Test on the causes of cracking of welded steel pipe flattening test

Welded steel pipe is a steel pipe that is bent and deformed into a square or round shape by bending the steel plate or strip, and then welded. This kind of steel pipe surface are with seams, the use of billet steel or steel plate mainly. As the welded steel pipe plays a great role in the field of engineering and construction, so the community has been paying great attention to the quality control of steel pipe, and focus on the steel pipe flattening test analysis. The article will focus on the welded steel pipe flattening test cracking causes, expect to be able to provide some help to the use of welded steel pipe.

Flattening test is a common metal pipe defect detection means, will be implemented through the flattening of metal pipe processing, the detection of defects. In the specific test process, the test specimen will be placed in the parallel plate, through the use of presses or other means, the specimen is uniformly processed until it reaches the required size, stop applying pressure. Will be checked by the bending deformation of the specimen to determine whether the specimen is qualified, is an effective means of testing the quality of welded steel pipe. In order to better launch the flattening test cracking analysis, researchers should first study the basic situation of welded steel pipe construction.

1. Welded steel pipe construction

Welded steel pipe (hereinafter referred to as welded pipe), with the advantages of high production efficiency and a variety of specifications, is more widely used in field engineering construction and other fields, but due to the impact of processing, welded pipe welding position is the weakness of the pipe, welded pipe strength is lower than ordinary seamless steel pipe [1]. After years of development, the welded pipe construction process has been significantly advanced, and the level of welded pipe processing technology such as spiral welded pipe as well as straight seam welded pipe has been effectively improved. The direction of the weld seam of the two types of welded pipes is a visual representation of the difference in performance, where the straight seam welded pipe is obtained directly by welding construction, while the spiral welded pipe is implemented in accordance with the angle of the spiral line billet rolled into and completed the welding task. In terms of reliability, straight seam welded pipe is more reliable. As a unique fiber organization form of welded pipe seam, metal flow line is one of the important indicators for judging the welding squeeze. If the angle of rise of the metal flow line is relatively large, it indicates that the welding squeeze pressure is high and relatively more molten metal is squeezed out, through which it can be determined that there is a problem with the metal weld on the edge of the plate, which can easily appear cold welding conditions [2].

2. Analysis of the factors affecting the results of the steel pipe flattening test and the causes of test cracking

2.1 Factors affecting the test results

  • (1) Test equipment. Each region in the implementation of welded pipe flattening test process, the test equipment used and test processing is not the same, the more common modern flattening test technology, mainly based on the two-end flattening technology. In the specific development of the test, through the use of pressure stretching machine or universal testing machine, the implementation of the two ends of the welded pipe cracking test processing, but some regions will be implemented in the middle of the welded pipe flattening test way to start the corresponding test [3]. In different extrusion parts and extrusion volume and other factors interference, the test results obtained will appear a certain deviation, need to do a good job of test equipment control.
  • (2) Material quality. Material quality is also one of the main factors affecting the degree of accuracy of the experimental results, which will affect the chemical properties of the material, physical properties and other indicators set at the same time, the construction of quality indicators to form a certain interference. When conducting the test, the quality of the material may be different, and the choice of different indicator parameters, will directly affect the test results. For example, in the implementation of low-pressure fluid transport, will be carried out through the scientific use of welded pipe, low-pressure fluid transport. Emphasis on the implementation of heating steam and water and other fluid transportation process, the need to try to choose to meet the relevant standard requirements of the material. According to the standard requirements, the outer diameter of the selected material needs to be kept above 60.3 mm, and the flattening test test should be implemented for resistance welded pipes, and the final test results should be obtained through the testing of specimens longer than 64 mm in order to make relevant judgments and planning designs. In this process, if there are cracks or fissures at the weld, it will have an impact on the test results, so the need to do a good job of quality control of test materials.
  • (3) Personnel operation. The impact of personnel operation is mainly reflected in two aspects: First, because the professional ability and comprehensive quality of personnel is not the same, so in the implementation of the flattening experimental operation, will also show different experimental results, the stronger the professional ability of personnel, the more accurate the experimental results obtained, the sample selection is also more representative, to achieve accurate assessment of the manufacturing level of welded pipe production and welding process Secondly, the degree of personnel specification, will have a direct impact on the reliability of the final results. Because the experimental design operating procedures are more cumbersome and complex, if personnel operations are not effectively standardized, operating behavior does not form a unified, it is easy to cause the experimental results to be affected by the differences in personnel operating habits [4].

