Residual Stress Measurement and Process Optimization of Cold-Drawn Steel Tubes

Literature Overview

This paper by Wang Jiagong and colleagues from Xuzhou XCMG Hydraulic Parts Co., Ltd., published in the journal Steel Pipe in 2017, presents a systematic investigation of residual stress distribution in cold-drawn steel tubes used for hydraulic cylinder barrels. The study employs the blind hole method (also known as the incremental core drilling method) to measure surface residual stresses at various stages of the cold-drawing process. The research is particularly relevant to hydraulic cylinder manufacturing, where residual stress state directly influences fatigue life, dimensional stability, and corrosion resistance of the finished product.

Core Technical Content

The investigation covers the complete cold-drawing process chain for hydraulic cylinder tubes, including equal-diameter cold drawing, multi-pass drawing, and intermediate annealing. The blind hole method, standardized in GB/T 17042 and ASTM E837, involves drilling a small blind hole at the measurement point and measuring the strain relief around the hole using strain gauges. The measured strains are then converted to residual stresses using calibration factors determined by the removal method or finite element analysis.

Residual Stress Distribution Characteristics

The study reveals several important findings. First, equal-diameter cold drawing produces very small surface residual stresses, all of which are compressive in nature, resulting in good comprehensive mechanical properties. Second, adding an intermediate annealing between two cold-drawing passes does not significantly reduce the final surface residual stress. Third, when the total deformation is kept constant, increasing the deformation in the first pass and decreasing it in the second pass yields products with smaller surface residual stresses.

Metallurgical Analysis and Process Interpretation

From a metallurgical standpoint, the residual stress distribution in cold-drawn tubes is governed by the plastic deformation mechanism and the associated microstructural evolution. During cold drawing, the outer surface of the tube undergoes significant plastic deformation, while the core remains relatively undeformed. This differential deformation creates a residual stress gradient with compressive stresses at the surface and tensile stresses in the interior, maintaining equilibrium.

The blind hole method measurement results must be interpreted in the context of the tube's microstructural state. After cold drawing, the surface layer exhibits work hardening with increased dislocation density, grain refinement, and possibly some martensitic transformation in high-carbon steels. The compressive residual stress at the surface is beneficial for fatigue resistance because it opposes applied tensile stresses, delaying crack initiation and propagation.

Process Optimization Recommendations

The finding that intermediate annealing does not effectively reduce final residual stress is counterintuitive but can be explained by the subsequent cold-drawing pass re-introducing residual stresses. The annealing process relieves stresses accumulated in the first pass, but the second pass generates new stresses that dominate the final stress state. This insight has important implications for process design: intermediate annealing should be used primarily for microstructural recovery rather than residual stress control.

Quality Control Implications

For hydraulic cylinder tube manufacturers, the residual stress state is a critical quality parameter that directly affects product performance. High tensile residual stresses at the surface can lead to premature fatigue failure, stress corrosion cracking (SCC), and dimensional instability during subsequent machining operations. The following quality control measures are recommended based on the study's findings:

1. Implement residual stress measurement at critical process stages using the blind hole method or X-ray diffraction method as specified in GB/T 17042.
2. Establish process windows that limit the deformation ratio per pass to control residual stress levels.
3. Conduct stress relief annealing as the final heat treatment operation to achieve the target residual stress state.
4. Perform metallographic examination of the HAZ and surface layer to verify microstructural integrity.
5. Correlate residual stress measurements with fatigue test results to establish acceptance criteria.

The blind hole method, while accurate, is destructive and time-consuming. For routine quality control, X-ray diffraction (XRD) methods based on the sin²ψ technique may be more practical, although they measure only near-surface stresses (typically within 50-100 μm depth). For deeper stress profiling, the incremental core drilling method provides a complete stress gradient across the tube wall thickness.

Engineering Practice Cases

In hydraulic cylinder manufacturing, residual stress-related failures have been documented in the field. A common failure mode is fatigue cracking at the cylinder tube surface, often initiated at stress concentration sites such as tool marks or microstructural inhomogeneities. The study's findings suggest that optimizing the cold-drawing process to minimize surface residual stresses can significantly improve fatigue life.

For example, a hydraulic cylinder manufacturer producing tubes from 45 steel or 40Cr steel can apply the study's optimization strategy by adjusting the deformation distribution between drawing passes. If the total cross-sectional area reduction is 20%, instead of distributing it equally (10% per pass), the process can be optimized to 15% in the first pass and 5% in the second pass. This asymmetric distribution reduces the final surface residual stress while maintaining the same total deformation and mechanical properties.

The study also highlights the importance of considering the interaction between residual stress and other quality factors such as dimensional accuracy, surface finish, and microstructural uniformity. A comprehensive quality management system should integrate residual stress control with these other parameters to ensure consistent product quality.

Study Insights and Implications

This research provides practical, actionable guidance for cold-drawn tube manufacturers seeking to optimize their processes for improved product performance. The key insight is that residual stress control in multi-pass cold drawing is primarily achieved through deformation distribution optimization rather than intermediate heat treatment. This finding simplifies process design and reduces manufacturing cost by eliminating unnecessary annealing operations.

From a broader perspective, the study underscores the importance of understanding the relationship between processing history and residual stress state in cold-worked steel products. Engineers involved in tube selection and specification should request residual stress data from suppliers and incorporate stress state considerations into design calculations. For critical applications such as hydraulic cylinders, aerospace components, and pressure vessels, residual stress measurement should be a mandatory part of the quality assurance program.

In conclusion, the research by Wang Jiagong et al. demonstrates that strategic optimization of cold-drawing deformation distribution is an effective and economical approach to controlling surface residual stresses in hydraulic cylinder tubes, and that residual stress measurement using the blind hole method provides reliable data for process development and quality assurance.