- Email: [email protected]
Influence of water cavitation peening with aeration on fatigue behaviour of SAE1045 steel
Applied Surface Science 253 (2007) 9342–9346 www.elsevier.com/locate/apsusc
Influence of water cavitation peening with aeration on fatigue behaviour of SAE1045 steel B. Han a, D.Y. Ju b,*, W.P. Jia c a Graduate School of Saitama Institute of Technology, Fusaiji 1690, Fukaya, Saitama 369-0293, Japan Department of Material Science and Engineering, Saitama Institute of Technology, Fusaiji 1690, Fukaya, Saitama 369-0293, Japan c Department of Material Science and Engineering, University of Science and Technology Liaoning, Anshan, Liaoning 114051, China b
Received 26 February 2007; received in revised form 28 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract Water cavitation peening (WCP) with aeration is a recent potential method in the surface enhancement techniques. In this method, a ventilation nozzle is adopted to improve the process capability of WCP by increasing the impact pressure, which is induced by the bubble collapse on the surface of components in the similar way as conventional shot peening. In this paper, fatigue tests were conducted on the both-edgenotched flat tensile specimens to assess the influences of WCP on fatigue behaviour of SAE1045 steel. The notched specimens were treated by WCP, and the compressive residual stress distributions in the superficial layer were measured by X-ray diffraction method. The tension–tension (R = Smin/Smax = 0.1, f = 10 Hz) fatigue tests and the fracture surfaces observation by scan electron microscopy (SEM) were conducted. The experimental results show that WCP can improve the fatigue life by inducing the residual compressive stress in the superficial layer of mechanical components. # 2007 Elsevier B.V. All rights reserved. PACS : 61.10.Nz; 62.20.Mk; 68.37.Hk; 68.47.De Keywords: Water cavitation peening; X-ray diffraction; Residual stress; Fatigue life
1. Introduction Cavitation impact can cause severe damage in the components of hydraulic machinery [1–3]. Therefore, most studies have focused on the damage mechanism of cavitation. However, cavitation technology can be utilized for enhancing the fatigue strength of mechanical components by introducing residual stress in the superficial layer of metallic components in a similar way as conventional shot peening [4,5]. Water cavitation peening (WCP) or cavitation shotless peening (CSP) is a recent surface enhancement technique. In the method, a high-speed submerged water jet pressurized by a plunger pump is used. When the high pressure water is jetted through a jet nozzle to the metallic component, the uniform big bubble cloud can be generated, and the bubble collapse on the surface of the component will produce impact effect just as shot
* Corresponding author. Tel.: +81 48 5856826; fax: +81 48 5856826. E-mail address: [email protected] (D.Y. Ju). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.076
peening. Recently, water cavitation peening technology has been developed by some researchers. Such as Soyama has achieved a cavitating jet in air by injecting a high-speed water jet into a low-speed water jet using a concentric nozzle in air, which can introduce the higher compressive stress into the surface of components [6,7]. Qin 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, and the tremendous pressure gradient between the upstream and the downstream flow has formed [8]. Sahaya Grinspan and Gnanamoorthy have accomplished an oil cavitation jet by injecting a high-speed oil jet into an oil-filled tank [9,10]. Several recent investigations have revealed that WCP can improve the fatigue strength by introducing the compressive residual stress in the surface of metallic components [11–18]. Compared with conventional shot peening, WCP has the following advantages: (1) complicated and narrow surface can be treated easily; (2) smoother surface can be obtained; (3) there is no thermal effect on the material surface; (4) it is clean, inexpensive and nontoxic; (5) the impact pressure and the
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
process capability of WCP are isotropic [19]. However, WCP can also lead to the stress corrosion and other surface damages of material, which are also factors influencing on the improvement degree of fatigue strength. So, the further investigations on fatigue life of product parts and the process optimization still need to be conducted. In this paper, the tension–tension specimens of SAE1045 steel, as an example, were treated by WCP with aeration. The residual stress distributions in the superficial layers and the fatigue behaviour were investigated.
