| The suspension spring steel as an important damping elastic element of suspension system is demanded higher elastic limit and relax resistance when working in periodic alternating stress. At the same time, the stress of spring steel cross section along the radial increases gradually from center to edge during use, which lead to the surface under the largest impact load. It is easy for spring steel with high carbon to appear serious surface decarburization phenomenon under high temperature conditions, which significantly influences the hardness, anti-fatigue performance, service life of the steel. if there is a complete decarburization layer in the steel surface, the fatigue life of spring steel will reduce 50%. The degreen of decarburization is a strict index for automobile suspension spring steel, therefore controlling the decarburization layer depth and decarburization type is an important question for the study of spring steel decarburized, and it is one of the difficult problems to solve for production of high quality spring steel.This paper studied the influence of heating temperature, holding time, cooling rates,cooling modes and heating medias on spring steel decarburization and the influence of quenching temperature of decarburization steel after rolling on decarburization using the spring steel 55 SiCrV and 60Si2 Mn as experimental material. Experiment materials were processed into ? 6×9 mm sample, then doing decarburization experiments. The samples after experiment were embedded into metallographic specimens by using metallographic inlaying machine. The surface decarburization layer depth of specimens were measured by using metallographic microscope metallographic and the software of ImageJ after a series of coarse grinding, fine grinding, polishing process, corrosion, etc. At the same time, the decarburization mathematical model of heating temperature and holding time was established by using the fick’s law and high temperature oxidation kinetics formula.The research results show that the higher heating temperature and the longer holding time plays a role in promoting the formation of spring steel decarburization phenomenon.When the heating temperature is in the range of 750 ~ 1100℃, the depth of the decarburization layer of the spring steel shows a trend of increasing firstly and then decreasing with the rise of heating temperature and the peak temperature of 55 SiCr V appears at about 1000℃ as well as the peak temperature of 60Si2 Mn appears at about1050℃. When the holding time is in the range of 20 ~ 100 min, the depth of the decarburization layer of the spring steel shows a trend of increasing with the extension of holding time, but the magnitude of increasing decreases gradually. The depth of decarburization layer of the spring steel with Cr, V elements decreases significantly compared with the spring steel without Cr, V elements. The increase of cooling rate can effectively reduce the depth of decarburization layer, When the cooling rate is more than10℃ / s, there is no decarburization layer in spring steel surface. The different ways of cooling and heating medium have a significant effect on the depth of decarburization layer,while water cooling can significantly reduce the depth of decarburization layer and reducing atmosphere can hinder the generation of the decarburization layer. The rising of quenching temperature is beneficial to reduce the depth of decarburization layer of decarburized steel.That using decarburization mathematical model analyzed the influence of heating temperature and holding time on decarburization in oxidation reaction is verified by using the experimental data, Which can improve the reliability of the model and also provide theoretical basis for the formulating of production process. |
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