The choice of spring material should be determined according to factors such as the nature of the spring load, the stress state, the magnitude of the stress, the working temperature, the environmental medium, the service life, the requirements for conductive and magnetic conductivity, process performance, material source and price.
When determining the cross-sectional shape and size of materials, the series of sizes specified by national standards and ministerial standards should be selected first, and materials with non-standard series specifications should be avoided as far as possible.
Small and medium-sized springs, especially spiral tension springs, should be given priority to steel wires that have been strengthened, lead bath isothermal cold drawn steel wires and oil-quenched tempered steel wires, which have higher strength and good surface quality, and have higher fatigue properties than ordinary quenched steel wires. Tempered steel wire, simple processing, good manufacturability and stable quality.
Carbon spring steel wire and piano steel wire produce large residual stress after cold drawing. After processing the spring, there is a large residual stress, and the size changes after tempering, which makes it difficult to control the dimensional accuracy. Oil-quenched and tempered steel wire is modulated and strengthened after the steel wire is drawn to the specified size. There is basically no residual stress. After the spring is formed, it is tempered at low temperature, the size change is small, and the heat resistance stability is better than cold drawn Strengthen the steel wire.
For large and medium-sized springs, cold-drawn or cold-drawn polished steel should be used for high load accuracy and stress. For springs with lower load accuracy and stress, hot rolled steel can be used.
Leaf springs generally use flat steels of 55Si2Mn, 60Si2MnA, 55SiMnVB, 55SiMnMoV, 60CrMn, 60CrMnB, etc.
For the material cross section of the coil spring, a circular cross section should be preferred. Square and rectangular cross-section materials have strong bearing capacity and good impact resistance. It can also miniaturize the spring, but there are few material sources. And the price is high, except for special needs, generally try not to choose this material. In recent years, the development of flattened round steel wire instead of trapezoidal steel wire has achieved good results.
Spring materials that work at high temperatures require good thermal stability, resistance to relaxation or creep, resistance to oxidation, and resistance to certain medium corrosion.
As the working temperature of the spring increases, the elastic modulus of the spring material decreases, resulting in a decrease in stiffness and a decrease in bearing capacity. Therefore, the spring working at high temperature must understand the rate of change (value) of the elastic modulus, and calculate the impact of the drop in the load capacity of the spring on the performance. According to the regulations of GB1239, when the working temperature of ordinary coil spring exceeds 60℃, the shear modulus should be corrected. The formula is: Gt=KtG where G is the modulus of elasticity at room temperature; Gt is the cut at working temperature t Variable modulus; Kt——The temperature correction coefficient is selected according to Table 2-98.
The spring material used at low temperature should have good low temperature toughness. Carbon spring steel wire, piano steel wire and austenitic stainless steel spring steel wire such as 1Cr18Ni9, copper alloy and nickel alloy have good low temperature toughness and strength.
Table 2-98 Temperature correction coefficient Kt
Grade
Working temperature/℃
≤60
150
200
250
Kt
50CrVA
60Si2Mn
1Cr18Ni9Ti
0Cr17Ni7A1
QBe2
1
1
1
1
1
0.96
0.99
0.98
0. 95
0.95
0. 95
0.98
0.94
0.94
0.94
0.94
0.98
0. 90
0.92
0.92
At low temperatures, the brittleness of the material is very sensitive to surface defects, so the surface quality of the material should be strictly required.
At low temperatures, the environmental media has much less corrosion to materials than in the greenhouse, and cadmium and zinc plating are likely to cause cold brittleness.
At low temperatures, the elastic modulus and expansion coefficient of the material do not change much, so it can be ignored in the design.
For springs made of spring steel, the hardness (that is, strength) should be selected according to the spring bearing properties and stress. However, the hardness has a great relationship with the plane strain fracture toughness.
It can be seen from the curve relationship that as the hardness increases, the plane strain fracture toughness (KIC) value decreases significantly. This means that when determining the hardness value of the spring, it should be based on the premise of meeting the requirements of the spring characteristics, and the hardness value of the spring should be lower.
When selecting spring materials, pay attention to the hardenability of the steel. Whether the cross-section of the spring material is hardened and the degree of hardening have a great bearing on the quality of the spring.
Electrical springs that use the spring itself as a conductor or springs that work in an environment with variable humidity, such as water (including sea water) and steam, generally use copper and gold materials.
Springs that work under acid contact with other corrosive media generally use corrosion-resistant materials such as stainless acid-resistant steel or nickel alloy. For springs used under general environmental conditions, ordinary spring steel is selected, and the surface of the spring is made of anti-rust coating or electroplating (galvanized, cadmium, copper) to prevent corrosion.
For springs used in weighing instruments and meters, in order to ensure that their accuracy is not affected by temperature changes, constant elastic alloys with minimal changes in elastic modulus and expansion coefficient are generally used.
Reinforced plastics can be used for special-purpose springs that require light weight, insulation, anti-collision, anti-corrosion, etc. At present, the more suitable plastic springs are thermosetting reinforced plastic GFRP with epoxy resin and phenolic resin as the matrix and glass fiber reinforced. Various types of rubber springs can also be made of anti-vibration rubber.