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Cavitation shotless peening for improvement of fatigue strength of carbonized steel
International Journal of Fatigue 25 (2003) 1217–1222 www.elsevier.com/locate/ijfatigue
Cavitation shotless peening for improvement of fatigue strength of carbonized steel Dan Odhiambo, Hitoshi Soyama ∗ Tohoku University, Graduate School of Engineering, Department of Mechanical Engineering, Aoba-yama 01, Aoba-ku, Sendai 980-8579, Japan
Abstract Cavitation shotless peening (CSP) is a surface enhancement technique which makes use of cavitation impact to induce compressive residual stress on the metallic materials, thereby increasing the fatigue life of components. The technique is similar to shot peening except that shots are not used and that is why we refer to it as shotless peening. In case of CSP a submerged high-speed water jet with cavitation, i.e. a cavitating jet is used. To explore the potentials of CSP as a means of improving fatigue strength, carbonized chrome-molybdenum alloy steel (JIS SCM415) has been analyzed in the non-peened, shot-peened and CSP conditions with respect to processing times, residual stress and cyclic-stress curves. The residual stress was measured by an X-ray diffraction method. Experimental results confirmed that the rotating beam fatigue strength of a CSP specimen was stronger than non-peened and shotpeened specimens by 79 and 27 MPa, respectively. 2003 Elsevier Ltd. All rights reserved. Keywords: Peening; Residual stress; Fatigue strength; Cavitation; Processing time
1. Introduction Cavitation impacts have been intensely investigated using hydraulic machinery. Experimental interest stems from the time it was discovered that cavitation was a cause of erosion in ship propellers [1]. Most studies on cavitation have focused on the damage mechanism behind the erosion problem [2–6]. In this study, we discuss the turn-around point of cavitation impacts and focus on the positive engineering applications. Cavitation shotless peening is a new technology for surface enhancement of metallic materials. In CSP, cavitation impact is induced by a submerged high-speed water jet with cavitation, i.e. a cavitating jet. The collapsing cavitation bubble causes plastic deformation on the surface, thereby creating a compressive layer which impedes fatigue crack initiation, as well as improves the strength of the material. The cavitating jet intensity and the occurring region are controlled with parameters such as upstream pressure and nozzle size. Soyama et al. [7– 9] discovered the optimum cavitating jet conditions for
Corresponding author. Tel.: +81-22-217-6891; fax: +81-22-2176893. E-mail address: [email protected] (H. Soyama). ∗
0142-1123/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0142-1123(03)00121-X
introducing compressive residual stress in metallic materials. The authors have already proved that the surface finish of CSP specimen are known to be of a better quality finish than shot-peened specimen due to the fact that there are no solid body collisions [10–12]. This paper is in two parts: First, we illustrate the effects of processing times on the residual stress and effect of residual stress changing with depth by comparing the non-peened, shot-peened and cavitation shotless peened carbonized chrome-molybdenum alloy steel (JIS SCM415) specimens. Secondly, the paper demonstrates the improvement of fatigue strength using CSP. Note that this is the first evidence to compare CSP with shot peening.
2. Experimental facilities and procedures Fig. 1 shows the experimental set-up of the cavitating jet apparatus for CSP. The test chamber was filled with tap water. The specimen was mounted horizontally along the motor axis for both transverse and rotational movements. The cavitating jet would then impinge the specimen at 90° to its axis, while the motor gave a scanning speed of 1 mm/s. A water jet pressurized by a plunger pump was passed through a nozzle having a throat diam-
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D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
n t⫽ . v
Fig. 1.
