Mechanical properties of spring wire drawn and tempered at lower bainite region

Abstract

Medium carbon spring steel containing Si (SWOSC-V) was aus-drawn and tempered at 543 to approximately 573 K (lower bainite region) to obtain the wire with ausformed bainite structure. Aus-forming refined bainitic ferrite. Especially near surface, bainitic ferrite tended to align in one direction because of a large amount of redundant shear strain in drawing. The redundant shear strain also resulted in larger hardness near the surface especially in the cases of low drawing velocities and large die-cone angles. At center regions, the aus-formed bainite wires showed smaller hardness than that of a conventional oil quenched and tempered wire with tempered martensite structure possibly due to higher transformation temperature by heat generation during drawing. The aus-formed bainite wires had large amount of retained austenite up to 25%. The retained austenite transformed to martensite during shot-peening or fatigue testing, so that the aus-formed bainite wires showed good mechanical properties and fatigue strength comparable to those of the conventional wires. These results indicate that the present new and simple process (aus-drawing+tempering) could be applicable to practical use for spring wire production.