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Research and analysis on the processing technology of spring steel wire
In addition to tensile strength, the toughness of steel wire is primarily evaluated through its torsion properties. During the torsion test, the wire surface must remain intact without cracks, and the cross-section should be even after testing. The overall toughness of the wire depends not only on the manufacturing process but also on its inherent material quality. For example, non-metallic inclusions can significantly degrade the mechanical performance of the wire. Surface defects such as folds, rolling marks, dents, or severe corrosion can compromise product quality. Additionally, the chemical composition—especially carbon content—should be as uniform as possible, with strict control over sulfur and phosphorus levels. To ensure consistent quality, wires are grouped during production, and tailored processes are applied accordingly.
Heat treatment plays a critical role in achieving high strength and toughness. Lead quenching is commonly used to produce a uniform sorbite structure. This involves heating the wire above the AC3 temperature for a set period, allowing it to form uniform austenite, followed by controlled cooling to decompose the austenite into sorbite. In continuous lead treatment, the wire is typically heated to around 10–20°C above AC3. This higher temperature accelerates austenite growth and enhances its stability, ensuring that the transformation occurs close to isothermal, resulting in a uniform microstructure. However, excessively high temperatures may cause surface decarburization, especially in high-carbon wires. Therefore, when using a high line temperature process, the cooling rate must be carefully adjusted to ensure complete decomposition of austenite in the lead bath.
Selecting the appropriate lead bath temperature is essential for obtaining a uniform sorbite structure. Lead quenching works by supercooling austenite, allowing it to undergo isothermal transformation into a fine pearlitic structure. The lead bath temperature is determined based on factors like carbon content, wire diameter, and the thermal characteristics of the bath. The final microstructure—such as the spacing of pearlite layers and the amount of pre-eutectoid ferrite—directly influences the mechanical properties of the finished wire.
During drawing, wires with a high total reduction ratio, multi-pass processing, and small-diameter sections require lead quenching and optimized compression ratios. A well-planned drawing sequence and improved working conditions are crucial for achieving the desired results. The drawing equipment, cooling system, lubrication, and mold geometry all have strict requirements. Poor cooling can cause the wire to overheat, leading to strain aging and making the wire brittle. The flatness of the wire is closely related to the condition of the mold and the pulling stress after drawing. Installing a suitable straightener at the exit of the final pass helps reduce residual stresses, ensuring a smooth and flat final product.
The immersion oil process is also vital, particularly for spring steel wires. The oil temperature has a significant impact on the aging behavior of the wire. An improperly designed oil immersion process can lead to hardening and brittleness. Therefore, a well-structured oil immersion procedure must be established to maintain the desired properties.