Computer Finite Element Simulation of Steel Tube Cold Drawing Process

Literature Overview

The paper by Han Baoyun, Jin Ming, and Zhong Qianxia (2000), published in Steel Pipe, Vol. 29, No. 4, pp. 23-27, presents a comprehensive computer finite element simulation study of the steel tube cold drawing process. Conducted at the Central Iron and Steel Research Institute (Ministry of Metallurgical Industry), this research involved over 500 simulation runs under various process parameters to investigate transverse cracking, longitudinal cracking, and dimensional changes during drawing. The study resulted in the development of over 100 mathematical models for the major process variables and the creation of a CAD system for steel tube cold drawing process design.

Scope and Methodology

This paper is a companion to the study on Chinese-style die drawing (Topic 4 above) but focuses on the more general cold drawing process, which includes a wider range of die geometries and process configurations. The 500-plus simulation runs represent a significant computational effort for the year 2000 and demonstrate the authors' commitment to thorough process investigation.

The simulation methodology involves:

The process parameters studied include die entry angle, die exit angle, reduction ratio, friction coefficient, tube dimensions, material properties, and lubrication conditions. The large number of simulations allows for the identification of parameter interaction effects and the development of robust mathematical models.

Defect Analysis: Transverse and Longitudinal Cracking

The study specifically investigates two major defect types in tube drawing:

Transverse Cracking

Transverse cracks occur perpendicular to the tube axis and are typically initiated at the die entry or exit where the stress state is most severe. The simulation results reveal that:

• Transverse cracking is promoted by high reduction ratios, large die entry angles, and high friction coefficients.
• The critical stress state for transverse cracking is characterized by a high ratio of axial tensile stress to radial compressive stress.
• The material's strain hardening exponent plays a crucial role: materials with higher n-values exhibit greater formability and are less susceptible to transverse cracking.
• Pre-existing surface defects or inclusions can act as crack initiation sites, particularly under high strain conditions.

Longitudinal Cracking

Longitudinal cracks occur parallel to the tube axis and are associated with the circumferential stress state:

• Longitudinal cracking is promoted by high hoop tension, which can result from excessive internal pressure or from the constraint of the mandrel.
• The risk of longitudinal cracking increases with the reduction ratio and with the wall thickness-to-diameter ratio.
• Materials with lower ductility (lower elongation) are more susceptible to longitudinal cracking.
• The presence of a mandrel can either increase or decrease the risk of longitudinal cracking, depending on the mandrel geometry and the friction conditions at the mandrel-tube interface.

Mathematical Models and CAD System Development

The development of over 100 mathematical models represents a significant achievement in process modeling. These models cover:

1. Drawing force models: Predicting the force required for drawing as a function of reduction ratio, die geometry, friction, and material properties.
2. Dimensional change models: Predicting the final dimensions (outer diameter, inner diameter, wall thickness) as a function of the initial dimensions, reduction ratio, and springback.
3. Defect prediction models: Predicting the likelihood of transverse cracking, longitudinal cracking, and other defects as a function of the process parameters and material properties.
4. Surface quality models: Predicting the surface roughness and surface integrity as a function of the die geometry, friction, and material behavior.

The CAD system developed from these models enables engineers to:

• Input the desired product specifications (dimensions, tolerances, surface finish).
• Select the appropriate die geometry and process parameters.
• Predict the drawing force, dimensional accuracy, and defect risk.
• Optimize the process parameters to minimize costs while maintaining quality.
• Generate die drawings and process documentation.

Engineering Practice Integration

For practitioners in the steel tube industry, the findings from this study have several practical implications:

• Process selection: The simulation results help in selecting the appropriate drawing process (free drawing, constrained drawing, or Chinese-style die drawing) based on the material grade, product specifications, and quality requirements.
• Die design optimization: The mathematical models enable the optimization of die geometry to minimize drawing force, maximize productivity, and ensure dimensional accuracy. The entry and exit angles, bearing lengths, and die material selection are all critical design parameters.
• Quality assurance: The defect prediction models can be used as a quality assurance tool, allowing engineers to identify potential defects before production and to implement preventive measures. This is particularly important for high-value applications such as automotive tubing, hydraulic cylinder tubes, and structural tubing.
• Cost reduction: By optimizing the process parameters and reducing the number of physical trials, the CAD system can significantly reduce development time and costs. The ability to simulate new process conditions without physical trials is a major advantage.
• Standards compliance: The simulation results can be used to verify that the process parameters comply with relevant standards (ASTM A519, ASTM A513, EN 10210, GB/T 8162, etc.) for tube dimensions, mechanical properties, and surface quality.

Key Questions and Reflections

The scale of this simulation study (over 500 runs) was ambitious for the year 2000 and represents a significant contribution to the understanding of steel tube cold drawing. The development of over 100 mathematical models and a CAD system demonstrates the practical value of simulation in process engineering. However, several limitations should be noted:

1. The accuracy of the simulation depends on the material model used, and the elastic-plastic models employed may not fully capture the complex behavior of real materials, particularly at high strains and elevated temperatures.
2. The friction model is a simplification of the complex tribological conditions at the die-tube interface, and the actual friction coefficient can vary significantly with speed, temperature, and lubricant condition.
3. The simulation does not account for all process variables, such as die wear, tube surface condition, and environmental factors, which can affect the actual process performance.
4. The CAD system, while valuable, requires validation against production data to ensure its predictions are accurate for specific production conditions.

Despite these limitations, the study provides a solid foundation for process optimization and quality control in steel tube cold drawing. The systematic approach of simulating a wide range of process conditions and developing mathematical models is a methodology that remains relevant today, even as computational capabilities have advanced significantly. For engineers entering the field, this work demonstrates the power of simulation as a tool for understanding complex forming processes and for developing efficient, high-quality production methods.

Summary

These five literature studies collectively represent important contributions to the fields of steel tube concrete structures, bridge engineering, seamless pipe production, and tube forming processes. Topic 1 addresses the structural optimization of rectangular steel tube concrete beams through non-equal wall thickness design, providing a validated theoretical framework for section optimization. Topic 2 demonstrates the superior seismic performance of composite steel tube concrete bridge piers compared to conventional reinforced concrete piers, supporting their adoption in hydraulic-constrained bridge applications. Topic 3 presents a technological advancement in seamless pipe production, enabling the manufacture of ultra-thick-wall pipes with D/t ratios of 5 or less through micro-expansion piercing technology. Topics 4 and 5 represent pioneering applications of finite element simulation to steel tube forming processes, providing detailed insights into stress and deformation states, defect mechanisms, and process parameter optimization. Together, these studies reflect the multidisciplinary nature of steel pipe and fitting engineering, spanning structural mechanics, materials science, process engineering, and computational methods, and they provide a valuable knowledge base for engineers working in these fields.