What are the properties of the spring material

High strength: In order to improve the ability of the spring to resist fatigue damage and anti-relaxation, the spring material should have a high yield strength and elastic limit, especially a high yield ratio. Under normal circumstances

High strength: In order to improve the ability of the spring to resist fatigue damage and anti-relaxation, the spring material should have a high yield strength and elastic limit, especially a high yield ratio. In general, the elastic limit of the material is proportional to the yield strength, so spring designers and manufacturers always want the material to have a high yield strength. The tensile strength and yield strength of the spring material are close, such as about 90% of the cold-drawn carbon steel wire: because the tensile strength is easier to measure than the yield strength, the tensile strength provided in the material delivery is generally used as a basis in the design and manufacture. However, the tensile strength of the material is not the higher the better, and too high strength will reduce the plasticity and toughness of the material and increase the brittleness tendency. The tensile strength of the material is related to its chemical composition, metallographic structure, heat treatment, cold working (drawing or rolling) degree and other strengthening processes. There is also a certain relationship between tensile strength and fatigue strength, when the material is below 1600MPa, its fatigue strength increases with the increase of tensile strength.

Good plasticity and toughness: in the spring manufacturing process, the material needs to undergo different degrees of processing deformation, so the material is required to have a certain plasticity. For example, the carabiner and torsion arm of the complex stretching and twisting spring, when the radius of curvature is very small, the spring material shall not have cracks, losses and other defects when processing winding or stamping bending. At the same time, when the spring is subjected to impact load or variable load, the material should have good toughness, which will be of great benefit to improving the service life of the spring.

Excellent surface state and fatigue performance: the surface of the spring is under large stress when working, and the fatigue damage is often started from the surface of the steel wire, for the springs used in important occasions such as valve springs, valve springs and suspension springs are required to have millions of times, tens of millions of times or even longer cycle life, which puts high requirements on the fatigue performance of the material. There are many factors that affect the fatigue properties of materials, such as the chemical composition of materials, hardness, purity of steel, surface quality and metallographic organization, especially the surface quality of materials. Surface defects of the material, such as cracks, folds, scales, rust, pits, scratches and indentations, are easy to cause stress concentration in the working process of the spring. The part where the stress is concentrated is often the fatigue source that causes fatigue failure. The fatigue source is also easy to occur first in the part of the surface decarburization, so strictly controlling the depth of the decarburization layer is also a very important quality index. In order to improve the surface quality of the spring material, the surface of the material can be polished or polished, and a layer of the material skin can be removed by peeling process before the wire drawing, so that most of the surface defects can be removed. The spring heat treatment can be controlled atmosphere or vacuum heat treatment to prevent surface decarbonization and oxidation.

Strict dimensional accuracy: many springs have higher requirements for load accuracy, such as the valve spring load deviation should not be greater than 5%-6% of the specified load, with a round wire pull, pressure spring as an example, if the wire diameter deviation is 1%, the load will produce a deviation of about 4%. It can be seen that strict dimensional accuracy is also very important to ensure the quality of the spring.

Good uniformity: The uniformity of materials refers to the chemical composition, mechanical properties, dimensional deviation and other indicators of the material requirements of uniformity and stability. If the performance of all aspects of the material is inconsistent, it will bring great difficulties to the spring production, resulting in the dispersion of product geometry, hardness, load and other parameters, and serious non-uniformity will even cause waste products.

To meet the above performance requirements, spring steel must have excellent metallurgical quality, including strictly controlled chemical composition, high purity, low impurity content, low non-metallic inclusion content, and control its shape, particle size and distribution. In addition, the uniformity and stability of the steel are required. Spring steel should also have good surface quality and high precision shape and size.

Continuous casting has been widely used in spring steel production. Compared with die casting, continuous casting can reduce the segregation of steel and improve the uniformity of steel through electromagnetic stirring and low temperature casting. Reduce the secondary oxidation, improve the surface decarburization of steel; Make the structure and performance of steel stable and uniform; Improve yield and productivity; The oxygen content is reduced and the chemical composition of steel is controlled effectively in combination with the off-furnace refining technology.

In the case of the dimensional deviation, section shape, surface quality and the micro-structure uniformity along the length of the spring steel, the longitudinal continuous rolling mill is widely used abroad, and it is constantly improved to achieve high-speed, torsion-free and tension-free rolling. In order to ensure the dimensional accuracy of steel, short stress lines or prestressed rolling mills are used.