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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 assessed 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 the inherent quality of the material. 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 negatively impact product quality. Moreover, the chemical composition—especially carbon content—must be as uniform as possible, with strict control over sulfur and phosphorus levels.
During production, wires are grouped according to their specifications, and tailored production processes are developed. Heat treatment plays a critical role in achieving high strength and toughness. Lead quenching is commonly used to produce a uniform sorbite structure, followed by cold working at high temperatures. In this process, the steel wire is heated above the AC3 temperature for a specific time to achieve uniform austenite, then cooled under controlled conditions to transform the austenite into sorbite. In continuous sootite treatment, the heating temperature is typically set 10–20°C above AC3, which helps accelerate austenite growth and increase its stability. This allows for near-isothermal decomposition during cooling, resulting in a more uniform microstructure.
However, excessively high temperatures may lead to surface decarburization, especially in high-carbon wires. Additionally, higher carbon content increases austenite stability, so when using elevated line temperatures, the cooling rate should be adjusted accordingly to ensure complete decomposition of austenite in the lead bath. Selecting the correct lead bath temperature is essential for achieving a consistent sorbite structure after quenching. Lead quenching involves supercooling austenite, allowing it to undergo isothermal transformation into a uniform sorbite structure. The lead bath temperature is determined based on factors such as carbon content, wire diameter, and the thermal characteristics of the bath.
The final microstructure of the wire—such as the spacing of pearlite layers and the number of pre-eutectoid ferrites—directly influences its mechanical properties. When drawing wires with a high total reduction ratio, multiple passes, and small final dimensions, lead quenching, carbon compression, and optimized drawing routes are necessary. Drawing equipment, cooling conditions, lubrication, and mold geometry and quality all have strict requirements. Poor cooling can cause the wire to heat up, leading to strain aging and making the wire hard and brittle. Wire flatness is closely related to mold quality and condition, particularly the pull-out stress after drawing. A suitable straightener should be installed at the exit of the finished product to reduce drawing stress and ensure the wire remains flat and well-formed.
Immersion oil also plays an important role in the aging behavior of spring steel wire. An improper oil immersion process can result in excessive aging, making the wire brittle. Therefore, a rational oil immersion procedure must be established to maintain optimal performance.