on controlling these three aspects. The mechanical properties of steel strips have a significant impact on the quality of welded pipes. Commonly used steel grades for welded pipes are carbon structural steels, with main grades including Q195, Q215, Q235, SPCC, SS400, SPHC, and others. If the yield point and tensile strength of the steel strip are too high, it will cause difficulties in forming the steel strip, especially when the pipe wall is thick, leading to large deformation stress during welding and easy crack formation in the weld. When the tensile strength of the steel strip exceeds 635 MPa and the elongation is less than 10%, cracks are prone to occur in the weld during the welding process. If the tensile strength is less than 300 MPa, wrinkles are prone to occur on the surface of the steel strip during forming due to the soft material. Therefore, the mechanical properties of the material have a significant impact on the quality of the steel pipe, and effective control should be carried out from the perspective of material strength.

Impact of Surface Defects of Steel Strips on Pipe Quality

Common surface defects of steel strips include sickle bends, waves, longitudinal shear nibbling, etc. Sickle bends and waves generally occur during the cold rolling process of steel strips due to improper control of the amount of reduction. During the pipe forming process, sickle bends and waves can cause the strip to deviate or flip, easily leading to overlap welding in the weld seam and affecting the quality of the steel pipe. Edge nibbling of the steel strip (i.e., the phenomenon where the edge of the steel strip appears jagged and uneven) generally occurs on longitudinal sheared strips due to dull or blunt cutting blades of the longitudinal shearing machine. Edge nibbling leads to local thinning of the steel strip, making it prone to cracks and affecting the stability of the weld seam quality during welding.

Impact of Geometric Dimensions of Steel Strips on Pipe Quality

When the width of the steel strip is smaller than the allowable deviation, the extrusion force during welding of the steel pipe decreases, resulting in weak welding at the weld seam or open pipes. When the width of the steel strip exceeds the allowable deviation, the extrusion force during welding of the steel pipe increases, leading to welding defects such as sharp edges, overlap welding, or burrs at the weld seam. 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 that require the wall thickness to not exceed a specified value, i.e., steel pipes that require high uniformity in wall thickness, fluctuations in the thickness of the steel strip will transfer the allowable deviation of the thickness of the same coil of steel strip to the wall thickness deviation of the finished steel pipe, resulting in a large number of steel pipes being rejected due to thickness exceeding the allowable deviation. Thickness fluctuation not only affects the thickness accuracy of the finished steel pipe but also, due to the uneven thickness of the steel strip, unstable extrusion force and welding temperature occur during welding, resulting in unstable weld seam quality during welding. Additionally, material defects such as laminations, impurities, blowholes, etc., inside the steel also significantly impact the quality of the steel pipe. Therefore, before welding the steel strip, the surface quality and geometric dimensions of each coil of steel strip should be inspected, and production should not be carried out if the quality of the steel strip does not meet the standard requirements to avoid unnecessary losses.

Impact of High-Frequency Welding on Pipe Quality

During the high-frequency welding process of steel pipes, the control of welding process and process parameters, placement of induction coils and impeders, etc., has a significant impact on the welding quality of steel pipe welds.

Control of Weld Seam Gap

After the steel strip enters the welded pipe unit and is formed by forming rolls and guided rolls, it forms a round pipe blank with an open gap. Adjust the compression amount of the squeeze rolls to control the gap between the weld seams to 1~3mm and keep the ends of the weld flush. If the gap between the weld seams is too large, it will result in poor welding of the weld seam, leading to lack of fusion or cracking. If the gap between the weld seams is too small, due to excessive heat, the weld seam may burn, and molten metal may splash, affecting the stability of the weld seam quality.

Adjustment of Induction Coil Position

The induction coil should be placed on the same centerline as the steel pipe. The distance from the front end of the induction coil to the centerline of the squeeze roll should be as close as possible to the specifications of the steel pipe without burning the squeeze roll. If the induction coil is too far from the squeeze roll, the effective heating time is longer, the heat-affected zone is wider, and the strength of the weld seam of the steel pipe is reduced or not welded through during welding; conversely, the induction coil is easily destroyed.

