Finishing defect of seamless carbon steel pipe and its prevention

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, meet the needs of the special purpose of the product, and clarify the “identity” of the product. The finishing of steel pipe mainly includes: straightening of steel pipe, cutting end (chamfering, sizing), inspection and inspection (including surface quality inspection, geometric dimension inspection, non-destructive inspection and hydraulic test, etc.), grinding, Processes such as length measurement, weighing, painting, printing and packaging. Some special-purpose steel pipes also require surface shot peening and machining. Anti-corrosion treatment, etc.

Straightening quality defect of seamless pipe

steel pipe straightening machine can be divided into pressure straightening machine, inclined roll straightening machine and tension straightening machine. The process of straightening the steel pipe is to make the steel pipe undergo repeated elastic-plastic! The quality defects of the steel pipe during the straightening process mainly include: the steel pipe is not straightened (including the pipe end goose head bend), concave straightening, square straightening, crack straightening, surface scratches and indentations. There are many factors affecting the straightening quality of steel pipes, mainly including: straightening machine type, pass shape and pass adjustment, steel pipe characteristics (original curvature, size and material) and so on. In 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 is seriously deviated from the straightening center line, the outer surface of the steel pipe will also be scratched, Corrected concave or can not properly bite. In general, the inner diameter of the guide cylinder should be 30-50mm larger than the outer diameter of the straightened steel pipe, and the centerline of the guide cylinder should be consistent with the straightening centerline. Quick opening and trapezoidal speed straightening are the characteristics of our company.

The geometric size (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, and the steel pipe is prone to bending and flattening deformation during straightening, and the straightness of the steel pipe after straightening is relatively high. High and small ovality; if the straightening force is too large, the steel pipe is prone to concave straightening and flattening: if the D/S value is too small, the elastic deformation and elastic recovery of the steel pipe are large, so The required straightening force is also large, and the straightness of the straightened steel pipe will be reduced. 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 become worse.

The higher the strength of the steel pipe and the greater the original bending, the greater the required straightening force. The 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 initial 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 a skew roll straightening machine, the reduction and flattening amount should be controlled, and then the straightening process should be carried out. Fine straightening of the initially straightened steel pipe is required.

Grinding and cutting defects of steel pipes and their prevention

The purpose of grinding the surface defects of the steel pipe is to remove the surface defects that are allowed by the standard of the steel pipe but must be cleaned, so as to improve the surface quality 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 exceed 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 defects of the steel pipe, the wall thickness of the grinding place shall not be less than the negative deviation of the nominal wall thickness of the steel pipe, and the outer diameter of the grinding place 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 keep the surface of the steel pipe surface smooth and curved, 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.

Surface Machining Defects of steel pipe and Its Prevention

The surface processing of steel pipe mainly includes: shot peening on the surface of steel pipe, overall surface grinding and mechanical processing. Its purpose is to further improve the surface quality or dimensional accuracy of steel pipes. Shot peening on the surface of steel pipe: Shot peening on the surface of steel pipe is to spray iron shot or quartz sand shot (collectively referred to as sand shot) of a certain size on the surface of the steel pipe at a high speed, and knock off its surface. Iron oxide scale to improve the surface finish of 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 for easy removal. The size and hardness of the sand shot and the injection speed are important factors affecting the quality of the shot peening on the surface of the steel pipe. If the sand shot is too large, 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, but it may also hit a large number of pits of different sizes on the surface of the steel pipe. to form pockmarks. On the contrary, the iron oxide scale may not be removed cleanly. In addition, the thickness and density of the iron oxide scale on the surface of the steel pipe will also affect the effect of shot peening.

The thicker and denser the iron oxide scale on the surface of the steel pipe, the worse the cleaning effect of the iron oxide scale 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 machine tools. For the overall grinding of the inner surface of the steel pipe, the grinding wheel or the inner mesh grinding machine are used for grinding. After the surface of the steel pipe is ground as a whole, it can not only completely remove the iron oxide scale on the surface of the steel pipe, improve the surface finish of the steel pipe, but also remove some small defects on the surface of the steel pipe, such as small cracks, Hair lines, pits, abrasions, etc. Grinding the surface of the steel pipe as a whole with the abrasive belt or grinding wheel may cause the main quality defects: black skin on the surface of the steel pipe, excessive wall thickness, plane (polygon), pits, burns and wear marks, etc. 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, the surface quality of steel pipe will be better if the abrasive belt is used to grind the steel pipe as a whole, but the efficiency will be lower.