9343
Table 1 The chemical compositions of SAE1045 steel by wt.% C Mn Si P S Cr Mo Cu
0.46783 0.64016 0.26524 0.01472 0.00663 0.02348 0.00132 0.00220
2. Experimental procedures 2.1. WCP equipment The schematic of the water cavitation peening equipment which can jet ultrahigh speed water bubble cloud with cavitation is illustrated in Fig. 1. Water is supplied by a pump to the water jet from the reservoir tank. The pressure and the flux of the plunger pump are 30 MPa and 16 L/min, respectively. The nozzle with the inner diameter of 1 mm can be pressed by a small air pump, and its flux is adjustable from 0 to 1.0 L/min. Test specimen is positioned near the bottom of test water tank, and the jet direction is normal to the surface of test specimen. During the process of WCP, the test tank is filled with water. The temperature of the working water is kept 293–297 K. When the high pressure water is jetted in the vertical direction from a jet nozzle to the top of tank, the uniform big bubble cloud can be generated and the bubble collapse on the surface of the specimen will produce impact effect just as shot peening. 2.2. Test specimen and WCP conditions The chemical compositions of the SAE1045 steel specimens are shown in Table 1. The schematic figure of test specimen is shown in Fig. 2. WCP can also lead to stress corrosion and other surface damages of material. The degree of fatigue improvement is strongly dependent on process conditions of WCP, such as the
Fig. 2. The geometry and dimensions of the fatigue test specimen.
flux of aeration, the standoff distances and the WCP time. Therefore, we need to determine the process conditions before treating the test specimens. The impact pressure distributions with various fluxes of aeration induced by the bubble collapse are diverse in the vertical direction, and there is an obvious change rules between impact pressure of bubble collapse and the standoff distance. Therefore, the standoff distance is an important parameter in the WCP technology, which is the distance from the outlet of nozzle throat to the surface of test specimen, and it is adjustable. In order to obtain the optimal standoff distances with various fluxes of aeration, a pressure sensitive film produced by FUJI PHOTO FILM Co. Ltd. had been used to measure the pressure distribution of bubble cloud along the jet direction in Qin’s recent study [8]. The color density of the film will be changed due to the different pressures. The color density can be evaluated by FPD-305E color indicator. By the soft of FPS-307E, the pressure can be drawn out from the change of color density. In this study, a new high pressure sensitive film was adopted. The measurement pressure of the film ranges from 130 to 300 MPa. The precision is less than 10%, and the time of retention at the pressure is 5 s. As shown in Fig. 3, the support of pressure sensitive film is installed in the center of the
Fig. 1. The schematic of the water cavitation peening equipment.
9344
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
5410LV scanning microscope was used for all testing. The fracture surface morphology of the fatigue test specimens was examined by SEM to assess mode and origin of fatigue failure. 3. Results and discussion 3.1. Optimal WCP conditions
Fig. 3. The schematic of the impact pressure measurement equipment and an example of the pressure sensitive film treated by WCP.
vertical-type test water tank bottom. An example of pressure sensitive film treated by WCP is also given in Fig. 3. When the fluxes of aeration were 0.2, 0.4, 0.6, 0.8, and 1.0 L/min, respectively, the impact pressure distributions were measured. The optimal standoff distances were determined by the method mentioned above. According to authors’ recent study on the fatigue performances of SPA-H and SPCC steel, the dot pitting and preferential corrosion near crack tip appeared after the WCP time extends 15 min. Therefore, the WCP time of 15 min was adopted, and the peening area at the target was near the notch tip of the sample in this study. 2.3. Residual stress measurement The residual stress distributions in the superficial layer of the different test specimens were measured by X-ray diffraction stress analysis with conventional sin2 C method. The X-ray tube of Cr Ka type was operated at 40 kV and 40 mA with 0.5 mm slit in diameter. The shift of a-Fe (2 1 1) diffraction profile was detected. The residual stress sx along the tension– tension direction was determined near the notched tip. In order to investigate the depth distribution of residual stress, the superficial layer of test point was removed by electrolytic polishing step by step by an electrolytic polisher with a Proto Electrolytic Polisher-Model 8818 produced by Proto Manufacturing Ltd. The depth distributions of the residual stress were obtained by measuring the residual stress at each step.
