Diameter expansion is a pressure processing process that uses hydraulic or mechanical methods to apply force from the inner wall of the steel pipe to expand the steel pipe radially outward. The mechanical method is simpler and more efficient than the hydraulic method. It is adopted in the diameter expansion process of several advanced large-diameter straight seam welded pipe pipes in the world. The process is: mechanical diameter expansion uses the fan-shaped blocks at the end of the expander to expand radially, so that the tube blank can be plastically deformed in sections along the length direction in a step-by-step manner. It is divided into 5 stages.
1. Initial rounding stage. The fan-shaped blocks are opened until all the fan-shaped blocks contact the inner wall of the steel pipe. At this time, the radius of each point in the inner round tube of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded.
2. Nominal inner diameter stage. The fan-shaped blocks start to reduce the movement speed from the front position until they reach the required position. This position is the inner circumference position of the finished pipe with quality requirements.
3. Springback compensation stage. The fan-shaped block starts to further reduce the speed at the position of stage 2 until it reaches the required position, which is the inner circumferential position of the steel pipe before rebounding required by the process design.
4. Pressure-maintaining and stable stage. The fan-shaped block remains stationary at the inner circumferential position of the steel pipe before rebounding for a period of time. This is the pressure-maintaining and stable stage required by the equipment and the diameter expansion process.
5. Unloading and returning stage. The fan-shaped block starts to shrink rapidly from the inner circumferential position of the steel pipe before rebounding until it reaches the initial diameter expansion position. This is the minimum shrinkage diameter of the fan-shaped block required by the diameter expansion process.
