What is ERW steel pipe welding rod?

What is ERW steel pipe welding rod?

Steel pipes often need to be flattened, bent, punched, pressed, etc. in use. These processes have very strict requirements on the welding of our pipes. Let's first understand the process of pipe production.

High-frequency straight seam welded steel pipe is formed by rolling a certain specification of long steel strip into a round pipe through the front row of the pipe-making unit and welding it through high-frequency induction welding. The first half is called forming, and the second half is called sizing. . The style of the steel pipe can be round, square or special-shaped, which depends on the sizing and rolling after welding.

The main materials of welded steel pipes are: low carbon steel and low boron steel with σs≤300N/mm2, σs≤500N/mm2 or other steels. The influence of raw materials on the welding quality of steel pipes The factors affecting the quality of raw materials mainly include three aspects: unstable mechanical properties of steel strips, surface defects of steel strips, and large deviations in geometric dimensions. Therefore, key control should be carried out from these three aspects.

1) The influence of the mechanical properties of the steel strip on the quality of the steel pipe The commonly used steel grades for welded steel pipes are carbon structural steels, and the main grades are Q195, Q215, Q235 SPCC SS400 SPHC and so on. If the yield point and tensile strength of the steel strip are too high, it will be difficult to form the steel strip, especially when the pipe wall is thick, the resilience of the material is large, the steel pipe has a large deformation stress during welding, and the weld is prone to cracks. When the tensile strength of the steel strip exceeds 635 MPa and the elongation is lower than 10%, the weld seam of the steel strip is prone to cracking during welding. When the tensile strength is lower than 300MPa, the surface of the steel strip is prone to wrinkle due to the soft material during the forming process. It can be seen that the mechanical properties of materials have a great influence on the quality of steel pipes, and the quality of steel pipes should be effectively controlled from the aspect of material strength.

2) The influence of steel strip surface defects on the quality of steel pipes The common surface defects of steel strips include camber, wave shape, longitudinal shear and gnawing edge, etc. camber bend and wave shape generally appear in the rolling process of cold-rolled steel strip, and are Caused by improper control of the amount of reduction. During the forming process of the steel pipe, the camber and wavy shape will cause the deviation or overturn of the strip steel, which will easily cause lap welding of the steel pipe weld seam and affect the quality of the steel pipe. The gnawing edge of the steel belt (that is, the jagged and uneven phenomenon on the edge of the steel belt) generally occurs on the slitting belt, and the reason is that the disc blade of the slitting machine is blunt or not sharp. Due to the edge gnawing of the steel strip, there is a local lack of meat from time to time, which makes the steel strip prone to cracks and cracks during welding, which affects the stability of the weld quality.

3) The influence of the geometric dimensions of the steel strip on the quality of the steel pipe When the width of the steel strip is less than the allowable deviation, the extrusion force when welding the steel pipe is reduced, which makes the weld of the steel pipe weld weak, cracks or open pipes appear; when the steel strip When the width of the steel pipe is greater than the allowable deviation, the extrusion force when welding the steel pipe will increase, and welding defects such as sharp mouth, lap welding or burrs will appear at the weld of the steel pipe. Therefore, the fluctuation of the width of the steel strip not only affects the accuracy of the outer diameter of the steel pipe, but also seriously affects the surface quality of the steel pipe. For steel pipes that require that the wall thickness difference of the same section does not exceed the specified value, that is, steel pipes that require a high degree of wall thickness uniformity, fluctuations in the thickness of the steel strip will transfer the allowable value of the excess thickness difference of the same coil of steel strip to the wall thickness difference of the finished steel pipe. , so that the thickness of a large number of steel pipes exceeds the allowable deviation and is scrapped. The fluctuation of the thickness not only affects the thickness accuracy of the finished steel pipe, but at the same time, due to the different thickness of the steel strip, the extrusion force and welding temperature of the steel pipe are unstable during welding, resulting in unstable weld quality when the steel pipe is welded.

In addition, due to the existence of interlayers, impurities, trachoma and other material defects inside the steel, it is also an important factor affecting the quality of the steel pipe. Therefore, before the steel strip is welded, it is necessary to check the surface quality and geometric dimensions of each coil of steel strip. If the quality of the steel strip does not meet the standard requirements, do not produce it to avoid unnecessary losses.

Influence of High Frequency Welding on Steel Pipe Quality

In the process of high-frequency welding of steel pipes, the welding process and the control of process parameters, the placement of induction coils and impedances, etc. have a great influence on the welding quality of steel pipe welds.

