What Causes Uneven Wall Thickness Of The Steel Pipe? What Should Be Noted?

The unevenness of the wall thickness of steel pipes can be divided into lateral wall thickness unevenness (the difference between the maximum and minimum wall thickness within the same cross-section) and longitudinal wall thickness unevenness (the fluctuation of wall thickness along the length of the pipe). It is one of the most common dimensional accuracy defects in steel pipe production. Its causes are strongly related to the production process. The main causes differ significantly between seamless pipes and welded pipes. At the same time, it is influenced by multiple factors such as raw materials, equipment, and operations.

1. Inherent deviations of raw materials

• Seamless steel pipes: The diameter tolerance of the pipe billet is large, the roundness is poor, there is central porosity/oven shrinkage or composition segregation, which directly causes eccentricity in the wall thickness after piercing; if the heating temperature of the pipe billet is uneven (with significant temperature differences between the inner and outer walls, and the head and tail sections), the deformation resistance is inconsistent, and after the piercing deformation, the wall thickness deviation will be further amplified.
• Welded steel pipes: The thickness tolerance of the steel strip/steel plate is excessive. There is "edge thinning" (the edge thickness is less than the middle part) in the same steel strip roll, or the thickness fluctuation at the head and tail of the steel strip, which directly converts into longitudinal and transverse wall thickness inhomogeneity of the pipe body; when the steel strip has a crescent bend, the fluctuation in the forming angle will also indirectly cause wall thickness deviation.

2. Process deviations in forming/rolling procedures

•  Seamless pipe - piercing process: This is the main source of uneven wall thickness across the pipe. Wear and eccentricity of the piercing machine head, bending of the push rod, inaccurate centering of the pipe billet, asymmetry in the inclination angle of the rolls, and unreasonable setting of the push rod extension amount, all can cause "deviation of the center" during the piercing of the raw pipe, resulting in eccentricity of the wall thickness of the entire steel pipe.
• Seamless pipe - continuous rolling and sizing reduction process: Inconsistent adjustment of the roll gap in the continuous rolling machine, inconsistent wear of the die, unstable rolling tension (pulling steel/stacking steel), can cause longitudinal fluctuations in wall thickness along the pipe length; excessive reduction amount in the sizing reduction machine, improper tension matching, not only will it amplify the original wall thickness deviation, but also will result in typical thickening of the pipe end wall thickness (with the end wall thickness being greater than the middle part of the pipe body).
• Cold-drawn steel pipe: Wear and damage of the outer mold and core head, insufficient centering accuracy, unstable drawing speed, uneven lubrication, can lead to inconsistent wall thickness reduction amount, resulting in longitudinal or lateral wall thickness deviations.
• Welded pipe - forming and sizing process: Inconsistent pressure adjustment of the forming rolls, inconsistent bending deformation amount at the edges, can cause deformation differences in the wall thickness of both sides after welding; Inconsistent centering of the die in the sizing machine will further exacerbate the lateral wall thickness difference; Inappropriate control of the extrusion amount in the weld area can also result in local wall thickness abnormalities.

3. Insufficient accuracy of equipment and tooling

• The processing accuracy of rolling rolls and die profiles does not meet standards. After long-term production, they may experience uneven wear or local collapse, which cannot ensure stable forming dimensions.
• The alignment accuracy of guide devices, mandrels/headers is poor. Or the mandrels may be bent or wear unevenly, causing the steel pipe to deviate during rolling and resulting in asymmetric force deformation.
• The rigidity of the main equipment is insufficient. During the rolling of large-sized thick-walled pipes, the bouncing amount fluctuates greatly, and the actual value of the reduction amount deviates significantly from the set value.

4. Insufficient operation and process control

• After changing specifications, changing rolls/switching nozzles, insufficient debugging was conducted. The first batch of steel pipes were produced without fully verifying the wall thickness.
• The temperature distribution in the heating furnace was uneven, and the heating rhythm was unreasonable. The temperature difference of the pipe billets exceeded the allowable range of the process.

1. Key Points for Production Control

• Strictly control the entry of raw materials: Conduct standard inspections on the diameter, roundness, surface quality of the billets, as well as the thickness tolerance and edge thickness difference of the steel strips. Unqualified raw materials shall not be put into production; ensure the centering accuracy before billet piercing.
• Regular calibration and replacement of tools: Establish wear detection standards for rolls, heads, cores, and dies. Replace them promptly if they exceed the limits; after each tool change or specification change, conduct centering adjustment and first-piece wall thickness measurement, and only proceed with batch production if it is qualified.
• Optimize the stability of process parameters: Control the uniformity of billet heating, reduce the temperature difference between the head and tail and the inside and outside; precisely match the rolling speed, tension, and reduction amount to avoid significant fluctuations; for the problem of tail thickening in fixed reduction heads, the tension system and end thinning compensation process can be optimized to improve the situation.
• Configure online detection loops: Install ultrasonic or radiographic online thickness measurement systems to monitor the lateral and longitudinal wall thickness deviations in real time, and adjust the rolling parameters in a coordinated manner to promptly correct deviations.

2. Precautions for Product Acceptance

• Strictly verify the allowable deviation of wall thickness in accordance with the applicable standards (such as GB/T 8163, API 5L, ASTM A106, DIN 2448, etc.): The tolerance for the wall thickness in conventional industrial pipes is typically ±10% to ±12.5%, while for high-precision pipes, it can reach ±5% to ±8%.
• The inspection should cover key positions such as the middle part of the pipe body, both ends, and weld seams (welded pipes), not only measuring the single-point wall thickness, but also detecting the maximum/minimum wall thickness of the same section and calculating the wall thickness unevenness.
• For steel pipes with critical wall thickness deviations, additional verification of roundness is required to avoid misjudgment of wall thickness due to "compression deformation".
• There was no on-line wall thickness monitoring method. During production, parameters could not be adjusted in real time, and deviations continued to accumulate.

3. Post-processing and Usage Precautions

• Before conducting secondary processing such as threading, bending, flaring, and cold drawing, it is necessary to recheck the wall thickness deviation. When processing severely unevenly thick steel pipes, problems such as cracking, inappropriate thread profile, and excessive thinning on the outer side of the bent pipe may occur.
• For pressure-bearing pipelines and structural load-bearing scenarios, special attention should be paid: Uneven wall thickness can cause stress concentration, reducing the overall pressure-bearing capacity and structural safety. Under high-pressure and corrosive conditions, the risk of local failure will be exacerbated.
• During storage and transportation, avoid heavy objects squeezing or knocking, and prevent local deformation of the pipe end that leads to measurement deviation of the wall thickness.