Abstract: This article analyzes the factors causing the cracking of
welded steel pipes in flattening tests from the aspects of experimental equipment, material quality, and personnel operation. The main reasons for the cracking of welded pipes in flattening tests are identified, and treatment strategies for cracking issues are proposed, providing theoretical references for further improving the quality of welded steel pipes.
Flattening tests are one of the common methods for inspecting welding seam defects in welded steel pipes, as shown in Figure 1. By flattening welded steel pipes, their defects can be inspected. Place the defective welded pipe in parallel plates. The defective welded pipe is placed between parallel plates. A press or another method is used to uniformly compress the welded pipe until it reaches the target size. The press is then stopped, and the deformation of the welded pipe is inspected. Based on the inspection results, it is determined whether the welded pipe meets the standard requirements. Since the cracking of welded pipes is related to the production site, it is necessary to understand the conditions at the construction site of the welded pipe.
Figure 1 Welded steel pipes
Construction of welded pipes
The production process of welded pipe is simple and has the advantages of high production efficiency, low cost, rapid development, and a wide variety of specifications. It is widely used in fields such as engineering site construction. However, due to the welding process, the joint at the welding position is the weakest part, resulting in its strength being lower than that of seamless steel pipe of the same specification. In recent years, significant improvements in the construction process of welded pipes have greatly enhanced the welding process of both straight seam and spiral welded pipes. The direction of the weld in straight seam and spiral welded pipe determines the performance difference between the two. The straight seam welded pipe is obtained by direct welding, while the spiral welded pipe is made by rolling the pipe blank at a specific spiral angle. Therefore, the straight seam welded pipe generally has better reliability.
During the welding process, the fiber organization mode of the welding seam is one of its characteristics, and the important factor for judging the welding extrusion force is the metal streamline. If the rising angle of the metal streamline is large, it indicates that the welding extrusion force is high, resulting in more metal being extruded and melted. This can indicate a problem with the welding of the plate edge metal, leading to the occurrence of cold welding.
2.
Factors affecting the experimental results and analysis of cracking causes
2.1
Factors affecting the experimental results
2.1.1
Influence of experimental equipment and experimental methods
In various flattening test schemes for welded pipes, the experimental equipment and methods are not exactly the same. Most flattening tests are conducted by applying pressure at both ends of the welded pipe. During the pressure application process, a universal testing machine (Figure 2) or a pressure stretching machine is mainly used to perform cracking experimental analysis on both ends of the welded pipe; a small number of experiments apply pressure to the middle section of the welded pipe. As a result, the test results exhibit varying degrees of deviation.
2.
Universal testing machines
2.1.2
Influence of experimental material quality
Another significant cause of experimental deviation is the quality of the material. It affects the physical and chemical performance indicators of the welded pipe, as well as its quality indicators. Therefore, when conducting experiments, different target parameters are selected according to the quality differences of the materials, which also cause deviations in the experimental results. When different target parameters are selected, the experimental results will be directly affected. For example, when transporting low-pressure fluids, welded pipes should be used appropriately. When conveying low-pressure fluids such as heating steam and water, materials that meet the corresponding standards should be selected. According to the GB/T 3091-2015 standard, welded pipe samples with an outer diameter of at least 60.3 mm and a length of 64 mm can be selected for flattening tests, with corresponding judgments and planning designs made based on the experimental results. During the experiment, if cracks occur at the weld, it will affect the experimental results. Therefore, high-quality materials should be selected for the experiment.
2.1.3
Influence of personnel operation during the experiment
The experimental results are closely related to personnel operation, and improper operation can distort the results.
The influence of personnel operation on the experimental results is evident in the following two aspects:
1) Different operating habits and professional abilities of the operators can bias the experimental results during flattening tests. If the operator's professional ability is strong and the operating process is standardized, the experimental results will be highly accurate, and the selection of welded pipe samples will be representative, reflecting an accurate evaluation of the welding process on the production level of welded pipes.
2) The degree of standardization in the operator's actions during the experiment directly affects the reliability of the experimental results. Given the complexity of the experimental procedures, if the operator's actions are not standardized and do not strictly follow the procedures, deviations or even erroneous results can easily occur.
