Steel pipe finishing defects and their prevention.
The finishing process of steel pipes is an important process to remove steel pipe defects, further improve the quality of steel pipes, meet the needs of special product uses, and clarify the “identity” of products. Steel pipe finishing mainly includes steel pipe straightening, cutting ends (chamfering, sizing), inspection and testing (including surface quality inspection, geometric dimension inspection, non-destructive inspection, and hydraulic test, etc.), grinding, length measurement, weighing, painting, printing, and packaging. Some steel pipes for special purposes also need surface shot peening, mechanical processing, anti-corrosion treatment, etc.

1. Quality defects of steel pipe straightening
Steel pipe straightening machines can be divided into pressure straightening machines, inclined roller straightening machines, and tension straightening machines. The process of steel pipe straightening is to make the steel pipe undergo repeated elastic-plastic bending and flattening deformation, to achieve the purpose of reducing the curvature and ovality of the steel pipe. The quality defects of steel pipes during the straightening process mainly include the un-straightening of steel pipes (including goose-head bends at the ends of the pipes), concave correction, square correction, crack correction, surface scratches, and indentations, etc.

Many factors affect the quality of steel pipe straightening, mainly including straightening machine type, hole shape, hole adjustment, steel pipe characteristics (original curvature, size, and material), etc. During the straightening process of steel pipes, attention should also be paid to the size and quality of the inlet and outlet guide cylinders of the straightening machine and their adjustment.

When the inner surface of the guide cylinder is not smooth or the inner diameter is too small, the outer surface of the steel pipe may be scratched; once the center line of the guide cylinder deviates seriously from the straightening center line, the outer surface of the steel pipe may be scratched, concave correction or failure to bite normally. In general, the inner diameter of the guide cylinder should be 30-50mm larger than the outer diameter of the steel pipe to be straightened, and the center line of the guide cylinder should be consistent with the straightening center line.

The geometric dimensions (outer diameter and wall thickness), material, and original curvature of the steel pipe have a very close relationship with the straightening quality of the steel pipe. When the D/S value of the steel pipe is large, the required straightening force is small, the steel pipe is prone to bending and flattening during straightening, and the steel pipe after straightening has higher straightness and a smaller ovality; if the straightening force is too large, the steel pipe is prone to concave and flattening: If the D/S value is too small, the elastic deformation and elastic recovery of the steel pipe are large, the required straightening force is also large, and the straightness of the steel pipe after straightening will decrease. It is generally believed that the straightening effect of the steel pipe is best when the D/S value is in the range of 9 to 10; if it deviates from this range, the straightening effect of the steel pipe will deteriorate. The higher the strength of the steel pipe, the greater the original curvature, and the greater the required straightening force. A larger straightening force will increase the residual stress of the steel pipe, which is easy to cause surface scratches and cracks on the steel pipe. Therefore, when straightening steel pipes with high alloy content and large original curvature, a straightening process combining primary straightening and fine straightening can be used. During the primary straightening, it can be carried out on a pressure straightening machine to eliminate the “large bend” of the steel pipe. If it is carried out on an inclined roller straightening machine, the amount of pressure reduction and flattening must be controlled, and then the steel pipe that has been initially straightened is finely straightened according to the straightening process requirements.

2. Grinding and cutting defects of steel pipes and their prevention
The purpose of grinding surface defects of steel pipes is to remove surface defects that are allowed to exist in the steel pipe standard but must be ground clean, to improve the surface quality of the steel pipe. The defects caused by grinding the surface of steel pipes are mainly that the depth and shape of the grinding point after grinding exceed the requirements specified in the standard, causing the outer diameter or wall thickness of the steel pipe to exceed the negative deviation or irregular shape. The surface grinding of steel pipes should generally meet the following requirements.
(1) After the surface defects of the steel pipe are ground, the wall thickness of the grinding point shall not be less than the negative deviation of the nominal wall thickness of the steel pipe, and the outer diameter of the grinding point shall meet the requirements of the outer diameter of the steel pipe.
(2) After the steel pipe surface is ground, the ground part of the steel pipe surface must be kept smooth and curved. The ratio of the grinding depth, width, and length should be 1:6:8.
(3) When the steel pipe is ground as a whole, there should be no overburning and obvious polygonal marks on the steel pipe surface.
(4) The number of grinding points on the steel pipe surface should not exceed the number specified in the standard.

3. Surface processing defects of steel pipes and their prevention
The surface processing of steel pipes mainly includes steel pipe surface shot peening, surface overall grinding, and mechanical processing. Its purpose is to further improve the surface quality or dimensional accuracy of steel pipes.

Steel pipe surface shot peening: Steel pipe surface shot peening is spraying iron shots or quartz sand shots (collectively referred to as sand shots) of a certain size on the surface of the steel pipe at a high speed to knock off the iron oxide on its surface to improve the surface finish of the steel pipe. While the iron oxide on the steel pipe surface is broken and fallen off, some surface defects that are not easy to detect with the naked eye will also be exposed and easy to remove. The size, hardness, and spraying speed of the sand shot are important factors affecting the quality of shot peening on the steel pipe surface. If the sand shot is too large, too hard and the spraying speed is too fast, it is easy to break the iron oxide scale on the steel pipe surface, but it may also hit a large number of pits of varying sizes on the steel pipe surface to form a rough surface. On the contrary, the iron oxide scale may not be removed completely. In addition, the thickness and density of the iron oxide scale on the steel pipe surface will also affect the effect of shot peening. The thicker and denser the iron oxide scale on the steel pipe surface, the worse the effect of cleaning the iron oxide scale under the same conditions.

Overall grinding of steel pipe surface: The tools for overall grinding of the outer surface of the steel pipe are mainly sanding belts, grinding wheels, and grinding machines. The overall grinding of the inner surface of the steel pipe is done by a grinding wheel grinding or internal mesh grinding machine. After the steel pipe surface is completely ground, the iron oxide scale on the steel pipe surface can be completely removed, the surface finish of the steel pipe can be improved, and some small defects on the steel pipe surface such as small cracks, hair lines, pits, scratches, etc. can also be removed. The quality defects that may be caused by the overall grinding of the steel pipe surface with abrasive belts or grinding wheels are black skin on the steel pipe surface, excessive wall thickness, planes (polygons), pits, burns, and wear marks. The black skin on the steel pipe surface is caused by too little grinding or pits on the steel pipe surface. Increasing the grinding amount can eliminate the black skin on the steel pipe surface.

Generally speaking, the surface quality of the steel pipe will be better when the steel pipe is ground with abrasive belts, but the efficiency will be lower.