1) Control of weld seam gap of steel pipe The steel strip enters the welded pipe unit and is formed by the forming roller and oriented by the guide roller to form a circular steel pipe billet with an opening gap. Adjust the extrusion amount of the extrusion roller so that the weld seam gap is controlled at 1 ~ 3mm, and keep both ends of the welding joint flush. If the weld gap is controlled too large, the weld will be poorly welded and cause infusion or cracking; if the weld gap is controlled too small, the weld will be burned and the molten metal will splash due to excessive heat, which will affect the welding quality of the weld. .

2) The position of the high-frequency 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 determined according to the specifications of the steel pipe under the premise of not burning the extrusion roller. Get as close as possible. If the induction coil is far away from the extrusion roller, the effective heating time will be longer and the heat-affected zone will be wider, so that the strength of the weld seam of the steel pipe will decrease or the penetration will not be penetrated; otherwise, the induction coil will easily burn the extrusion roller.

3) Adjustment of the position of the impedance (that is, the magnetic rod or carbon rod we are talking about), the impedance is one or a group of special magnetic rods for welded pipes, and the cross-sectional area of the impedance should usually not be less than 70% of the inner diameter of the steel pipe. Its function is to make the induction coil, the weld edge of the tube blank and the magnetic rod form an electromagnetic induction loop, which produces a proximity effect, and the eddy current heat is concentrated near the weld edge of the tube blank, so that the edge of the tube blank is heated to the welding temperature. The resistor should be placed in the heating section of the V-shaped area, and the front end should be at the center of the extrusion roller, so that its centerline is consistent with the centerline of the tube. If the position of the impedance is not well placed, it will affect the welding speed and welding quality of the welded pipe, and cause cracks in the steel pipe.

High-frequency welding process parameters - control of input heat The heat input by the high-frequency power supply to the weld part of the steel pipe is called the input heat. When converting electrical energy into thermal energy, the formula for input heat is: Q=KI2 Rt

(1) In the formula, Q—the heat of input tube; K—energy conversion efficiency; I—welding current; R—loop impedance; t—heating time, heating time: t=Lv.

(2) In the formula, L—the center distance from the induction coil or the front end of the electrode head to the squeeze roller; v—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 still maintains its solid structure and cannot be welded, forming cracks that are not fused or not 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 is prone to overheating or even overburning, causing weld breakdown, causing metal spatter and forming shrinkage cavities.

From the formulas (1) and (2), it can be known that the high-frequency welding current (voltage) or the welding speed can be adjusted to control the high-frequency input heat, so that the weld of the steel pipe can be welded without penetration. Welding through to obtain steel pipes with excellent welding quality. Now more popular is the solid-state high frequency, which is 25% to 30% more efficient than the tube.

The effect of roll adjustment on steel pipe quality From the causal analysis diagram of steel pipe waste, it can be seen that roll adjustment is an operation process for steel pipes. During the production process, when the rolls are damaged or worn seriously, some rolls need to be replaced on the unit, or the entire set of rolls needs to be replaced if a certain variety has been produced in sufficient quantity. At this time, the roll should be adjusted to obtain good steel pipe quality. If the rolls are not adjusted properly, it is easy to cause defects such as twisting of the pipe seam, lap welding, edge waves, bulges, indentations or scratches on the surface of the pipe body, and large ellipticity of the steel pipe. Therefore, the adjustment of the rolls should be mastered when changing the rolls. Skill.

1) When changing the specification of the steel pipe, the whole set of rolls is generally replaced. The method of roll adjustment is: use a steel wire to pull a center line from the entrance to the exit of the unit, and adjust it so that the passes of each frame are on a center line, and make the forming bottom line meet the technical requirements. After changing the specifications of the rolls, first make a comprehensive adjustment to the forming rolls, guide rolls, extrusion rolls, and sizing rolls, and then focus on the adjustment of the closed pass, guide rolls, and extrusion rolls of the forming rolls.

2) The role of the guide roller is to control the pipe seam direction of the steel pipe and the bottom line height of the pipe blank, relieve the edge extension, control the rebound of the pipe blank edge, and ensure that the pipe seam is straight without twisting and enters the extrusion roller. If the adjustment of the guide roller is not good, it is easy to cause welding defects such as torsion, lap welding, and edge waves of the steel pipe seam during the welding process of the steel pipe.

3) The extrusion roller is the key equipment of the welded pipe unit, and its function is to complete the pressure welding of the pipe body whose edge is heated to the welding temperature under the extrusion force of the extrusion roller. In the production process, it is necessary to control the size of the opening angle of the extrusion roller. If the extrusion force is too small, the strength of the weld metal will decrease, and cracks will occur after being stressed; when the extrusion force is too large, the welding strength will be reduced, and the amount of external burrs will increase, which will easily cause welding defects such as lap welding.

4) During the slow start of the welded pipe unit, pay close attention to the rotation of the rolls in each part, and adjust the rolls at any time to ensure that the welding quality and process size of the welded pipe meet the specified requirements.