The impact pressure distributions along the jet direction due to the different fluxes of aeration rate are shown in Fig. 4. The trend of the impact pressure distributions curve is similar to the results in [8]. Nevertheless, the impact pressure values are higher due to the new high pressure sensitive film, and the maximal pressure value is up to about 320 MPa. According to the impact pressure distributions along the jet direction, the optimal standoff distances can be directly determined. When the fluxes of aeration are 0.2, 0.4, 0.6, 0.8, and 1.0 L/min, the coverage of the impact pressure is about from 100 to 320 MPa, the range of standoff distances with the extreme impact pressure value is from 70 to 95 mm. The corresponding optimal standoff distances are around 75, 85, 90, 85, and 80 mm, respectively. It is obvious that the tremendous pressure gradient along the jet direction is induced with the 0.4 L/min flux of aeration. Because the flux of the plunger pump is 16 L/min, and then 0.4 L/min flux of aeration is the air concentration of 2.5%. It can be explained well by Huang and Xia in [20], impact pressure increases gradually before the air concentration reaches 2.43%, and decreases gradually after the air concentration exceeds 2.43%. Therefore, that the flux of aeration was 0.4 L/min and the standoff distance of 85 mm were adopted in this study. 3.2. Residual stress The typical residual stress distributions in the superficial layers of the test specimens are shown in Fig. 5. Compared with the original state and quenched state, the superficial layers of the original with WCP and the quenched with WCP state specimens have apparent compressive residual stress layers. The highest compressive residual stress values of the original with WCP and the quenched with WCP state are found on the surface, and the maximum values are up to about 215 and 535 MPa, respectively. As the depth is extended, the
2.4. Fatigue testing The test specimens with various states (original, original with WCP, quenched and quenched with WCP) were tested completely in tension–tension mode at the room temperature until fracture. In this study, the temperature of quenching was 1093 K, and the medium of quenching was oil with 313 K. Load control (R = Smin/Smax = 0.1) using a sinusoidal waveform at 10 Hz was conducted for all testing. A Shimadzu servohydraulic fatigue test machine (15 kN force and 25 mm displacement capacities) with in situ observation by JSM-
Fig. 4. The relationship between standoff distance and impact pressure of WCP with various fluxes of aeration.
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
9345
Fig. 6. The S–N curves of the different test specimens. Fig. 5. The depth distributions of the residual stress of the different test specimens.
compressive residual stress values decrease gradually, and the depth of the compressive residual stress zone is up to 110 and 140 mm, respectively. 3.3. Fatigue behaviour It can be seen from Fig. 6 that the maximums of improvement of the fatigue limits in the original with WCP and the quenched with WCP state specimens are about 15–20% when compared with that of the original and the quenched state specimens. The improvement of high cycles fatigue life is especially apparent. The fatigue limits of original with the WCP and the quenched with WCP state specimens increase from 170 to 200 MPa, and
from 300 to 350 MPa, respectively. The SEM fracture morphologies are shown in Fig. 7. Fig. 7(a) is typical toughness fracture surface, and Fig. 7(b) is typical brittleness fracture surface, fatigue striation patterns can be clearly observed on the toughness fracture surface. The spacing of the striation in the quenched with WCP specimen is less than that of the quenched one, which indicate the slower fatigue crack grow rate. It can be seen from Fig. 7(c) that a crack is initiating from the outer surface of the quenched specimen. However, in Fig. 7(d) a fatigue crack initiation site is shown about 125 mm beneath the surface for the quenched with WCP specimen. The results above can be attributed to the introduction of compressive residual stress in the superficial layers of the test specimens by the WCP.
Fig. 7. The SEM fractographies of (c) the quenched test specimen and (a, b and d) the quenched with WCP test specimens.
9346
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
4. Conclusion The main results show that the WCP with aeration can induce the residual compressive stress in the superficial layer of the specimens. The depth of the compressive residual stress zone can be up to around 140 mm, which can prevent the fatigue crack from initiating in the surface of specimen and reduce the crack growth rate. Accordingly, the improvement of the fatigue life by WCP with aeration can be achieved. However, the degree of fatigue life improvement is dependent on the WCP process conditions, and the further investigations on fatigue life and process optimization still need to be conducted. Acknowledgement This research is partially supported by High-Tech Research Center in Saitama Institute of Technology. References [1] C.E. Brenne, Cavitation and Bubble Dynamics, Oxford University Press, Oxford, 1995, pp. 201–219. [2] Y.J. Chen, L.W. Huang, T.S. Shih, Mater. Trans. 44 (2003) 327.
