What is spring steel

What is spring steel?

Spring steel refers to the steel specially used for manufacturing springs and elastic elements due to its elasticity in the quenched and tempered state. The elasticity of steel depends on its ability of elastic deformation, that is, within the specified range, the ability of elastic deformation makes it bear a certain load, and there is no permanent deformation after the load is removed. Spring steel shall have excellent comprehensive properties, such as mechanical properties (especially elastic limit, strength limit and yield ratio), elastic reduction resistance (i.e. anti elastic degradation performance, also known as anti relaxation performance), fatigue performance, hardenability, physical and chemical properties (heat resistance, low temperature resistance, oxidation resistance, corrosion resistance, etc.). In order to meet the above performance requirements, spring steel has excellent metallurgical quality (high purity and uniformity), good surface quality (strict control of surface defects and decarburization), and accurate shape and size.
According to the standard of steel classification (GB/T 13304), spring steel belongs to the steel for mechanical structure according to the basic performance and service characteristics I; According to the quality grade, it belongs to special quality steel, that is, steel requiring special strict control of quality and performance in the production process. According to Chinese custom, spring steel belongs to special steel. When making spring steel, the technical requirements are relatively high, and excellent technology directly determines the quality.

Classification of spring steel

Classification by chemical composition

According to GB/T 13304 standard, spring steel is divided into non alloy spring steel (carbon spring steel) and alloy spring steel according to its chemical composition.

Carbon spring steel

The carbon content (mass fraction) of carbon spring steel is generally 0.62% ~ 0.90%. According to its manganese content, it is divided into general manganese content (mass fraction) (0.50% ~ 0.80%), such as 65, 70 and 85, and higher manganese content (mass fraction) (0.90 ~ 1.20%), such as 65Mn.

Alloy spring steel

Alloy spring steel is a kind of steel which can improve the mechanical properties, hardenability and other properties of steel by properly adding one or several alloy elements on the basis of carbon steel, so as to meet the properties required for manufacturing various springs.
The basic composition series of alloy spring steel include silicon manganese spring steel, silicon chromium spring steel, chromium manganese spring steel, chromium vanadium spring steel, tungsten chromium vanadium spring steel, etc. On the basis of these series, some brands have added alloy elements such as molybdenum, vanadium or boron in order to improve their performance in some aspects.
In addition, some grades are selected from other steels, such as high-quality carbon structural steel, carbon tool steel, high-speed tool steel and stainless steel.

Classification according to production and processing methods

1. Hot rolled (forged) steel includes hot rolled round bar, square bar, flat bar, steel plate, forged round bar and square bar.
2. Cold drawn (rolled) steel includes steel wire, steel strip and cold drawn material (cold drawn round bar).

Classification according to steel delivery status

1. Hot rolled (forged) steel

  • A. The steel delivered in hot rolling (forging) state is hot formed into spring, and then quenched and tempered.
  • B. The steel delivered in annealed condition is cold formed into spring, and then quenched and tempered.

The above shall comply with the provisions of corresponding delivery state hardness.
2. Cold drawn (rolled) steel
A. Steel wire

  • ① Salt bath isothermal quenching cold drawn steel wire (also known as paidengtuo treated cold drawn steel wire) after the steel wire is made into spring, it only needs to be tempered at low temperature to eliminate stress.
  • ② After the oil quenched and tempered steel wire is cold drawn to the required size, it shall be continuously heated, continuously oil quenched and lead tempered. After the steel wire is made into a spring, only the bottom warm fire is needed to eliminate the stress.
  • ③ Cold drawn steel wire (i.e. steel wire not quenched and tempered)

a. Delivery in cold drawn condition.
b. Delivered in annealed, normalized or tempered condition.
The steel wires delivered in the above two states shall be quenched and tempered after being made into springs.
B. Steel strip

  • ① After the spring is delivered in cold rolling state, it shall be tempered at low temperature to eliminate stress.
  • ② After the spring is delivered in quenched and tempered state, it shall be tempered at low temperature to remove the stress.
  • ③ Quenching and tempering treatment shall be carried out after the spring is delivered in the annealing state.

C. Cold drawn steel
The steel delivered in annealed condition is cold formed into spring, and then quenched and tempered.

Other classification methods

In addition to the above, there are other classification methods, such as:
According to different delivery conditions, it can be divided into delivery according to chemical composition (mechanical properties) and delivery according to hardenability.
According to the working conditions of the spring, it can be divided into spring steel bearing static load, spring steel bearing impact load, high (low) temperature resistant spring and corrosion resistant spring steel.

