In the production of
high-frequency welded steel pipes, ensuring product quality meets technical standards and customer needs requires an analysis of factors affecting product quality.
Based on statistics of defective products from a 76mm high-frequency welded steel pipe production line in a particular month, it was determined that seven factors influencing steel pipe quality during production are raw materials, welding process, roll adjustment, roll material, equipment failure, production environment, and other factors. Raw materials accounted for 32.44%, the welding process for 24.85%, and roll adjustment for 22.72%, representing a combined 80.01% of the total, making them the primary factors. Roll material, equipment failure, production environment, and other factors, however, accounted for 19.99% of the impact on steel pipe quality, making them relatively minor factors. Therefore, in the steel pipe production process, raw materials, the welding process, and roll adjustment should be the key factors to be controlled.
First, the impact of raw materials on steel pipe welding quality.
The main factors affecting raw material quality are unstable mechanical properties of the steel strip, surface defects, and large geometric dimensional deviations. Therefore, these three aspects should be focused on.
1. The Impact of the Mechanical Properties of the Steel Strip on Steel Pipe Quality.
Carbon structural steel is commonly used for welded steel pipes, with major grades including Q195, Q215, Q235, SPCC, SS400, and SPHC. Excessively high yield points and tensile strengths of the steel strip make forming difficult. This is especially true when the pipe wall is thick, as the material's resilience is high. This creates significant deformation stress during welding, making weld cracks more likely. When the tensile strength of the steel strip exceeds 635 MPa and the elongation is less than 10%, the weld is prone to cracking during welding. When the tensile strength is less than 300 MPa, the surface of the steel strip is prone to wrinkling during forming due to its soft material. As can be seen, the mechanical properties of the material significantly impact the quality of steel pipes, and effective control of steel pipe quality should be implemented from the perspective of material strength.
2. The Impact of Steel Strip Surface Defects on Steel Pipe Quality.
Common steel strip surface defects include camber, ripples, and slitting edge defects. Camber and ripples typically occur during the cold-rolling process and are caused by improper reduction control. During the steel pipe forming process, camber and ripples can cause the strip to deviate or flip, potentially resulting in overlapping welds and affecting pipe quality. Slitting edge defects (i.e., jagged, uneven edges) typically occur on slitting strips and are caused by blunt or unsharp blades in the slitting machine. Slitting edge defects often result in localized gaps in the strip, making it susceptible to cracks and fissures during welding, impacting weld quality and stability.
3. The Impact of Steel Strip Geometric Dimensions on Steel Pipe Quality. When the width of the steel strip is less than the allowable deviation, the extrusion pressure during pipe welding is reduced, resulting in a weak weld and cracks or split pipes. When the width of the steel strip is greater than the allowable deviation, the extrusion pressure increases, causing weld defects such as beaks, lap welds, or burrs. Therefore, fluctuations in the steel strip width not only affect the accuracy of the steel pipe's outer diameter but also seriously affect its surface quality. For steel pipes that require wall thickness variations within the same section to not exceed the specified value, i.e., those that require high wall thickness uniformity, fluctuations in the steel strip thickness can transfer excess thickness variations within the same roll of steel strip to the wall thickness variations of the finished steel pipe, causing a large number of pipes to exceed the allowable deviation and be rejected. Thickness fluctuations not only affect the thickness accuracy of the finished steel pipe, but also, due to the varying thickness of the steel strip, the extrusion pressure and welding temperature during welding are unstable, resulting in unstable weld quality. Furthermore, internal defects such as interlayers, impurities, and sand holes in the steel are also a significant factor affecting steel pipe quality. Therefore, before welding a steel strip, the surface quality and geometric dimensions of each roll must be inspected. Steel strips that do not meet standard requirements should not be produced to avoid unnecessary losses.
Second, the Impact of High-Frequency Welding on Steel Pipe Quality
During high-frequency welding of steel pipes, the welding process and process parameter control, as well as the placement of the induction coil and resistor, significantly influence the weld quality of the steel pipe.
1. Controlling the Weld Gap of the Steel Pipe.
After the steel strip enters the steel pipe welding unit, it is formed by forming rollers and oriented by guide rollers to form a round steel pipe billet with an open gap. The extrusion of the extrusion rollers is adjusted to maintain the weld gap between 1 and 3 mm and ensure that both ends of the weld are flush. Excessively large weld gaps can result in poor welds, resulting in incomplete fusion or cracking. Excessively small weld gaps can lead to excessive heat, causing burns and molten metal splashing, which can affect weld quality.
