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Defects of Rolled Tubes

Posted: 05/23/2022 07:28:03 Hits: 75

The tube's rolling process is the main process to determine the wall thickness of the seamless steel tube, and its function is to reduce the wall of the perforated billet. The main defects are uneven wall thickness of pierced billets, cracks on surfaces, pits, scratches, puncturing and so on.

1. Defects of automatic rolling tubes
When the automatic pipe rolling machine rolls the billet, the defects of the pierced billet include uneven wall thickness, ears and folds, straight lines, pits on the inner surface, cracks and scratches on the surface and other defects.

The uneven wall thickness of the pierced billet: The main defect produced by automatic pipe rolling mills is not only related to the uniform wall thickness and temperatures of pierced billets, but also structure of rolling mills, shapes of roll passes, deformation, shapes of plugs, operating technology and other factors.

Ears and folds of pierced billets: they are mainly caused by unreasonable distribution of deformation, incorrect hole shapes, tool wear and so on. Any condition that is not conducive to the axial extension of the pierced billet tube and promotes the transversal expansion of the metal is likely to cause ears and folds in the pierced billet.

Straight lines and pits on the inner surface of the pierced billet: The inner straight line is a defect on the inner surface of the pierced billet that often occurs in automatic pipe rolling mills, and it is also a difficult problem to be solved. In most cases, the straight lines in the pierced billet are not enough to make the pipe scraped. The inner straight line is a linear scratch along the axial direction on the inner surface of the pierced billet, which is mainly generated in the reduced wall area of the pierced billet. The surface state of the top, the scale of iron oxide on the inner wall of the pierced billet, the distribution of deformation, and the lubrication conditions are all related to the inner straight lines in the pierced billet. Spray lubricant onto the inner hole of the pierced billet to improve the lubrication effect of the top, which can reduce the rolling pressure, the wear of the top, and prevent the plug from sticking to the steel, thereby reducing the occurrence of inner lines for the pierced billet. The inner pit is manifested as an uneven pit in a certain area of the inner surface of the pierced billet. The main reason is that the numerous iron oxide scales on the inner surface of the pierced billet tube have not been blown away, or it may be caused by the accumulation of antioxidants or lubricants in a certain area of the inner surface of the pierced billet tube after being wet and agglomerated. To reduce the inner pit of the pierced billet, the angle of the tuyere used in the iron oxide scale removal device should be properly adjusted and sufficient wind pressure should be ensured to completely remove the iron oxide scale.

Cracks and scratches on the surface of the pierced billet: There are two types of cracks on the surface of the pierced billet; one is caused by the pierced billet, and the other is generated in the automatic tube rolling process. Surface cracks will occur when the deformation is too great. The distribution of the deformation is unreasonable, and the pass shape is adjusted improperly or the surface of the rolling tool is severely worn. All measures to reduce the uneven wall thickness of the pierced billet and the uneven deformation of the metal are beneficial to reduce the surface cracks of the pierced billet. The surface scratches of the pierced billet are caused by the rough surface of the deformation tool or the relative movement between the pierced billet and the workpiece being in contact with the surface of the pierced billet. To reduce the surface scratch of the pierced billet, the inspection and adjustment of the pipe rolling tools and workpieces should be strengthened.

2. Defects of periodic rolling tubes
There are two reasons for the defect of periodic rolling tubes. One is caused by the pierced billet, and the other is produced in the rolling process. The resulting defects include uneven wall thickness such as uneven transverse wall thickness and uneven longitudinal wall thickness, surface defects such as surface scratches, internal and external scarring, internal lines, cracks and internal pits.

The uneven wall thickness of the pierced billet: It is the main quality defect produced by the periodic rolling mill, including the uneven horizontal wall thickness, uneven longitudinal wall thickness, uneven wall thickness of convex hulls and bamboo joints.

Factors affecting the uneven wall thickness of the pierced billet:
A. The shape of the roll pass
B. Process parameters
C. Pipe rolling tools
D. Operation technology

Defects on the surface of the pierced billet: they are mainly surface scratches, inner straight lines, inner pits, inner and outer scars, etc. The inner and outer surface scratches of the pierced billet are caused by the surface defects of the mandril and roller, and the scratches on the surface of the pierced billet are caused by the guide groove after rolling. The inner straight is a straight groove formed due to the mandril’s surface sticking to steel, severe wear, losing metal and oxide iron scale and iron filings remaining on the inner surface of the pierced billet tube, which scratches the inner surface of the pierced billet tube in the rolling process. To reduce the scratches on the surface and the inner straight line of the pierced billet, the surface of the mandrel and the roller should be smooth, and the cooling of the roll and the lubrication of the mandril should be strengthened.

3. Defects of continuous rolling tubes
The distinguishing feature of a continuous tube rolling mill is its large production capacity and high production efficiency. The rolled pierced billet has a long length, good quality and a wide range of specifications. However, the investment is great; the relevant technical requirements are many and the degree of automation is high. The defects of continuously rolled pierced billet are mainly: uneven wall thickness of the pierced billet, surface defects, etc.

4. Defects of skew-rolled tubes
There are two reasons for defects of skew-rolled tubes. One is the pierced billet and the other is the rolling process. The resulting defects are related to the shape and adjustment of the skew rolling mill, deformation process parameters and adjustment, quality of the tool, and lubrication conditions of mandrels.

The resulting defects: surface defects, uneven wall thickness and irregular tail shapes.

To prevent the quality defects of the pierced billet produced in the skew rolling process, we should not only improve the quality of the tube blank, but also the quality of heating and perforation of the tube blank, ensuring the quality of the perforated pierced billet tube, and eliminating the rolling folds, cracks and pits existing on the surface of the pierced billet. In addition, it is necessary to reasonably design the pass shape, optimize the deformation process parameters, carefully adjust the pass shape according to the process parameters, ensure the quality of the rolling tools and improve the lubrication conditions of the tools.

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