Process properties of spring steel

Spring is used under impact, vibration or long-term cross stress, so spring steel is required to have high tensile strength, elastic limit and high fatigue strength. In terms of process, spring steel is required to have certain hardenability, not easy to decarburize and good surface quality, that is, high-quality carbon structural steel with carbon content WC in the range of 0.6% – 0.9%. Alloy spring steels are mainly silicon manganese series steels, and their carbon content is slightly lower. Their properties are mainly improved by increasing silicon content WSI; In addition, there are chrome, tungsten and vanadium alloy spring steels. In recent years, combined with Chinese resources and according to the requirements of new technologies for automobile and tractor design, a new steel type with boron, niobium, molybdenum and other elements added to silicon manganese steel has been developed, which has prolonged the service life of the spring and improved the quality of the spring.

Performance requirements of spring steel

Spring is used under impact, vibration or long-term cross stress, so spring steel is required to have high tensile strength, elastic limit and high fatigue strength. In terms of process, spring steel is required to have certain hardenability, not easy to decarburize and good surface quality, that is, high-quality carbon structural steel with carbon content WC in the range of 0.6% – 0.9%. Alloy spring steels are mainly silicon manganese series steels, and their carbon content is slightly lower. Their properties are mainly improved by increasing silicon content WSI; In addition, there are chrome, tungsten and vanadium alloy spring steels. In recent years, combined with Chinese resources and according to the requirements of new technologies for automobile and tractor design, a new steel type with boron, niobium, molybdenum and other elements added to silicon manganese steel has been developed, which has prolonged the service life of the spring and improved the quality of the spring.

Production process of spring steel

Spring steel includes hot rolled steel, cold drawn steel, cold rolled steel strip, etc. The smelting of spring steel is very important. We must ensure excellent metallurgical quality, not only accurate chemical composition, but also high purity, low content of sulfur, phosphorus, oxygen and nitrogen, and good steel uniformity and stability. The metallurgical quality of spring steel shall reach the level of high-quality steel and high-grade high-quality steel.
The surface quality and dimensional accuracy of spring steel have a great influence on the performance and service life of spring. Because the surface of the spring steel in the delivery state is no longer processed except for surface shot peening, that is, the working surface of the finished spring. Various defects on the original surface are also left to the spring. Therefore, the surface requirements of spring steel are very high, and the restrictions on decarburization, crack, folding, scarring, inclusion and delamination are very strict. Various surface defects will cause early damage and shorten the service life of the spring. Taking the steel wire with circular section as an example, the strength and stiffness of the finished spring are directly proportional to the third and fourth power of the steel wire diameter respectively. Small changes in the steel wire diameter will lead to great fluctuations in the spring performance, so the shape and dimensional tolerance of the steel should be strictly controlled. In this way, the processing deformation of spring steel (hot rolling, cold rolling, cold drawing, etc.) is very important and must be taken seriously.
Hot rolled spring steels include round steel, square steel, flat steel, etc. The surface quality of steel is required to be high. It can be delivered as hot rolled or after heat treatment, so as to ensure that the hardness does not exceed the specified value and facilitate spring making. Hot rolled materials can be made into coil springs in cold state, and those with large section size need to be made into springs after heating. After the spring is formed, it is quenched and tempered at medium temperature. The tempering temperature is about 350 ~ 550 ℃, so as to obtain high yield ratio and good comprehensive mechanical properties.
Due to the large section size of hot rolled material, the brand with high hardenability shall be adopted to ensure the core hardenability. If there are non martensitic transformation products in the core, such as ferrite and bainite, the properties after tempering will be reduced, especially the fatigue properties.
Cold drawn (rolled) spring steel includes steel wire and steel strip. The surface quality and dimensional accuracy of this kind of steel are particularly strict. The production process of steel wire includes oil quenching and cold drawing.
Oil quenched steel wire is cold drawn to the specified size and then heated, oil quenched and lead bath tempered. Therefore, the steel wire has the required mechanical properties when leaving the factory. After making the spring, it can be used as long as it is tempered to eliminate stress. The advantage is that the whole batch has uniform performance, stability and good straightness. It is suitable for mass production and can manufacture all kinds of important springs, especially valve springs. Brands include 65Mn, 50CrVA, 60Si2Mn, 55CrSi, etc.
Alloy spring steel wire can also be delivered in cold drawing state, or after annealing, normalizing and tempering treatment, and then quenched and tempered after being made into spring. Its performance is not as uniform and stable as oil quenched steel wire, low production efficiency and high cost. It is mostly used in small batch spring production.
In addition, the process of treating high-strength spring steel wire with high-frequency induction heating has also been studied recently. Cold drawn steel wire is quenched and tempered on continuous induction heating device. Its internal structure is uniform and fine. Its plasticity and toughness, elastic reduction resistance, fatigue limit and fracture toughness are obviously better than those of conventional treated steel wire. This kind of steel wire has been produced in the United States and Japan. It produces suspension springs, engine valve springs of automobiles and motorcycles, emergency braking springs of trucks and trailers and various high-strength mechanical springs. The application effect is satisfactory. In addition, induction heating treatment has also been applied in the production of spring flat steel.
There is also a steel wire production process, namely lead bath isothermal quenching. After austenitizing, the steel is isothermal in lead bath to form 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 and cold deformation of the steel. The spring can be used as long as the processing stress is eliminated without heat treatment. Carbon steel wires are produced by this process, but there are also a few alloy steel wires (such as 65Mn).
Cold rolled spring steel strip is generally supplied after cold rolling or annealing, and then quenched and tempered after making spring, or only tempered to eliminate processing stress.

