Microstructure and depth of decarburization layer of high-strength high-quality spring steel

(1) Microstructure

In order to reduce the weight of the car body, automobile spring steel is developing towards high strength, but it cannot destroy its plasticity, so its fine structure control is very important. High-strength spring steel requires the hot-rolled material to have uniform ferrite and pearlite structure and fine grains. There should be no reticulated ferrite and quenched structure. The higher the sorbitization rate, the better.

(2) Decarburization Depth (Guide: Encyclopedia of Fastener Standards and Specifications)

Decarburization of 0.1mm on the surface of spring steel will significantly reduce its fatigue limit. If a ferrite fully decarburized layer appears, the fatigue limit can be reduced by 50%. As the depth of the decarburization layer on the steel surface increases, the fatigue life decreases. This is because after the steel is quenched, the surface decarburized layer cannot reach the required hardness and mechanical strength, and the expansion coefficients of different parts of the surface layer are different during quenching, resulting in the transition zone between the fully decarburized layer and the semi-decarburized layer of the part. Micro cracks, these visible or invisible cracks become stress concentration areas, and as the origin of the cracks continue to develop, causing failure and fracture of the spring. High-strength high-quality spring steel requires the lower the decarburization tendency, the better, the ferrite full decarburization layer structure is not allowed, and the total decarburization layer depth of the spring steel is limited.

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