Cavitating jet apparatus for CSP.
eter of 1.8 mm. The upstream and downstream pressures at the nozzle were measured by the pressure transducers. The cavitating conditions for the test specimen were as shown in Table 1. The main parameter for cavitating jet was the cavitation number, which is defined as a measure of the resistance of the flow to cavitation [13,14]. In nozzles and orifices, the flow velocity depends on the pressure difference between upstream and downstream pressures. Hence the cavitating number s is given by: s⫽
p2⫺pv p1⫺p2
(1)
where p1, p2, and pv are upstream, downstream and vapour pressures, respectively. Since p 1 ⬎ p 2 ⬎ pv, Eq. (1) can be rewritten as: s⫽
p2 . p1
(2)
The maximum intensity of the cavitation impacts induced by the cavitating jet occurs at s = 0.014, with constant pressure difference, ⌬p = p1⫺p2 [4]. Therefore the cavitating condition with s = 0.014 was chosen. The standoff distance s, a variable of major concern, is defined as the distance between the upstream corner of the nozzle throat and the surface of the specimen under test. Hence, the optimum standoff distance sopt was determined qualitatively by the erosion test changing with standoff distance. The residual stress was measured at different exposure time per unit length t to determine the optimum scanning speed vopt. Thus, the exposure time per unit length is expressed as the ratio of number of scans n to scanning speed v.
(3)
In this paper, the processing times of 2, 20, and 100 s/mm were used to deduce the exposure effects. The tested material was carbonized chrome-molybdenum alloy steel (JIS SCM415) with a standoff distance of 55 mm. The shape of specimen was designed according to Japanese Industrial Standards (JIS Z2274), as shown in Fig. 2. The chemical composition of experimental specimen is as shown in Table 1. The test specimen was first heat-treated at 1193 K for 3.5 h and then at 1123 K for 30 min, and oil-quench hardened. It was annealed at 443 K for 2 h. This gave a Rockwell hardness HRC of 58– 62 and hardness layer of 0.45–0.8 mm. For the determination of compressive residual stress changing with depth, a blind slot of 3 × 5 mm on test specimen was removed by electrochemical polishing. The fatigue strength was evaluated by a rotating bending fatigue test. In order to evaluate the peening effect, the residual stress sR in the surface was measured by a side inclination method using X-ray diffraction. The stress was measured parallel to the longitudinal direction of the specimen, since the direction of the applied maximum bending stress during the rotating bending fatigue test was in this direction. A Cr tube operated at 30 kV and 8 mA was used for producing Ka1 X-rays. The angle of the solar slit was 1° and the slit of 3 × 5 mm. X-rays were counted for 2 s at each step using a scintillation counter at angles of y = 11.8, 21.8, 26.8, 31.8, 41.8 and 51.8°. Here, y is the angle between the normal to the specimen surface and the normal to the diffractive face. The lattice plane hkl was (211). The diffractive angle 2q without strain was 156.4° and the stress factor of the Xray diffraction method was –318.0 MPa/deg. The diffractive angle was determined by the half value method. Residual stress was calculated using the sin2q⫺sin2y method, i.e. the gradient of the line from six points on the diagram using the least square method. For the purpose of comparing the peening effect on shot peened and CSP specimens, an A-type Almen strip, 76 × 19 × 1.3 mm thick was scanned at a speed of 20 mm/min. Then the arc height was measured using an
Table 1 Composition of JIS SCM415 by weight % C
Si
Mn
P
S
Cu
Ni
Cr
Mo
0.16
0.18
0.64
0.012
0.018
0.019
0.11
0.99
0.15
Fig. 2. Geometry of test specimen for rotating bending fatigue test.
D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
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Table 2 Cavitation shotless peening condition for JIS SCM415 Upstream pressure (MPa)
Downstream pressure (MPa)
Cavitation number
Nozzle diameter (mm)
Standoff distance (mm)
30
0.42
0.014
1.8
55
Table 3 Shot peening condition for JIS SCM415 Project amount (kg/min)
Shot pressure (MPa)
Peening Standoff time (sec) distance (mm)
Arc height (mmA)
Coverage (%)
13.5
0.3
18
0.55
300
200
Almen gauge according to the required standard [15]. The shot peening material was casting steel of diameter 0.6 mm (SAE S280). Tables 2 and 3 show the CSP and shot peening conditions, respectively, for the test specimen.
3. Results In order to determine the appropriate processing time, the residual stress on the surface was measured changing with processing time. The initial specimen in the nonpeened condition had a tensile residual stress of 240 MPa on the surface. When the specimen was subjected to CSP condition at different exposure times, a gradual receding plot was obtained as shown in Fig. 3. There was an increase of residual stress up to ⫺560 MPa. After a processing time of 10 s/mm, the residual stress on the surface reached the saturation level.
