### Characteristics of Spring Steel
Spring steel must first possess a **high elastic limit** and a **high yield-to-tensile strength ratio** to prevent permanent deformation of the spring under high loads. Secondly, it should have a **high fatigue limit** and **high tensile strength** to avoid fatigue failure due to long-term vibration and alternating load stresses. Additionally, it requires a certain degree of **impact toughness** and sufficient **plasticity**. Furthermore, it should have **good hardenability** and **low decarburization sensitivity**, as well as **good surface quality**, making it easy to form in both hot and cold states and facilitating effective heat treatment processes.
Spring steel typically operates within the elastic limit range, and no plastic deformation is permitted under load. Therefore, spring steel is required to have the highest possible elastic limit and yield-to-tensile strength ratio (≥0.90) after quenching and medium-temperature tempering.
To achieve the desired performance of springs, spring steel is manufactured with a relatively high carbon content. Carbon spring steels generally have a carbon content ranging from **0.60% to 0.85%**. For example, steels like **65** and **70** have poor hardenability, and sections larger than 12mm cannot be fully hardened in oil, while water quenching may lead to cracking. Springs with larger cross-sectional dimensions or that bear heavier loads are made from **alloy steel**.
Alloy spring steels generally contain **0.50% to 0.70% carbon**, and alloying elements such as **Si, Mn, Cr, V**, etc., are added. These elements primarily improve the steel’s **hardenability** and **tempering stability**, strengthen the ferrite, and refine the grain structure, thus significantly improving the mechanical properties of spring steel. **Si** and **Mn** mainly increase hardenability and yield-to-tensile strength, with Si having the most pronounced effect. However, Si promotes surface decarburization during heat treatment, and Mn makes the steel prone to overheating, leading to coarse grains. For critical applications, spring steels must include elements such as **Cr, V, W**, etc. **Si-Cr spring steel** is less prone to decarburization on the surface, while **Cr-V spring steel** is less prone to overheating, with fine grain structure and resistance to grain growth.