Common problems and preventive measures in production of HFW welded pipe
In order to improve the product quality and production efficiency of HFW welded pipe, the common problems in the production process of HFW welded pipe, such as steel band edge bulge, steel tube surface scratch, weld seam misalignment and steel band deviation, are analyzed, the causes of the problems are found out, and the corresponding preventive measures are given. The actual production results show that by controlling the size of raw materials, designing reasonable roll pass and process parameters, installing accurate roll position and using lubricating fluid reasonably, the problems of steel strip edge bulge, steel tube surface scratch, weld seam stagger and steel strip deviation can be solved, and the product quality and production efficiency of HFW welded pipe can be effectively improved.
In the production process of HFW welded pipe, forming is the core process of welded pipe. If the forming process design is not reasonable, there will be bulge, scratch, wrong edge and other problems. If the forming process is controlled stably, the residual stress of HFW welded pipe body can be reduced properly to ensure the product quality of welded pipe. The forming quality control of HFW welded pipe is not only related to the adjustment of forming equipment, but also related to the actual work experience of operators. This study summarizes the common problems in the forming process of HFW welded pipe, analyzes the causes of these problems, and gives the specific preventive measures.
Causes and preventive measures of plate edge bulging
In the forming process of HFW welded pipe, the most fundamental reason for bulge is that the plastic stretch of strip edge exceeds the elastic ultimate elongation of the material. In actual production, the plastic stretch of strip edge can be reduced by controlling the size of raw material, designing reasonable roll pass and process parameters, so as to prevent the occurrence of bulge.
- (1) Raw materials. Raw materials meet the requirements, strip edge is neat, sickle bend is strictly controlled within 3 m m/M. If the tolerance range of strip size is strictly controlled, the occurrence of bulge can be prevented.
- (2) Roll pass. Ensure that the rollers are in good condition and the bearings are not damaged, minimize the radial run out of the pass, and ensure that the forming is stable and reliable.
- (3) Process parameters. When adjusting the process parameters of each frame, it is necessary to ensure the accuracy of the parameters, and the position of the roll must meet the relevant requirements of “downhill” molding.
Causes and preventive measures of surface scratch of welded pipe
The scratch on the surface of welded pipe mainly occurs at the contact position between the roller edge and the strip steel. The main reason for the scratch is that there is an on-line speed difference between the linear motion of the tube blank and the circular motion of the roller when the roller is forming, which causes the sliding friction between the roller and the strip steel. The larger the pressing force is, the deeper the scratch will be. In actual production, it can be analyzed and prevented from the following four aspects.
(1) Roll pass. The hole type clearance is shown in Figure 1. The reasonable roll pass design can reduce the pressure on the surface of the strip steel and reduce the surface scratch. In addition, four roll forming can be used in the fine forming part to reduce the linear velocity difference of each rolling surface, so as to reduce the relative sliding and sliding friction between the strip steel and the roll surface.
Fig.1 Diagram of roll pass clearance
(2) Roll installation. The installation position of the roll shall be accurate and ensure that the roll center is consistent with the rolling center of the unit. If the roll center deviates from the rolling center, the stress on one side of the roll will be too large, resulting in the surface scratch of the tube body. The deviation between the roll center and the rolling center of the unit is shown in Figure 2.
Fig.2 Diagram of deviation between roll center and unit rolling center
(3) Roll parameters. When adjusting the roll parameters of each frame, it is necessary to ensure the accuracy of the parameters. The reducing amount of each frame shall be controlled within the appropriate range and the uniform reducing shall be realized as far as possible. The excessive local reducing amount will cause the surface scratch of the pipe body. In addition, set a reasonable roll speed, the actual production of strip steel will have a certain amount of extension, in the speed setting, the speed of each roll is increasing in turn.
(4) Reasonable use of lubricating fluid. The reasonable use of lubricating fluid in the forming process can reduce the friction coefficient between the roller and the strip steel, thus reducing the possibility of the surface scratch of the welded pipe. The reasonable use of lubricating fluid can also extend the service life of the roller. However, the concentration of the lubricating fluid shall be appropriate, and it shall be maintained and replaced regularly, such as cleaning up the particles, iron powder, floating oil and other impurities in the lubricating fluid to prevent the deterioration of the lubricating fluid.
Causes and preventive measures of weld misalignment
The staggered edge defect of HFW welded pipe usually refers to the welding defect caused by the dislocation of two edges of the tube blank during welding, as shown in Figure 3. In actual production, it can be analyzed and prevented from the following three aspects.
Fig.3 Schematic diagram of weld misalignment
(1) Poor molding quality. The parameters of the forming roll are not set properly. Before the tube blank enters the extrusion roll, the two edges of the strip steel change in a wavy shape. After entering the extrusion roll, there will be staggered edges.
