Sizing of seamless steel pipes: How to reduce residual stress?

Sizing is a key step in the production process of seamless steel pipes. However, the seamless pipe sizing operation may cause residual stress in the pipe body, thereby reducing its mechanical strength. In order to ensure the geometry of the pipe body and improve its mechanical properties, we need to deeply study the relationship between the reduction amount, temperature and residual stress distribution of sizing rolling to obtain the best sizing effect.

First of all, the reduction amount is an important parameter in the sizing process. Too much reduction may cause deformation of the pipe body, while too little reduction may not achieve the specified geometry. Therefore, it is necessary to find a suitable reduction amount that can meet the requirements of the geometry and reduce the generation of residual stress.

Secondly, temperature also has an important influence on the sizing process. Too high a temperature may cause the pipe body material to soften, thereby increasing the possibility of deformation; while too low a temperature may harden the material and increase the difficulty of sizing. Therefore, it is necessary to control the temperature during the sizing process to ensure the quality and performance of the pipe body.

Finally, the distribution of residual stress directly affects the mechanical strength of the pipe body. If the residual stress is too large, it may cause cracks or deformation in the pipe body during use. Therefore, studying the relationship between the reduction amount and temperature of sizing rolling and the residual stress distribution can help us find the best sizing process, thereby improving the mechanical strength of the pipe body.

Causes and control methods of residual stress after cold drawing of thick-walled seamless pipes:

1. Generation of cold drawing stress
During the seamless tube cold drawing process, thick-walled seamless pipes will produce uneven deformation due to additional stress, thereby forming residual stress after cold drawing. This high-precision thick-walled seamless steel pipe is widely used in many fields, such as pipelines or structural parts for transporting liquids. Its hollow cross-section design makes it relatively light in weight under the same bending, torsion and compressive strength, making it an economical cross-section steel.

In the process of generating cold drawing stress, the metal material on the surface of the thick-walled seamless steel pipe first enters the plastic deformation state, and its radial metal fluidity is not hindered by friction, resulting in uneven radial deformation along the wall thickness direction. This phenomenon causes the natural tensile strength of the surface layer of the steel pipe to be significantly lower than that of the inner layer. Therefore, in the surface layer of thick-walled seamless steel pipe, radial and axial tensile deformation and additional stress reach the maximum value; while in the opposite direction, additional reduction deformation and reduction stress also reach the maximum value in the pipe. These additional stresses are converted into residual stress after deformation and remain in the pipe. In summary, the asymmetric deformation of thick-walled seamless steel pipe after drawing is the main reason for the increase of its residual stress.

2. Production process and its influence
The production process of thick-walled seamless steel pipe mainly includes cold drawing and hot rolling. The hot rolling process is relatively complicated and requires multiple processes, such as three-roll mill extrusion molding, die inspection, surface initialization and cutting. During this process, if cracks or other defects appear on the surface of the steel pipe, it will be precisely cut by a cutting machine to ensure product quality.

3. Factors and influences of residual stress
The residual stress distribution and size of thick-walled seamless steel pipe are closely related to its asymmetric deformation level. This deformation characteristic is mainly attributed to specific conditions in the cold drawing process, such as lack of mandrel support and deformation differences caused by wall thickness changes. During the deformation process, the metal material on the inner surface of the thick-walled seamless steel pipe first enters the plastic deformation state, and its radial metal fluidity is not hindered by friction, resulting in uneven radial deformation along the wall thickness direction. This phenomenon causes the natural tensile strength of the steel pipe surface layer to be significantly lower than that of the inner layer.

Therefore, in the surface layer of the thick-walled seamless steel pipe, the radial and axial tensile deformations and additional stresses reach the maximum value; in the opposite direction, the additional reduction deformation and reduction stress also reach the maximum value in the pipe. These additional stresses are converted into residual stresses after deformation and remain in the pipe.

In summary, the uneven deformation of the thick-walled seamless steel pipe after drawing is the main reason for the increase in its residual stress. Therefore, it is necessary to explore methods that can effectively reduce the residual stress and reduce the vertical gap damage during the pipe drawing process.

Read more: Methods for stress elimination of pipeline engineering or Seamless Steel Pipe Manufacturing Process