Introduction

Galvanized steel wire is widely used in oil and gas exploration cables, where high tensile strength and corrosion resistance are critical. Cold drawing is an effective method to improve the mechanical properties of steel wires. However, its impact on the microstructure and corrosion resistance of galvanized layers has been less studied.

This paper investigates how secondary cold drawing affects the microstructure, tensile strength, and corrosion resistance of galvanized steel wire through scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrochemical performance testing.

1. Experimental Process and Materials

1.1 Test Materials

• Raw Material: 82B steel wire with a diameter of 5.5 mm and 82% carbon content.
• Intermediate Process: Heat-treated and hot-dip galvanized to a diameter of 4.3 mm.
• Final Product: Cold-drawn galvanized steel wire with a diameter of 1.26 mm and a strain of 2.46.

1.2 Testing Equipment

2. Results and Analysis

2.1 Tensile Strength Improvement

After secondary cold drawing, the tensile strength increased significantly from 1,050 MPa to 2,035 MPa, while elongation decreased from 18.2% to 2.5%.

➡ Key Finding: Cold drawing significantly enhances tensile strength but reduces ductility.

2.2 Microstructural Evolution

Cold drawing led to the transformation of the steel matrix into a fibrous structure along the drawing direction.

• Before Cold Drawing: Equiaxed pearlite structure
• After Cold Drawing: Fibrous structure aligned with the drawing direction

2.3 Changes in Galvanized Coating Structure

• The coating thickness decreased significantly due to deformation.
• The η (free zinc layer) phase experienced the greatest reduction.
• The δ (intermetallic layer) and ξ (intermediate phase) phases also thinned, but at a slower rate.

2.4 Corrosion Resistance Enhancement

Cold drawing reduced the corrosion current density from 6.080 × 10⁻⁵ A·cm⁻² to 0.861 × 10⁻⁵ A·cm⁻², indicating improved corrosion resistance.

➡ Explanation: Cold drawing reduces the grain boundary area, leading to denser microstructures and better corrosion resistance.

3. Mechanism Behind Strength and Corrosion Resistance Improvement

3.1 Strength Enhancement

• Microstructure Refinement: Cold drawing transforms the pearlite structure into a fibrous structure, which strengthens the material.
• Work Hardening Effect: Dislocation density increases, further improving tensile strength.

3.2 Corrosion Resistance Improvement

• Reduction in Grain Boundaries: Fewer grain boundaries reduce the path for corrosion.
• Denser Galvanized Layer: The coating becomes more compact, limiting the penetration of corrosive substances.

4. Engineering Significance and Application

5. Conclusion

Tensile strength increased from 1,050 MPa to 2,035 MPa, but elongation decreased to 2.5%.
Cold drawing transformed the microstructure into a fibrous structure, improving strength.
The corrosion current density decreased by 86%, enhancing corrosion resistance.
The improvement is attributed to grain boundary reduction and a denser galvanized layer.

Keywords

• Galvanized Steel Wire
• Cold Drawing Process
• Tensile Strength Enhancement
• Corrosion Resistance Improvement
• Microstructural Evolution

References

1. Wu Alin, Tao Manhua, Wu Guifeng, Shen Aihong, Qiao Wenwei (2024). Effect of Cold Drawing on the Microstructure and Properties of Galvanized Steel Wire. Wire & Cable, 2024(6), 9-12. DOI: 10.16105/j.dxdl.1672-6901.202406002
2. GB/T 228.1-2010 – Metallic Material Tensile Test Method
3. ISO 9227-2017 – Corrosion Testing Standard for Metallic Coatings