直缝焊管扩径是一种利用液压或机械方式从焊管内壁加力使焊管沿着径向向外扩胀成型的压力加工工艺。机械方式比液压方式，设备简单且效率更高，在世界上最先进的几条大口径直缝焊管制管线扩径工序都被采用，其工艺为： 机械扩径利用扩径机端部的分瓣的扇形块沿径向扩张，使管坯沿长度方向以步进方式，分段实现全管长塑性变形的过程。分为5个阶段。
Diameter enlargement of straight seam welded pipe is a kind of pressure processing technology, which uses hydraulic or mechanical method to force the welded pipe from the inner wall to make the welded pipe expand radially outward. The mechanical method is simpler and more efficient than the hydraulic method. In the world, the most advanced expanding process of large diameter straight seam welded pipe line is adopted. The process is that the mechanical expanding expands along the radial direction by using the sector block at the end of the expander, so that the billet can realize the plastic deformation of the whole length of the pipe in a step-by-step manner. It is divided into five stages. 1、初步整圆阶段。扇形块打开直到所有扇形块都接触到焊管内壁，此时步长范围内焊管内圆管中各点半径大小都几乎一致，焊管得到初步整圆。
1. Preliminary rounding stage. The sector block opens until all the sector blocks touch the inner wall of the welded pipe. At this time, the radius of each point in the welded pipe is almost the same within the step size range, and the welded pipe is initially rounded.
2、名义内径阶段。扇形块从前段位置开始降低运动速度，直到抵达要求位置，这个位置是质量要求的成品管内圆周位置。
2. Nominal inner diameter stage. The fan-shaped block reduces its motion speed from the front position until it reaches the required position, which is the circumferential position of the finished pipe required by the quality.
3、弹复补偿阶段。扇形块在2阶段的位置开始进一步将低速度，直到抵达要求位置，这个位置是工艺设计要求的弹复前焊管内圆周位置。
3. Rebound compensation stage. The sector block will start at a lower speed at the position of phase 2 until it reaches the required position, which is the circumferential position of the pre-spring welded pipe required by the process design.
4、保压稳定阶段。扇形块在弹复前焊管内圆周位置一段时间保持不动，这是设备和扩径工艺要求的保压稳定阶段。
4. Pressure-holding and stabilization stage. The sector block remains stationary for a period of time in the circumference of the welded pipe before resilience, which is the pressure-holding and stabilizing stage required by the equipment and the expanding process.
5、卸荷回归阶段。扇形块从弹复前焊管内圆周位置开始迅速回缩，直到抵达初始扩径的位置，这是扩径工艺要求的扇形块最小收缩直径。 在实际应用中，工艺简化中，2、3步骤可以合并简化，这对焊管的扩径质量没有影响。
5. Return stage of unloading. The sector block shrinks rapidly from the circumferential position of the welded pipe before resilience until it reaches the initial enlargement position, which is the minimum shrinkage diameter of the sector block required by the enlargement process. In practical application, 2 and 3 steps can be combined and simplified in the process simplification, which has no effect on the quality of welded pipe diameter enlargement.