[3] M.W. Lee, Y.K. Kim, Y.M. Oh, Y. Kim, S.H. Lee, H.S. Hong, S.J. Kim, Wear 255 (2003) 157. [4] D.Y. Ju, T. Uchiyama, H. Machida, T. Kanawa, R. Oba, Y. Sunayama, H. Tsuda, Mater. Sci. Forum 404 (2002) 635. [5] D.Y. Ju, H. Tsuda, V. Ji, T. Uchiyama, R. Oba, Surface improvement of a quenched gear using water peening by a cavitation jet technique, in: Proceedings of the Third International Conference on Advanced Technology in Experimental Mechanics, 2003, pp. 399–405. [6] H. Soyama, ASME J. Fluids Eng. 127 (2005) 1095. [7] H. Soyama, ASME J. Eng. Mater. Technol. 126 (2004) 123. [8] M. Qin, D.Y. Ju, R. Oba, Surf. Coat. Technol. 200 (2006) 5364. [9] A. Sahaya Grinspan, R. Gnanamoorthy, Appl. Surf. Sci. 253 (2006) 989. [10] A. Sahaya Grinspan, R. Gnanamoorthy, Appl. Surf. Sci. 253 (2006) 997. [11] H. Soyama, D. Odhiambo, K. Saito, Cavitation shotless peening for improvement of fatigue strength, in: Proceedings of the Eighth International Conference on Shot Peening, 2003, pp. 436–440. [12] H. Soyama, K. Kusaka, M. Saka, J. Mater, Sci. Lett. 20 (2001) 1263. [13] D. Odhiamabo, H. Soyama, Int. J. Fatigue 25 (2003) 1217. [14] Y. Tan, G. Wu, J.M. Yang, T. Pan, Fatigue Fract. Eng. Mater. Struct. 27 (2004) 649. [15] M. Ramulu, S. Kunaporn, M. Jenkins, M. Hashish, J. Hopkins, J. Press. Vess. Trans. ASME 124 (2002) 118. [16] H. Soyama, K. Saito, M. Saka, J. Eng. Mater. Technol. 124 (2002) 135. [17] D.Y. Ju, M. Qin, T. Koubayashi, R. Oba, Surf. Eng. 22 (2006) 219. [18] H. Soyama, J.D. Park, M. Saka, J. Manuf. Sci. Eng. Trans. ASME 122 (2002) 83. [19] M. Qin, D.Y. Ju, R. Oba, Surf. Coat. Technol. 201 (2006) 1409. [20] X.B. Huang, W.H. Xia, J. Hydrodyn. 19 (2A) (2004) 158.
Influence of water cavitation peening with aeration on fatigue behaviour of SAE1045 steel B. Han a, D.Y. Ju b,*, W.P. Jia c a Graduate School of Saitama Institute of Technology, Fusaiji 1690, Fukaya, Saitama 369-0293, Japan Department of Material Science and Engineering, Saitama Institute of Technology, Fusaiji 1690, Fukaya, Saitama 369-0293, Japan c Department of Material Science and Engineering, University of Science and Technology Liaoning, Anshan, Liaoning 114051, China b
Received 26 February 2007; received in revised form 28 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract Water cavitation peening (WCP) with aeration is a recent potential method in the surface enhancement techniques. In this method, a ventilation nozzle is adopted to improve the process capability of WCP by increasing the impact pressure, which is induced by the bubble collapse on the surface of components in the similar way as conventional shot peening. In this paper, fatigue tests were conducted on the both-edgenotched flat tensile specimens to assess the influences of WCP on fatigue behaviour of SAE1045 steel. The notched specimens were treated by WCP, and the compressive residual stress distributions in the superficial layer were measured by X-ray diffraction method. The tension–tension (R = Smin/Smax = 0.1, f = 10 Hz) fatigue tests and the fracture surfaces observation by scan electron microscopy (SEM) were conducted. The experimental results show that WCP can improve the fatigue life by inducing the residual compressive stress in the superficial layer of mechanical components. # 2007 Elsevier B.V. All rights reserved. PACS : 61.10.Nz; 62.20.Mk; 68.37.Hk; 68.47.De Keywords: Water cavitation peening; X-ray diffraction; Residual stress; Fatigue life
1. Introduction Cavitation impact can cause severe damage in the components of hydraulic machinery [1–3]. Therefore, most studies have focused on the damage mechanism of cavitation. However, cavitation technology can be utilized for enhancing the fatigue strength of mechanical components by introducing residual stress in the superficial layer of metallic components in a similar way as conventional shot peening [4,5]. Water cavitation peening (WCP) or cavitation shotless peening (CSP) is a recent surface enhancement technique. In the method, a high-speed submerged water jet pressurized by a plunger pump is used. When the high pressure water is jetted through a jet nozzle to the metallic component, the uniform big bubble cloud can be generated, and the bubble collapse on the surface of the component will produce impact effect just as shot
* Corresponding author. Tel.: +81 48 5856826; fax: +81 48 5856826. E-mail address: [email protected] (D.Y. Ju). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.076
