Spring is the most commonly used part in mechanical industry and daily life. The spring should return to its original position after unloading. The smaller the plastic deformation, the better. Therefore, the steel wire should have high elastic limit, yield strength and tensile strength. The higher the yield ratio is, the closer the elastic limit is to the tensile strength, so the higher the strength utilization ratio is, the stronger the spring elasticity is.
The spring absorbs the impact energy by elastic deformation, so the spring steel wire does not have to have high plasticity, but at least it must have the plasticity that can bear the spring forming and enough toughness to bear the impact energy.
Spring usually works under alternating stress for a long time, so it should have high fatigue limit and good creep and relaxation resistance.
Spring used in specific environment, there will be some special requirements for steel wire, for example: the spring used in corrosive medium must have good corrosion resistance. The spring used in precision instrument should have long-term stability and sensitivity, low temperature coefficient, high quality factor, small aftereffect effect and constant elastic modulus. The spring working at high temperature is required to keep enough elastic limit and good creep resistance at high temperature.
In addition, the forming process and heat treatment process of spring steel wire should also be considered. Both cold drawn spring steel wire and oil quenched and tempered spring steel wire are directly wound with steel wire in supply state, and the spring is directly used after stress relief treatment after forming. The tensile strength of cold drawn spring steel wire is slightly higher than that of oil quenched and tempered steel wire. The elasticity of large size cold drawn steel wire is too large, and it is difficult to wind spring. Therefore, the specification of cold drawn spring steel wire is generally less than 8.0 mm, and that of oil quenched and tempered steel wire is generally less than 13.0 mm. In fact, spring steel wire in light drawing state is mostly used for spring with diameter of 13.0 mm, which is cold drawn, wound and then quenched and tempered. Most of the steel wires with diameter above 15.0 mm are made by heating winding process.
The spring can be divided into static spring and dynamic spring according to the running state. Static spring refers to the spring with limited vibration times in service, such as safety valve spring, spring pad, scale spring, fixed load spring, mechanical spring, watch hairspring, etc. Dynamic spring refers to the spring whose vibration times are more than 1 × 106, such as engine valve spring, vehicle suspension spring, shock proof spring, coupling spring, elevator buffer spring, etc. Static spring material selection mainly considers tensile strength and stability, while dynamic spring material selection mainly considers fatigue, relaxation and resonance performance.
According to the load condition, the spring can be divided into three states: light load, general load and heavy load. Light load refers to the spring which bears static stress, low stress and small deformation, such as spring for safety device, spring for vibration absorption, etc. The designed service life is 103 ~ 104 times.
The general load refers to the common spring which has a design life of 105 ~ 106 times and is used under the condition of vibration frequency of 300 times / min. The lower the load stress, the longer the life.
Heavy load refers to the spring which works for a long time and vibrates frequently. Such as valve spring, air hammer, press, hydraulic controller spring, its load is high, often less than 10% of the allowable stress, service life is more than 1 × 106 times, usually 107 times.
The principle of spring material selection is: first, meet the functional requirements, then the strength requirements, and finally consider the economy.
Carbon spring steel is widely used in spring steel and its consumption is the largest. The steel contains 0.60% ～ 0.90% carbon and 0.3% ～ 1.20% manganese, so the cost is relatively low. After proper processing or heat treatment, high tensile strength, sufficient toughness and good fatigue life can be obtained. However, the carbon steel wire has low hardenability, poor relaxation resistance and corrosion resistance. The temperature coefficient of elastic modulus is large (up to 300 × 10-6 / ℃). It is suitable for manufacturing springs with small cross-section and low working temperature (120 ℃ >).
Alloy spring steel generally contains 0.45% – 0.70% carbon and a certain amount of Si, Mn, Cr, V, W and B alloy elements. The addition of alloying elements can improve the relaxation resistance and toughness of spring steel, and significantly improve the hardenability and service temperature of the steel. It is suitable for manufacturing springs with larger cross-section and higher temperature. The comparison of common grades of spring steel at home and abroad is shown in Table 1. Table 1 Comparison of common grades of spring steel at home and abroad