Properties of Induction Heat-Treated High-Strength Spring Wire

In order to increase the fuel economy of automobiles, efforts are continuing at weight saving. For suspension springs, a new high-strength Si-Cr steel (SAE 9254) wire were developed by an application of induction heat treatment technique. The wires (ITW) with diameters of 7 to 14 mm with fine micro structure, good surface and less decarburization were attained by rapid heating and short hold times during austenitization. In spite of very high tensile strength from 1770 to 2060 MPa., ITW shows high ductility in tensile, torsion and impact test, and these properties provide excellent cold formability. Coil springs cold worked from ITW showed superior fatigue life (torsion stress Շ=637±490 MPa., over 3 x 10⁵ cycles) and high sag resistance. This high performance makes it possible to produce suspension springs that have the same performance as conventional ones but are 10~20 % lighter. Both suspension and emergency brake springs made from ITW have been confirmed by laboratory studies in the United States, to conform to the respective standards and offer weight saving.