ショットピーニングによって生成したナノ結晶相の疲労強度に及ぼす影響

Abstract

The shot peening process produces a nanocrystalline layer on the surface of carbon steel. This nanocrystalline surface layer is harder than the matrix phase. Therefore, there are expectations that this nanocrystalline layer could become a new solution for surface hardening. However, little information exists on the effect this nanocrystalline surface layer has on fatigue properties. In this study, coil springs prepared with this nanocrystalline surface layer were investigated and were compared with springs of the same physical properties without this nanocrystalline surface layer. Coil springs were made from oil-tempered steel wire with chemical composition of 0.6C, 1.4Si, 0.7Mn and 0.7Cr(mass%) and were formed into compressive coil springs. Two types of springs were manufactured using different shot peening conditions. These springs had the same surface hardness and residual stress destruction, one with a nanocrystalline surface layer, and the other without. Fatigue testing was carried out on a spring fatigue test machine operated over 5×107 cycles. This test method has the merit of reproducing almost exactly the actual working condition of the valve springs. The results of the fatigue test showed that the spring with a nanocrystalline layer had a fatigue limit of τm±τa=600±531 MPa at 107 cycles, whereas the other spring had a limit of τm±τa=600±489 MPa. Thus, it was evident that this nanocrystalline surface layer could increase the fatigue life by 8%.