Views:1 Author:Site Editor Publish Time: 2020-10-29 Origin:Site
In the mechanical expansion process of welded pipe, the fan-shaped block of the expansion die is in the initial state, and there is a gap between the working surface of the die and the inner side of the tube blank before the expansion. After the start of the diameter expansion, the diameter expansion head acts on the die sector block to move it radially outwards, tending to separate from each other, and the size of the circumscribed circle of the die gradually increases. After a period of idle stroke, the working surface of the mold first contacts the inner surface of the tube blank on the side of the short axis of the ellipse. According to the ratio of the diameter of the circular arc section of the mold working surface to the inner diameter of the tube blank, there are two cases. When the ratio is greater than 1, the fillet of the mold edge first contacts the inner diameter of the tube blank, and when the ratio is less than 1, the mold first Contact at the middle of the working arc. After the expanding die is in contact with the tube blank, the die applies a radial load to the tube blank, which causes the tube blank to undergo elastic and plastic deformations through the process of rounding and expanding. In the numerical simulation, the process shows that when the rigid mold is given radial displacement, the deformed body gradually enters into plastic deformation from elastic deformation.
In actual production, after the expansion stage, the mold needs to stay in the original position for a period of time, that is, the pressure-holding and stable stage. After the pressure is maintained, the die begins to return to the initial position in the radial direction and returns to the collapsed state, and the diameter of the tube blank is slightly reduced. In the numerical simulation, the process is shown as giving the rigid mold reverse radial displacement, and the deformed body undergoes elastic recovery.
From a macro point of view, the deformation law of the longitudinally welded tube blank during the expansion process is basically the same. Before diameter expansion, the tube blank has an initial ellipticity, and the gap between the long axis and the mold is larger than the short axis side. The deformation process during mechanical expansion shows three stages: full circle, expansion and unloading:
(1) Full circle stage In the production process of straight seam welded pipes, after the tube blank has undergone multiple processes such as forming and welding, its cross-sectional shape is mostly approximately elliptical depending on the forming method, and there are also straight sections and edges and corners. The rounding stage is to change the shape of the tube blank into a perfect circle. At this stage, the geometry of the tube blank changes, but no plastic deformation occurs. The inner wall of the tube blank first contacts the working surface of the mold at the short diameter, and the geometric shape changes under the action of the radial movement of the mold, the long axis is shortened, and the short axis is elongated.
(2) Diameter-expanding stage Diameter-expanding die is used to expand the inner wall of the steel pipe after expanding in cross section. There is a gap between every two fan-shaped block molds. As the mold expands, the gap gradually increases. Therefore, some suspended deformation zones that cannot be supported by the mold and transitional deformation zones with the same deformation characteristics are formed on the cross section of the part. Some special local deformation features. In the expansion stage, the tube blank is in the elastic deformation stage first, and when the stress reaches yield, the tube blank begins to enter the plastic deformation stage.
(3) During the unloading stage, the deformation of the blank tube at this stage is mainly elastic recovery. The welded pipe retains the local characteristics before unloading, while the cross-sectional diameter of the welded pipe is reduced, and the ovality of the welded pipe has been significantly reduced compared with the original billet. Therefore, the size and shape of the welded pipe are changed compared with that before processing.