2.2 Analysis of cracking causes

  • (1) The presence of impurities inside the welded pipe. In order to avoid cracking or other problems in the implementation of the test process, to ensure that the overall performance of the welded pipe can be consistent with the experimental requirements, in the implementation of the cracking problem analysis, the need to start from the material elements of the cross-sectional layer, the cross-sectional layer sulfide analysis, clear sulfide content distribution specifics. At the same time, the need to implement a full range of testing of steel impurities to ensure that the quality of materials can be consistent with the corresponding standard requirements, in order to control the possibility of cracking welded pipe to a minimum.
  • (2) Welded pipe material differences are obvious. In accordance with the relevant basis for the implementation of the steel division process, the forging temperature will be the main judgment detection basis. According to the common form of steel division, steel can be divided into γ-iron, α-iron and β-iron. Among them, α iron has the highest degree of cracking, and through the use of relevant technology, it can be found that α iron cracking section will form longitudinal cracks after cracking. The other two types of iron, although there are also cracking conditions, but the cracking transverse distribution is more prominent, and there is no more serious longitudinal cracking. As far as the degree of cracking of iron and steel is concerned, its value and the overall degree are closely related, and the two show an inverse proportional relationship, i.e., the higher the overall degree, the smaller the value is indicated.

3. Cracking problem treatment strategy

3.1 Enhance the welding level of welded pipe

As the level of welding process processing will have a direct impact on the quality of the flattening experiment, it is one of the main reasons for the occurrence of cracking problems. So in order to effectively reduce the probability of cracking problems, the need to do a good job of welding process strengthening. Not only to implement the standardization of personnel welding operations, professional and targeted training, to ensure the quality of personnel operations, but also to implement accurate control of welding conditions, in accordance with the specific circumstances of the welding technology, the amount of welding extrusion and temperature to carry out reasonable control, to ensure that the weld will not be cracked because of external influences, to ensure that the welded pipe machinability and pressure resistance [5]. In addition, the need to optimize the existing welding technology, should be in accordance with the specific construction requirements and performance indicators and other content, the welding process to adjust and improve the processing plan, in order to achieve the best welding treatment effect.

3.2 Do a good job impurity pre-treatment

Implementation of the impurity pretreatment process of welded pipe, the need to do a good job of the specifications and requirements of the analysis, not only to implement the chemical treatment operation specifications, but also in the beneficiation stage to do a good job of the overall composition analysis, in order to determine the subsequent treatment in accordance with the analysis results, to do a good job of impurities such as oxides and sulfides treatment. At the same time, in the process of iron making operation, should also be appropriate to enhance the overall operation process monitoring efforts, should ensure that each link operation standardization and scientific, to ensure that in the process of iron making can effectively reduce the amount of impurities contained in steel. In addition, in the steel forming stage, do a good job of analysis and use of compound characteristics, according to the use of steel needs, selective implementation of harmful components to eliminate.

3.3 Focus on production process control

Welded pipe flattening experimental operations before the experimental technicians need to start the analysis and screening of the flattening experimental program. According to the specific performance requirements, the production process of welded pipe should be strictly controlled to ensure that it can achieve the goal of effectively strengthening the chemical and physical properties of welded pipe from the source to prevent it from cracking during the test. Not only to do a good job of welded pipe composition ratio mix analysis, according to the results of the material composition of the chemical properties and physical properties of the index construction, to provide accurate standards to support the test, while the need to optimize the implementation of steel forging and rolling process, should be through the use of a variety of advanced production equipment and processes to ensure the overall quality of welded pipe.

4. Conclusion

As welded steel pipe has a non-negligible position in the modern steel market, so the relevant institutions need to further strengthen the research efforts on the way of welded steel pipe flattening experiments. Should be clear on the basis of the basic situation of welded steel pipe, in accordance with the flattening experimental objectives and operational specifications, scientific development of experimental cracking factors affecting control, to ensure the accuracy of the final results and reliability, to ensure that personnel can be in accordance with the experimental cracking, accurate judgment of the quality of steel pipe problems, clear whether there are defects in steel pipe, in order to lay a good foundation for the use of high-quality welded steel pipe.

Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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