One type is cold-strengthened carbon steel wire, also known as cold-drawn spring steel wire. Cold-drawn carbon spring steel wire is first lead-quenched to obtain a sorbitic structure, then surface-phosphated, and drawn to the finished size with a large reduction in surface area. The wire structure is fibrous, possessing high tensile strength and elastic limit, as well as good bending and torsional properties. Cold-drawn spring steel wire has high dimensional accuracy, a smooth surface, no oxidation or decarburization defects, and relatively stable fatigue life, making it the most widely used spring steel wire.
Another type of carbon spring steel wire is martensitic strengthened carbon steel wire, also known as oil-quenched and tempered carbon steel wire.
Through quenching and tempering, carbon steel wire can obtain good comprehensive mechanical properties. When the wire size is small (φ≤2.0mm), the strength indicators of oil-quenched and tempered carbon steel wire are lower than those of cold-drawn carbon steel wire after martensitic treatment. When the steel wire diameter is large (φ≥6.0mm), it is impossible to achieve the required strength by using a large reduction ratio in the surface area of sorbitic steel wire. However, oil-quenched and tempered steel wire can achieve higher performance than cold-drawn steel wire as long as it is fully quenched. Under the same tensile strength conditions, martensitic steel wire has a higher elastic limit than cold-formed steel wire. The microstructure of cold-drawn steel wire is fibrous and significantly anisotropic, while the microstructure of oil-quenched and tempered steel wire is uniform tempered martensite and almost isotropic. At the same time, oil-quenched and tempered steel wire has better resistance to relaxation than cold-drawn steel wire, and its operating temperature (150～190℃) is also higher than that of cold-drawn steel wire (≤120℃). Large-diameter oil-quenched and tempered steel wire is showing a strong trend of replacing cold-drawn steel wire.
