What Is Spring Steel?

Spring steel refers to the special alloy steel used in the manufacture of various springs and other elastic components. According to performance requirements and use conditions, it can be divided into ordinary alloy spring steel and special alloy spring steel. Spring steel has excellent comprehensive properties. Spring steel has excellent metallurgical quality (high purity and uniformity), good surface quality (strict control of surface defects and decarburization), precise shape and size. ^[1]

Spring steel refers to steel that is specifically used to make springs and elastic elements due to its elasticity in the quenched and tempered state. The elasticity of steel depends on its ability to deform elastically, that is, within the specified range, the ability of elastic deformation to withstand a certain amount of

I. Classification according to chemical composition

According to GB / T 13304 standard, spring steel is divided into non-

The spring is used under shock, vibration or long-term cross stress, so the spring steel is required to have high

The spring is used under shock, vibration or long-term cross stress, so spring steel is required to have high tensile strength, elastic limit, high

Spring steel includes hot-rolled steel, cold-drawn steel, and cold-rolled steel strip. The smelting of spring steel is very important. It must ensure excellent metallurgical quality, not only accurate chemical composition, but also high purity, low content of sulfur, phosphorus, oxygen, nitrogen, etc., and uniformity and stability of steel. Sex is better. The metallurgical quality of spring steel should reach the level of high-quality steel and high-quality steel.

The surface quality and dimensional accuracy of spring steel have a great impact on the performance and life of the spring. Because the surface of the spring steel is not processed except for the surface shot peening, it is the working surface of the finished spring. Various defects on the original surface are left to the spring. Therefore, the surface requirements of spring steel are extremely high, and restrictions on decarburization, cracking, folding, scarring, inclusion, and delamination are extremely strict. Various surface defects can cause early damage to the spring and shorten its life. Taking a steel wire with a circular cross-section as an example, the strength and stiffness of the finished spring are respectively proportional to the third and fourth powers of the wire diameter. Small changes in the wire diameter can cause large fluctuations in spring performance, so the shape and size of steel must be strictly controlled tolerance. In this way, the processing deformation (hot rolling, cold rolling, cold drawing, etc.) of spring steel is very important and must be taken very seriously.

Hot rolled spring steel includes round steel, square steel and flat steel. The surface quality of the steel is relatively high, and it can be delivered in the hot rolled state or after heat treatment to ensure that the hardness does not exceed the specified value, which is convenient for making the spring. The hot-rolled material can be made into a coil spring in a cold state. For larger cross-section sizes, it is necessary to make a coil after heating. After the spring is formed, it is then quenched and tempered at medium temperature. The tempering temperature is about 350 ~ 550 ° C to obtain a higher yield ratio and good comprehensive mechanical properties.

Due to the large cross-section size of hot-rolled material, a grade with sufficiently high hardenability should be used to ensure core hardening. If non-martensitic transformation products such as ferrite and bainite appear in the heart, the properties after tempering are reduced, especially the fatigue properties.

Cold-drawn (rolled) spring steel includes steel wire and steel strip. The surface quality and dimensional accuracy of this type of steel are extremely stringent. There are two types of steel wire production processes: oil quenching and cold drawing.

Oil quenched steel wire is cold drawn to a specified size and then heated, oil quenched and lead bath tempered. Therefore, the steel wire has the required mechanical properties when it leaves the factory. As long as it is made of spring, it can be tempered to eliminate stress. The advantage is that the whole batch has uniform performance, stability, and good straightness, which is suitable for mass production. It can manufacture various important springs, especially valve springs. Grades are 65Mn, 50CrVA, 60Si2Mn, 55CrSi and so on.

Alloy spring steel wire can also be delivered in the cold-drawn state, or after annealing, normalizing, and tempering, and then quenched and tempered after being made into a spring. Its performance is not as uniform and stable as oil-quenched steel wire, its production efficiency is also low, and its cost is high, and it is mostly used for small batch spring production.

In addition, a process for treating high-strength spring steel wires with high-frequency induction heating has also been recently studied. The cold-drawn steel wire is quenched and tempered on a continuous induction heating device. Its internal structure is uniform and fine, and its plasticity and toughness, as well as its elastic resistance, fatigue limit, and fracture toughness, are significantly better than those of conventionally processed steel wires. This kind of steel wire has been produced in the United States and Japan. It is used to manufacture suspension springs for automobiles and motorcycles, valve springs for engines, emergency brake springs for trucks and trailers, and various high-strength mechanical springs. The results are satisfactory. In addition, induction heating is also used in the production of spring flat steel.

There is also a steel wire production process, which is isothermal quenching of a lead bath. After austenitizing the steel, the steel is moderately heated in a lead bath to generate fine pearlite structure, and then cold drawn to the specified size. The mechanical properties of the steel wire are controlled by adjusting the carbon content in the steel and the amount of cold deformation. No heat treatment is required after the spring is made, as long as the processing stress is eliminated, it can be used. All carbon steel wires are produced by this process, but there are also a few alloy steel (such as 65Mn) steel wires.

Cold-rolled spring steel strips are generally cold-rolled or annealed and supplied. After being made into a spring, they are quenched and tempered, or only tempered to eliminate processing stress.

