The Reasons For The Bending Of Longitudinal Welded Pipes And How To Straighten Them

The reasons for the bending of straight seam steel pipes and the methods for straightening involve material science, processing techniques and mechanical principles.

1. Materials and Residual Stress

• Rolling Residual Stress: During the rolling process of the steel plate, due to uneven cooling or deformation, residual stress is formed within the plate. After the pipe is rolled, the stress is released, causing the pipe to bend.
• Chemical component segregation: The distribution of elements such as carbon and manganese in the steel billet is uneven, resulting in local differences in strength. This leads to deformation during subsequent processing.

2. Forming and Welding Process

• Forming Deviation: Uneven roller pressing during the rolling process or wear of the mold causes inconsistent curvature of the tube billet.
• Welding thermal influence: For longitudinal seam welding, high-frequency welding (ERW) or submerged arc welding (SAW) is used. The weld area is not heated evenly, and after cooling, the stress distribution is unbalanced. Especially in single-sided welding, it is more prone to bend towards the weld side.
• Asymmetry in weld fillet height: The difference in height between the inner and outer surfaces of the weld creates a "rib effect", causing the pipe body to deform when cooled.

3. Cooling and Heat Treatment

• Inhomogeneous Cooling: Differences in cooling rates during quenching or normalizing processes, resulting in microstructural stresses (such as the different volumes of martensite and ferrite during transformation).
• Incorrect temperature during straightening: When performing hot straightening, the temperature was lower than the recrystallization temperature (for carbon steel, approximately 600°C). This resulted in insufficient material plasticity, and the elastic recovery after straightening led to secondary bending.

4. Storage and Transportation

• Improper Support: When stacking in multiple layers, the spacing between supports was too large, causing the middle part of the pipe to sag under its own weight and undergo plastic deformation.
• Temperature gradient: Due to the temperature difference between the sunny side and the shady side during outdoor storage, thermal stress causes bending.

1. Mechanical straightening (cold straightening)

• Roller straightening machine:

1. Principle: By using multiple sets of interleaved straightening rolls to repeatedly perform plastic-bending on the steel pipe, the curvature is gradually eliminated.
2. Key parameters: Roll spacing (usually 0.8 - 1.2 times the pipe diameter), reduction amount (should exceed the yield strength but be lower than the tensile strength), straightening speed (for carbon steel, it is typically 20 - 40 m/min).
3. Key  points for adjustment: Use the "reverse bending method". Adjust the reduction amount of the corresponding roll group according to the position of the bending point. For large bends, it is necessary to pass through multiple times.

• Pressure Straightening Machine:

It is applicable to local hard bends. It adopts the three-point bending principle, applying pressure on the convex side of the bend, and simultaneously using a micrometer to monitor the straightness in real time (the target is ≤ 1.5mm/m).

2. Hot straightening
• Flame straightening:

1. Method: Use an oxygen-acetylene flame to apply heat in a band-like manner on the convex curved side (temperature 700 - 850℃, showing a dark red color), then cool and retract to straighten.
2. Technical points: The heating width should be no more than twice the pipe wall thickness, and the depth should not exceed one-third of the wall thickness. Avoid excessive heating that may cause grain coarsening.

• Overall heat treatment straightening:
For pipes with high residual stress, stress-relieving annealing (heating to 580-650℃, holding for a period, and then cooling slowly) combined with mechanical straightening yields better results.

1. Online monitoring:

Use a laser scanner to continuously measure straightness (such as the LAP laser measurement system), and the data is fed back to the straightening machine's PLC for automatic adjustment.

2. Process optimization:

• During welding, perform double-sided synchronous welding to reduce the asymmetry of heat input.
• Control the final rolling temperature above the Ar3 phase transformation point (approximately 850°C for low alloy steel), and avoid two-phase zone rolling.

3. Storage Specifications:

 Use V-shaped saddle supports. The spacing between the supports should be no more than 1/3 of the pipe length. Avoid long-term single-side exposure to direct sunlight.

For high-strength steel (such as X70 or above), the straightening process should control the deformation rate to be less than 3% to prevent the expansion of micro-cracks.

For thin-walled tubes (with a diameter/thickness ratio greater than 40), an internal supporting mold is required during straightening to prevent rounding errors.

After straightening, it is recommended to conduct ultrasonic testing (UT), especially to inspect the heat affected zone of the weld.