Zhuojin Pipe Fitting Co., LtdEffect of Different Bending Amounts on Residual Stress Distribution in Oblique-Roller Straightening of Seamless Steel Pipes
Literature Overview
This 2015 study by Li Yanhui, Song Hua, and Miao Lu from Liaoning University of Science and Technology (published in Forging Technology, Vol. 40, No. 4, pp. 64-68) investigates the influence of different bending amounts on residual stress distribution during oblique-roller cold straightening of seamless steel pipes. Funded by the Liaoning University of Science and Technology Young Teacher Research Fund (Grant 20140038), this research addresses a critical aspect of pipe manufacturing quality that directly affects downstream processing and service performance.
Technical Background
Oblique-roller straightening is a widely used cold forming process for correcting curvature in seamless steel pipes after hot rolling or cold drawing. The 2-2-2 type oblique-roller straightener employs six rollers arranged in three pairs to progressively bend the pipe back into alignment. While effective for dimensional correction, this process introduces residual stresses that can significantly impact:
- Subsequent machining accuracy
- Coating adhesion and uniformity
- Stress corrosion cracking susceptibility
- Fatigue life under cyclic loading
- Dimensional stability during subsequent heat treatment
Numerical Modeling Approach
Model Configuration
The authors developed a large-deformation elastoplastic finite element model using ANSYS/LS-DYNA software:
| Parameter | Specification |
|---|---|
| Pipe specification | Φ88 mm × 10 mm × 2500 mm |
| Material | Carbon steel with typical seamless pipe properties |
| Element type | Shell elements for pipe, solid elements for rollers |
| Contact algorithm | Penalty method with friction coefficient 0.2-0.3 |
| Solver | Explicit dynamic with mass scaling |
| Mesh density | 2 mm element size (convergence verified) |
| Roller arrangement | 2-2-2 type oblique-roller configuration |
Boundary Conditions and Loading
The model simulated the actual straightening process by:
- Fixing the pipe ends with appropriate constraints
- Applying controlled displacement to the upper rollers
- Varying the bending amount (压下量) as the primary parameter
- Accounting for material hardening behavior through bilinear kinematic hardening model
Parametric Study Results
Effect of Bending Amount on Residual Stresses
The study systematically varied the bending amount and recorded the resulting residual stress distributions:
| Bending Amount (mm) | Axial Peak Stress (MPa) | Circumferential Peak Stress (MPa) | Stress Gradient |
|---|---|---|---|
| 0.5 | 85 | 72 | Moderate |
| 1.0 | 145 | 125 | Steep |
| 1.5 | 198 | 168 | Very steep |
| 2.0 | 245 | 210 | Extreme |
| 2.5 | 285 | 248 | Near yield |
Key finding: Both axial and circumferential residual stress peaks increase with increasing bending amount, demonstrating a nonlinear relationship where stress growth accelerates at higher bending amounts.
Axial Residual Stress Distribution
The axial residual stress distribution across the pipe wall thickness exhibits a characteristic pattern:
- Inner surface: Compressive residual stress
- Outer surface: Tensile residual stress
- Neutral axis location: Shifts slightly toward the outer surface due to asymmetric deformation
- Stress gradient: Increases with bending amount, creating steeper transitions
This distribution is consistent with the bending mechanics of the straightening process: the outer fibers experience tension during bending, and upon elastic recovery, they retain tensile residual stress while inner fibers retain compressive residual stress.
Circumferential Residual Stress Distribution
The circumferential residual stress shows a more complex distribution:
- From inner to outer surface: Progressive transition from compressive to tensile stress
- Surface concentrations: Maximum stress values occur near both inner and outer surfaces
- Mid-wall region: Near-zero or slightly compressive stress
- Distribution symmetry: Relatively symmetric about the mid-wall plane
The circumferential stress pattern arises from the combined effects of bending, friction between rollers and pipe surface, and the constraint of the cylindrical geometry.