peening. Recently, water cavitation peening technology has been developed by some researchers. Such as Soyama has achieved a cavitating jet in air by injecting a high-speed water jet into a low-speed water jet using a concentric nozzle in air, which can introduce the higher compressive stress into the surface of components [6,7]. Qin 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, and the tremendous pressure gradient between the upstream and the downstream flow has formed [8]. Sahaya Grinspan and Gnanamoorthy have accomplished an oil cavitation jet by injecting a high-speed oil jet into an oil-filled tank [9,10]. Several recent investigations have revealed that WCP can improve the fatigue strength by introducing the compressive residual stress in the surface of metallic components [11–18]. Compared with conventional shot peening, WCP has the following advantages: (1) complicated and narrow surface can be treated easily; (2) smoother surface can be obtained; (3) there is no thermal effect on the material surface; (4) it is clean, inexpensive and nontoxic; (5) the impact pressure and the
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
process capability of WCP are isotropic [19]. However, WCP can also lead to the stress corrosion and other surface damages of material, which are also factors influencing on the improvement degree of fatigue strength. So, the further investigations on fatigue life of product parts and the process optimization still need to be conducted. In this paper, the tension–tension specimens of SAE1045 steel, as an example, were treated by WCP with aeration. The residual stress distributions in the superficial layers and the fatigue behaviour were investigated.
9343
Table 1 The chemical compositions of SAE1045 steel by wt.% C Mn Si P S Cr Mo Cu
0.46783 0.64016 0.26524 0.01472 0.00663 0.02348 0.00132 0.00220
2. Experimental procedures 2.1. WCP equipment The schematic of the water cavitation peening equipment which can jet ultrahigh speed water bubble cloud with cavitation is illustrated in Fig. 1. Water is supplied by a pump to the water jet from the reservoir tank. The pressure and the flux of the plunger pump are 30 MPa and 16 L/min, respectively. The nozzle with the inner diameter of 1 mm can be pressed by a small air pump, and its flux is adjustable from 0 to 1.0 L/min. Test specimen is positioned near the bottom of test water tank, and the jet direction is normal to the surface of test specimen. During the process of WCP, the test tank is filled with water. The temperature of the working water is kept 293–297 K. When the high pressure water is jetted in the vertical direction from a jet nozzle to the top of tank, the uniform big bubble cloud can be generated and the bubble collapse on the surface of the specimen will produce impact effect just as shot peening. 2.2. Test specimen and WCP conditions The chemical compositions of the SAE1045 steel specimens are shown in Table 1. The schematic figure of test specimen is shown in Fig. 2. WCP can also lead to stress corrosion and other surface damages of material. The degree of fatigue improvement is strongly dependent on process conditions of WCP, such as the
Fig. 2. The geometry and dimensions of the fatigue test specimen.
flux of aeration, the standoff distances and the WCP time. Therefore, we need to determine the process conditions before treating the test specimens. The impact pressure distributions with various fluxes of aeration induced by the bubble collapse are diverse in the vertical direction, and there is an obvious change rules between impact pressure of bubble collapse and the standoff distance. Therefore, the standoff distance is an important parameter in the WCP technology, which is the distance from the outlet of nozzle throat to the surface of test specimen, and it is adjustable. In order to obtain the optimal standoff distances with various fluxes of aeration, a pressure sensitive film produced by FUJI PHOTO FILM Co. Ltd. had been used to measure the pressure distribution of bubble cloud along the jet direction in Qin’s recent study [8]. The color density of the film will be changed due to the different pressures. The color density can be evaluated by FPD-305E color indicator. By the soft of FPS-307E, the pressure can be drawn out from the change of color density. In this study, a new high pressure sensitive film was adopted. The measurement pressure of the film ranges from 130 to 300 MPa. The precision is less than 10%, and the time of retention at the pressure is 5 s. As shown in Fig. 3, the support of pressure sensitive film is installed in the center of the
Fig. 1. The schematic of the water cavitation peening equipment.
