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.
Spring steel requires higher strength and fatigue limit, which is usually used in the state of quenching + tempering at medium temperature to obtain higher elastic limit. Heat treatment technology has an important influence on the internal quality of spring. Therefore, how to further improve the fatigue life of spring needs further research, especially the chemical surface modification heat treatment, shot peening strengthening and so on have an important impact on the spring fatigue life.
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 nitrocarburizing treatment after forming, and then shot peened. For example, in Japan, the fatigue limit of f4mm Si CR oil quenched steel wire can be increased by 240 MPa by comparison with that of low temperature bulk carbonitriding at 450 ℃× 4.5H and tempering at 400 ℃× 15min. At the same time, the high temperature strength of the valve spring after nitriding and low temperature liquid carbonitriding is increased.
The deformation amount at 150 ℃ is 0.2% (the specified value is 0.5%), and the deformation amount at 250 ℃ is 0.56%. The thermal stability and anti relaxation stability of valve spring are improved, but the time of nitriding and liquid carbonitriding should be Otherwise, the network sulfide and network nitride will be formed, and the fatigue strength will be reduced.
Two kinds of shot peening can be used to improve the strength of valve spring. One is 0.8 mm in diameter and its microhardness is 720hv0.2, and the other is 0.25 mm in diameter and its microhardness is 800hv0.2. The third shot peening can achieve better strengthening effect and improve the surface quality.