Development of High Strength Valve Spring Steel with Excellent Fatigue Life Performance

Abstract

In order to respond to the needs for high-stress valve spring designing, we developed a new valve spring steel which has superior fatigue property and sag resistance without sacrificing the cold-coiling performance of wire. This paper describes the following results:1) The tensile strength of newly developed steel was 200MPa higher than that of conventional SAE9254 after heat treatment which simulated the nitriding process.2) Austenite grain size number of the developed steel was 2 point higher than that of SAE9254 after oil tempering process.3) Fracture toughness of the developed steel was 5MPa√m higher than that of SAE9254 compared at the same hardness level. This is thought to be the effect of Ni addition and higher tempering temperature.4) It was confirmed that conventional cold coiling process being adopted to SAE9254 is applicable to this newly developed steel.5) The developed steel is less sensitive against inclusions during fatigue test. If the inclusion size is same level, 16% higher cyclic stress can be applied for the developed steel than for SAE9254.6) We have also developed the inclusion control technique for high Si steel, and high fatigue performance are being stably obtained.