Fukaya, , Saitama, Japan. Japan. Keywords: Water cavitation peening, Impact pressure, Arc height value, Residual stress

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1 Key Engineering Materials Online: ISSN: , Vols , pp doi:1.428/ 28 Trans Tech Publications, Switzerland Experimental Investigation on Intensity and Residual Stress in Superficial Layers Induced by Water Cavitation Peening with Aeration D.Y. Ju 1, a, B. Han 2, b 1 Department of Material Science and Engineering, Saitama Institute of Technology, Fusaiji 169, Fukaya, , Saitama, Japan 2 Graduate School of Saitama Institute of Technology, Fusaiji 169, Fukaya, , Saitama, Japan a dyju@sit.ac.jp, b q62fea@sit.ac.jp Keywords: Water cavitation peening, Impact pressure, Arc height value, Residual stress Abstract. Water cavitation peening (WCP) with aeration is a novel surface enhancement method. A new ventilation nozzle with aeration is adopted to improve the process capability of WCP by increasing the impact pressure induced by the bubble collapse on the surface of components. In this study, in order to investigate the process capability of the WCP with aeration, a standard N-type almen strips of spring steel SAE 17 was treated by WCP with various process conditions, and the arc height value and the residual stress in the superficial layers were measured by X-ray diffraction method. The optimal fluxes of aeration and the optimal standoff distances were achieved. Introduction Cavitation normally cause the severe cavitation erosion damage in hydraulic machinery [1, 2], therefore, most previous studies on cavitation have focused on the damage mechanism. However, it can also induce the residual compressive stress in the superficial layer of the specimens, which can improve the fatigue life of mechanical components by a similar way as conventional shot peening [3-6], and such method is named water cavitation peening (WCP). In the past, in order to protect hydraulic structures against cavitation damage, air is normally added by aerators close to the bottom region where the cavitation number falls below a critical value [7, 8]. However, for the aerated jet-flow with high speed, appropriate aeration may accelerate the cavitation instead of preventing cavitation. During the process of the aerated jet-flow jetting out through a short pipe, if the flow velocity is close to the sonic velocity and the air concentration of aerated jet-flow is from 1.5 % to 4 %, the pressure gradient between the inlet and outlet part in the short pipe will become tremendous [9]. According to their simulation results, D.Y. Ju et al. has designed a new ventilation nozzle by which the suitable air can be aerated into the extra high-velocity flow in the nozzle throat, the tremendous pressure gradient between the upstream and the downstream flow has formed [1]. In this paper, in order to further verify the process capability of WCP with aeration, a standard N-type almen strip of spring steel SAE 17 is treated by WCP with various process conditions. The arc height values and the depth distributions of residual stress were investigated. Experimental procedures Measurement of impact pressure. The standoff distance is an important parameter in the WCP process, which is the distance from the outlet of nozzle throat to the treated surface of test specimen, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-9/5/16,22:3:8)

2 Key Engineering Materials and it is adjustable. In order to obtain the optimal process capability of WCP, we need to determine the optimal standoff distance before treating the specimen. A pressure sensitive film produced by FUJI PHOTO FILM Co., Ltd. can be used to measure the pressure distribution of bubble cloud along the jet direction. By the soft of FPS-37E, the pressure can be drawn out from the change of color density of the treated film. In this study, the measurement pressure range of the film was from 13 to 3 MPa; the precision was less than 1%; the time of retention at the pressure was 5 s. Measurement of arc height and residual stress. According to the results of the impact pressure measurement, a standard N-type almen strip of spring steel SAE 17 was treated by WCP with the optimal process conditions. A convex surface was formed on the almen strip after WCP, and the arc height is a scale standard of the WCP intensity. The arc height is measured by an almen gauge in accordance with the measurement standard of the conventional shot peening. 4 Impact pressure, MPa 3 2.8L/min.4L/min.2L/min 1.L/min.6L/min Standoff distance, mm Fig.1 Relationship between standoff distance and impact pressure with various fluxes of aeration Arc height, um WCP time, min.2 L/ min, 75 mm.6 L/ min, 9 mm.8 L/ min, 85 mm 1. L/ min, 8 mm.4 L/ min, 75 mm.4 L/ min, 85 mm.4 L/ min, 95 mm.4 L/ min, 15 mm Fig.2 Relationship between arc height and WCP time with various process conditions Moreover, in order to more estimate the capability of the WCP, the residual stress distributions in the superficial layer of test specimen were measured by X-ray diffraction stress analysis with Arc height, um WCP time, min conventional sin 2 ψ method. The X-ray tube of CrK α type was operated at 4kV and 4mA with 1mm slit in diameter. In order to investigate the influence of the aeration on the depth distribution of residual stress, the superficial layer of test point was removed by electrolytic polishing step by step with an Proto Electrolytic Polisher-Model 8818 produced by Proto Manufacturing Ltd..

