High-frequency straight seam welded pipe online weld quality rapid evaluation and diagnosis

High-frequency straight seam welded pipe has the advantages of excellent dimensional accuracy, good surface quality, high production efficiency, less investment, short construction period, low production cost, and quick results. It is widely used in various fields of production and life. In the actual production of high-frequency straight seam welded pipes, in addition to the design factors of related equipment and processes such as raw materials, forming, and welding, the debugging technology level of the welded pipe unit directly determines the weld quality and production efficiency of the welded pipe. Since the production of high-frequency straight seam welded pipes in existing small and medium-sized enterprises in my country mainly relies on the experience of the operator to control the weld quality, and the equipment status and human factors, it is often difficult to quickly debug the pipeline unit and continuously and stably ensure the weld quality. The author has studied a set of online weld quality rapid evaluation and diagnosis solutions for small and medium-sized enterprises to easily operate, efficiently, and stably control the weld quality of high-frequency straight seam welded pipes.

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 width and wall thickness of the plate, and comparison charts or special tools are used to quickly detect the quality of the plate edge. Generally, the inspection frequency is determined according to the furnace number or the roll number, and the head and tail of the plate are measured and recorded.
1.2 Forming inspection: The key to plate and strip forming is to prevent the edge of the strip from generating excessive tensile stress 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. 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, resistors, and sensors, the state of the forming liquid and the environmental factors such as the air pressure value, to meet the startup requirements determined by the process specifications. The relevant measurements are mainly based on the operator's experience, supplemented by a tape measure or special equipment, and quickly measured and recorded.
1.4 In-welding detection: During welding, focus on the values of the main parameters such as welding power, welding current voltage, and welding speed. Generally, the corresponding sensors or auxiliary instruments in the unit directly read and record them. According to the relevant operating procedures, ensure that the main welding parameters meet the requirements of the process specifications.
1.5 Post-welding detection: Post-welding detection needs to pay attention to welding phenomena such as welding spark state and post-welding 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 all key detection items. It is mainly based on the operator's actual production experience, and visual monitoring 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 detection: Compared with other detection links, metallographic detection 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 link
1.6.2 Optimization of sample preparation link
1.6.3 Optimization of corrosion link
1.6.4 Optimization of detection link
1.7 Large sample detection: According to the small sample detection 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 is required for a small sample process test. The process performance test includes a flattening test, bending test, expansion test, crimping 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 detection: All the above-mentioned tests are sampled according to the relevant specifications or standards, so it is inevitable that missed inspections will occur. 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 inspection personnel 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 is no missed inspection, and isolate the defective welded pipes exceeding the standard in time.

2 Rapid assessment and diagnosis of online weld quality
The rapid assessment of online weld quality is based on the rapid detection of the above-mentioned processes, and it can be quickly determined by comparing the process specifications of the relevant processes. Due to the differences in pipe specifications, forming processes, welding methods, technical requirements, and differences in equipment processes, production processes, and technologies of various enterprises, the specific acceptance scope of the process specifications of each relevant process needs to be determined in combination with the actual situation of each pipeline. In general, the main influencing factors of weld quality are the consistency of the temperature of the inner and outer edges of the plate and the matching of the extrusion amount and the heating temperature.
2.1 Rapid assessment and diagnosis in the initial adjustment stage: The main assessment indicators in the initial adjustment stage include dimensional variables, instrument variables, and visual variables. 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 a quick assessment and diagnosis based on the operator's experience.
2.1.1 Quick assessment and diagnosis of welding sparks: Generally, the welding state without a large amount of spark splashing and not darkening is a normal state. When it is dark, it can be diagnosed as too small welding power or too fast welding speed; a large amount of splashing can be diagnosed as too large welding power or too small a distance between the welding point and extrusion point or welding angle.
2.1.2 Quick assessment and diagnosis of welding burrs: The weld just out of the extrusion roller is orange-red. The red and white color can be judged that the temperature (power) is too high, and the dark red color can be judged that the temperature (power) is too low. The weld is straight and uniform, the burr width is large, the height is small, the top is shiny and round, and the convex points with slightly 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 forked or cracked, and the tool position is normal, it can be judged that the plate is misaligned.
2.1.3 Rapid evaluation and diagnosis of HAZ color: After removing the outer burr, there is a clear and continuous blue straight thin line on both sides of the heat-affected zone. The evaluation standard is that the color in the area between the two lines gradually fades and the axial direction is uniform. The uniform blue color of the HAZ indicates that the welding temperature is too high; the lighter color indicates that the welding temperature is too low. The width or shape of the outer weld after the burr is removed changes, which can be inferred as a misalignment of the plate.
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. Adverse consequences Thick fusion line Welding temperature is too high, metal surface decarburization increases, in most cases due to insufficient extrusion pressure Obvious gray spots or oxide inclusions are often produced in the center of the fusion line Causes of bad shape Diagnosis Thin fusion line Extrusion pressure is too large, molten metal is squeezed out excessively Welds are prone to cold welding and flattening test failures Irregular fusion line Highly unbalanced extrusion pressure There are fusion lines or S-shaped fusion lines tilted in different directions, complex thermal deformation, high internal stress Fusion line has oxide inclusions or gray spots 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.
Adverse consequences: The streamline angle is too large. The extrusion pressure is too large during welding. The larger extrusion pressure can squeeze out more molten metal, making the metal on the edge of the plate poorly welded, and it is easy to produce cold welding. Causes of bad shape diagnosis: The streamline angle is too small. The extrusion pressure is too small, the streamline display is unclear, and even the fusion line in the middle of the weld cannot be seen. It is often accompanied by more oxide inclusions, which become the crack source of the weld cracking. The unbalanced extrusion pressure can easily cause the streamlined shape to change. Some of the rise angles are too large, and the streamlines are very thick; some of the rise angles are too small, and the streamlines are thin and unclear.
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. The streamline angle is asymmetric. The plate edge parallelism is not good, and it is easy to have a positive "V" shape and an 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 a positive "V" shape appears on the edge of the plate, 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 rise angle of the metal streamline of the inner wall that contacts first is larger, while the rise angle of the metal streamline of the outer wall is smaller, and in severe cases, the streamline is not even displayed.
On the contrary, when an inverted "V" shape appears on the edge of the plate, 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 to form 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.
2.3 Rapid assessment and diagnosis of large sample and full-line inspection stages: Large sample and full-line inspection 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 made based on the 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 the prospect of system structure
The online weld quality rapid assessment and diagnosis system of high-frequency straight seam welded pipe can be divided into four stages: preliminary machine adjustment assessment and diagnosis, small sample assessment and diagnosis, large sample assessment and diagnosis, and full-line assessment and diagnosis. Among them, the preliminary adjustment stage ensures that the values of each process control point meet the requirements of the corresponding process specifications; the small sample evaluation stage is to further optimizes the adjustment data according to the metallographic test data. If the small sample test data after the preliminary adjustment has met the process specification requirements, mass production can be directly started. Otherwise, further fine-tuning is performed within the specification range of the preliminary 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 excluding 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 inspection 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 the welded pipes shipped is all 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 adopted in the whole stage until the requirements are met, and then the large sample inspection is confirmed, and the full-line inspection 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 mass-produced, the control data recorded before can be repeated or imitated, and the machine adjustment can often be completed in one stage. The subsequent small samples, large samples, and full-line evaluation stages are more for repeated confirmation or real-time monitoring. The actual adjustment and production efficiency advantages are more obvious.
In the process of full-stage evaluation and diagnosis, 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 by 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 of pipelines, which will help to further improve production quality and efficiency.