1 Online weld quality rapid detection
1.1 Feeding detection: The steel strip entering the welded pipe forming unit is focused on detecting its size and plate edge quality to ensure that the plate width, wall thickness, and feeding direction meet the process requirements. Generally, digital calipers, digital wall thickness micrometers, and tape measures are used to quickly measure the plate width and wall thickness, and the plate edge quality is quickly detected by comparison charts or special tools. Generally, the inspection frequency is determined according to the furnace number or the volume number, and the head and tail of the plate are measured and recorded. If conditions permit, the edge of the steel strip must also be inspected to ensure that there are no defects such as delamination or cracks on the steel strip and its processed edges. At the same time, the raw materials with processed edges must also be prevented from mechanical damage to the edge of the steel strip when they are transported to the welded pipe production line.
1.2 Forming detection: The key to plate and strip forming is to prevent excessive tensile stress on the edge of the strip to avoid the formation of wave bends. The relevant inspection items in the installation and commissioning of the forming unit include the rapid inspection and recording of the dimensions and gaps of the forming, finishing and sizing rollers, the circumference variables of the strip, the curling of the strip edge, the welding angle, the plate edge docking method, the extrusion amount, etc. Digital calipers, angle gauges, feeler gauges, tape measures, tape measures, and corresponding special tools are often used for rapid measurement to ensure that each control variable is within the range required by the production process specifications.
1.3 Pre-welding inspection: After adjusting and recording the various parameters of the forming unit, the pre-welding inspection mainly determines the specifications and positions of the internal and external burr cutters, impedance devices, and sensors, the state of the forming liquid and the air pressure value and other environmental factors to meet the startup requirements determined by the process specifications. The relevant measurements are mainly based on the operator’s experience, supplemented by tape measures or special instruments, and are quickly measured and recorded.
1.4 In-welding inspection: During welding, focus on the values of the main parameters such as welding power, welding current voltage, and welding speed. Generally, they are directly read and recorded by the corresponding sensors or auxiliary instruments in the unit. According to the relevant operating procedures, it is sufficient to ensure that the main welding parameters meet the requirements of the process specifications.
1.5 Post-weld inspection: Post-weld inspection needs to pay attention to welding phenomena such as welding spark state and post-weld burr morphology. Generally, the weld color, spark state, internal and external burr morphology, hot zone color, and wall thickness variables at the extrusion roller during welding are key inspection items. It is mainly based on the actual production experience of the operator, and the naked eye is monitored and supplemented by relevant comparison maps to quickly measure and record, and ensure that the relevant parameters meet the requirements of the process specifications.
1.6 Metallographic inspection: Compared with other inspection links, metallographic inspection is difficult to carry out on-site, generally takes a long time, and directly affects production efficiency. Therefore, it is of great practical significance to optimize the metallographic inspection process, improve inspection efficiency, and achieve rapid evaluation.
1.6.1 Optimization of sampling links: In the selection of sampling points, there are generally finished pipe sampling, flying saw point sampling, and pre-sizing sampling. Considering that cooling and sizing have little effect on weld quality, it is recommended to sample before sizing. In terms of sampling methods, gas cutting, metal saws, or manual grinding wheels are generally used. Due to the small sampling space before sizing, it is recommended to use electric grinding wheels to cut samples. For thick-walled pipes, gas-cutting sampling efficiency is higher, and each company can also design relevant special tools to improve sampling efficiency. In terms of sampling size, in order to reduce the inspection area to improve sample preparation efficiency, on the premise of ensuring the integrity of the weld, the sample is generally 20 mm × 20 mm and above. For upright microscopes, when sampling, the inspection surface should be parallel to its opposite side as much as possible to facilitate focusing measurement.
1.6.2 Optimization of sample preparation: The sample preparation process generally uses manual grinding and polishing of metallographic samples. Because the hardness of most welded pipes is low, 60 mesh, 200 mesh, 400 mesh, and 600 mesh sandpaper can be used for water grinding, and then 3.5 μm diamond spray particle canvas is used for rough polishing to remove visible scratches, and then water or alcohol-moistened woolen polishing cloth is used for fine polishing. After obtaining a clean and bright inspection surface, it is directly dried with hot air from a hair dryer. If the relevant equipment is in good condition, sandpaper, and other materials are properly prepared, and the processes are connected conveniently, the sample preparation can be completed within 5 minutes.