3 756 Surface Engineering residual st ress, MPa L/min, 75mm.4L/min, 95mm.2L/min, 75mm.8L/min, 85mm L/min, 85mm.4L/min, 15mm.6L/min, 9mm 1.L/min, 8mm dist ance from surface, um original 4min 8min 16min 32min 64min Fig.3 Distributions of residual stress in the superficial compressive with the various WCP time and various process conditions Results and discussion Impact pressure. The distributions of impact pressure along the jet direction with the different fluxes of aeration rate are shown in Fig. 1, and the optimal standoff distance can be directly determined. When the fluxes of aeration are.2,.6,.8, and 1. L/min, the corresponding optimal standoff distances are around 75, 9, 85, and 8 mm, respectively. When the flux of aeration is.4

4 Key Engineering Materials L/min, the huge impact pressure is induced, and the corresponding optimal standoff distances are around from 75 mm to 15 mm. Arc height. The relationship between the WCP time and the arc height of test specimens with various process conditions is investigated, and the results are shown in Fig. 2. It can be seen that the arc height values increase slowly after 48 minutes, therefore, 48 minutes can be considered as the saturated time. When the fluxes of aeration are.8, 1. L/min, the process capability is weaker; however, when the fluxes of aeration are.2,.4,.6 L/min, the process capability is obviously stronger. Especially, when the flux of aeration is.4 L/min, the process capability of the standoff distances from 75 mm to 15mm are all stronger, the maximum of the arc height value can reach around 15µm. Residual stress. The distributions of residual stress in the superficial compressive with the various WCP time and various process conditions are shown in Fig. 3. It can be seen that the maximum of the compressive residual stress is always found in the surface of the specimen; as the depth is extended, the compressive residual stress values decrease gradually. The maximum of the compressive residual stress and the depth of compressive residual stress layer are up to around -65 MPa and 165 µm, respectively. The process capability to residual stress of WCP with various process conditions is similar to the process capability to arc height values of WCP. Conclusion WCP with aeration can induce not only compressive residual stress in the superficial layers of specimens but also the convex surface; the maximum values and the depth of compressive residual stress are up to around -65 MPa and 165 µm, respectively, and the arc height value of the standard N-type almen strips of spring steel SAE 17 can reach around 15µm. The optimal process conditions of WCP with aeration can be obtained, the optimal flux range of aeration is around.4 L/min, and the range of the optimal standoff distance is from 75 mm to 15 mm. References [1] C.E. Brenne: Cavitation and Bubble Dynamics (Oxford University Press, Oxford 1995). [2] Y. Tomita and A. Shima: J. Fluid Mech., Vol. 169 (1986), p. 535 [3] D.Y. Ju, M. Qin, T. Koubayashi, R. Oba: Surf. Eng. Vol. 22 (26), p. 219 [4] H. Soyama, K. Saito, M. Saka: J. Eng. Mater. Technol. Vol. 124 (22), p. 135 [5] D. Odhiamabo, H. Soyama: Int. J Fatigue Vol. 25 (23), p [6] B. Han, D. Y. Ju, W. P. Jia: Appl. Surf. Sci. Vol. 253 (27), p [7] H. Chanson: J. Hydraul. Res. Vol. 27(4) (1989), p. 519 [8] S.O. Russel, G.J. Sheehan: Can. J. Civil Eng. Vol. 1 (1974), p. 97 [9] X.B. Huang, W.H. Xia: J. Hydrodynamics, Vol. 19(2A) (24), p. 158 [1] M. Qin, D.Y. Ju, R. Oba: Surf. Coat. Tech. Vol. 2 (26), p. 5364

5 Surface Engineering 1.428/ Experimental Investigation on Intensity and Residual Stress in Superficial Layers Induced by Water Cavitation Peening with Aeration 1.428/