The cold expansion of large diameter direct welded pipe is to push the pipe from the steel wall by the pressure moulding process of hydraulic or mechanical device expanding outward along the radial direction. Mechanical mode is simpler than hydraulic mode, the device is simpler, the use efficiency is higher, the steel is more precise under high precision processing conditions, and the large diameter pipe joint of reservoir emptying does not need to be used directly. At present, the most advanced large diameter straight seam steel in the world adopts pipe expanding process, the process is as follows:


Mechanical expansion is a process of plastic deformation in which the longitudinal direction of the tube is stepped and the whole length of the tube is realized by using the radial diameter of the enlarged part of the split side of the expansion section. It is divided into five stages.
1. The initial stage of the whole circle. The section opens until all sections are exposed to the tube wall. At this time, the step size within the radius of the steel tube at each point is almost the same, and the steel plate is initially circular.
2. The nominal inner diameter of the stage. The front section reduces the motion speed from the starting position until it reaches the desired position, which is in the circumferential position required for the quality of the finished pipeline.
3. Rebound compensation stage. Section 2 at the position of the initial stage is further low-speed until it reaches the desired position, which is within the requirements of the design process for elastic recovery before the circumferential position of the pipeline.
4. Stable stage of pressure. Stable phase packaging equipment and extended process requirements are maintained for a period of time before playback of complex inner tube circumference position.
5. Return stage of unloading. The initial position of circumferential segment retracts rapidly from the inner tube bullet in front of the complex to the initial position of enlarged diameter, which is the requirement of reducing the diameter of the smallest segment in the process of enlargement.