1.6.3 Optimization of the corrosion process: The metallographic inspection of the weld mainly detects the center width and streamlined angle of the fusion line in the weld area. In practice, a supersaturated picric acid aqueous solution is heated to about 70°C and corroded until the light disappears before taking it out. After wiping the stains on the corrosion surface with absorbent cotton in the water flow, it is rinsed with alcohol and blown dry with hot air from a hair dryer. To improve the preparation efficiency, picric acid can be poured into a large beaker, added with water and a little detergent or hand soap (to act as a surface active agent), and stirred evenly to make a supersaturated aqueous solution at room temperature (with obvious crystal precipitation at the bottom) and placed for use. When actually used, after stirring and the bottom precipitation rises, the suspension is poured into a small beaker for heating and can be used. To improve the corrosion efficiency, the corrosion solution can be heated to the specified temperature in advance according to the production sample delivery time point before the test and kept warm for use. If corrosion needs to be further accelerated, the heating temperature can be increased to about 85°C. A skilled tester can complete the corrosion process within 1 minute. If the measurement of organization and grain size is required, a 4% nitric acid alcohol solution can also be used for rapid corrosion.
1.6.4 Optimization of the inspection process: The metallographic inspection process includes fusion line inspection, streamline inspection, waist drum morphology inspection, metallographic organization and banded organization evaluation of the base material and heat affected zone, and grain size rating. Among them, fusion line inspection includes fusion line inclusion, inner, middle, and outer width, fusion line skew, etc.; streamline inspection includes upper, lower, left, and right streamline angles, streamline angle extreme value, streamline center deviation, hook pattern, streamline double peak, etc.; waist drum morphology inspection includes inner, middle and outer width, burr tolerance, misalignment, etc. Waist drum morphology and fusion line can both characterize welding energy and extrusion pressure characteristics, while waist drum shape is also related to steel strip thickness, edge state, welding periodicity, etc., and it is difficult to accurately identify the measurement boundary after corrosion, and there are measurement errors. The metallographic structure and banded structure rating of the parent material, the grain size rating of the parent material, etc. have been inspected during the acceptance of incoming raw materials, and can also be used as reference items during online weld inspection. In order to improve the inspection efficiency, it is necessary to optimize the relevant inspection items according to product requirements. It is recommended to give priority to the inspection of fusion lines and streamline morphology, especially to grasp the two core indicators of the center width of the fusion line and the streamline angle. Under the metallographic microscope, the streamline angles of the four directions of the upper, lower, left, and right of the weld zone are generally measured at 1/4 of the wall thickness, and the center width of the fusion line is measured by magnifying it about 100 times. In order to improve the inspection efficiency, it is recommended to configure the metallographic microscope with corresponding analysis and measurement software for rapid measurement of length and angle. If it cannot be configured, it can be measured with an eyepiece scale or the picture can be printed at a fixed magnification and then measured with a ruler or gauge. The measurement of the above two core data normally takes about 1 minute for the experimenter. Other data can also be quickly measured according to the corresponding specification requirements.
1.7 Large sample inspection: According to the small sample inspection data, the pipeline is further refined, and after adjusting the relevant parameters and meeting the requirements of the process specifications, a steel pipe sample of a specified size needs to be taken for a small sample process test. The process performance test includes a flattening test, bending test, expansion test, curling test, torsion test, longitudinal pressure test, expansion test, water pressure test, internal pass test, etc. Generally, according to the standards or user requirements, samples are taken and tested near the production line according to the operating procedures, and visual judgment is sufficient.
