Alloy Spring Steel

• Types of Spring Steel:Spring steel comes in various forms, including hot-rolled, cold-drawn, and cold-rolled steel strips.
• Importance of Metallurgical Quality: The production of spring steel is critical, as it requires excellent metallurgical quality. This involves ensuring accurate chemical composition, high purity, and low levels of impurities such as sulfur, phosphorus, oxygen, and nitrogen. The steel should exhibit uniformity and stability, meeting the standards for high-quality and superior-grade steel.
• Surface Quality and Dimensional Precision:The surface quality and dimensional precision of spring steel significantly impact the performance and lifespan of the springs. Since the surface of the delivered spring steel is typically not further processed beyond surface shot blasting, any existing defects on the surface affect the final product. Thus, stringent controls are essential to avoid decarburization, cracks, folds, scars, inclusions, and delamination. Surface defects can lead to premature failure and reduced lifespan of the springs. For example, in round-section steel wires, the strength and stiffness of the finished springs are proportional to the cube and fourth power of the wire diameter, respectively. Therefore, maintaining strict control over the steel’s shape and dimensional tolerances is crucial.
• Hot-Rolled Spring Steel:Hot-rolled spring steel is available in round, square, and flat sections. The surface quality requirements are high, and the steel may be delivered in its hot-rolled state or after heat treatment to ensure hardness within specified limits, facilitating spring formation. Hot-rolled steel can be used to manufacture helical springs in cold conditions, but larger cross-sections may require heating before forming. Post-formation, the springs undergo quenching and tempering at temperatures around 350-550°C to achieve a high strength-to-yield ratio and good overall mechanical properties.
• Cold-Drawn Spring Steel:This category includes steel wires and strips. The surface quality and dimensional accuracy of cold-drawn spring steel are exceptionally strict. The production methods include oil quenching and cold drawing.

Oil-Quenched Steel Wire: After cold drawing to the required size, the steel wire is heated, oil-quenched, and lead-bath tempered. The wire is ready for use with the desired mechanical properties upon delivery, with only tempering required post-spring formation to relieve stress. This method ensures uniform performance across batches, stability, and straightness, making it suitable for mass production of critical springs like valve springs. Common grades include 65Mn, 50CrVA, 60Si2Mn, and 55CrSi.

Alloy Spring Steel Wire: Alloy spring steel can be delivered in a cold-drawn state or after annealing, normalizing, and tempering, with additional quenching and tempering required after spring formation. This type has less uniform performance compared to oil-quenched wire, lower production efficiency, and higher costs, typically used for small-batch spring production.

• Advanced Processing Techniques:Recent developments include high-frequency induction heating for high-strength spring steel wires. In this process, cold-drawn wires are quenched and tempered using continuous induction heating, resulting in a more uniform, finer internal structure with superior plasticity, toughness, fatigue resistance, and fracture toughness compared to conventional treatments. This technology is used in the US and Japan for automotive, motorcycle, and truck springs, among others. Induction heating is also applied in the production of flat spring steel.
• Lead-Bath Isothermal Quenching:This process involves austenitizing the steel and then isothermally quenching it in a lead bath to form a fine pearlitic structure, followed by cold drawing to the specified size. This method adjusts carbon content and cold deformation to control the wire’s mechanical properties, eliminating the need for further heat treatment after spring formation. This technique is generally used for carbon steel wires but occasionally for alloy steels like 65Mn.
• Cold-Rolled Spring Steel Strips:These are typically delivered cold-rolled or after annealing, with subsequent quenching and tempering (or just tempering) post-spring formation to relieve processing stress.