New developments in spring materials. Ferrous metal materials for valve springs and suspension springs have been widely used in Si-Cr steel. To improve fatigue strength and relaxation resistance. Hot-formed springs with V and Mo added to Si-Cr steel are generally larger in size. In order to ensure that springs with large cross-sections have high elasticity and excellent comprehensive properties, two types of alloy elements are added. The purpose of adding these elements is to stabilize the state of the organization, thereby improving the resistance and relaxation resistance of the spring material to small amounts of plastic deformation<1>.
Alloying elements and their effects The alloying elements act on Si, Mn, Cr, Ni, Mo, W, VB, Ti, Zr, Nb, Re to strengthen the matrix and increase the hardenability of trace active elements. In 1999, BMW set up a special project to study 11 types High-stress and high-strength spring steel, compare the size and distribution of various inclusions and the relationship between fatigue life. After detailed statistics and analysis of various failure modes and fatigue fracture mechanisms, a database of fatigue performance of high-stress springs has been formed, and the largest factors affecting fatigue performance have been found, as well as the limit of expansion without instability under certain high-stress conditions The crack length and other results provide a reference for the future use and selection of ultra-pure spring steel, which will be applied to future automotive springs <2>.
Non-ferrous metal material spring design has begun to develop towards high stress. Generally, clutch springs from Polaris and Ski-Doo have been stressed more than 70% of the material’s tensile strength. At this time, high-stress spring steel has only a slight surface In the presence of scratches or corrosion spots, stress fracture will occur. The sensitivity to these surface defects is called /notch sensitivity. In recent years, the design of clutch springs has surpassed the limit of spring steel, and Ti alloys with small steering notch sensitivity are used to make springs.
As one of the best spring materials, Ti alloy, a non-ferrous metal material, has the characteristics of light weight, small modulus of elasticity, and good corrosion resistance, and has been widely used. Its tensile strength is lower than steel, and its modulus of elasticity is more than half that of steel. This means that using a slightly larger diameter Ti spring will double the expansion and contraction of a steel spring, while reducing the weight and volume by half. The notch sensitivity of Ti alloy spring is very small. When driving in the desert, there can be large rock pits on the whole spring, and it can still work without loss of performance.
Non-metallic materials Non-metallic materials for springs are mainly anti-vibration rubber and reinforced plastics. The shape of the rubber spring is not limited, the stiffness in each direction can be freely selected, and it can bear loads from various aspects. Air springs can be made of rubber. The air springs produced by the world’s ** Fire-stone Air Spring Company are used in trucks, trailers, new Lincoln city cars and Navigator<4>.
Reinforced plastic has the characteristics of light weight, high specific strength, more energy absorption than spring steel, corrosion resistance, etc., and has been used in the manufacture of leaf springs and disc springs. Currently used in the manufacture of plastic springs are mainly thermosetting resin plus glass fiber or other fiber-reinforced composite materials. Glass fiber reinforced plastic (GFRP) leaf springs are widely used in transverse suspensions in Britain, Japan, the United States and other countries. With the same quality of GFRP and spring steel, the capacity of GFRP to store elastic deformation energy is 7 times that of spring steel, that is to say, under the same stiffness and plate width, the spring made of GFRP, its deformation and maximum load are spring steel 1.7 times that of steel, and the mass can be reduced to 41% of steel. At present, a stronger carbon fiber reinforced plastic (GFRP) suspension spring has been successfully developed, the mass is 80% of the metal leaf spring<1>.
Cold coil/hot coil and induction treatment fatigue test, the maximum stress mostly occurs on the surface of the spring, so the surface quality of the spring has a great influence on the fatigue strength. Different rolling methods will cause different surface conditions. Generally, the smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. When the spring is hot coiled, the surface of the spring becomes rough due to oxidation and decarburization occurs, which reduces the fatigue strength of the spring, which is detrimental to the fatigue life. Therefore, coil springs with simpler shapes are all cold rolled <3>.
In addition, the spring steel used for cold coils is manufactured after induction hardening and induction tempering. The required mechanical properties have been reached before leaving the factory, and cold rolls only change the shape, but do not improve the performance. One of the benefits of induction processing with rapid heating is grain refinement. Grain refinement is a heating method that does not sacrifice strength and improves toughness. The conventional electric furnace heating method, the general grain size can reach 8 grades, through induction heating, it can be refined to more than 10 grades<4>.
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