Causes and Countermeasures of Outward Bending and Hairline of Seamless Steel Tube

This article introduces the causes and countermeasures of outward bending and hairline of seamless steel tubes. Outward bending, also known as ovalization, is a common defect, which is caused by the uneven surface stress of the tubular material during the stretching process of the tube drawing or by the large lateral force in the tubing during the normal transportation process. Hairline is a common defect caused by plastic deformation of the tube material under the action of too high stress or temperature. The countermeasure for both the outward bending and the hairline of seamless steel tube should be focused on the surface stress during the stretching process and on the temperature during the working process.
    
Keywords: Seamless steel tube, Outward Bending, Hairline, Countermeasures
    
1 Introduction:
Seamless steel tubes are widely used in many important industries like military, aerospace, and energy fields due to its relatively higher strength, heat resistance, flexibility and durability. However, it often shows outward bending or hairline defects. Outward bending, which is also known as ovalization, is a common defect caused by uneven surface stress of the tubular material during the stretching process of tube drawing or by the large lateral force in the tubing during the normal transportation process. Hairline is another common defect caused by plastic deformation of the tube material under the action of too high stress or temperature. Therefore, it is necessary to understand the causes of these two kinds of defects and take necessary countermeasures to ensure the quality and performance of seamless steel tubes.
    
    2 Causes of Outward Bending and Hairline
    
    2.1 Causes of Outward Bending
    
    Outward bending of seamless steel tube is a common defect caused by uneven surface stress. The uneven surface stress is usually caused by non-uniform effect of force during the stretching process of the tube drawing. Usually, the force acts on the inner and outer wall of the tubing simultaneously, so the surface stress is different between the inner and outer wall of the tubing. If the force on the inner wall exceeds that on the outer wall, there will be an inward deformation of the tube, leading to an outward bending. In addition, during the transportation process of the tubing, due to certain external factors, such as sharp inclines and sudden brakes, the tubing may also be subjected to large lateral force which results in an uneven surface stress of the tubing and thus the outward bending.
    
    2.2 Causes of Hairline
    
    Hairline is a common defect of seamless steel tube caused by plastic deformation under too high stress or temperature. When the stress or temperature exceeds the yield strength of the tubing material, the tubing will tend to "give", resulting in plastic deformation of the tubing material. Under the action of too high stress or temperature, the tubing material will deform and become uneven, thus leading to hairline defects. In addition, if the stress or temperature exceeds the allowable limit of the tube material, the tubing material may fracture and form a "crack" which is also known as hairline defect.
    
    3 Countermeasures
    
    3.1 Countermeasures for Outward Bending
    
    The countermeasure for outward bending should be focused on the surface stress during the stretching process of the tube. It is important to ensure that the force acting on the inner and outer wall of the tubing is well distributed during the stretching process of the tube drawing. In addition, when installing and transporting the tubing, we should also pay attention to avoid the deformation caused by lateral force.
    
    3.2 Countermeasures for Hairline
    
    Countermeasure for hairline should focus on controlling the stress and temperature that the tube material is exposed to. During the working process of the tubing, stress and temperature should be strictly controlled within the allowable range of the tubing material to ensure no plastic deformation or fracture of the material.
    
    4 Conclusion
    
    Outward bending and hairline of seamless steel tubes are common defect caused by uneven surface stress or plastic deformation under too high stress or temperature. The countermeasures should be focused on the surface stress during the stretching process and the temperature during the working process. It is necessary to take appropriate measures to ensure the quality and performance of seamless steel tubes.

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