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The gas-nitriding characteristics of valve springs made from three different grades of oil-tempered wires were investigated as a preliminary study, and the springs made from the most heat resistant grade of oil-tempered wire (SOTHS) were selected for further study. As a result, fatigue limit strength as high as 736MPa (mean stress) +/- 665MPa (amplitude stress) at a stress repetion of 5×107 cycles was obtained by applying gas-nitriding, followed by multi-stage shot-peening, the last stage of which was very fine steel bead peening with the average size less than 50 μm (SS-Treatment). The reasons for the high fatigue endurance obtained are also discussed.

The peening of fine steel beads with diameters less than 80μm (referred to as SS Treatment in this paper) on the surface of nitrided and then conventionally shot-peened (with 0.6mm diameter cut steel wires) high-strength helical springs made from developed high strength valve spring wire, was found to effectively improve spring fatigue strength. This improved fatigue strength is mainly the result of very high compressive residual stress caused by work hardening of the spring surface layer without inducing deleterious surface qualities such as harmful surface micro-fissuring or surface roughness.

The peening of fine steel beads with diameters less than 80 micrometers onto the surface of a spring (referred to as SS Treatment in this paper) was found to improve the fatigue limit of the spring effectively since the surface compressive residual stress and surface hardness were improved significantly. As an example, by applying this technology to non–nitrided high–strength valve springs made from the so–called high–strength oil tempered wire, the fatigue limit was increased to the nearly equivalent level as the nitrided and conventionally shot–peened valve springs made from the same high–strength wire. This means that the SS Treatment can be used as a substitute to the nitriding process which has been applied in order to get improved fatigue property of high–strength valve springs.