2. Controlling the Position of the High-Frequency Induction Coil. The induction coil should be placed on the same centerline as the steel pipe. The distance between the front end of the induction coil and the centerline of the squeeze roller should be as close as possible to the pipe's specifications, without damaging the squeeze roller. If the induction coil is too far from the squeeze roller, the effective heating time will be longer and the heat-affected zone will be wider, resulting in reduced weld strength or incomplete penetration. Conversely, the induction coil may damage the squeeze roller.
3. Adjusting the Impeder Position.
The impeder is a magnetic rod or a group of magnetic rods specifically designed for steel pipe welding. The cross-sectional area of the impeder should generally be no less than 70% of the pipe's inner diameter. Its function is to create an electromagnetic induction loop between the induction coil, the weld edge of the pipe, and the magnetic rods, generating a proximity effect. This concentrates eddy current heat near the weld edge of the pipe, heating the pipe edge to the welding temperature. The impeder should be placed in the V-shaped heating section, with the front end at the center of the squeeze roller, so that its centerline aligns with the centerline of the pipe barrel. Improper impeller placement can affect the welding speed and quality of the pipe and may cause cracks in the pipe.
Third, high-frequency welding process parameters - control of input heat
The heat input to the weld of the steel pipe by the high-frequency power supply is called input heat. When converting electrical energy into thermal energy, the formula for input heat is: Q=KI2Rt(1) where Q is the heat input to the pipe blank; K is the energy conversion efficiency; I is the welding current; R is the loop impedance; and t is the heating time. Heating time: t=Lv(2) where L is the center distance from the front end of the induction coil or electrode head to the extrusion roller, and 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 steel pipe body cannot reach the welding temperature, and the steel still maintains its solid structure and cannot be welded, forming cracks that are not fused or not welded through. When the high-frequency input heat is too large and the welding speed is too slow, the edge of the heated steel pipe body exceeds the welding temperature, which is prone to overheating or even overburning, causing the weld to break down, resulting in metal splashing and shrinkage holes. From formulas (1) and (2), it can be seen that the 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 is both fully welded and not welded through, and a steel pipe with excellent welding quality is obtained.
1. The influence of roller adjustment on steel pipe quality.
Roller adjustment is an operating process for steel pipes. During the production process, when the rollers are damaged or severely worn, some rollers need to be replaced on the unit, or when a certain variety is continuously produced in sufficient quantity, the entire set of rollers needs to be replaced. At this time, the rollers should be adjusted to obtain good steel pipe quality. If the rollers are not adjusted well, it is easy to cause defects such as twisting of the steel pipe seam, lap welding, edge waves, bulging, indentations, or scratches on the steel pipe body surface, and large ovality of the steel pipe. Therefore, the skills of roller adjustment should be mastered when changing rollers.
2. When changing the steel pipe specifications, the entire set of rollers is generally replaced. Roller adjustment is accomplished by drawing a centerline from the mill inlet to the outlet using a steel wire. Adjustments are then made to align the pass profiles of the various rolls and ensure the bottom line of the forming rollers meets technical requirements. After changing roll specifications, the forming rolls, guide rolls, squeeze rolls, and sizing rolls should be fully adjusted first. Then, adjustments should be made to the closing pass profiles of the forming rolls, the guide rolls, and the squeeze rolls.
The guide rolls control the direction of the pipe seam and the bottom line height of the tube billet, mitigate edge extension, and control edge springback, ensuring a straight pipe seam without twisting before entering the squeeze rolls. Improper guide roll adjustment can easily lead to weld defects such as twisting, lap welds, and edge ripples during welding. The squeeze rolls are key components of the steel pipe welding line. Their function is to pressure weld the pipe body, heated to welding temperature, under the extrusion force of the squeeze rolls. During production, the squeezing roll opening angle must be carefully controlled. If the extrusion pressure is too low, the weld metal strength decreases, leading to cracking under stress. If the extrusion pressure is too high, the weld strength is reduced, and the amount of external burrs increases, which can easily cause weld defects such as lap welds.
During the slow start-up of the steel pipe welding unit, the rotation of the rollers at various locations should be closely monitored and adjusted continuously to ensure that the weld quality and process dimensions of the welded steel pipe meet specified requirements.
