Zhuojin Pipe Fitting Co., LtdDeformation Characteristics of Skew Rolling Expansion for Seamless Steel Tubes
Literature Overview
Lü Qinggong, Xu Wenjing, and Mu Renling (2017) from the University of Science and Technology Beijing published a study in Metal World that employs finite element simulation to analyze the deformation characteristics of skew rolling expansion of seamless steel tubes. The paper compares the deformation behavior of skew rolling expansion with traditional skew rolling elongation, examining primary deformation, additional deformation, and equivalent deformation across the deformation process. The research is particularly relevant to the production of high-quality, large-diameter, medium-to-thin-wall seamless steel tubes.
Technical Background on Skew Rolling
Skew rolling is a rotary forging process used to produce seamless steel tubes. The workpiece (a steel billet or pre-formed tube) is placed between two counter-rotating rollers with specific groove profiles. As the rollers rotate, the workpiece simultaneously rotates and advances axially, undergoing complex plastic deformation.
Comparison of Skew Rolling Modes
| Parameter | Skew Rolling Elongation | Skew Rolling Expansion |
|---|---|---|
| Primary deformation | Longitudinal elongation with wall thinning | Diameter expansion with minimal elongation |
| Wall reduction | Significant | Negligible (almost zero) |
| Longitudinal extension | Large | Very small |
| Circumferential shear | Moderate | Present but smaller than elongation |
| Longitudinal shear | Moderate | Present but smaller than elongation |
| Torsional deformation | Significant | Negligible (effectively zero) |
| Total equivalent deformation | Baseline | Comparable |
| Total additional deformation | Higher | Significantly lower |
| Application | Standard tube production | Large-diameter, medium-thin-wall tubes |
Deformation Analysis
Primary Deformation
In skew rolling expansion, the primary deformation is fundamentally different from skew rolling elongation. In elongation mode, the material is stretched longitudinally while the wall thins proportionally. In expansion mode, the material is forced outward radially, increasing the diameter while maintaining approximately constant wall thickness. This is achieved through specific roller groove geometry that directs the deformation primarily in the radial direction.
The FE simulation reveals that the radial strain rate in expansion mode is approximately 3–5 times higher than the longitudinal strain rate, confirming that the deformation is predominantly diametral.
Additional Deformation Components
Additional deformation refers to the non-ideal deformation components that do not directly contribute to the desired shape change. These include:
- Circumferential shear deformation: Caused by the differential velocity between the roller surface and the workpiece surface at different angular positions
- Longitudinal shear deformation: Arising from the non-uniform axial velocity distribution across the workpiece cross-section
- Torsional deformation: Resulting from asymmetric contact forces between the rollers and the workpiece
The key finding is that while skew rolling expansion does exhibit circumferential and longitudinal shear deformation, the magnitude is significantly lower than in traditional skew rolling elongation. The torsional deformation in expansion mode is negligible and can be considered zero for practical purposes.
Equivalent Deformation
The total equivalent deformation (von Mises strain) in both modes is comparable, which means the overall material work input is similar. However, the total additional deformation in expansion mode is substantially lower than in elongation mode. This has important implications for:
- Material quality: Lower additional deformation means less internal damage accumulation, fewer micro-cracks, and better mechanical properties
- Surface quality: Reduced shear deformation leads to smoother surface finish
- Dimensional accuracy: Less non-ideal deformation means better dimensional control
- Material utilization: Less additional deformation means less material waste from trimming and rework
Process Advantages and Limitations
Advantages of Skew Rolling Expansion
| Advantage | Description |
|---|---|
| High quality | Lower additional deformation preserves material integrity |
| Large diameter capability | Suited for producing large-diameter tubes |
| Medium-thin wall capability | Wall thickness can be maintained during expansion |
| Flexible production | Suitable for small-batch, multi-variety production |
| Reduced energy consumption | Lower additional deformation means less energy wasted |
Limitations and Considerations
| Limitation | Description |
|---|---|
| Limited elongation | Not suitable when significant length extension is required |
| Specialized roller grooves | Requires custom roller design for each product specification |
| Setup time | Changeover between product variants requires roller adjustment |
| Initial investment | Higher tooling cost for specialized expansion rollers |
Engineering Practice Integration
For seamless steel tube manufacturers considering skew rolling expansion, the following practical considerations apply:
- Product selection: This process is best suited for large-diameter (typically >300 mm), medium-to-thin-wall seamless tubes where high quality is paramount, such as high-pressure boiler tubes, heat exchanger tubes, and large-diameter hydraulic cylinders.
- Roller groove design: The groove geometry must be carefully optimized through FE simulation before production. The contact angle, groove depth, and groove profile directly influence the deformation distribution.
- Temperature control: The rolling temperature should be maintained in the range of 1050–1150 °C for carbon steel, ensuring adequate plasticity while avoiding excessive grain growth.
- Quality verification: Post-expansion inspection should include ultrasonic testing (UT) for internal defects, dimensional measurement of diameter and wall thickness uniformity, and hardness testing to verify the mechanical properties.
Study Insights
This research provides a valuable quantitative comparison between two skew rolling modes, and the conclusion that skew rolling expansion is superior for high-quality large-diameter tube production is well-supported by the deformation analysis. The negligible torsional deformation in expansion mode is particularly significant, as torsional deformation in elongation mode can induce residual stresses and distortion that affect downstream processing.
The finding that total equivalent deformation is comparable between the two modes but additional deformation is much lower in expansion mode is the key insight. In essence, skew rolling expansion achieves a more "pure" deformation state, where a higher proportion of the total deformation contributes to the desired shape change rather than to non-productive deformation modes. This translates directly to better material quality and higher product yield.
For manufacturers transitioning from elongation to expansion processes, the investment in specialized roller tooling and process development is justified by the improved product quality and reduced defect rates, particularly for high-value products where quality is critical.