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Quality defect and prevention of hot expension seamless pipe
- Jan 09, 2018 -

There are two methods of hot rolling and expanding pipe for hot-rolled pipe expansion. The quality defects of longitudinal rolling and expanding pipe mainly include the surface crack of the pipe and the crack of the pipe body, the uneven wall thickness, the inner straight, the inner hemp Pit, the crease and the bending, etc. The quality defects of the skew-rolling pipe expander are basically the same as those of the two-roll skew tube mill, including the uneven wall thickness of the pipe, the surface crack and the abrasion, the rolling, the spiral road and the internal fracture of the pipe.

The deformation process of longitudinal rolling and expanding pipe has the drawing and pushing pipe expanding process. The deformation process is divided into three stages: under the action of the head cone, the tube wall of the billet tube first takes on the bending deformation, and with the increasing of the diameter of the head, the tube billet is enlarged, the wall is reduced, and the diameter of the pipe is determined by the head cylindrical section. Obviously, the enlargement is mainly in the second stage. The main defects in the process of longitudinal rolling and pipe expanding are: the surface cracking of the pipe and the crack of the pipe body, uneven wall thickness, inner straight, crease, inner hemp pit, bending and so on.

The pipe-expanding process of skew rolling stems from two times perforation process. It is an oblique tube mill with two conical rolls and horizontally arranged. The diameter of the roll is gradually increased from the inlet of the deformation area to the outlet. The roll axis and the rolling line are spatially intersecting, which constitute the feeding angle and rolling angle. Two rolls for single support, respectively by 2 motor drive, transmission mechanism is located in the exit side of the pipe mill. The position of the roll can be adjusted along the rolling line axis. The guide plate is arranged up and down. The deformation area is divided into three parts: the flattening reduction area, the reduction area and the round area.

Flattening reduction deformation: the billet tube First contact with the cantilever beam state of the cone roll, under the action of friction. At this point, the billet tube only contact with the roll, not with the head contact. The deformation is mainly the deformation of flattening and reducing diameter. When the diameter is too large, the pipe wall is sometimes thickened. The size of the flattening reduction area is related to the position of the head, and the position adjustment of the head is mainly for the smooth realization of one bite of the blank tube and the creation of good mechanical and kinematic conditions for two bites and wall enlargement.

Wall-Reducing deformation: the main deformation area of skew rolling and expanding pipe. The blank tube in this area has been reduced and expanded. As the elongation coefficient is close to 1, according to the law of the volume of the metal, the thin wall of the tube becomes almost entirely a metal that needs to be replenished due to the increase in OD. The deformation of metal mainly occurs in the circumferential and radial direction of the billet tube. As the rollers and the head can be moved along their respective axes, so long as the position of the roller and the head can be changed, the size of the extended pipe is changed. When the position of the roll and the head is adjusted, the deformation of the wall-reducing area is not changed.

Rotary deformation: The billet tube after the wall expansion deformation, no longer contact with the head into the circular zone. Under the action of surface friction, the pipe is successfully separated from the deformation zone.

The extension coefficient of oblique rolling and expanding pipe is 1. In the event of main deformation, some extra deformation is unavoidable. The additional deformation of the metal will cause the defect of the pipe, and increase the energy consumption of the rolling. Additional deformation of metal includes additional torsional deformation, additional axial shear deformation and additional circumferential shear deformation. The cause of additional deformation and the process of deformation are the same as that of the two-roll tapered roll piercing machine with the feeding angle and the rolling angle, and the pipe mill. However, the length of the roller is relatively short because of the larger cone angle of the pipe and the fixed guide plate instead of the active one. Therefore, the three kinds of additional deformations produced by skew rolling and expanding tubes are different from those of two-roll skew-rolling piercing machine and pipe-rolling machine, that is, the additional torsional deformation is smaller and the additional axial shear is larger. The inclined rolling and expanding pipe is basically the same as that of the two-roll extension machine, and the pipe-expanding process and the deformation characteristic are similar to the two-roll skew-pipe mill. The difference between the two is that the latter want the billet tube to be deformed by the wall, deformed metal is mainly the axial extension, and control its spread, the former requires the billet tube in the wall deformation, deformation of metal should be mainly horizontal spread and control of its axial extension, elongation coefficient of 1, or even less than 1, The tube diameter of the billet tube is enlarged but the length is constant or shortened. So the cone angle of the pipe expander head will be very large, and the rolling angle and the feeding angle are also relatively large.

The defects of skew rolling and expansion pipe: the thickness of the pipe is uneven, surface crack and abrasion, rolling, spiral Road and spiral inner fold, inner hemp pits, etc. Because of the large diameter of the inclined tube, the probability of crack on the surface of the pipe is higher. In addition, the helical channel is more serious when the tube is rolled over. Therefore, in the design of the hole, we can consider the appropriate length of the lengthened roll and design the corresponding wall zone. In the homogeneous area, the normal distance between the head surface and the roll surface is equal. At the same time, the appropriate reduction of the size of the pipe is also conducive to the improvement of wall thickness accuracy. Although the roller shape design and adjustment can be used to reduce the pipe wall thickness uneven and the surface spiral, but after all, the deformation of skew rolling tube is very large and the length of the roll is short. There are still different degrees of uneven wall thickness and spiral path in the pipe. For this reason, the wall thickness accuracy of the pipe will be improved obviously after the cross rolling process is finished.

Generally speaking, the billet tube in the heating process, both inside and outside the surface will occur oxidation. The iron oxide skin of the billet tube can be descaling by high-pressure water before the head, in order to improve the inner surface quality and the head life of the pipe.