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A β-titanium alloy wire with a tensile strength of 210 kgf/mm2 (2060 MPa) was made for automotive valve springs. The fatigue of the β-titanium valve springs were heavily affected with the existence of the oxide layer on the surface. The repeated contact of coils to the ends also gives rise to heavy abrasion during the service, indicating the necessity of some measures to prevent it. Through this investigation, technological basis for β-Ti valve springs was established.

For the requirements of high power output, lightweight and improved fuel consumption of motorcycles to respond to global environmental needs, high fatigue strength to cope with high stress is becoming more important than ever for valve springs. To satisfy such needs, a new alloy steel that softens less in tempering (1),(2) and nitriding that increases surface hardness of valve springs has been developed using oil-tempered silicone-chrome steel wire as the base material. Also, with regards to shot-peening to create compression residual stress on the surface, studies are being performed for multi-stage and high-hardness peening. The research reported in this paper is aimed at an increase of internal hardness after nitriding while maintaining the cold-coiling-ability by adding elements that reduce softening from tempering while reducing non-dissolved carbide in the material.