Characterization of nanocrystalline surface layer induced by shot peening and effect on their fatigue strength
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T3.7.1
Characterization of nanocrystalline surface layer induced by shot peening and effect on their fatigue strength. Hideo Mano1,2, Kondo Satoru1, Akihito Matsumuro2, Toru Imura3 TOGO SEISAKUSYO CORPORATION, Aichi-gun, Aichi, Japan 2 Nagoya university, Nagoya-shi, Aichi, Japan 3 Aichi institute of technology, Toyota-shi, Aichi, Japan 1
Abstract 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%. Introduction Shot peening process of normal condition given compressive residual stress on material surface without phase transformation. But the shot peening process is known to produce a hard layer, known as the white-layer on the surface of carbon-steel, under intensify peening conditions; intensify peening velocity and using high hardness shot media. This white-layer have high hardness than matrix phase. Therefore white-later had been thought composing martensite-phase. Recently, It was suggested white-layer is nanocrystalline phase, composed nano-ferrite [1-5]. In this study, coil springs with nanocrystalline surface layer were manufactured. Moreover fatigue examination carried out with the coil springs. Experimental Test springs were made from oil-tempered wire; its chemical composition is shown in Table 1. It was compressive coil spring. Table 2 shows dimensions of test springs, and photograph of accomplished test spring is shown Figure 1. Table 1 Chemical composition of wire (mass%). C 0.57
Si 1.43
Mn 0.7
P 0.02
S 0.006
Cu 0.01
Cr 0.68
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Table 2 Dimensions of test springs. W ire diam eter (m m ) M ean diam eter of coil (m m ) A ctive num ber of coil Totalnum ber of coil Free height (m m ) Spring rate (N /m m )
φ3.2 20.0 4.0 6.0 47.0 32.0
Figure1 Photograph of manufactured spring. Test springs manufactured same process until end grinding. But, shot peening condition are different. Manufacturing process of springs is shown in Figure 2. Two kinds of springs with which shot peening conditions are different were produced. Manufactured two kinds of springs are spring A and Spring B. Spring A is a spring with a surface nanocrystalline layer, and spring B is a spring without the nanocrystalline layer. Shot peening condition is shown in Table 3. The sho
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