Finishing defects of seamless steel pipes and their prevention

The finishing process of seamless steel pipes is an important process to remove defects of seamless steel pipes, further improve the quality of seamless steel pipes, meet the needs of special uses of products, and clarify the "identity" of products. The finishing of seamless steel pipes mainly includes: straightening of seamless steel pipes, 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 seamless steel pipes for special purposes also require surface shot peening and mechanical processing. Anti-corrosion treatment, etc.

1. Seamless steel pipe straightening quality defects:
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 pipe ends), 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, it will also cause the outer surface of the steel pipe to 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 seamless steel pipes have a very close relationship with the straightening quality of steel pipes. 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 straightness of the steel pipe after straightening is high and the ovality is small; 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. It is easy to cause scratches and cracks on the surface of 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 initial straightening, it can be carried out on a pressure straightening machine to eliminate the "big bend" of the steel pipe. If it is carried out on an inclined roller straightening machine, the amount of pressing and flattening must be controlled, and then the initially straightened steel pipe is finely straightened according to the straightening process requirements.

2. Grinding and cutting defects of seamless steel pipes and their prevention: The purpose of grinding surface defects of seamless steel pipes is to remove surface defects that are allowed to exist in the seamless steel pipe standard but must be polished clean, to improve the surface quality of seamless steel pipes. The defects caused by the surface grinding of seamless 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 seamless steel pipe to exceed the negative deviation or irregular shape. The surface grinding of seamless steel pipes should generally meet the following requirements.

(1) After the surface defects of seamless steel pipes are polished, the wall thickness of the grinding point shall not be less than the negative deviation of the nominal wall thickness of the seamless steel pipe, and the outer diameter of the grinding point shall meet the requirements of the outer diameter of the seamless steel pipe.
(2) After the surface of the seamless steel pipe is ground, the surface of the seamless steel pipe must be kept smooth and curved, and the ratio of the depth, width, and length of the grinding should be 1:6:8.
(3) When the seamless steel pipe is ground as a whole, the surface of the seamless steel pipe shall not have overburning and obvious polygonal traces.
(4) The number of surface grinding points of the seamless steel pipe shall not exceed the number specified by the standard.

3. Surface processing defects of seamless steel pipes and their prevention:
The surface processing of seamless steel pipes mainly includes: seamless steel pipe surface shot peening, surface overall grinding, and mechanical processing. Its purpose is to further improve the surface quality or dimensional accuracy of seamless steel pipes. Seamless steel pipe surface shot peening: Seamless 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 seamless steel pipe at a high speed to knock off the surface oxide scale to improve the surface finish of the seamless steel pipe. While the surface oxide scale of the seamless steel pipe is broken and falls 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 surface of seamless steel pipes. If the sand shot is too large, too hard and the spraying speed is too fast, it is easy to break and fall off the iron oxide scale on the surface of the seamless steel pipe, but it may also hit a large number of pits of varying sizes on the surface of the seamless steel pipe to form a pockmarked 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 surface of the seamless steel pipe will also affect the effect of shot peening.

The thicker and denser the iron oxide scale on the surface of the seamless steel pipe, the worse the effect of cleaning the iron oxide scale under the same conditions.
Overall grinding of seamless steel pipe surface: The tools for overall grinding of the outer surface of seamless steel pipes are mainly sand belts, grinding wheels, and grinding machines. The overall grinding of the inner surface of seamless steel pipes is done by grinding wheels or internal mesh grinders. After the surface of the seamless steel pipe is completely ground, the iron oxide scale on the surface of the seamless steel pipe can be completely removed, the surface finish of the seamless steel pipe can be improved, and some small defects on the surface of the seamless steel pipe 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 seamless steel pipe surface with abrasive belts or grinding wheels are: black skin on the surface of the seamless steel pipe, excessive wall thickness, plane (polygon), pits, burns and wear marks. The black skin on the surface of the seamless steel pipe is because the grinding amount is too small or there are pits on the surface of the seamless steel pipe. Increasing the grinding amount can eliminate the black skin on the surface of the seamless steel pipe.

Generally speaking, the surface quality of the seamless steel pipe will be better if the abrasive belt is used to grind the seamless steel pipe as a whole, but the efficiency will be lower.