It is afraid that increasing the strength of oil-tempered steel wire for valve springs might result in a loss of workability, fatigue strength and creep resistance. The committee, therefore, investigated the effects of the chemical composition, wire strength, annealing condition, residual stress induced by shot peening, etc., on the mechanical properties of wires and valve springs.
The material used for this study was made of the conventional oil-tempered high tension steel wire (JIS G 3566) by increasing its carbon level to the JIS-prescribed upper limit and adding some vanadium. It was then made into two specimens by tempering to tensile strength levels of 2,000 and 2,100MPa, respectively. For the purpose of comparison, two other specimens were made of conventional oil-tempered wire (JIS G 3566) by tempering to strength levels of 1,900 and 2,000MPa, respectively.
In conclusion, high-tension steel wire of this study has been found to possess equal workability with improved fatigue strength, and better creep resistance, when compared with the standard steel wire (JIS G 3566), thus demonstrating the superiority of high-tension steel wire for valve springs. To further improve fatigue strength and creep resistance, it has been found more important to make changes in chemical compositions, and to make further improvements in shot peering process and in other surface-treating technologies, rather than only increasing the tensile strength of the wire.