Carbon spring steel wire should possess high tensile strength, elastic limit, toughness, and fatigue strength, and be resistant to impact and vibration. Ensuring strength and toughness, especially preventing torsional cracking, is crucial in the production of spring steel wire. The internal and surface quality of the wire rod directly affects the wire's performance. Carbon spring steel wire is manufactured from high-carbon, high-quality carbon structural steel or carbon tool steel wire rod, and its chemical composition, gas content, and non-metallic inclusions must be strictly controlled according to the spring's application. To reduce surface defects and decarburized layers, the steel billets used for wire rod production must be surface-ground, and peeling may be necessary. Wire rods must undergo normalizing or martensitizing treatment; for larger specifications, spheroidizing annealing is used instead.
Jarious heat treatments, especially before the finished product drawing process, widely employ martensitizing. Decarburization must be prevented during heat treatment. After heat treatment, pickling with sulfuric acid or hydrochloric acid is used to remove iron oxide scale. Coatings (see Lubricating Carrier) can be applied by lime dipping, phosphating, borax treatment, or copper plating. The drawing process of the finished product has a significant impact on its performance. Generally, a large total reduction in area (around 90%, see area reduction rate) and a small reduction in area per pass (approximately ≤23%) are used to ensure the product's toughness. For high-strength spring steel wire, the exit temperature of the wire in each pass should be controlled below 150℃ during drawing to prevent torsional cracks caused by strain aging, which is a major defect leading to wire scrap. Therefore, good lubrication and sufficient cooling are essential during drawing. Using a smaller reduction in area per pass and a lower drawing speed helps reduce the temperature rise of the wire. After drawing, the steel wire contains significant residual stress, affecting its performance. This can be eliminated by online straightening or low-temperature (180–370℃) heating.
