First welding a polygonal polygonal polygonal ring shell or closed at both ends of the polygonal fan shell, the internal pressure is filled with pressure medium, the internal pressure under the action of the cross section from polygon gradually become round, and eventually become a circular ring shell. As required, a circular shell can be cut into 4 90° elbows or 6 60° elbows or other sizes. This process is suitable for manufacturing large elbows of any size with the ratio of middle diameter to inner diameter of elbows greater than 1.5 times. It is an ideal method for manufacturing large carbon steel elbows.
