The shot peening process in normal conditions produces a compressive residual stress on the surface of a material without phase transformation. Instead, shot peening induces a change in phase (e.g. nanoferrite layer and metal flow layer) on the surface of carbon steel under the intensified peening conditions associated with higher peening velocities and the use of a high hardness shot media.This study investigated the effect of shot peening time on changes in surface microstructure and its effect on fatigue strength. The test specimens were compressive coil springs made from oil tempered wire. The test springs were manufactured by the same process as that used for the test specimens except for the shot peening condition. A total of seven shot peening times were employed: 100, 300, 500, 1000, 2000, 3000, 6000 s. The projected material was 0.25 mm diameter steel cut wire. The test springs were measured for surface roughness and residual stress distribution observed using an optical microscope and scanning electron microscope.Fatigue tests were conducted using a spring fatigue test machine at a frequency of 30 Hz for 2×10 7 cycles and a test stress of t m ±t a ＝600±540 MPa.Values of the residual stress distribution for 300 6000 s were similar, as was surface roughness of all the springs as peening was performed using the same media.The spring for the 300 s projection consisted of the matrix phase and the surface metal flow phase. Fatigue strength increased until a shot peening time of 1000 s, after which levels remained constant. The spring microstructure for the 1000 s projection revealed a white layer on part of the surface. Durations exceeding 3000 s were associated with the occurrence of the white layer, which covered entire the surface and was not associated with any increase in fatigue strength.

The shot peening process in normal conditions produces a compressive residual stress on the surface of a material without phase transformation. But, the shot peening process produces a phase transformation(e.g., nanoferrite-phase and metal flow layer) on the surface of carbon steel under the intensified peening conditions of higher peening velocity and the use of a high-hardness shot media. Previously, we reported that the fatigue strength of springs having a nanocrystalline phase were higher than without nanocrystalline phase. This study investigates the microstructural surface layer induced by shot peening and its effect on fatigue strength. The test specimens were compressive coil springs made of oil-tempered wire. The test springs were manufactured by the same process except for the shot peening time. The shot peening time was varied as 100, 300, 500, 1000, 2000, 3000, 6000 s. Shot media was steel cut wire of diameter 0.25 mm. Surface roughness and residual stress distribution were measured. Microstructures were observed with an optical microscope and a scanning electron microscope. And, fatigue tests were carried out by using a spring fatigue test machine. The experimental values of the residual stress distribution and the surface roughness s are almost the same for 300-6000. The spring shot-peened for 300 s consists of the matrix phase and the surface metal flow phase. The spring shot-peened for 1000 s contains the white layer on a part of the surface. For the shot-peened time greater than 3000 s, the white layer almost covered full of surface. The fatigue strength increased with peening time until 1000 s. For the specimens shot-peened for more than 1000 s, the fatigue strength remained at a constant level.

The shot peening process is known to produce a hard layer, known as the white layer" on the surface of coil springs. However, little is known about the fatigue properties of this white-layer.In this study, coil springs with a white-layer were manufactured. The surface of these springs was then examined using micro Vickers hardness, FE-SEM etc. to test fatigue strength of the springs. From the results obtained, a microstructure of the white-layer with grain size of 50-100 nm was observed, with a Vickers hardness rating of 8-10 GPa. Tow category springs were manufactured utilizing a double-peening process. These springs had the same residual stress destruction and surface roughness. Only one difference was observed: one spring had a nanocrystalline layer on the surface, while the other did not. The results of the fatigue test realized an increase in the fatigue life of the nanocrystalline surface layer by 9%.

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 5×10 7 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 t m ±t a ＝600±531 MPa at 10 7 cycles, whereas the other spring had a limit of t m ±t a ＝600±489 MPa. Thus, it was evident that this nanocrystalline surface layer could increase the fatigue life by 8.

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