高強度13Crマルテンサイト系ステンレス鋼の疲労特性に及ぼすイオン注入による表面改質効果

抄録

Effect of surface modification by ion implantation on fatigue behaviors was investigated on two sorts of 13Cr martensitic stainless steels with high tensile strength of about 1900MPa, under an out of plane bending load condition. Nitrogen ions were implanted into the surface of thin plate specimen of 1mm thick with several energy levels from 0.8 to 1.2MeV and with doses in the range of 10¹³-10¹⁶ions/cm². Effect of post heat-treatment on the fatigue strength was also investigated for the specimens ion-implanted with several different conditions. Results of a series of fatigue tests revealed that the surface modification by ion implantation raised fatigue resistance of the materials by choosing appropriate implantation conditions. Furthermore, a stable increase in fatigue strength could be attained when the specimen was annealed after ion implantation. Observations of initiation and propagation of surface fatigue cracks indicated that the thin surface layer modified by ion implantation had an arresting effect against crack initiation though it gave no effect on the propagation of large crack. Characteristics of the modified surface layer were evaluated from several viewpoints; penetration depth of nitrogen, hardness and residual stress by X-ray stress measurement method. The results revealed the following features: 1) The surface layer thickness modified by ion implantation was less than 1μm in the case as implanted but was extended to 1-2μm by the post heat-treatment, 2) the hardness at the surface layer showed a trend to rise with an increase in annealing temperature, and 3) the high compressive residual stress was measured at the implantation dose of 10¹⁶ions/cm².