Surface condition and fatigue properties of spring wire

Excellent surface state and the fatigue performance: the spring under the surface of the work stress is larger, and the fatigue failure often starts from the surface of steel wire, for use in the important occasion of the spring, such as car engine valve spring fatigue life of general requirements - 3.0 x 2.3 x 107 times 107 times, high-grade car suspension spring general requirements - 5.0 x 2.0 x 106 times 106 times longer fatigue life, even it is puts forward high demands on the fatigue property of materials. There are many factors that affect the fatigue properties of materials, such as chemical composition, hardness, purity of steel, surface quality and metallographic structure of materials, especially the surface quality of materials. Material surface defects, such as cracks, folds, scales, corrosion, pits, scratches and indentation, are easy to make the spring in the process of stress concentration. The stress concentration is often the source of fatigue failure. The fatigue source is also easy to occur first at the site of surface decarburization, so strictly controlling the depth of decarburization layer is also a very important quality index. In order to improve the surface quality of the spring material, the material surface can be polished or polished, before the steel wire drawing process to remove a layer of material skin, this can be removed most of the surface defects. Controlled atmosphere or vacuum heat treatment can be used to prevent surface decarburization and oxidation.