Fig. 4.
Surface roughness vs processing time.
Fig. 4 shows the plots of the surface roughness Ra vs. the processing time of the test specimen. The values of the surface roughness for the tested specimen are shown in Table 4. The surface roughness of peened specimen by CSP decreased with processing time up to 5 s/mm. Consequently, there was a gradual increment of surface roughness value up to processing times of 20 s/mm. In the limits of 20–60 s/mm, the surface roughness value was uniform. After a processing time of 60 s/mm, the surface roughness increased sharply. Fig. 5 shows the surface micrographs for the non-peened, shot peened and CSP specimens treated at different exposure times at a magnification of 50×. The surface roughness value of the specimen treated by CSP at low exposure times was the same as the non-peened specimen. This is because CSP only caused plastic deformation on the material surface and not morphological damage. From Table 4 and Fig. 5, it is shown that shot-peened specimen had the roughest and most uneven surface finish. Fig. 6 shows the compressive residual stress changing with depth z. At depth z = 0 µm, i.e. on the specimens surface, the residual stress for CSP treated specimens were ⫺359, ⫺560 and ⫺566 MPa for processing times of 2, 20 and 100 s/mm, respectively. The shot peened specimen had a surface residual stress of ⫺386 MPa. Thus, the skin effect was better in CSP specimens than in Table 4 Surface roughness values for tested specimen
Fig. 3.
Effect of processing time on residual stress.
Specimen
Roughness value Ra (µm)
Non-peened Shot peened CSP t = 2s / mm CSP t = 20 CSP t = 100
0.25 1.26 0.23 0.25 0.49
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D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
Fig. 5. Micrographs of tested specimen.
Fig. 6.
Residual stress vs. depth from the surface.
shot-peened specimen. In CSP, the compressive residual stress is maximum on the surface due to the pressure distribution of cavitation impacts. One of the reasons is that the pressure distribution of individual cavitation impact is cone-shaped with maximum intensity at the cone centre. We have also shown that the cavitation bubbles consist of multiple continuous shots; large shots responsible for high compressive residual stress and vice versa [12]. In the case of the shot peened specimen, the maximum residual stress was attained at a depth z = 75 µm. This is due to the fact that the maximum residual stress occurs below the center of the arc radius of each dimple. Okada et al. [16] have shown that the spherical indenter of shot peening produces Hertzian stress distribution. In order to determine the fatigue strength, the fatigue limit was considered to be at 107 cycles for each
exposure time as shown in Fig. 7. The specimens which failed as a result of inclusions are shaded black, while those which failed due to surface defects are unshaded. Applying Little’s method on estimating the median fatigue limit [17], CSP specimen treated at 2, 20 and 100 s/mm had an increase in fatigue limit by 79, 88 and 99 MPa, respectively in comparison with non-peened specimen. This accounted for more than 11% increase in fatigue limits. Similarly, the fatigue limit was a function of exposure time, i.e. longer exposure time had higher fatigue limit. The increase in fatigue limit was due to delay in crack initiation and retardation of fatigue crack growth by the residual compressive stress induced as a result of cavitation bubble collapse. The shot peened specimen gave an increment of 52 MPa (7%) in fatigue limit compared to the non-peened specimen despite the fact that it had a higher maximum compressive residual stress of 1189 MPa.
Fig. 7.
Cyclic-stress curves.
D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
Fig. 8 shows the fractographs of the non-peened, shot peened and CSP specimens. Fracture in all specimens were examined optically to ascertain the fatigue origin locations relative to the processed surface. The fatigue failures occurred near or at the centre of the specimens’ gauge length. Fatigue cracks are known to nucleate when the applied stress is higher than the fatigue limit. The presence of the surface flaw either as intrusions marked ‘I’ or as extrusions marked ‘E’ (see Fig. 8) provided sites for crack initiation. From the fractographs, it can be seen that the fatigue crack on the non-peened specimen were initiated at the subsurface. Macropersistent
Fig. 8.
