There is a diameter ranging phenomenon in the cross section of round pipe, that there is not necessarily perpendicular to the maximum diameter and the minimum diameter, the maximum diameter and the minimum diameter of the difference is the ellipticity (or roundness). In order to control ovality, and some steel standard specifies the allowable ovality index, generally defined as no more than 80% of diameter tolerance.
In the pipe forming process, as the strip during rolling poor control of dimensional accuracy of the resulting "crescent bend", leading to instability of the molding seam, forming the wrong side (or slit). In order to eliminate the wrong side as much as possible, we must continue to ensure the proper trim molding molding angle joints, which can cause fluctuations in diameter, while the formation of ovality. In addition, as Disc Shear blade gap adjustment properly, sheared wide and narrow strip on the side, resulting in forming the tight loose joints, can lead to instability molding, forming ovality. On the whole, the impact diameter ovality of many factors, in addition to the strip "crescent bend" to changes in the width of the strip, the strip intensity inhomogeneity, molding and forming roll control mode, the weld head strip impact etc will result in the generation of ovality.
Pipe bending, the inside of the compressive stress, the lateral tensile stress, the stress on the neutral layer is equal to zero in theory, pipe in uneven stress state. Bending direction parallel to the smaller diameter of the trend, perpendicular to the bending direction has a tendency to increase in diameter, so the tube by the bending moment, especially when thin-walled steel pipe, oval prone to deformation. The elliptical nozzle causes mainly due to its free state in the axial direction, poor steel nozzle, even when not subject to considerable radial force, deformation also occurs oval, thin-walled large diameter steel pipe is particularly evident.