Application examples of spring steel

1. Steel grades 65, 70 and 85
High strength, hardness and yield ratio can be obtained, but the hardenability is small, the heat resistance is poor, and the ability to bear dynamic load and fatigue load is low. It is widely used, but it is mostly used for small springs with low working temperature or less important large springs. Such as springs for automobiles, tractors, railway vehicles and general machinery.
65Mn has simple composition, better hardenability, comprehensive mechanical properties, decarburization and other process properties than carbon steel, but it is sensitive to overheating, temper brittleness, easy to crack after quenching, low price and large amount. It can also make valve springs, spring rings, shock absorbers, clutch springs, brake springs, etc.
2. 55Si2Mn60Si2Mn60Si2MnA
Silicon content (WSI) is high (up to 2.00%).
High strength and good elasticity. Good tempering resistance and stability. Easy decarburization and graphitization. Low hardenability.
The main spring steel is widely used. Manufacture all kinds of springs, such as leaf springs and coil springs of automobiles, locomotives and tractors, cylinder safety valve springs and some important springs working under high stress, and seriously worn springs.
The hardenability of 55si2mnb is significantly improved due to its boron content. Front and rear suspension springs and auxiliary springs of light and medium-sized vehicles.
55si2mnb steel grade developed by China has better hardenability, comprehensive mechanical properties and fatigue properties than 60Si2Mn steel. It mainly manufactures leaf springs for small and medium-sized vehicles with good application effect. It can also make other leaf springs and coil springs with medium section size.
3. 60Si2CrA, 60Si2CrVA
High strength spring steel. High hardenability and good heat treatment process performance. Due to the high strength, the internal stress shall be eliminated in time after the spring is rolled, and the important large spring with large load shall be manufactured. 60si2cra can produce steam turbine gland spring, regulating spring, condenser support spring, high-pressure water pump Belleville spring, etc. 60Si2CrVA steel also makes very important springs, such as spring of conventional weapon take-off hook and spring of crusher.
4. 55CrMnA
The outstanding advantage of 60CrMnA is good hardenability. In addition, it has good hot working performance, comprehensive mechanical properties and decarburization resistance. It is an important spring with large section, such as large leaf spring and coil spring of automobile and locomotive.
5. 60CrMnMoA
Among the existing spring steels, the hardenability is the highest. Mechanical properties and tempering stability are also good for super large springs used in large civil buildings, heavy vehicles and machinery. The thickness of steel plate can reach more than 35mm, and the diameter of round steel can exceed 60mm.
6. 50CrVA
A small amount of vanadium improves elasticity, strength, yield ratio and elastic reduction resistance, refines grains and reduces decarburization tendency. The carbon content is small, and the plasticity and toughness are better than other spring steels. Various important coil springs with high hardenability and good fatigue performance are especially suitable for springs with high working stress amplitude and strict fatigue performance requirements, such as valve spring, fuel nozzle spring, cylinder expansion ring, safety valve spring, etc.
7. 60CrMnBA
The hardenability is higher than 60CrMnA, and other leaf springs, spiral springs, torsion springs, etc. with similar properties and larger dimensions.
8. 30W4Cr2VA
High strength heat-resistant spring steel. Good hardenability. High temperature relaxation resistance and hot working performance are also very good. Heat resistant springs with working temperature below 500 ~ C, such as steam turbine main steam valve spring, steam seal spring, boiler safety valve spring, 400t boiler disc valve spring, etc.

