Spring is an important part of various machines and instruments widely used. Its shape can be divided into leaf spring and spiral spring. The main functions of the spring are shock absorption and energy storage. When the spring works, it will produce great elastic deformation, absorb the impact energy and ease the impact, such as the buffer spring on the car and other vehicles; the spring can also release the absorbed energy to make other parts complete certain actions, such as the valve spring and instrument spring on the engine.
1. Characteristics of spring steel
First of all, spring steel must have high elastic limit and high yield ratio to avoid permanent deformation of spring under high load; secondly, it should have high fatigue limit and high tensile strength to avoid fatigue failure of spring under long-term vibration and alternating load stress; thirdly, it should have certain impact toughness and sufficient plasticity; At the same time, it also requires good hardenability and low decarburization sensitivity, which greatly reduces the elastic limit; and good surface quality, easy to be processed in hot and cold state and good heat treatment process.
Spring steel is generally used in the range of elastic limit, and plastic deformation is not allowed when bearing load. Therefore, it is required that the spring steel should have the highest elastic limit and yield strength ratio (≥ 0.90) after quenching and tempering at medium temperature.
In order to obtain the required properties of spring, high carbon content is used in spring steel, which is usually in the range of 0.60% – 0.85%. For example, the hardenability of 65 and 70 steel is poor. When the cross-section size is more than 12 mm, it can not be quenched in oil. If it is quenched with water, it is easy to crack. The spring with large section size and heavy load is made of alloy steel.
The carbon content of alloy spring steel is generally between 0.50% and 0.70%, and the alloy elements include Si, Mn, Cr and V. their main functions are to improve the hardenability and tempering stability of the steel, strengthen the ferrite and refine the grain size, so as to effectively improve the mechanical properties of the spring steel. Si and Mn mainly improve hardenability and yield ratio, and Si plays the most important role, but it promotes surface decarburization during heat treatment, while Mn makes steel easy to overheat and causes coarse grain. The important spring steel must be added with Cr, V, W and other elements. The surface of Si — Cr spring steel is not easy to decarbonize; Cr — V spring steel is not easy to overheat, has fine grain, is not easy to grow and coarsen, has good impact resistance and high temperature strength, among which CR, V and W are also conducive to improve the high temperature strength of spring steel.
For the brand of spring steel, please refer to the article “technical development and production use of spring steel” in 888 elastic zone, issue 85.
2. Characteristics of spring manufacturing process
The quality of spring after heat treatment is mainly determined by the service life of spring. The balance between elastic parameters and toughness parameters needs to be adjusted from the perspective of performance; the performance is closely related to the hardenability of spring steel.
At present, the spring manufacturing methods can be divided into three categories according to the different steel and process routes.
One is cold rolled steel strip and cold drawn steel wire, cold roll forming, quenching and tempering or low temperature tempering;
Second, hot rolled non annealed steel is used. After hot forming, quenching and tempering are carried out. Generally, this method is used in the manufacture of leaf spring and large coil spring;
Third, it adopts hot rolled annealed steel, cold coil forming, heating quenching and tempering heat treatment after stress shaping. Generally, medium coil spring with steel wire diameter of 6-12mm is used.
The heat treatment of spring steel can be divided into cold drawn steel wire heat treatment and hot rolling spring steel heat treatment.
The heat treatment of cold drawn steel wire is that the material is quenched and tempered at medium temperature to obtain tempered troostite structure. After forming, the steel wire is subjected to stress relief tempering below 150 ℃.
The heat treatment of hot-rolled spring steel is hot formed spring, which can be heated at 830-890 ℃ and then oil quenched and tempered at 400-480 ℃ to obtain tempered troostite structure. If the diameter of spring steel wire is too large (> 15mm) and the plate is too thick (> 8mm), the phenomenon of hardenability will occur, and the elastic limit and fatigue strength will be reduced.
The bending stress and rotation stress of spring in service are on the surface, so its surface state is very important. Oxidation and decarburization during heat treatment is the key point of prevention. The atmosphere in the furnace should be strictly controlled to shorten the heating time. After heat treatment, the spring should be shot peened to strengthen the surface and produce residual compressive stress on the surface to improve the fatigue strength.
The high-speed shot peening technology is a method to spray high-speed projectiles onto the spring surface, which causes the plastic deformation of the surface layer under the impact of the projectile, thus producing strengthening and surface compressive stress, and improving the fatigue resistance and stress corrosion resistance of the spring.
When the arc height is 0.15-0.60mma, the surface quality with surface roughness value > 4 μ M can be improved, the initiation period of contact fatigue crack on spring surface is prolonged, the propagation crack bifurcates several times, the propagation speed is reduced, and the bending fatigue crack becomes short and thin. Therefore, the formation of bending fatigue crack is delayed and the crack propagation speed is reduced, and the bending fatigue strength is increased by 42% – 5% 6%。
For this reason, spring parts are usually shot peened with high strength, and it should be noted that the determination of process index value of strong shot peening. JB / t10174-2000 stipulates that “the shot peening strength shall generally be more than 0.35mm a, and the surface coverage shall not be lower than 200%”. According to the effect of shot peening on the surface microstructure of spring, the shot peening strength of 0.35-0.43 mm a is suitable. Considering that under the same conditions, the shot peening strength of the surface of the leaf spring is only 1 / 3 of that of the round bar, and the surface coverage of the spring shot peening should be more than or equal to 250%.