The final product of seamless steel pipes often has the problem of having uneven wall thickness. But how this problem develops? It remains undisclosed most of the time. In this article, we will talk about cold-drawn, cold-rolled, and hot-rolled seamless pipes and the reasons behind developing uneven wall thickness or eccentricity in them.
The eccentricity primarily occurs in hot rolled steel pipes, and the resulting links are created during hot perforation.
As per the anatomical analysis of the steel pipe after going through the automatic pipe rolling machine, the uneven longitudinal wall thickness of the steel pipe after the perforated capillary is rolled by the automatic pipe rolling machine. This process retains the distribution characteristics of the uneven wall thickness of the perforated capillary. But the seamless steel pipes still may have a spiral wall thickness unevenness after rolling, and the lateral wall thickness unevenness is remarkably increased.
The major reasons behind the uneven wall thickness of the automatic pipe mill are:
- The presence and severity of uneven wall thickness of perforated capillary tube directly affect the existence and severity of uneven wall thickness of finally produced steel pipe after rolling.
- As the pipe is rolled on the automatic pipe rolling machine, the thickness of the head is shifted from the center of the hole due to the bending of the ram. The resulting uneven wall thickness, the maximum wall thickness, and minimum wall thickness of the cross-section of the steel pipe and the pipe head are almost fixed. The same is true as the inequality of the wall thickness from the tube end to the tube head is gradually increased. Thus, by lessening the residual bending of the ram and lowering the axial force of the ram during rolling, the degree of eccentricity of the pipe wall can be reduced significantly.
- The more the wall thickness is removed, the more serious the wall thickness of the waste pipe is. When the wall reduction quantity is small, the automatic pipe rolling machine reduces the wall thickness unevenness of the perforated pipe.
- This also can happen if the hole type adjustment is not correct and the roll gap is not parallel; the wall thickness of the waste pipe will be amplified.
Top seamless steel tube manufacturers of India perform a Fourier transform on the measured wall thickness data of Φ400mm automatic tube rolling mill, perforation, secondary perforation (extension), automatic rolling, and uniform rolling process, and obtained uneven wall thickness. Based on the quantitative analysis and the reasons for its formation, here are a few suggested ways to improve the uneven wall thickness of steel pipes:
- After the secondary perforation or extension, the distribution of the uneven wall thickness on the waste pipe is retained to the finished seamless steel pipe. Hence improving the secondary perforation (extension) is the key link to enhance the wall thickness accuracy of the finished pipe. The main measure should be improved in the first place. The tool is designed to improve the concentricity of the ram and the pipe head with the rolling line during the rotation process.
- Improving the uneven thickness of the capillary tube after perforation is a vital part. The principal measures are to improve the heating uniformity of the tube blank, improve the precision of the centering hole, extend the length of the entire belt and the length of the reverse cone, and improve the ejector and the head. That can improve the concentrically with the rolling line during the rotation.
- Although there is a severe symmetry wall thickness unevenness in the rolling tube, it does not have a specific influence on reducing the spiral wall thickness. Therefore, two tubes should be rolled during the rolling process, and the waste pipe should be turned over 90° between the passes.
The eccentricity of many steel pipes is generated at the perforation site, so it is essential to control this during perforation strictly. In addition, finished steel pipe and Cold drawn steel pipe can also cause eccentricity problems due to perforation:
Any factor that ruins the geometrical accuracy of the deformation zone created during the roll, the head, and the guide plate will increase the wall thickness unevenness of the capillary.
- Head shape design, the ideal head rolling cone should be parallel with the roll exit cone. If the head is designed as per the traditional Matveev formula, the rolling cone of the head is not parallel with the exit cone of the roll; the metal that gets enlarged in between the gap will inevitably result in an insufficient tube wall rolling. Thus, the uneven wall thickness of the capillary tube and the wall thickness unevenness of the capillary tube is more severe with the increased feeding angle.
- Due to the inadequate rigidity of the ram, bending happens during the perforation process, and as a result, the head cannot maintain the centering position. Thus, the wall thickness of the pierced capillary becomes uneven.
- Uneven wear or damage to the head can also cause uneven wall thickness of steel pipes.
(2) Guide plate:
- In cases where the distance between the guide plates is too large, the eccentricity can occur in the pipes. During the perforation process, the centering of the perforations is sustained by the restriction of the guide plates. So, the distance between the guide plates is truly significant. The heads are considerably changed in the upper and lower positions, making the head unstable, resulting in irregular wall thickness of the capillary tubes.
- Uneven wear of the upper and lower guide plates can also increase the degree of wall thickness unevenness.
- Roll centerline deflection also is a major cause of uneven wall thickness. In the production process, due to the inaccurate installation of the screws on both sides of the puncher, or the horizontal deflection between the two rollers due to the thread and bearing wear, the feed angles of the two rolls become inconsistent. This distortion of the deformation zone results in uneven wall thickness.
- Large feed angles bring the head in more parallel with the rolling cone of the roll.
- Improper roll speed can also affect the wall thickness accuracy of the steel pipes.
(4) Centering and heating of the tube blank:
- Centering hole eccentricity and uneven heating may cause uneven wall thickness.
(5) Stiffness, structure, and adjustment of the punch:
- The rigidity of the punching machine is not enough, and the locking mechanism on it is not always dependable.
- If the centering device of the ejector is not accurate, the operation is unreliable, and the distance from the fuselage increases.
- The adjustment of the rolling centerline is generally lower than the centerline of the rolling mill. The objective is to improve the stability of the rolled product. If the adjustment is too large, the relative relationship between the tools in the deformation zone will change asymmetrically after the rolling line moves down, which can also affect the uneven wall thickness of the capillary.
Here, it can be concluded clearly that the uneven wall thickness of the seamless steel pipes will affect the overall performance during applications. This is the sole reason the leading seamless steel pipe manufacturers of India concentrate on producing products that maintain the accuracy of the wall thickness to compete in the global market. In this course, more advanced technology and types of machinery are being adopted by those piping mills.