9344
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
5410LV scanning microscope was used for all testing. The fracture surface morphology of the fatigue test specimens was examined by SEM to assess mode and origin of fatigue failure. 3. Results and discussion 3.1. Optimal WCP conditions
Fig. 3. The schematic of the impact pressure measurement equipment and an example of the pressure sensitive film treated by WCP.
vertical-type test water tank bottom. An example of pressure sensitive film treated by WCP is also given in Fig. 3. When the fluxes of aeration were 0.2, 0.4, 0.6, 0.8, and 1.0 L/min, respectively, the impact pressure distributions were measured. The optimal standoff distances were determined by the method mentioned above. According to authors’ recent study on the fatigue performances of SPA-H and SPCC steel, the dot pitting and preferential corrosion near crack tip appeared after the WCP time extends 15 min. Therefore, the WCP time of 15 min was adopted, and the peening area at the target was near the notch tip of the sample in this study. 2.3. Residual stress measurement The residual stress distributions in the superficial layer of the different test specimens were measured by X-ray diffraction stress analysis with conventional sin2 C method. The X-ray tube of Cr Ka type was operated at 40 kV and 40 mA with 0.5 mm slit in diameter. The shift of a-Fe (2 1 1) diffraction profile was detected. The residual stress sx along the tension– tension direction was determined near the notched tip. In order to investigate the depth distribution of residual stress, the superficial layer of test point was removed by electrolytic polishing step by step by an electrolytic polisher with a Proto Electrolytic Polisher-Model 8818 produced by Proto Manufacturing Ltd. The depth distributions of the residual stress were obtained by measuring the residual stress at each step.
The impact pressure distributions along the jet direction due to the different fluxes of aeration rate are shown in Fig. 4. The trend of the impact pressure distributions curve is similar to the results in [8]. Nevertheless, the impact pressure values are higher due to the new high pressure sensitive film, and the maximal pressure value is up to about 320 MPa. According to the impact pressure distributions along the jet direction, the optimal standoff distances can be directly determined. When the fluxes of aeration are 0.2, 0.4, 0.6, 0.8, and 1.0 L/min, the coverage of the impact pressure is about from 100 to 320 MPa, the range of standoff distances with the extreme impact pressure value is from 70 to 95 mm. The corresponding optimal standoff distances are around 75, 85, 90, 85, and 80 mm, respectively. It is obvious that the tremendous pressure gradient along the jet direction is induced with the 0.4 L/min flux of aeration. Because the flux of the plunger pump is 16 L/min, and then 0.4 L/min flux of aeration is the air concentration of 2.5%. It can be explained well by Huang and Xia in [20], impact pressure increases gradually before the air concentration reaches 2.43%, and decreases gradually after the air concentration exceeds 2.43%. Therefore, that the flux of aeration was 0.4 L/min and the standoff distance of 85 mm were adopted in this study. 3.2. Residual stress The typical residual stress distributions in the superficial layers of the test specimens are shown in Fig. 5. Compared with the original state and quenched state, the superficial layers of the original with WCP and the quenched with WCP state specimens have apparent compressive residual stress layers. The highest compressive residual stress values of the original with WCP and the quenched with WCP state are found on the surface, and the maximum values are up to about 215 and 535 MPa, respectively. As the depth is extended, the
2.4. Fatigue testing The test specimens with various states (original, original with WCP, quenched and quenched with WCP) were tested completely in tension–tension mode at the room temperature until fracture. In this study, the temperature of quenching was 1093 K, and the medium of quenching was oil with 313 K. Load control (R = Smin/Smax = 0.1) using a sinusoidal waveform at 10 Hz was conducted for all testing. A Shimadzu servohydraulic fatigue test machine (15 kN force and 25 mm displacement capacities) with in situ observation by JSM-
Fig. 4. The relationship between standoff distance and impact pressure of WCP with various fluxes of aeration.
