Surface Defects of Seamless Steel Pipes and their Causes

In the production of seamless pipes (SMLS), there are as many as 10 surface defects of steel pipes caused by various reasons. These defects mainly include: surface cracks (cracks), hairlines, inner folds, outer folds, crushing, inner straight roads, outer straight roads, separation layers, scars, pits, convex hulls, hemp pits (pockmarked surfaces), abrasion (scratch), internal spiral, external spiral, blue line, concave correction, roll printing, etc.

Among the surface defects of the above-mentioned steel pipes, some defects are very harmful to the performance of the steel pipe, which are called dangerous defects, such as steel pipe cracks (cracks), inner folds, outer folds, rolling breaks, delamination scars, and dents. , convex hull, etc.; some defects have relatively little impact on the performance of steel pipes, which are called general defects, such as steel pits (surfaces), blue lines, scratches (scratches, bumps), slight inner straights and outer straights, Slight internal and external spirals, concave correction, roll printing, etc.

Although some general surface defects that are very slight and have little effect on the use of steel pipes can remain on steel pipes, the standards still have very strict restrictions on the depth and length (size) of their defects. For dangerous steel pipe surface defects, it must be completely removed by cutting or grinding. When grinding those steel pipe surface defects that allow grinding, the depth of the specified grinding point and the shape of the grinding point must meet the requirements specified in the standard. In order to improve the surface quality of the steel pipe, shot peening (sand), sand grinding or machining turning is sometimes performed on the inner and outer surfaces of the steel pipe.

There are two main reasons for the surface defects of seamless steel pipes.

On the one hand, it is caused by surface defects or internal defects of the tube blank. On the other hand, it is produced in the production process. If the rolling process parameters are not designed correctly, the surface of the tool (mold) is not smooth, the lubrication conditions are not good, the pass design and adjustment are unreasonable, etc., may cause the surface quality of the steel pipe to appear. Problem; or during the heating, rolling, heat treatment and straightening process of the tube blank (steel pipe), if the heating temperature is not properly controlled, the deformation is uneven, the heating and cooling speed is unreasonable, or the straightening deformation is too large, resulting in excessive residual stress, it is also likely to cause surface cracks in steel pipes.

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