(2) Too much extrusion pressure. In general, in the production of thin-walled tube, the rigidity of thin-walled tube blank is relatively poor, and the possibility of staggering caused by excessive extrusion pressure is relatively high. In actual production, the size of extrusion pressure is usually measured by the amount of extrusion. Generally, it is suitable to control the extrusion amount at 50% of the wall thickness of the steel pipe. The proper extrusion quantity can not only prevent the wrong edge, but also effectively control the welding quality.
(3) The extrusion roll moves. The bearing damage, excessive bearing wear, and excessive bearing clearance of the extrusion roller will cause the extrusion roller to move, resulting in uneven extrusion pressure, resulting in misalignment.
Causes of strip deviation and preventive measures
Strip deviation is also known as strip turnover. Strip deviation usually occurs in the process of rough forming, specifically in the process of blank forming, the height difference between the two edges is inconsistent (as shown in Figure 4). When it is serious, it will cause the tube blank to turn over and can’t enter into the fine forming guide ring smoothly, which will force the production line to stop for processing, resulting in product degradation or scrap, and seriously affecting the operation efficiency of the production line. In actual production, it can be analyzed and prevented from the following four aspects.
Fig.4 Diagram of strip deviation
(1) The roll pass center is not correct. During the forming process, the deviation of the pass center of the upper and lower rolls will make the stress of the tube blank uneven, thus causing the tube blank to deviate from the rolling center line (as shown in Figure 5). Therefore, before production, it is necessary to strictly check the positioning parameters of the roller to ensure that the working position of the roller is accurate.
Figure.5 Deviation diagram of pass center of upper and lower rolls
(2) The bearing is damaged. The damage of the roller bearing will cause the change of the reduction at both ends of the roller, and in serious cases, it will cause the roller to tilt, so that the tube blank will run off in the process of movement. When the bearing is seriously jammed, it may damage the relevant parts of the drive end, such as the coupling, reducer, etc. Therefore, before using the roller, the bearing of the roller should be strictly checked to ensure the normal use of the roller.
(3) Insufficient pressure. The insufficient pressure usually refers to the insufficient lower pressure of the upper roll, which is mainly manifested in the excessive gap between the tube blank and the roll pass, resulting in unstable forming and the tube blank swinging back and forth, thus causing the tube blank to deviate.
(4) Roll axial movement. Excessive wear of the roller bearing, excessive bearing clearance and loose fixed parts of the roller will cause roll movement. In the production of small-diameter thin-walled tube, because of the poor rigidity of the tube blank, it is easy to be affected by the roll movement, and the phenomenon of deviation is more likely to occur.
Factors influencing the straightness of HFW welded pipe
Straightness of HFW welded pipe is one of the important indexes to measure the quality of steel pipe. Higher straightness is more conducive to the control of pipe end groove in pipe end chamfering process, and also to improve the efficiency of hydraulic test. It can be seen that straightness will have a certain impact on subsequent processes. In actual production, the straightness of welded pipe can be adjusted and controlled from the following three aspects.
(1) Straightening frame adjustment. The key to control the straightness of HFW welded pipe is to adjust the position of straightening roller correctly. According to the requirements of API spce 5l-2011 standard, the straightness of steel pipe must be controlled within 0.002l (L is the total length of steel pipe). The adjustment amount of straightening roller is mainly affected by the pipe diameter, steel grade, reducing amount of sizing and other factors. This requires accumulation of experience in production practice and mastering the law of change, and continuous fine-tuning in the production process until the straightness meets the customer’s requirements.
(2) The temperature difference between the upper and lower surfaces of the steel pipe is too large. Under normal circumstances, the surface temperature of the steel pipe shall be uniformly reduced to room temperature after it comes out of the cold water tank to meet the requirements of cold sizing. But sometimes the temperature difference between the upper and lower surface of the steel pipe is large, which will lead to thermal expansion and cold contraction, resulting in bending of the steel pipe. In addition, when changing the specification, pay attention to the surface temperature of the steel pipe after water cooling in time to avoid the bending of the steel pipe.
(3) Insufficient sizing. Sometimes the straightness of the steel pipe meets the requirements before the hydrostatic test, but after the hydrostatic test, the straightness of the steel pipe changes or even exceeds the tolerance, which is often caused by the insufficient sizing of the steel pipe and the excessive residual stress. If the sizing amount is insufficient and the residual stress near the weld is too large, the residual stress near the weld will further increase, even exceed the yield limit of the steel pipe, and plastic deformation will occur, resulting in bending of the steel pipe and out of tolerance of straightness. This bending phenomenon can be solved by appropriately increasing the sizing amount, which is generally controlled at about 1%.
In view of the problems in the production of HFW welded pipe, the causes of these problems are analyzed, and the preventive measures are given. The practice shows that these measures and methods can improve the product quality of HFW welded pipe. In addition, the production efficiency of HFW welded pipe is high, but the scrap rate is relatively high. Therefore, in the production process of HFW welded pipe, it is necessary to observe and deal with various problems in the production process at any time, so as to effectively ensure the product quality of HFW welded pipe.
Source: China HFW welded pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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