1.8 Full line inspection: All the above-mentioned inspections are carried out according to the sampling of relevant specifications or standards, so it is inevitable that missed inspections will occur. In order to ensure the quality of finished welded pipes, special attention should be paid to the application of online non-destructive testing technology. In the production of welded pipes, the commonly used non-destructive testing methods are ultrasonic testing, eddy current testing, magnetic testing, and radioactive testing. Various flaw detection equipment has a complete detection system, and the application of digital control technology and electronic computers also ensures the reliability of the test results. The inspectors only need to ensure that the inspection equipment works normally according to the relevant operating procedures, monitor the stability of the welding quality, ensure that there are no missed inspections, and isolate the defective welded pipes that exceed the standard in time.
2 Rapid assessment and diagnosis of online weld quality
2.1 Rapid assessment and diagnosis in the initial machine adjustment stage: The main assessment indicators in the initial machine adjustment stage include dimensional variables (such as plates, tubes, gaps, extrusion volume, component positions, heights, and angles, etc.), instrument variables (molding liquid conditions, power, current voltage, and speed, etc.) and visual variables (plate connection methods and welding forms, etc.). Dimensional variables and instrument variables can be directly judged by comparing the measured values according to the numerical range required by the actual process specifications. Visual variables generally require the operator to compare the relevant descriptions or reference drawings during processing and make rapid assessments and diagnoses based on the operator’s actual experience.
2.1.1 Rapid assessment and diagnosis of welding sparks: Generally, a welding state without a large number of sparks and no darkening is a normal state. The darkening can be diagnosed as too low welding power or too fast welding speed; a large amount of splashing can be diagnosed as too high welding power or too small a distance between the welding point and the extrusion point or welding angle.
2.1.2 Rapid assessment and diagnosis of welding burrs: The color of the weld just out of the extrusion roller is orange-red. Red and white can be judged as too high temperature (power), and dark red can be judged as too low temperature (power). The weld is straight and uniform, the burr width is large, the height is small, the top is shiny and smooth, and the convex points with slight discontinuous distribution on the line can be judged as moderate temperature and extrusion. According to whether the size of the burrs protruding inside and outside the weld is similar, it can be judged whether the heating of the edge of the material is consistent. If the outer protrusion of the weld is thicker, the heating temperature of the outer edge is higher than that of the inner edge; conversely, the temperature of the inner edge is higher. When the molten material extruded by the outer burr is not in the middle or the inner burr is intermittently split or cracked, and the tool position is normal, it can be judged that the plate joint has a wrong edge.
2.1.3 Rapid evaluation and diagnosis of HAZ color: After removing the external burrs, there is a clear and continuous blue straight thin line on each side of the heat-affected zone. The evaluation standard is that the color in the area between the two lines gradually fades and the axial uniformity is consistent. If the HAZ color is uniformly blue, the welding temperature is too high; if the color is lighter, the welding temperature is too low. If the width or shape of the external weld bead changes after the burrs are removed, it can be inferred that the plate is connected at the wrong edge.
2.2 Rapid evaluation and diagnosis of small sample testing:
2.2.1 Rapid evaluation and diagnosis of fusion line: At present, there is no unified regulation on the control of fusion line width in various countries. The existing standards are generally the internal control standards of each enterprise. For example, Nippon Steel of Japan stipulates that the fusion line width is 0.02~0.2 mm, Kawasaki of Japan is 0.07~0.13 mm, Germany stipulates that it is 0.02~0.12 mm, and PSP of South Korea requires it to be 0.05~0.3 mm. my country’s welded pipe industry once believed that it was most appropriate to control the fusion line width at 0.02-0.11 mm. Some literature also suggested that the fusion line width standard be set as the standard value: fn=0.02-0.14 mm, f0≈fi=1.3-3fn; warning value: fn=0.01-0.02 mm or fn=0.14-0.17 mm, f0≈fi=3-4fn; prohibited value: fn＜0.01 mm or fn＞0.17 mm, f0≈fi＞4fn. The evaluation standard for fusion line deflection or distortion is S≤t/10. Generally, it is not allowed that the length of a single inclusion in the fusion line area is ≥0.05t and inclusions are not allowed in the 15% area close to the inner and outer surfaces. The specific acceptance standards can be formulated by each enterprise after discussion and analysis based on its own production practice. The fusion line shape is closely related to the parameters such as the welding input energy, the size of the welding extrusion force, and the welding speed, and is an important indicator for measuring the quality of the weld.
