The significance of the flattened tube test for seamless steel tubes in China’s steel tube standards

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The last edit to this post was made by Desert Fish on 2013-2-18 at 17:14. The significance of the flattening test for seamless steel pipes in China’s steel pipe standards: Method GB246-63 for the flattening test states that the purpose of this test is to evaluate the deformation properties of metal pipes when compressed to specified dimensions, as well as to identify any defects present. ”Based on the stress and deformation characteristics experienced by the specimen during crushing, the crushing test should demonstrate the tube’s resistance to longitudinal cracking (caused by tensile stress) and circumferential cracking (caused by shear stress), as well as reveal its internal and surface defects. The flanging test provisions in China’s steel pipe standards specify a certain flat plate distance, or compression to a value calculated using the H formula, or compression to an inner wall distance of 2S or 4S, or to 0.4 times or 0.5 times the inner diameter, or to 0.5 times the outer diameter ； And there are no regulations specifying “coinciding inner diameters” or “inner wall contact”. This shows that the purpose of including flattening criteria in China’s steel pipe standards is not to replace tests such as \"flaw detection,\" \"tensile strength,\" \"microstructure examination,\" or \"fracture analysis,\" but rather to assess the \"deformation performance\" of steel pipes. At the same degree of compression, those with poor deformation properties do not develop cracks or fissures ； It occurs when the deformation performance is poor. Evaluating the \"deformation performance\" means assessing the crack resistance of the surface layer of the steel pipe. This property is related to its heat treatment condition and the density of the metal. Under fixed conditions of steel grade and forging ratio, improving the degree of heat treatment (raising the final rolling temperature, establishing an appropriate annealing regime, and enhancing plasticity without causing overheating or overburning) to achieve good cold bending properties is the fundamental measure for increasing \"deformation performance\". In China’s steel pipe standards, does the flattening test serve any purpose in detecting surface defects? It should be considered that there is none. This is because, in the manufacturing process, the specimens used for the flattening test are taken before the visual inspection of the steel pipes, and it is inevitable that there will be certain surface defects; moreover, every steel pipe standard permits the presence of certain surface defects. If a surface defect happens to be located at the site of the maximum tensile stress during flattening, it becomes the source of cracks or fissures. If such cracks and fissures are used as a basis for determining that a specimen is \"not suitable for flattening,\" it essentially means denying the provisions regarding surface quality, i.e., allowing no surface defects at all. This incorrect method of judgment not only puts the flattening clause in opposition to the surface quality clause, but more importantly, it leads production units to adopt unnecessary heat treatment measures, resulting in waste in production and a decline in the surface quality of the steel pipes.