０．２％Ｃ炭素鋼レーザ表面焼入切欠き材の疲労特性

Abstract

Although some case hardening methods, e.g. high frequency induction quenching, cementation and nitriding, are being applied to increase the fatigue strength of carbon steels, these methods require a troublesome treatment. Laser case hardening method has been developed to avoid the trouble and to increase hardness at a small desired area.The present study was planned to investigate the effect of laser case hardening on fatigue strength by using 0.2%C carbon steel specimens with five different types of U and V notches and by applying laser case hardening treatment on the notch part of each specimen. Since it is well known that the fatigue strength in impact fatigue is lower than that in non-impact fatigue, most of the experiments were conducted under impact fatigue load conditions in the wide stress cycles range of 10-107 cycles.The S-N curves under the non-impact load condition and those of the annealed specimens under the impact and non-impact load conditions were also obtained, and the effect of case hardening by laser surface quenching on fatigue strength were discussed by comparing these S-N curves each other, especially from the viewpoint of dependence of strength increase by laser case hardening on the stress concentration factor.The results showed that the laser case hardening method was effective to increase the fatigue strength of notched specimens whose stress concentration factor αis in the range of α<4. In other words, an increase in fatigue life of structual elements can be expected sufficiently by applying this method at locations considered as the initiating part of fatigue failure, because the stress concentration factor scarcely exceeds the value of 4 in service structural elements.