Fine Structures and Mechanical Properties of Thermally Cycled SUJ 2 Steel

The purpose of this work is to present results on the mechanical properties of SUJ 2 bearing steel improved by a cyclic heat treatment. The structure of thermally cycled steel consists of fine-grained ferrite particles of the size under about 2 μm containing fine cementite particles of the size about 0.2μm. Thermally cycled steel was found to be superplastic at 710°C, just below A1 temperature, at an initial strain rate of 1.67 × 10 -4s -1, i.e. elongation to failure was about 500% and the strain rate sensitivity, m value, was about 0.4. The activation energy of about 180 kJ/mol, which is nearly equal to that of grain boundary diffusion of Fe atoms in α-iron, was obtained for this plastic deformation. Furthermore the rolling contact fatigue life test was performed at room temperature under constant load. The fatigue life of SUJ 2 steel thermally cycled was found to be prolonged (about 2.5 times) as compared with that of conventional one.