1. Non-uniform wall thickness: the thickness of the tube wall can not penetrate the same objective lens wall thickness range, its cross-section and longitudinal tube body exists, that is, the thickness is not uniform. In order to control this non-uniformity, the allowable non-uniform wall thickness index stipulated in some steel plate standards generally does not exceed 80% of the wall thickness tolerance (after consultation between the two sides).


2. Ellipticity: The phenomenon of circular section of pipe outer diameter exists, that is, there is not necessarily the maximum and minimum outer diameters perpendicular to each other, and the difference between the maximum outer diameter and the outer diameter is ellipticity (or roundness). In order to control the ellipticity, some steel standards stipulate the allowable ellipticity index, which is generally defined as not exceeding 80% of the diameter tolerance (after implementation, it is negotiated by both sides).


3. Bending: The bending shape of steel pipe in the longitudinal direction. The degree of its digital display curve is called curvature. The curvature specified in the standard is generally divided into the following two types:
A, Local Curvature: Measure the chord height (m m) by using the meter length scale * the maximum elbow, which is the local curvature value in mm/m, expressed as 2.5 mm/m. The method is also applicable to the bending of pipe ends.
B, Total Curvature of Full Length: Measure the maximum height (mm) of the bending string of the pipe by tensing one string from both ends of the pipe, and then convert the percentage of the growth (m) to the total length curvature length of the pipe in the direction of length.


4. Differential size: Dimensional tolerance or standard size exceeding allowable deviation. Here, "size" mainly refers to the outer diameter and wall thickness of steel pipe. Usually people call the excessive habit "tolerance deviation", and this deviation is called the inaccuracy of tolerance equivalence, which should be called "deviation spectrum". The deviation here may be either "positive" or "negative", and there are few "positive" and "negative" deviations from the outliers in the same batch of steel tubes.