Zhuojin Pipe Fitting Co., LtdStress Analysis and Structural Optimization of Cold-Drawing Seamless Steel Pipe Heads
Literature Overview
This paper by Zhang Shangyi, Shen Zhengjie, Pang Deyu, and Wang Mingjia from Yangzhou Chengde Steel Pipe Co., Ltd. and Yanshan University was published in Steel Pipe in 2021 (Vol. 50, No. 5, pp. 77–79). The study addresses a critical manufacturing defect—cracking and breakage of cold-drawing mandrel heads (cold-drawing dies/plugs)—during the production of seamless steel pipes. The research employs ANSYS/LS-DYNA for numerical simulation and proposes structural optimization solutions.
Core Technical Content
Problem Statement and Manufacturing Context
Cold-drawing is a primary forming process for seamless steel pipes, where a heated or cold pipe blank is pulled through a die set while a mandrel (plug) is held inside to control the inner diameter. The cold-drawing head (the mandrel plug or die head) is subjected to extreme contact stresses during the forming operation. Cracking and fracture of these critical tool components lead to:
- Production line stoppages
- Product surface defects
- Economic losses from scrapped pipes
- Safety hazards from sudden tool failure
Numerical Simulation Approach
| Simulation Parameter | Specification |
|---|---|
| Software | ANSYS/LS-DYNA |
| Product specification | Φ385 mm × 50 mm |
| Material grade | 4140 alloy steel |
| Mandrel type | Short mandrel (short plug) |
| Analysis type | Explicit dynamic contact simulation |
| Output | Stress distribution, strain concentration |
The simulation models the full cold-drawing process including:
- Die-plate contact conditions
- Mandrel-pipe internal contact
- Material flow during plastic deformation
- Stress concentration identification at geometric discontinuities
Key Finding: Stress Concentration Location
The simulation revealed that stress concentration occurs at both sides of the arc apex (the rounded transition region) of the cold-drawing head. This is consistent with classical stress concentration theory where geometric discontinuities create local stress amplification factors that can exceed the nominal stress by 2–4 times.
Structural Optimization Design
Three alternative cold-drawing head structures were designed and evaluated:
Optimization Scheme Comparison
| Scheme | Structural Feature | Stress Reduction | Manufacturing Complexity |
|---|---|---|---|
| Scheme 1 | Modified arc radius with gradual transition | Moderate reduction | Low |
| Scheme 2 | Chamfered transition with reduced stress gradient | Significant reduction | Medium |
| Scheme 3 (separated type) | Disconnected/detached head structure | Maximum reduction | Higher |
The separated cold-drawing head structure (Scheme 3) demonstrated the lowest stress concentration and was confirmed through both simulation and production verification to be the most effective solution against head cracking and fracture.
Engineering Practice Analysis
Root Cause Analysis Using FMEA Approach
Applying Failure Mode and Effects Analysis (FMEA) to the cold-drawing head failure:
| Failure Mode | Cause | Severity | Occurrence | Detection | RPN |
|---|---|---|---|---|---|
| Head cracking | Stress concentration at arc apex | 9 | 7 | 6 | 378 |
| Head fracture | Cumulative fatigue from repeated use | 10 | 6 | 5 | 300 |
| Surface marking | Head wear during forming | 7 | 8 | 4 | 224 |
The high RPN values for cracking and fracture confirm that the stress concentration issue is the primary concern requiring immediate engineering intervention.
Material and Process Considerations
For 4140 alloy steel (42CrMo4 equivalent) cold-drawn to Φ385 mm × 50 mm, the wall thickness of 50 mm represents a substantial reduction ratio. The material flow during cold drawing generates significant contact pressures at the die-mandrel interface. The short mandrel design, while offering advantages in pipe straightness and dimensional accuracy, concentrates the forming load onto a smaller area, exacerbating stress concentration at the mandrel head.
Key Reflections and Study Insights
This paper exemplifies the practical application of explicit dynamic FEA to manufacturing process optimization. The choice of LS-DYNA is appropriate for this problem because cold drawing involves large deformations, complex contact conditions, and potentially rate-dependent material behaviour—all of which are well-suited to explicit dynamic solvers.
The progression from problem identification through simulation to structural redesign and production verification follows a rigorous engineering methodology. The decision to implement three alternative designs rather than a single solution demonstrates good engineering practice—providing options for different production scenarios and manufacturing capabilities.
The separated cold-drawing head structure represents an innovative approach that fundamentally changes the load path rather than merely modifying local geometry. By creating a structural discontinuity, the design allows controlled stress redistribution that prevents crack initiation at the most vulnerable location. This philosophy—changing the fundamental structural topology rather than incrementally modifying dimensions—is often more effective in solving severe stress concentration problems.
For steel pipe manufacturers dealing with large-diameter alloy seamless pipe production, this study provides a clear methodology for diagnosing and solving tool failure issues. The combination of numerical simulation with production verification creates a reliable engineering approach that can be replicated for other tooling problems in pipe manufacturing operations.