Research progress of spring steel

The strength level of traditional spring steel is difficult to meet the requirements of modern industrial development. As we all know, the mechanical properties of spring steel depend on the heat treatment process on the premise of material quality assurance, and the heat treatment process should also be determined according to the materials used. An important way of high strength of spring steel is to give full play to the role of alloy elements to achieve the best alloying effect.

Heat treatment

Spring steel requires high strength and fatigue limit. It is generally used in the state of quenching + medium temperature tempering to obtain high elastic limit. Heat treatment technology has a vital impact on the internal quality of spring. Therefore, how to further improve the spring fatigue life needs further research, especially chemical surface modification, heat treatment and shot peening have an important impact on the spring fatigue life. In order to further strengthen the surface strength of the valve spring, increase the compressive stress and improve the fatigue life, after the valve spring is formed, it should be further treated by nitriding, low-temperature liquid carbonitriding or sulfur nitrocarburizing, and then shot peening. For example, in Japan, f4mm Si CR oil quenched steel wire was quenched at 450 ℃ × 4.5H low temperature bulk carbonitriding and passing through 400 ℃ × Compared with 15 min medium temperature tempering, the fatigue limit can be increased by 240 MPa. The infiltration of nitrogen 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 after nitriding and low-temperature liquid carbonitriding is improved. The deformation at 150 ℃ is 0.2% (the specified value is 0.5%), and the deformation at 250 ℃ is 0.56%, which improves the thermal stability and anti relaxation stability of the valve spring, However, the time of nitriding and liquid carbonitriding should be strictly controlled, otherwise network sulfide and network nitride will be formed, which will reduce its fatigue strength.
Shot peening can also be used to improve the strength of valve spring. Through production practice, two kinds of shot can be used for surface valve spring shot peening, one with a diameter of 0.8mm and a microhardness of 720hv0.2, and the other with a diameter of 0.25mm and a microhardness of 800hv0.2. Three shot peening can achieve good strengthening effect and improve the surface quality.

Alloying

Carbon is the main strengthening element in steel, and its influence on spring steel is often more than other alloy elements. According to the application requirements, the spring steel material shall be medium and high carbon alloy steel. Nowadays, most of the carbon content of spring steel widely used in the world is 0.45% ~ 0.65%.
In order to overcome the problem that the toughness and plasticity of spring steel decrease after the strength is improved, there is also a trend to reduce the carbon content. China has conducted in-depth research on low-carbon martensitic spring steels, such as 28MnSiB and 35mnsib, with a carbon content of about 0.30%. The practice shows that these spring steels can be used under the plate martensite structure tempered at low temperature, and have sufficient strength and excellent comprehensive mechanical properties, especially excellent plasticity and toughness. The carbon content of several high strength spring steels developed in Japan, such as uhs1900, vhs2000, nd120s, nd250s, etc., is about 0.40%.
The main function of alloy elements in spring steel is to improve mechanical properties, process properties and give some special properties (such as high temperature resistance and corrosion resistance).
Silicon is the main alloy element in many spring steels. It is the alloy element that has the greatest impact on the elastic reduction resistance, which is mainly due to the strong solid solution strengthening effect of silicon; At the same time, silicon can inhibit the nucleation and growth of cementite during tempering, change the quantity, size and morphology of carbide precipitated during tempering, and improve the tempering stability of steel. WSI of domestic steel grades is 1.8% ~ 2.2%, which is the spring steel with the highest silicon content in the existing standards. However, if the silicon content is too high, it will promote the decarburization and graphitization tendency of steel in the process of rolling and heat treatment, and make smelting difficult and easy to form inclusions. Therefore, the use of spring steel with too 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 50CrV with chromium as the main strengthening element is widely used.
Manganese is the most effective alloying element to improve hardenability. It dissolves into ferrite and has the effect of solid solution. The research shows that WMN must be greater than 0.5% to make the core of spring steel completely change into martensite during quenching, but when WMN exceeds 1.5%, the toughness decreases obviously, which should be given priority in the selection of spring steel.
Molybdenum can improve the hardenability of steel, prevent temper brittleness and improve fatigue performance. There are few spring steels with molybdenum in the existing standards, and the addition amount is generally less than 0.4%.
Vanadium is a strong carbide forming element. The fine dispersed MC carbide precipitated in solid state has a strong precipitation strengthening effect. Adding 0.11% V to 35crmnb steel can significantly improve the hardenability of the steel. It is also found that vanadium can effectively reduce the decarburization sensitivity of 35simnb steel, which is related to vanadium reducing the effective solid solution carbon in the steel, preventing grain growth, preventing grain boundary diffusion and improving the oxidation resistance.

Source: Network Arrangement – China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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