Effect of Shot Peening on Improvement of Fatigue Strength for Metal Bellows

Author:  Okada*, Tange*, Ando**
Source:  Conf Proc: ICSP-8 Sept. 16-20, 2002 Garmisch-Partenkirchen, Germany
Doc ID:  2002044
Year of Publication:  2002
Authors H. Okada*, A. Tange*, Kotoji Ando** *NHK SPRING CO., LTD., Kanagawa-Pref, Japan **Yokohama National University, Kanagawa-Pref, Japn Introduction Generally metal bellows (it is hereafter described as a bellows) are known for a seal to defend a leak being elastic1). Recently small size bore bellows are getting to expect to a seal of pump which was small and can use under the circumstances of high pressure. However, until now, it would be difficult to achieve the fatigue strength for required specifications in most cases. Meanwhile it was known for that shot peening had an effect on improving fatigue strength2). It can be expected that it is difficult to obtain a large compressive residual stress on the inner surface of small bore size bellows by shot peening process. In order to obtain the effect of shot peening on the inner surface of bellows, a new shot peening processes by using a air peening machine and a reflective plate were developed. It was found that this method can apply the shots to the inner surface of bellows effectively, to improve the fatigue life of bellows, comparing with non-shot peening one. It can be also noted that the measurements of residual stress by X-ray diffraction is not applicable to the material of bellow, SUS304 stainless steel. The measurement of residual stress is essential to determine shot peening conditions. Since the structure of SUS304 bellows is mixed with austenite and stress-induced martensite by cold forming the signal to noise ratio is too small to show the peak of X-ray diffraction. In order to solve these problems, the SUS631 stainless steel bellows was selected because it was a good workability and had a lot of martensite. The optimum shot peening conditions were decided by using residual stress of SUS631 bellows.

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