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
9345
Fig. 6. The S–N curves of the different test specimens. Fig. 5. The depth distributions of the residual stress of the different test specimens.
compressive residual stress values decrease gradually, and the depth of the compressive residual stress zone is up to 110 and 140 mm, respectively. 3.3. Fatigue behaviour It can be seen from Fig. 6 that the maximums of improvement of the fatigue limits in the original with WCP and the quenched with WCP state specimens are about 15–20% when compared with that of the original and the quenched state specimens. The improvement of high cycles fatigue life is especially apparent. The fatigue limits of original with the WCP and the quenched with WCP state specimens increase from 170 to 200 MPa, and
from 300 to 350 MPa, respectively. The SEM fracture morphologies are shown in Fig. 7. Fig. 7(a) is typical toughness fracture surface, and Fig. 7(b) is typical brittleness fracture surface, fatigue striation patterns can be clearly observed on the toughness fracture surface. The spacing of the striation in the quenched with WCP specimen is less than that of the quenched one, which indicate the slower fatigue crack grow rate. It can be seen from Fig. 7(c) that a crack is initiating from the outer surface of the quenched specimen. However, in Fig. 7(d) a fatigue crack initiation site is shown about 125 mm beneath the surface for the quenched with WCP specimen. The results above can be attributed to the introduction of compressive residual stress in the superficial layers of the test specimens by the WCP.
Fig. 7. The SEM fractographies of (c) the quenched test specimen and (a, b and d) the quenched with WCP test specimens.
9346
B. Han et al. / Applied Surface Science 253 (2007) 9342–9346
4. Conclusion The main results show that the WCP with aeration can induce the residual compressive stress in the superficial layer of the specimens. The depth of the compressive residual stress zone can be up to around 140 mm, which can prevent the fatigue crack from initiating in the surface of specimen and reduce the crack growth rate. Accordingly, the improvement of the fatigue life by WCP with aeration can be achieved. However, the degree of fatigue life improvement is dependent on the WCP process conditions, and the further investigations on fatigue life and process optimization still need to be conducted. Acknowledgement This research is partially supported by High-Tech Research Center in Saitama Institute of Technology. References [1] C.E. Brenne, Cavitation and Bubble Dynamics, Oxford University Press, Oxford, 1995, pp. 201–219. [2] Y.J. Chen, L.W. Huang, T.S. Shih, Mater. Trans. 44 (2003) 327.
[3] M.W. Lee, Y.K. Kim, Y.M. Oh, Y. Kim, S.H. Lee, H.S. Hong, S.J. Kim, Wear 255 (2003) 157. [4] D.Y. Ju, T. Uchiyama, H. Machida, T. Kanawa, R. Oba, Y. Sunayama, H. Tsuda, Mater. Sci. Forum 404 (2002) 635. [5] D.Y. Ju, H. Tsuda, V. Ji, T. Uchiyama, R. Oba, Surface improvement of a quenched gear using water peening by a cavitation jet technique, in: Proceedings of the Third International Conference on Advanced Technology in Experimental Mechanics, 2003, pp. 399–405. [6] H. Soyama, ASME J. Fluids Eng. 127 (2005) 1095. [7] H. Soyama, ASME J. Eng. Mater. Technol. 126 (2004) 123. [8] M. Qin, D.Y. Ju, R. Oba, Surf. Coat. Technol. 200 (2006) 5364. [9] A. Sahaya Grinspan, R. Gnanamoorthy, Appl. Surf. Sci. 253 (2006) 989. [10] A. Sahaya Grinspan, R. Gnanamoorthy, Appl. Surf. Sci. 253 (2006) 997. [11] H. Soyama, D. Odhiambo, K. Saito, Cavitation shotless peening for improvement of fatigue strength, in: Proceedings of the Eighth International Conference on Shot Peening, 2003, pp. 436–440. [12] H. Soyama, K. Kusaka, M. Saka, J. Mater, Sci. Lett. 20 (2001) 1263. [13] D. Odhiamabo, H. Soyama, Int. J. Fatigue 25 (2003) 1217. [14] Y. Tan, G. Wu, J.M. Yang, T. Pan, Fatigue Fract. Eng. Mater. Struct. 27 (2004) 649. [15] M. Ramulu, S. Kunaporn, M. Jenkins, M. Hashish, J. Hopkins, J. Press. Vess. Trans. ASME 124 (2002) 118. [16] H. Soyama, K. Saito, M. Saka, J. Eng. Mater. Technol. 124 (2002) 135. [17] D.Y. Ju, M. Qin, T. Koubayashi, R. Oba, Surf. Eng. 22 (2006) 219. [18] H. Soyama, J.D. Park, M. Saka, J. Manuf. Sci. Eng. Trans. ASME 122 (2002) 83. [19] M. Qin, D.Y. Ju, R. Oba, Surf. Coat. Technol. 201 (2006) 1409. [20] X.B. Huang, W.H. Xia, J. Hydrodyn. 19 (2A) (2004) 158.