Influence of Process Parameters on Springback Behavior of High-Strength Stainless Steel Tubes During CNC Bending

Literature Overview

This study, published in the Journal of Forging Equipment and Technology in 2022 (Vol. 47, No. 11, pp. 137–145) by Fang Jun, Ouyang Fang, Shang Wenxuan, Lu Shiqiang, Wang Kelu, and Xiang Junhuai from Jiangxi Science and Technology Normal University and Nanchang Hangkong University, investigates the springback behavior of high-strength 0Cr21Ni6Mn9N stainless steel tubes during CNC bending operations. The research was funded by the Jiangxi Provincial Natural Science Foundation (Grant 20192BAB216022) and Jiangxi Provincial Department of Education Science and Technology Research Programs (GJJ201126, GJJ180615). The study addresses a critical manufacturing challenge in the production of high-strength stainless steel pipe components, where accurate springback prediction is essential for achieving dimensional accuracy in bent pipe assemblies.

Material Characterization and Elastic Modulus Degradation

A key innovation of this study is the characterization of the elastic modulus degradation of 0Cr21Ni6Mn9N stainless steel tubes under cyclic loading-unloading conditions. The material properties are as follows:

The cyclic loading-unloading tensile tests revealed that the elastic modulus of the high-strength stainless steel tube decreases progressively with accumulated plastic strain. This phenomenon, known as modulus degradation, is attributed to the evolution of dislocation structures and the development of geometrically necessary dislocations during plastic deformation. The researchers constructed a mathematical function relating the elastic modulus to the plastic strain and embedded this relationship into the ABAQUS finite element software to simulate the bending process with variable elastic modulus.

Finite Element Simulation and Process Parameter Analysis

The CNC bending process was simulated in ABAQUS with the variable elastic modulus model, and the following process parameters were systematically varied:

The sensitivity analysis revealed that the springback radius is more sensitive to process parameter variations than the springback angle. The sensitivity ranking for springback radius is: tube/die clearance > tube/die friction coefficient > boost speed > bending speed > core rod extension amount > tube/core rod clearance. The springback angle shows relatively low sensitivity to all process parameters.

Comparison of Constant and Variable Elastic Modulus Models

The study compared the springback predictions obtained using the constant elastic modulus model with those obtained using the variable elastic modulus model. The results showed that:

1. The trends of springback behavior with respect to process parameters are similar under both constant and variable elastic modulus conditions.
2. The springback values are consistently larger under the variable elastic modulus condition, indicating that ignoring modulus degradation leads to underestimation of springback.
3. The variable elastic modulus model provides more accurate predictions, particularly for processes involving significant plastic strain accumulation.

Engineering Practice Implications

For manufacturing engineers working with high-strength stainless steel tubes, the following practical guidelines emerge from this study:

• Die clearance optimization: The tube/die clearance is the most influential parameter on springback radius, and should be carefully controlled to minimize dimensional deviation.
• Friction management: The tube/die friction coefficient is the second most influential parameter, suggesting that lubrication strategy and die surface finish are critical process controls.
• Speed control: Both bending speed and boost speed affect springback, with higher speeds reducing springback due to increased strain rate effects and plastic deformation.
• Core rod design: The core rod extension amount and clearance influence springback, with optimal values dependent on the specific tube geometry and bend radius.
• Material model accuracy: Engineers should account for elastic modulus degradation when simulating or compensating for springback in high-strength stainless steel tube bending operations, as the constant modulus assumption leads to systematic underestimation.

Study Insights and Reflections

This research makes a significant contribution to the understanding of springback in high-strength stainless steel tube bending by incorporating the physically realistic phenomenon of elastic modulus degradation. The sensitivity ranking of process parameters provides a clear hierarchy for process optimization, enabling engineers to focus control efforts on the most influential variables. The finding that the springback radius is more sensitive to parameter variations than the springback angle has practical implications for quality control, as dimensional accuracy in the bend radius direction requires tighter process control. However, the study's reliance on numerical simulation, while validated against the material characterization data, would benefit from direct experimental validation of the springback predictions under the variable elastic modulus model. The methodology of embedding a modulus degradation function into FEA software is a transferable approach that could be applied to other high-strength materials and forming processes beyond tube bending.