Process performance of spring steel
Spring is used under impact, vibration or long-term alternating stress, so spring steel is required to have high tensile strength, elastic limit and high fatigue strength. In technology, spring steel is required to have certain hardenability, not easy decarbonization, good surface quality and other carbon spring steel, which means high quality carbon structural steel with carbon content of WC in the range of 0.6% - 0.9%. Alloy spring steels are mainly Silicon-manganese series steels, whose carbon content is slightly lower, mainly depending on increasing the silicon content of Wsi to improve performance. In addition, there are alloy spring steels of hammer, tungsten and vanadium. In recent years, a new kind of steel with boron, niobium and molybdenum added to Silicon-manganese steel has been developed, which extends the service life of springs and improves the quality of spring.
Generally spring steel can be produced in electric furnace, open hearth or oxygen converter, and high quality spring steel with good quality or special properties can be produced in electroslag furnace or vacuum furnace. The prescribed content range of carbon, manganese, silicon and other major elements in spring steel is relatively narrow, so chemical composition must be strictly controlled in smelting. When the silicon content is high, it is easy to form defects such as bubbles and so on. Therefore, raw materials for smelting must be dried to remove gases and inclusions as far as possible, and to avoid overheating of molten steel.
In spring rolling, special attention should be paid to decarburization and surface quality. When the surface of steel is decarburization, the fatigue limit of steel will be significantly reduced. For high silicon spring steel such as 70Si3MnA, graphitization should be avoided. Therefore, the stop rolling temperature should not be too low (> 850 C) during hot working, and the residence time should not be too long in the temperature range (650 - 800 C) where graphitization is easy to form.
The residual compressive stress on the spring surface can be produced by shot peening after the spring is made to offset part of the working stress on the surface and restrain the formation of surface cracks, which can significantly improve the fatigue limit of the spring.
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