Abstract
In order to improve the fatigue strength and the sag resistance in medium carbon spring steels, a novel thermomechanical processing, "aus-drawing", was developed. Aus-drawing is an aus-forming using wire drawing. The aus-drawing process was applied to two kinds of mediumcarbon spring steels containing Si, SWOSC-V, and SAE9254 (JIS SUP12). Three kinds of typical microstructures appeared in the aus-drawn wires depending on the drawing conditions:fully martensite structure, surface bainite structure, and fully bainite structure. It was clarified that mainly the acceleration of bainitic transformation kinetics by increase in plastic strain and additionally the heat generation during drawing cause the appearance of bainite. The preferential formation of bainite near the surface could be explained by the relatively large equivalent strain near the surface due to redundant shear strain during drawing. The aus-drawn wires with fully martensite structure showed superior fatigue strength and sag resistance to those of the conventional oil-tempered wires after tempering and shot peening. On the contrary, the aus-drawn wires with bainite showed inferior properties to that of the conventional wires. Therfore, it is quite important to decrease the redundant shear strain near surface to inhibit the bainitic transformation, in order to succeed the aus-drawing process.
