Abstract
Four kinds of surface hardened specimens having hardened thicknesses of 0.7∼1.8mm were prepared using a"Micropulse induction heating system". Rotation bending fatigue tests were performed with special focus on the effect of hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 107 cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000MPa. This is because the fatigue cracks originating from inside the hardened layer leads to final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved by forming thick hardened layers. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to the final fracture (hardened layer fracture mode).
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