Recent Progress with Nitrided Valve Springs for Automobile Internal Combustion Engines

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.