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Decarburization occurs on heat treated components. At leaf spring manufacturers, the occurrence of decarburization, which has a negative impact on fatigue, starts at hot rolled leaf spring steel raw materials' production process. The decarburization rate of the raw material, which is taken through a heat treatment, increases during the leaf spring manufacturing process. Through metallographic analyses and experimental tests, this study manifests how leaf spring's resistance to fatigue is affected by the correlation of ferrite structure occurred by decarburization on heat treated leaf springs, the surface hardness, and the permanent surface tension to be occurred during the shot peening process. This study conveys the techniques that can be applied to obtain low decarburization in leaf spring manufacturing, and the improvements achieved through optimization of high temperature treatment times of materials through use of thermographic survey at heat treatment furnaces.

The leaf spring manufacturer must supply high quality raw material at required strength for ensuring endurance rig tests. It's very important to maintain both inside and surface cleanliness of raw material. This study presents micro crack effects on material surface by evaluating residual stress values. Residual stress values on leaf springs are measured with X-ray diffractometer and different residual stress values are classified on the same raw material batch which have also the same material failures. Finally, micro cracks are measured in metrics. Micro crack standardization is performed considering the residual stress values and rig tests. The outputs in metrics which correlated with endurance rig tests can be taken as reference by the manufacturers of leaf spring and original equipment manufacturers.