1.65, 70, 85 steel

Can get high strength, hardness, yield ratio, but small hardenability, poor heat resistance, low ability to withstand dynamic and fatigue loads. It is widely used, but it is mostly used for small springs with low operating temperature or not too much. Important larger spring. Such as springs for automobiles, tractors, railway vehicles and general machinery.

The strength level of traditional spring steel is difficult to meet the requirements of modern industrial development. It is well known that the mechanical properties of spring steel depend on the heat treatment process under the premise of material quality assurance, and the heat treatment process should also be determined according to the material used. The important way is to give full play to the role of alloying elements to achieve the best alloying effect.

Heat treatment of spring steel

Spring steel requires higher strength and fatigue limit. It is generally used in the state of quenching + tempering to obtain a higher elastic limit. Heat treatment process technology has a crucial impact on the inherent quality of the spring. Therefore, how to further improve the fatigue life of the spring needs further research, especially the chemical surface modification heat treatment, shot peening and so on have an important impact on the spring fatigue life. In order to further strengthen the surface strength of the valve spring, increase compressive stress, and increase fatigue life, after the valve spring is formed, it is further subjected to nitriding, low-temperature liquid carbonitriding or sulfur-nitriding treatment, and then strengthened by shot peening. For example, Japan compared the f4mm si-cr oil quenched steel wire with carbonitriding at 450 × 4.5h and tempering at 400 × 15min to increase its fatigue limit by 240mpa. Nitrogen infiltration not only eliminates the adverse effects of decarburization, but also improves the residual compressive stress. At the same time, the high temperature strength of the valve spring subjected to nitriding and low temperature liquid carbonitriding is increased, and the deformation at 150 ° C is 0.2% (specified Value is 0.5%), and the deformation at 250 ° C is 0.56%, which improves the thermal stability and anti-relaxation stability of the valve spring, but the time of nitriding and liquid carbonitriding should be strictly controlled, otherwise network sulfide will be formed And reticulated nitrides, on the contrary, will reduce their fatigue strength.

The method for improving the strength of valve springs can also choose shot peening. According to production practice, two types of pellets can be used for surface spring shot peening, one is 0.8mm in diameter, its microhardness is 720hv0.2, and the other is 0.25mm in diameter. The micro-hardness is 800hv0.2. Three shot peening can achieve a better strengthening effect and improve the surface quality.

Spring steel alloying

Carbon is the main strengthening element in steel, and it often affects spring steel more than other alloying elements. According to the requirements of use, the spring steel material should be medium and high carbon alloy steel. The carbon content of spring steel, which is commonly used in countries all over the world, is between 0.45% and 0.65%.

In order to overcome the difficulty of reducing the toughness and plasticity after the strength of spring steel is increased, there is also a tendency to reduce the carbon content. China has conducted in-depth research on low-carbon martensitic spring steels, such as 28mnsib, 35mnsib, etc., whose carbon content is about 0.30%. Practice has shown that these spring steels can be used in low temperature tempered lath martensite structure, have sufficient strength and excellent comprehensive mechanical properties, especially excellent plasticity and toughness. Several high-strength spring steels researched and developed in Japan, such as uhs1900, vhs2000, nd120s, nd250s, etc., have carbon contents of about 0.40%.

The main role of alloying elements in spring steel is to improve mechanical properties, improve process properties, and impart certain special properties (such as high temperature resistance and corrosion resistance).

Many spring steels use silicon as the main alloying element, which is the alloying element that has the greatest impact on the elastic reduction resistance. This is mainly due to the strong solid solution strengthening effect of silicon; at the same time, silicon can inhibit the crystallization of cementite during tempering. Nucleation and growth, change the amount, size and morphology of precipitated carbides during tempering, and improve the tempering stability of steel. At present, wsi in domestic steel grades is 1.8% to 2.2%, which is the highest silicon steel in existing standards. However, if the silicon content is too high, it will promote the decarburization and graphitization tendency of the steel during rolling and heat treatment, and make smelting difficult and easy to form inclusions. Therefore, the use of spring steel with excessively high silicon content still needs to be cautious.

Because chromium can significantly improve the hardenability of steel, prevent the graphitization tendency of si-cr steel during spheroidizing annealing, and reduce the decarburization layer, it is a common alloy element in spring steel. Spring steel with chromium as the main strengthening element 50crv Widely used.

Manganese is the most effective alloying element for improving hardenability. It dissolves into ferrite and has a solid solution effect. Studies have shown that wmn must be greater than 0.5% in order to make the core of the spring steel completely martensite during quenching, but when wmn exceeds 1.5%, the toughness is significantly reduced, which should be given priority when selecting spring steel.

Molybdenum can improve the hardenability of the steel, prevent tempering brittleness, and improve fatigue performance. There are not many spring steels with molybdenum in the existing standards, and the amount is generally less than 0.4%.

Vanadium is a strong carbide-forming element. The finely dispersed mc-type carbides precipitated in the solid state have a strong precipitation strengthening effect. Adding 0.11% v to 35crmnb steel can significantly improve the hardenability of the steel. It was also found that vanadium can effectively reduce the decarburization sensitivity of 35simnb steel. It is believed that this and vanadium reduce the effective solid solution carbon in the steel, prevent grain growth and prevent it. Grain boundaries diffuse and improve oxidation resistance.