Factors Affecting the Measurement and Control of Steel Pipe Straightness

The factors affecting the accurate measurement and control of steel pipe straightness mainly include two aspects: production process and measurement method.

Production Process Causes of Steel Pipe Bending (poor straightness):

1. Improper Adjustment of Straightening Frame
Improper setting of straightening process parameters (such as roll gap, pressure, straightening model) leads to incomplete elimination of residual stress or the introduction of new stress.

Bending Phenomenon: The steel pipe bends after the sizing unit, and the bending direction is unidirectional.

2. Uneven Wall Thickness of Raw Material
The steel pipe itself has significant wall thickness unevenness (eccentricity), which causes bending due to inconsistent shrinkage/deformation during cooling or under stress.

For seamless steel pipes (SMLS), the source is the uneven wall thickness or eccentricity of the billet. During subsequent rolling and cooling processes, the thinner side cools faster or deforms more, causing bending towards the thicker side. This is an inherent material defect.
For welded pipes (such as ERW pipe), uneven thickness of the steel coil will cause bending.

3. Uneven Temperature on the Upper and Lower Surfaces of the Steel Pipe

Uneven cooling of the steel pipe (e.g., temperature difference between top/bottom or left/right) leads to different shrinkage rates in different parts of the pipe, causing thermal stress bending.

Bending Phenomenon: The steel pipe is straight after cutting during production, but bends during cooling on the finishing rack, and the overall bending direction is consistent.

4. Sizing and Residual Stress

Bending after hydrostatic testing is more commonly caused by large residual stress inside the steel pipe (from rolling, cooling, and straightening). During hydrostatic testing, the internal pressure causes slight plastic deformation of the steel pipe, releasing or redistributing residual stress, thus exposing potential bending.

Bending Phenomenon: The steel pipe is straight before hydrostatic testing, but bends after the test, and the bending direction is fixed. 

Technical Factors Affecting the Straightness Measurement of Steel Pipes:

1. Measurement Method and Reference Point

Factor: Whether the reference line used during measurement (e.g., taut steel wire, laser beam) is straight and stable.
Impact: Inaccurate reference points will distort all measurement results.

2. Measurement Equipment and Support

Factor: Whether the measurement platform or V-block is level, and whether the number and location of support points are reasonable (to avoid deflection due to its own weight).
Impact: Improper support can artificially introduce or conceal bending.

3. Ambient Temperature

Factor: The temperature difference between the measurement environment and the steel pipe production/storage environment.
Impact: Temperature differences cause thermal expansion and contraction of the steel pipe, affecting the actual length and shape during measurement, resulting in measurement errors.

Read more: Main quality testing items and methods of seamless pipes