Adverse consequences Thick fusion line The welding temperature is too high, and the decarburization of the metal surface increases. In most cases, it is caused by insufficient extrusion pressure. Obvious gray spots or oxide inclusions are often produced in the center of the fusion line. Poor shape Cause diagnosis Thin fusion line Extrusion pressure is too large, and the molten metal is squeezed out excessively. The weld is prone to cold welding and flattening test failure. Irregular fusion line Extrusion pressure is highly unbalanced There are fusion lines or S-shaped fusion lines inclined in different directions, complex thermal deformation, and high internal stress. There are oxide inclusions or gray spots in the fusion line. The parallelism of the plate edge is not good or the extrusion pressure is too small so that the oxidized metal surface layer of the plate edge cannot be effectively squeezed out. Gray spots or oxide inclusions often become the crack source of weld cracking.
2.2.2 Rapid assessment and diagnosis of welding flow line: The welding flow line is the most important metallographic feature in weld quality assessment. It is a special shape of the crystalline structure formed by the extrusion of locally molten or semi-molten metal under welding conditions. It is a comprehensive reflection of factors such as extrusion force size, extrusion direction, input heat, and welding speed during welding. There is no unified standard for the rise angle of streamlines in various countries. At present, each country uses its own internal control standard. For example, Japan’s Nippon Steel stipulates that it is 40°~70°, Germany stipulates that the inner wall is 60° and the outer wall is 65°, and the relevant information in my country points out that it is 50°~70°. There are also documents that propose that the evaluation standard of streamline angle can follow the following principles, that is, standard value: 45°~75°, extreme difference ≤10°; warning value: 40°~45° or 75°~80°, extreme difference 10°~15°; prohibited value: <40° or >85°, extreme difference ≥15°. There should be no hook-shaped segregation in the welding streamline area, and the distance between the streamline centerline and the wall thickness centerline should be <t/5. Each welded pipe enterprise can determine the evaluation standard suitable for its own product characteristics according to its own production practice. Adverse consequences Streamline angle is too large Extrusion pressure is too large during welding. Larger extrusion pressure can squeeze out more molten metal, making the metal welding of the plate edge poor and prone to cold welding bad shape Cause diagnosis Streamline angle is too small Extrusion pressure is too small, streamline display is not clear, or even invisible The fusion line in the middle of the weld is often accompanied by more oxide inclusions, which becomes the crack source of weld cracking Unbalanced extrusion pressure can easily cause changes in streamline shape. Some rise angles are too large and streamlines are very thick; some rise angles are too small, and streamlines are thin and not clearly displayed.
If the plate edges are not parallel, it is easy to produce misalignment on the weld, resulting in unidirectional loss of weld metal and stress concentration, and the probability of defects in the weld will also increase Streamline angle asymmetry The plate edge parallelism is not good and it is easy to have positive “V” shape and inverted “V” shape. If the plate edges are not parallel, the high-frequency voltage distribution is uneven, the local temperature difference is significant, and the plate edges cannot be synchronously contacted to achieve tight welding.
When the plate edge appears a positive “V” shape, the inner edge of the weld should contact the outer edge, so the current density of the inner edge should be larger, and the heating temperature should also be higher than the outer edge. Under the same extrusion pressure conditions, the metal streamlines rise angle of the inner wall that contacts first is larger, while the metal streamline rise angle of the outer wall is smaller, and in severe cases, no streamline is even displayed.
On the contrary, when the plate edge appears inverted “V” shape, the outer burr is larger than the inner burr, and its metal streamline rise angle is significantly larger than that of the inner wall of the welded pipe. The unreasonable parallelism of the plate edge may cause the edge of the rolled plate to bend, which makes it easy to make the edge wavy and increases the tendency of gray spots. At the same time, the weld may be dislocated during forming and continue to the weld point, which will cause the solidifying weld metal to be welded or cracked.
