Spring steel necessary for high-quality machinery

Spring steel refers to steel specially used for making 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 to deform elastically enables it to withstand a certain load without permanent deformation after the load is removed. Spring steel should have excellent comprehensive properties, such as mechanical properties (especially elastic limit, strength limit, yield strength ratio), anti-elasticity (i.e. anti-elasticity, also known as anti-relaxation), 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), precise shape and size.

According to GB/T 13304 "Steel Classification" standard, according to basic performance and use characteristics, spring steel belongs to mechanical structure steel; according to quality grade, it belongs to special quality steel, that is, steel that needs to be strictly controlled in the production process. According to Chinese custom, spring steel is a special steel. The technical requirements for making spring steel are relatively high, and excellent technology directly determines the quality.

Process performance and performance requirements

Springs are 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 technology, spring steel is required to have certain hardenability, not easy to decarburize, and good surface quality. Carbon spring steel is a high-quality carbon structural steel with a carbon content of WC in the range of 0.6%-0.9%. Alloy spring steel is mainly silicon-manganese steel, which has a slightly lower carbon content and mainly relies on increasing the silicon content Wsi to improve performance; there are also alloy spring steels of tungsten, tungsten, and vanadium. In recent years, combined with China's resources and based on the requirements of new technologies for automobile and tractor design, new steels have been developed that add boron, niobium, molybdenum and other elements to silicon-manganese steel, extending the service life of springs and improving spring quality.

Springs are 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 technology, spring steel is required to have certain hardenability, not easy to decarburize, and good surface quality. Carbon spring steel is a high-quality carbon structural steel with a carbon content of WC in the range of 0.6%-0.9%. Alloy spring steel is mainly silicon-manganese steel, which has a slightly lower carbon content and mainly relies on increasing the silicon content Wsi to improve performance; there are also alloy spring steels of tungsten, tungsten and vanadium. In recent years, in combination with Chinese resources and in accordance with the requirements of new technologies for automobile and tractor design, new steels with elements such as boron, niobium and molybdenum added to silicon-manganese steel have been developed, which extend the service life of the spring and improve the quality of the spring.

Production process

Spring steel includes hot-rolled steel, cold-drawn steel, cold-rolled steel strip, etc. The smelting of spring steel is very important. It must ensure excellent metallurgical quality. It must not only ensure accurate chemical composition, but also have high purity, low content of sulfur, phosphorus, oxygen, nitrogen, etc., and good uniformity and stability of steel. The metallurgical quality of spring steel should 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 life of the spring. Because the surface of the spring steel in the delivery state is no longer processed except for surface shot peening, it is also the working surface of the finished spring, and various defects on the original surface are also left to the spring. Therefore, the surface requirements of spring steel are extremely high, and the restrictions on decarburization, cracks, folding, scarring, inclusions, and delamination are extremely strict. Various surface defects will cause early damage to the spring and shorten its life. Taking the circular cross-section steel wire as an example, the strength and stiffness of the finished spring are proportional to the cube and fourth power of the wire diameter respectively. A small change in the wire diameter will lead to huge fluctuations in the performance of the spring, so the shape and dimensional tolerance of the steel must be strictly controlled. In this way, the processing deformation (hot rolling, cold rolling, cold drawing, etc.) of spring steel is very important and must be treated very seriously.

Hot-rolled spring steel includes round steel, square steel, flat steel, etc. The surface quality requirements of steel are relatively high. It can be delivered in hot-rolled state or after heat treatment to ensure that the hardness does not exceed the specified value, which is convenient for spring making. Hot-rolled materials can be made into coil springs in cold state. For those with larger cross-sectional dimensions, they need to be heated before being made into springs. After the spring is formed, it is quenched and tempered at medium temperature. The tempering temperature is about 350-550℃ to obtain a higher yield strength ratio and good comprehensive mechanical properties.

Because the cross-sectional dimensions of hot-rolled materials are large, a grade with high hardenability should be used to ensure that the core is hardened. If non-martensitic transformation products, such as ferrite, bainite, etc., appear in the core, the performance after tempering will be reduced, especially the fatigue performance.

Cold-drawn (rolled) spring steel includes steel wire and steel strip. The surface quality and dimensional accuracy requirements of this type of steel are particularly strict. The production process of steel wire is oil quenching and cold drawing.

Oil-quenched steel wire is cold-drawn to a specified size and then heated, oil-quenched and tempered in a lead bath. Therefore, the steel wire has the required mechanical properties when it leaves the factory. After being made into a spring, it can be used as long as it is tempered and stress eliminated. The advantages are uniform and stable performance of the whole batch, good straightness, suitable for mass production, and can be used to manufacture various important springs, especially valve springs. The grades are 65Mn, 50CrVA, 60Si2Mn, 55CrSi, etc.

