The selection of spring material shall be recognized according to the nature of the load borne by the spring, stress condition, stress size, operating temperature, environmental medium, service life, requirements for conductive and magnetic conduction, process function, material origin and price.
When recognizing the cross-sectional shape and standard of materials, the series of standards regulated by national standards and ministerial standards shall be preferentially selected, and non-standard series of materials shall be avoided as much as possible.
Medium and small-sized springs, especially spiral tension springs, should be preferentially used with steel wires that have been strengthened, lead bath isothermal cold drawn steel wires and oil quenched and tempered steel wires, which have high strength and excellent surface quality, fatigue function higher than that of ordinary quenched and tempered steel wires, simple processing, good workmanship and stable quality.
The carbon spring steel wire and Piano steel wire have a large residual stress after cold drawing. After the spring is processed, there is a large residual stress. After tempering, the standard changes greatly, and it is difficult to control the standard accuracy. The oil quenched and tempered steel wire is modulated and strengthened after the steel wire is drawn to the standard. Basically, there is no residual stress. After the spring is formed, it is tempered at low temperature. The standard change is very small, and the heat resistance and stability are better than the cold drawn and strengthened steel wire.
For large and medium-sized springs with high load accuracy and stress, cold drawn materials or polished steel after cold drawing shall be selected. For springs with low load accuracy and stress, hot rolled steel can be used.
Generally, flat steel of 55si2mn, 60Si2MnA, 55SiMnVB, 55simnmov, 60crmn, 60crmnb and other trademarks is selected for the leaf spring.
The material section of the coil spring shall preferably be circular. For square and rectangular section materials, the electronic spring has strong bearing capacity, good impact resistance, and can miniaturize the spring, but the material source is small. And the price is relatively high. Except for special needs, this material is generally not selected. In recent years, round steel wire is developed to replace trapezoidal steel wire, and good results have been achieved.
Spring materials operating under high temperature shall have good thermal stability, anti relaxation or creep ability, anti-oxidation ability, and resistance to certain medium corrosion.
When the working temperature of the spring rises, the elastic modulus of the spring material decreases, resulting in the decrease of the stiffness and the reduction of the bearing capacity. Therefore, the spring operating under high temperature must understand the change rate (value) of elastic modulus, and calculate the impact of the spring load reduction on the application function. According to gb1239, when the operating temperature of general coil spring exceeds 60 ℃, the shear modulus shall be modified. The formula is: GT = KTG, where G - elastic modulus at normal temperature; GT - shear modulus at operating temperature T; KT -- temperature modification coefficient is selected according to table 2-98.
For spring materials used at low temperature, the automobile interior spring shall have excellent low temperature resistance. Austenitic stainless steel spring wires such as carbon spring wire, piano wire and 1Cr18Ni9, copper alloy and nickel alloy have good low temperature resistance and strength.
At low temperature, the brittleness of materials is very sensitive to surface defects, so the surface quality of materials should be strictly required.
At low temperature, the corrosion degree of environmental medium to materials is much smaller than that in greenhouse, while cadmium plating and zinc plating are easy to cause cold embrittlement.
At low temperature, the elastic modulus and expansion coefficient of the material are not changed much and can not be considered in the planning.
For the spring made of spring steel, the hardness (i.e. strength) shall be selected according to the bearing property and stress of the spring. However, the hardness unevenness is closely related to the plane strain fracture resistance.
From the curve connection, it can be seen that the plane strain fracture resistance (KIC) decreases obviously with the increase of hardness. That is to say, when recognizing the hardness value of the spring, it should be based on the premise of satisfying the requirements of the spring characteristics, and the hardness value of the spring should be lower.
When selecting materials for springs, pay attention to the hardenability of steel. Whether the cross-section of spring material is quenched and the degree of quenching have great influence on the spring quality.
Copper and gold materials are generally used for electrical springs with springs as conductors or springs operating in water (including seawater) and water vapor environment under conditions with variable humidity.
For springs operating in acid touch extreme other corrosive media, generally stainless acid resistant steel or nickel alloy and other corrosion-resistant materials are selected. The spring used under the general environmental medium conditions shall be made of general spring steel. After the spring is made, the surface shall be coated with rust prevention or plated (galvanized, cadmium plated and copper plated) to prevent corrosion.
For springs used in weighing instruments and surfaces, in order to satisfy their accuracy without being affected by temperature changes, constant elastic alloys with minimal changes in elastic modulus and expansion coefficient are generally selected.
For springs requiring light weight, insulation, anti-collision, anti-corrosion and other special purposes, reinforced plastic can be selected. At present, the more suitable plastic spring is the thermosetting reinforced plastic GFRP with epoxy resin and phenolic resin as the matrix and glass fiber reinforced. Anti vibration rubber can also be used to make various types of rubber springs.
