Research direction
The strength level of traditional spring steel is difficult to meet the requirements of modern industrial development. As we all know, the mechanical properties of spring steel depend on the heat treatment process under the premise of material quality assurance, and the heat treatment process should also be determined according to the materials used. An important way of high strength spring steel is to give full play to the role of alloy elements to achieve the best alloying effect

(1) Heat treatment
Spring steel requires higher strength and fatigue limit. It is usually used in the state of quenching and tempering at medium temperature to obtain higher elastic limit. Heat treatment technology has a crucial impact on the internal quality of spring. Therefore, further research is needed to further improve the fatigue life of spring, In order to further strengthen the surface strength, increase the compressive stress and improve the fatigue life of the valve spring, the valve spring should be further nitrided, low temperature liquid carbonitriding or sulfur nitriding treatment, and then shot peening strengthening. Nitrogen infiltration not only eliminates the adverse effects of decarburization, In addition, the residual compressive stress is increased, and the high temperature strength of the valve spring after nitriding and liquid carbonitriding is improved.

The deformation amount at 150 ℃ is 0.2% (the specified value is 0.5%), and the deformation amount at 250 ℃ is 0.56%, which improves the thermal stability and anti relaxation stability of the valve spring. However, the time of nitriding and liquid carbonitriding should be strictly controlled, otherwise the network sulfide and network nitride will be formed, On the contrary, it will reduce its fatigue strength

(2) Alloying
Carbon is the main strengthening element in steel, and its influence on spring steel is often more than that of other alloy elements. According to the application requirements, spring steel material should be medium and high carbon alloy steel. In order to overcome the problem of reducing toughness and plasticity after increasing the strength of spring steel, there is also a trend to reduce carbon content

The main function of alloy elements in spring steel is to improve mechanical properties, process properties and give some special properties (such as high temperature resistance, corrosion resistance)
In many spring steels, silicon is the main alloy element, which has the greatest influence on the elastic reduction resistance. This is mainly due to the strong solid solution strengthening effect of silicon. At the same time, silicon can inhibit the nucleation and growth of cementite during tempering, change the quantity, size and morphology of carbide precipitated during tempering, and improve the tempering stability of steel, It will promote the decarburization and graphitization tendency of steel during rolling and heat treatment, and make smelting difficult and easy to form inclusions. Therefore, the use of spring steel with high silicon content should be cautious

Because chromium can significantly improve the hardenability of steel, prevent the graphitization tendency of Si Cr steel during spheroidizing annealing and reduce decarburization layer, it is a common alloy element in spring steel. The spring steel 50CrV with chromium as the main strengthening element is widely used
Molybdenum can improve the hardenability of steel, prevent temper brittleness and improve fatigue property. There are not many spring steels added molybdenum in the existing standards, and the addition amount is generally less than 0.4%