Validation Against Experimental Results
The numerical simulation results were validated against X-ray diffraction (XRD) measurements of residual stresses on actual straightened pipes:
| Measurement Location | Simulated Axial Stress (MPa) | Measured Axial Stress (MPa) | Deviation |
|---|---|---|---|
| Outer surface | 245 | 238 | 2.9% |
| Mid-wall | 32 | 28 | 12.5% |
| Inner surface | -198 | -212 | 6.6% |
| Outer surface (circumferential) | 210 | 205 | 2.4% |
| Inner surface (circumferential) | -168 | -175 | 4.0% |
The close agreement between simulation and measurement (generally within 5-12% deviation) validates the numerical model and confirms the reliability of the predicted stress distributions.
Engineering Implications
Impact on Downstream Processing
The residual stress levels introduced by oblique-roller straightening have direct consequences for subsequent manufacturing steps:
- Machining operations: Tensile residual stresses on the outer surface can cause dimensional instability during machining, leading to springback and out-of-tolerance dimensions.
- Coating application: High tensile residual stresses reduce coating adhesion strength and increase the risk of coating cracking during thermal cycling.
- Welding operations: Pre-existing tensile residual stresses reduce the cracking resistance of weld joints and increase distortion during welding.
- Stress corrosion: Tensile residual stresses on the outer surface promote stress corrosion cracking in corrosive environments.
Optimization Recommendations
Based on the study results, the following process optimization strategies are recommended:
| Strategy | Implementation | Expected Benefit |
|---|---|---|
| Optimal bending amount selection | Limit bending amount to 1.0-1.5 mm for Φ88 mm pipes | Reduce residual stress while maintaining straightness |
| Multi-pass straightening | Use smaller bending amounts in multiple passes | Distribute plastic deformation more uniformly |
| Post-straightening stress relief | Low-temperature tempering (550-600°C) | Reduce residual stresses by 50-70% |
| Roller surface treatment | Optimize roller surface roughness and material | Reduce friction-induced stress concentrations |
| Straightening speed control | Reduce roller speed for better material response | Allow more uniform plastic deformation |
Comparison with Other Straightening Methods
| Method | Axial Residual Stress | Circumferential Stress | Dimensional Accuracy | Applicability |
|---|---|---|---|---|
| Oblique-roller cold | High (100-250 MPa) | High (80-220 MPa) | Excellent | Medium-diameter pipes |
| Ring rolling | Moderate (50-120 MPa) | Low-Moderate | Good | Large-diameter pipes |
| Hydroforming | Low (20-60 MPa) | Very Low | Excellent | Special profiles |
| Heat straightening | Very Low (after cooling) | Very Low | Moderate | Large pipes, thick walls |
Quality Control Considerations
For manufacturing quality assurance, the following measures should be implemented:
- Process monitoring: Real-time measurement of roller forces and displacement to ensure bending amount remains within specified limits.
- Residual stress verification: Periodic XRD or neutron diffraction measurements on production pipes to validate process control.
- Acceptance criteria: Establish maximum allowable residual stress levels based on downstream application requirements.
- Process capability studies: Regular statistical analysis of straightening parameters to maintain process stability.
Key Reflections
This research provides valuable quantitative data on residual stress levels introduced during oblique-roller straightening, filling an important gap in manufacturing process knowledge. The finding that bending amount directly controls residual stress magnitude—with a nonlinear acceleration at higher amounts—provides a clear process optimization target. The validated finite element model serves as a powerful design tool for predicting residual stress patterns for different pipe specifications and straightening parameters, reducing the need for extensive trial production runs.
From a practical standpoint, the recommended strategy of multi-pass straightening with smaller bending amounts represents a significant improvement over conventional single-pass high-bending-amount practices. This approach reduces residual stresses by 30-50% while maintaining acceptable dimensional accuracy, ultimately improving product quality and reducing downstream processing problems. The research methodology—combining validated numerical simulation with experimental verification—provides a template for investigating residual stress effects in other cold forming operations for steel pipes.