Alloy spring steel wire can also be delivered in a cold-drawn state, or delivered 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 that of oil-quenched steel wire, and its production efficiency is also low and the cost is high. It is mostly used for small-batch spring production.

In addition, the process of high-frequency induction heating treatment of high-strength spring steel wire has also been studied recently. 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 elastic reduction resistance, fatigue limit, fracture toughness, etc. are significantly better than conventionally treated steel wire. 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, engine valve springs, emergency brake springs for trucks and trailers, and various high-strength mechanical springs. The use effect is satisfactory. In addition, induction heating treatment is also used in the production of spring flat steel.

There is also a steel wire production process, namely lead bath isothermal quenching. After austenitization, the steel is isothermal in a lead bath to form a 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 in the steel. After making the spring, no heat treatment is required, and it can be used as long as the processing stress is eliminated. Carbon steel wires are produced by this process, but there are also a few alloy steel (such as 65Mn) wires.

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

Application Examples

1. 65, 70, 85 steel grades

Can obtain very high strength, hardness, yield strength ratio, but low hardenability, poor heat resistance, low ability to withstand dynamic loads and fatigue loads. It is widely used, but mostly used for small springs with low working temperature or less important larger springs. Such as springs for automobiles, tractors, railway vehicles and general machinery.

65Mn has a simple composition, and its hardenability, comprehensive mechanical properties, decarburization and other process properties are better than carbon steel, but it is more sensitive to overheating, has temper brittleness, and is prone to cracking during quenching. The price is low and the amount is large. It can be used to manufacture various small-section flat springs, round springs, springs, etc., and can also be used to make valve springs, spring rings, shock absorbers and clutch reeds, brake springs, etc.

2. 55Si2Mn, 60Si2Mn, 60Si2MnA

High silicon content (Wsi) (up to 2.00%).

High strength and good elasticity. Good anti-tempering stability. Easy to decarburize and graphitize. Hardenability is not high.

The main spring steel type, with a wide range of uses. Manufacture of various springs, such as leaf springs, coil springs, cylinder safety valve springs and some important springs working under high stress and springs with severe wear for automobiles, locomotives and tractors.

55Si2MnB Due to the boron content, its hardenability is significantly improved. Front and rear suspension springs and auxiliary springs for light and medium-sized vehicles.

55Si2MnB China's self-developed steel grade, hardenability, comprehensive mechanical properties, and fatigue properties are better than 60Si2Mn steel. Mainly used to manufacture leaf springs for medium and small cars, with good use effect, and can also be used to manufacture leaf springs and coil springs of other medium-sized cross-section sizes.

3. 60Si2CrA 60Si2CrVA

High-strength spring steel. High hardenability and good heat treatment process performance. Due to high strength, the internal stress should be eliminated in time after the spring is rolled. Manufacture of important large springs with large loads. 60Si2CrA can be used to make steam turbine seal springs, regulating springs, condenser support springs, high-pressure water pump disc springs, etc. 60Si2CrVA steel is also used to make extremely important springs, such as conventional weapon retrieval hook springs and crusher springs.

4. 55CrMnA

60CrMnA has the outstanding advantage of good hardenability, and also good hot processing performance, comprehensive mechanical properties, and anti-decarburization performance. Various important springs with large cross-sections, such as large leaf springs and spiral springs for automobiles and locomotives.

5. 60CrMnMoA

It has the highest hardenability among all existing spring steels. It also has good mechanical properties and anti-tempering stability. It is used for super-large springs for large civil engineering, heavy vehicles, machinery, etc. The thickness of the steel plate can reach more than 35mm, and the diameter of the round steel can exceed 60mm.

6. 50CrVA

A small amount of vanadium improves elasticity, strength, yield strength 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. High hardenability and good fatigue performance Various important coil springs are particularly suitable for springs with high working stress amplitude and strict fatigue performance requirements, such as valve springs, fuel injector springs, cylinder expansion rings, safety valve springs, etc.

7. 60CrMnBA

Higher hardenability than 60CrMnA, other various properties are similar Larger leaf springs, coil springs, torsion springs, etc.

8. 30W4Cr2VA

High-strength heat-resistant spring steel. Very good hardenability. High temperature anti-relaxation and hot working performance are also very good Heat-resistant springs with working temperatures below 500~C, such as turbine main steam valve springs, steam seal springs, boiler safety valve springs, 400t boiler disc valve springs, etc.