In the welded pipe industry, people generally regard welded pipes with a ratio of wall thickness to outside diameter of 12%≤ t/D≤ 18% as thick-walled pipes. Regarding the forming difficulties of thick-walled pipes with used round pipe to square pipe machines, they clearly propose Solutions such as the edge double-semi-warp forming hole shape.

Difficulties in processing technology of thick-walled pipe deformation：

There are three processing technology problems for thick-walled welded pipe forming with the used round pipe to square pipe machines: 1、The large bending springback, 2、Is the actual deformation blind zone is wide, 3、The large difference between the inner and outer circumferences.

Bending resilience is large:

the tube blank will produce two kinds of deformation, elastic deformation and plastic deformation, in the process of bending the tube blank from a straight condition into a cylindrical shape. In addition, it is inevitable to produce springback, the key is how much. Thick-walled pipes have large deformation resistance and much springback, resulting in insufficient pipe blank deformation.

Affected by the mechanical properties of the tube blank, the wall-to-diameter ratio (t/D), the main parameters of the pass, the characteristics of the machine and equipment, the actual operation error, etc., the springback regularity is very different, so it is difficult to accurately predict and analyze the springback amount. Give precise springback compensation. At this stage, there are roughly two commonly used methods for characterizing springback:

• Actual measurement characterizatio: According to the evaluation of the chord length difference △b before and after springback at the same point on the edge of the deformed tube blank and the two index values of the semi-longitude difference △R before and after springback at the bending position, it is characterized by referring to the formula to calculate the following: specific accurate measurement of the characterization value is more intuitive , The most widely used.

• Function representation. According to the basic theory of bending neutralization layer and the basic theory of metal elastoplastic deformation, the correlation between the deformation of the deformed tube before and after springback can be calculated. Refer to the formula to calculate the following:

The theoretical significance of the second calculation formula is that according to the deformation half-warp of the tube blank after springback, the yield strength of the tube blank, the thickness of the tube blank, the springback angle and the metal modulus, the required pass bending deformation radius R can be directly calculated. In turn, the adverse effects of springback on the formed tube are eliminated.

Deformation blind zone width

Deformation dead zone, also known as deformation dead zone, means that in the process of tube blank deformation, no matter how it deforms, there is no way to deform at the edge of the tube blank and the area whose width is equal to the thickness of the tube blank. In fact, a part of the road sections adjacent to the deformation blind zone of the basic theory also belong to the range of the deformation blind zone. The existence of the deformation blind zone has its inevitable trend.

• There is an inevitable trend of deformation blind zone. The bending and deformation of the edge of the tube blank can be analyzed and studied with reference to the basic theory of bending stiffness. When a billet with a thickness of t has to bend and deform a billet with a length of L under the action of the forming and rolling force P, the positioning point of the section of the billet will definitely cause a vertical displacement y, that is, the billet will bend and deform. Calculate the bending deformation radius of the tube blank. When forming a thick-walled pipe of Φ60mm X 6mm, the calculation results show that in order to produce a bending deformation of only 0.001cm in the width of the thickness area (0.6cm), a forming force of 48kN is required. This is for welded pipe forming equipment. It is impossible to provide; what's more, according to the welding pipe forming process, the closer to the edge of the tube blank, the smaller the specific forming force acting on the tube blank, which in turn causes the specific deformation blind zone to be wider.

In addition, from the perspective of the specific deformation effect, a bending deformation of 0.001 cm should be completed in a length of 0.6 cm, and the bending radius R is equal to 30 cm. The arc length of 0.6 cm is the same as a straight line on an arc with a half meridian of 30 cm.

• Comparison of deformation blind zone. Both thick-walled pipes and thin-walled pipes have deformation blind zones; only thick-walled pipes' deformation blind zones account for a higher proportion of the total width of the tube blank. Taking Φ60mm X 3mm and Φ60mm X 6mm as examples, their blind zone width ratios of 2t /B (%) are 3.24% and 6.89%, respectively, and the latter is twice that of the previous one. Therefore, according to the calculation formula of the total opening width of the deformation blind zone, the total width of the trench on both sides of the wall thickness can be obtained as 0.64mm (tube inner diameter r2=27.87mm) and 2.79mm (tube inner diameter r2=25.08mm), the latter being 4.36 times of the former, the influence of such a wide groove on the thick-walled pipe of the used round pipe to square pipe machines on the weld strength is obvious.

Large difference in diameter between inside and outside

Still taking the Φ60mm X 6mm thick-walled pipe as an example, the difference between the inner and outer circumferences is 37.68mm, while the difference between the inner and outer circumferences of the Φ60mm X 3mm standard wall thickness pipe is only 18.84mm. A pipe with a large circumference difference represents a large amount of internal axial stress accumulated in the formed tube. The process of compressing the inner layer and stretching the outer layer requires a lot of deformation work, and its increased deformation resistance has an impact on the unit. Both the drag output power and the rigidity of the unit have special requirements. After welding, the large amount of compressive stress accumulated on the inner side of the neutral layer of the pipeline always tries to open the weld; in addition, the large amount of tensile stress accumulated on the outer side of the neutral layer is always pulling the weld, trying to get rid of the weld. In addition, the actual effects of these two stresses on the welding seam are the same, with superimposing effects. These accumulated stresses in thick-wall welded pipes cause serious stress corrosion damage to the weld seam, and have great potential safety hazards, which must be solved by the welded pipe forming process.

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