2.2.3 Rapid assessment and diagnosis of waist drum and other items: The width of the waist drum is related to the welding temperature, extrusion pressure, steel strip thickness, steel strip trimming, welding cycle, etc., and can be used as a reference indicator for weld quality assessment. An article suggests that the ideal waist drum shape is the center width hn= (1/4~1/3) t, and the inner and outer wall widths h0≈hi≈(1.5~2.2)hn. Similarly, each welded pipe enterprise can determine whether to include it in the assessment content or specify the assessment scope based on its own production reality.
2.3 Rapid assessment and diagnosis of large samples and full-line inspection stages: Large samples and full-line inspections are generally carried out according to the inspection standards specified in the product’s technical requirements. The operator can quickly complete the corresponding assessment and diagnosis by visual inspection or recording relevant inspection data. The focus of non-destructive testing assessment and diagnosis in full-line inspection is the defect calibration and standardized operation of the equipment. If quality problems are found in these two stages, the relevant departments such as design, process, and quality should be asked to comprehensively analyze the causes of the defects. If necessary, the possible problems in the design links such as raw materials, molding, and welding should be considered comprehensively, and the root cause analysis should be conducted in combination with actual production. Various measures including design optimization and process optimization should be taken to eliminate the quality defects that may occur at this stage.
3 Integration, optimization, and prospect of system structure
The online weld quality rapid evaluation and diagnosis system of high-frequency straight seam welded pipe can be divided into four stages: preliminary machine adjustment evaluation and diagnosis, small sample evaluation and diagnosis, large sample evaluation and diagnosis, and full-line evaluation and diagnosis. Among them, the preliminary machine adjustment stage ensures that the values of each process control point meet the corresponding process specification requirements; the small sample evaluation stage further optimizes the machine adjustment data according to the metallographic detection data. If the small sample detection data after the preliminary machine adjustment has met the process specification requirements, batch production can be started directly. Otherwise, further fine-tuning is performed within the specification range of the preliminary machine adjustment until the requirements are met; the large sample evaluation stage focuses on the verification of process performance such as weld strength and toughness. If it does not meet the relevant requirements, after eliminating accidental factors, it is necessary to conduct a full-link cause analysis of design, production, and testing, and supplement or improve relevant design equipment or process parameters to ensure that all subsequent production stages meet the requirements; the full-line detection stage is more focused on monitoring the quality of the weld, preventing welding defects caused by uncertain factors and marking and isolating them to ensure that the quality of all welded pipes leaving the factory is qualified.
In actual production, generally, only when a certain specification of welded pipe is produced for the first time, the initial adjustment, fine adjustment, and repeated adjustment are carried out in the whole stage until the requirements are met, and then the large sample is tested and confirmed, and the whole line detection and monitoring measures are taken to ensure the quality of the weld. With the continuous accumulation of actual production experience, when the same or similar pipes that have been produced before are produced in batches, the control data recorded before are actually repeated or imitated, and the machine adjustment can often be completed in one stage. The subsequent small sample, large sample, and full-line evaluation stage are more for repeated confirmation or real-time monitoring. The actual adjustment and production efficiency advantages are more obvious.
In the whole stage evaluation and diagnosis process, if the relevant operation methods recommended by this study can be applied, and continuous improvement and optimization can be carried out in combination with the actual production, the adjustment of relevant product parameters can be completed in an orderly, efficient and convenient manner to ensure the quality of online welds. If supplemented with relevant data statistics or software application tools, all data parameters can be automatically counted, analyzed, evaluated, and diagnosed directly on the pipeline production operation interface, further improving data processing efficiency and scientifically guiding the corresponding machine adjustment operation. At the same time, the continuous accumulation and improvement of relevant parameters and operating experience in the evaluation and diagnosis system at each stage will not only help to steadily improve the quality and efficiency of pipeline production but also serve as the data basis for the subsequent gradual promotion and application of automated production in the pipeline, helping to further improve production quality and efficiency.
Post time: Mar-12-2025