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Defects Caused by Finishing of Steel Pipes and Their Prevention

Posted: 07/14/2022 10:03:15 Hits: 14

The finishing process of the steel pipe is an indispensable and important process to remove the defects of the steel pipe, further improve the quality of the steel pipe, and meet the special needs of the product. Finishing of steel pipes mainly includes straightening of steel pipes, end cutting (chamfering and sizing), inspection (surface quality inspection, geometric dimension inspection, non-destructive inspection and hydraulic tests), grinding, length measurement, weighing, painting, and packaging. Some steel pipes used for special purposes also require shot blasting, machining and anti-corrosion treatment on surfaces.

Defects caused by straightening of steel pipes
Straightening machines of steel pipes can be divided into pressure straightening machines, inclined roll straightening machines and tension straightening machines. The process of straightening steel pipes is to make the steel pipe undergo repeated shaping. The process of bending and flattening can achieve the purpose of reducing the bending and ovality of the steel pipe. The quality defects of the steel pipe in the straightening process mainly include no straightening, concaves, squares, and cracks, surface scratches and indentations of the steel pipe. There are many factors affecting the straightening of steel pipes, mainly including straightening machine types, pass shapes and pass adjustment, characteristics of steel pipes such as original curvature, sizes and materials. In the straightening 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 centerline of the guide cylinder has seriously deviated from the straightening centerline, the outer surface of the steel pipe will be scratched, dented or not properly fit. In general, the inner diameter of the guide cylinder should be 30 to 50mm greater than the outer diameter of the steel pipe to be straightened, and the centerline of the guide cylinder should be consistent with the straightening centerline.

The geometric size such as outer diameters and wall thickness, materials and original curvature of the steel pipe has a very close relationship with the straightening quality of the steel pipe. When the D/S value of the steel pipe is great, the required straightening force is small, and the steel pipe is prone to bending and flattening during straightening. The straightened steel pipe has good straightness and less ovality. If the straightening force is too great, the steel pipe will be prone to concave straightening and flattening. If the D/S value is too small, the elastic deformation and elastic recovery of the steel pipe will be great; the required straightening force will also be great, and the straightness of the steel pipe will be decreased. It is generally believed that the straightening effect of the steel pipe is the 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 and the greater the original bending of the steel pipe are, the greater the required straightening force becomes. The greater 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 great original curvature, a straightening process combining initial straightening and fine straightening can be used. During the initial straightening, they can be carried out on a pressure straightening machine to eliminate the great bending of the steel pipe. If an oblique roll straightening machine is used, the reduction and flattening amount should be controlled, and then the initial straightening should be finely straightened according to the straightening process requirements.

8.2 Defects caused by grinding and cutting of steel pipes and their prevention
The purpose of grinding surface defects of steel pipes is to remove the surface defects that are allowed by the standard of the steel pipe but must be ground to improve the surface of the steel pipe. The defects caused by the surface grinding of the steel pipe are mainly that the depth and shape of the grinding point after grinding exceeds the requirements of the standard, resulting in the outer diameter or wall thickness of the steel pipe exceeding the negative deviation or irregular shape. The surface grinding of steel pipes should generally meet the following requirements:

(1) After grinding the surface of the steel pipe, the wall thickness of the grinding part shall not be less than the negative deviation of the nominal wall thickness of the steel pipe, and the outer diameter of the grinding part shall meet the requirements of the outer diameter of the steel pipe.
(2) After the surface of the steel pipe is ground, it is necessary to maintain a smooth curved surface at the grinding part on the surface of the steel pipe, and the ratio of the depth, width and length of the grinding should be 1:6:8.
(3) When the steel pipe is ground as a whole, the surface of the steel pipe shall not have over-burning and obvious traces of multilateral lines.
(4) The surface grinding points of the steel pipe shall not exceed the number specified in the standard.

3. Defects caused by machining of steel pipes and their prevention
Surface processing of steel pipes mainly includes shot blasting on the surface of steel pipes, overall surface grinding and machining. Its purpose is to further improve the surface quality or dimensional accuracy of the steel pipe.

Shot blasting on the surface of the steel pipe: it is a process to make the iron shot or quartz sand shot of a certain size to hit the surface of the steel pipe at a high speed to remove the iron oxide scale on its surface so as to improve the surface finish of the steel pipe. When the iron oxide scale on the surface of the steel pipe is crushed and peeled off, some surface defects that are not easily found by the naked eye will also be exposed and easily removed. The size and hardness of the sand shot and the injection speed are important factors that affect the quality of shot blasting on the surface of the steel pipe. If the sand shot is too big, the hardness is too high, and the injection speed is too fast, the iron oxide scale on the surface of the steel pipe will be easily broken and fall off. However, it may also make a large number of pits of different sizes on the surface of the steel pipe to form a pitted surface. On the contrary, the iron oxide scale may not be removed well. In addition, the thickness and density of the iron oxide scale on the surface of the steel pipe will also affect shot blasting. The thicker and denser the iron oxide scale on the surface of the steel pipe is, the worse the cleaning effect of the iron oxide scale becomes under the same conditions.

The overall grinding of the surface of the steel pipe: the tools for the overall grinding of the outer surface of the steel pipe mainly include abrasive belts, grinding wheels and grinding machines. For the overall grinding of the inner surface of the steel pipe, use a grinding wheel or an inner mesh grinder. After the surface of the steel pipe is ground as a whole, not only the iron oxide scale on the surface of the steel pipe can be completely removed and the surface finish of the steel pipe be improved, but also small defects on the surface of the steel pipe such as small cracks, hair lines, pits and scratches can be removed. The quality defects that may be brought about by the overall grinding of the steel pipe surface with an abrasive belt or grinding wheel mainly include black skin on the surface, excessive wall thickness, pits, burns and wear marks of steel pipes. The black skin on the surface of the steel pipe is because the grinding amount is too small or there are pits on the surface of the steel pipe. Increasing the amount of grinding can eliminate the black skin on the surface of the steel pipe. Generally speaking, using an abrasive belt to grind the steel pipe as a whole will improve the surface quality of the steel pipe, but the efficiency will be lower.

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