Adjustment of Impeder Position

The impeder is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impeder 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 and the edge of the pipe blank weld seam, generate proximity effects, and concentrate eddy current heat near the edge of the pipe blank weld seam, heating the edge of the pipe blank to the welding temperature. The impeder should be placed in the heating section of the V-shaped area, and the front end should be positioned at the center of the squeeze roll so that its centerline coincides with the centerline of the pipe body. If the impeder position is not well placed, it will affect the welding speed and welding quality of the welded pipe, causing cracks in the steel pipe.

Control of High-Frequency Welding Process Parameters—Managing Input Heat

The heat supplied to the welded pipe seam from the high-frequency power source is referred to as input heat. When converting electrical energy into thermal energy, the formula for input heat is as follows: Q = KI^2 Rt (1) where Q—heat input to the billet; K—energy conversion efficiency; I—welding current; R—circuit impedance; t—heating time. Heating time: t = Lv (2), where L—distance from the front end of the induction coil or electrode head to the center of the squeeze roll; v—welding speed. Insufficient input heat and excessive welding speed result in the heated edge of the pipe not reaching the welding temperature, causing the steel to remain in its solid-state and forming unfused or underwelded cracks. Conversely, excessive input heat and slow welding speed lead to overheating, causing the weld to burn through, resulting in metal splashing and shrinkage holes. From equations (1) and (2), it is evident that controlling the magnitude of high-frequency input heat can be achieved by adjusting the high-frequency welding current (voltage) or welding speed, ensuring the weld seam of the steel pipe is fully penetrated without burn-through, achieving high-quality welding.

Impact of Roll Adjustment on Pipe Quality

The impact of roll adjustment on pipe quality is evident from the cause-and-effect analysis chart of pipe defects. During the production process, when rolls are damaged or worn out, partial rolls need replacement, or a sufficient quantity of a particular variety has been continuously produced, requiring the replacement of the entire set of rolls. In such cases, roll adjustment should be carried out to ensure good pipe quality. Poor roll adjustment can lead to defects such as twisting, overlap welding, edge waves, bulging, surface indentation, or scratches on the pipe body surface, and large ellipticity of the pipe. Therefore, mastering the technique of roll adjustment is essential during roll replacement.

Generally, when changing pipe specifications, the entire set of rolls is replaced.

The method of roll adjustment involves pulling a wire from the entrance to the exit of the unit to establish a central line and adjust each stand hole to align on a single central line while ensuring the forming bottom line meets technical requirements. After changing the roll specifications, comprehensive adjustment is first performed on the forming roll, guide roll, squeeze roll, and sizing roll, followed by focusing on adjusting the closed hole shape of the forming roll, guide roll, and squeeze roll.

The function of the guide roll is to control the direction of the pipe seam and the height of the pipe billet bottom line, alleviate edge extension, control the rebound of the pipe billet edge, and ensure a straight pipe seam without twisting entering the squeeze roll. Poor adjustment of the guide roll can cause welding defects such as twisting, overlap welding, and edge waves during the welding process of the pipe.

The squeeze roll is a key equipment in the welded pipe unit, responsible for completing the pressure welding of the pipe body whose edge has been heated to the welding temperature under the action of the squeeze roll's extrusion force. During the production process, the size of the squeeze roll opening angle needs to be controlled. When the extrusion force is too small, the strength of the weld metal decreases, leading to cracking under stress; when the extrusion force is too large, it reduces the welding strength and increases the amount of external burrs, leading to welding defects such as overlap welding.

During the slow start-up of the welded pipe unit, close attention should be paid to the rotation of each roll position, and rolls should be adjusted promptly to ensure that the welding quality and process dimensions of the welded pipe meet specified requirements.