1221
slip bands (PSB) were visible in the protruded form (see Fig. 8a,b) or as a snapping intrusion, marked ‘C’ (see Fig. 8g). Also, local striations occurred in the annular regions marked ‘S’ on Fig. 8. The shot peened specimen had its crack originating on the surface leading to the intrusion ‘I’ (see Fig. 8d). This is due to the surface unevenness and ‘stress-raising’ regions caused by the shot dimples. The CSP specimens had crack initiated from the sub-surface as shown in Fig. 8e–h. The shot peened specimen had shorter fatigue limit in comparison to CSP specimens due to the surface roughness and low residual stress on the surface.
Fractographs of tested specimen.
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D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
4. Conclusions In the present study, a high-speed submerged water jet, cavitating jet, was used to produce cavitation bubbles. Upon collapse, the bubbles produced shock waves and micro-jet which impinged the carbonized chrome-molybdenum steel alloy (JIS SCM415) specimen to introduce compressive residual stress on the surface. The key points would be summarized as follows: 1. There was a rapid increase, as a function of time, in compressive residual stress up to the saturation level. Thus, longer processing times had profound effect on the near-surface microstructure of the material. 2. The rotating beam fatigue strength of carbonized chrome-molybdenum steel alloy (JIS SCM415) increased by 7 and 11% for shot peened and CSP specimens, respectively, in comparison with nonpeened specimen. The high increase in fatigue strength for the CSP specimen was attributed by good surface finish, thereby impeding the initiation and development of surface cracks. Shot peened specimen had lower fatigue strength in comparison to CSP due to the microcracks developed during peening and could only retard crack development. 3. CSP can introduce compressive residual stress on the material thereby prolonging its fatigue life. However, the intensity of residual stress is not the only parameter governing the fatigue strength, but surface finish plays a more significant role in extending the fatigue life of components as evidenced in the comparison of shot peened and CSP specimens.
Acknowledgements This work was partly supported by the Japan Society for the Promotion of Science under the Grant-in-aid for Scientific Research (B) (2) 13555022 and 14350049.
References [1] Besant WH. In: A treatise on hydromechanics. Part II: Hydromechanics. London: G Bell and Sons; 1913. p. 30–9. [2] Tomita Y, Shima A. Mechanisms of impulsive pressure generation and damage pit formation by bubble collapse. J Fluid Mech 1986;169:535–64. [3] Bourne NK, Field JE. A high-speed photographic study of cavity damage. J Appl Phys 1995;78:4425–7. [4] Soyama H. Material testing and surface modification by using cavitating jet. J Soc Mater Sci Japan 1998;47(4):381–7 [In Japanese]. [5] Soyama H, Kumano H. The fundamental threshold level—a new parameter for predicting cavitation erosion resistance. J Test Eval (in press). [6] Young FR. In: Cavitation. Imperial College Press; 1999. p. 187–290. [7] Soyama H. Improvement in fatigue strength of silicon manganese steel SUP7 by using a cavitating jet. JSME Int J: (Ser. A) 2000;43:173–8. [8] Soyama H, Park JD, Saka M. Use of cavitating jet for introducing compressive residual stress. Trans ASME J Manuf Sci Eng 2000;122:83–9. [9] Soyama H, Kusaka T, Saka M. Peening by the use of cavitation impacts for the improvement of fatigue strength. J Mater Sci Lett 2001;20:1263–5. [10] Soyama H, Saito K, Saka M. Improvement of fatigue strength of aluminium alloy by cavitation shotless peening. Trans ASME J Eng Mater Technol 2002;124:135–9. [11] Soyama H, Odhiambo D, Saito K. Cavitation shotless peening for improvement of fatigue strength. 8th International Conference on Shot Peening, 2002. p. 435–40. [12] Soyama H, Sasaki K, Odhiambo D, Saka M. Cavitation shotless peening for surface improvement of alloy tool steel. JSME Int J 2003;46(3):398–402. [13] Yamauchi Y, Soyama H, Adachi Y, Sato K, Shindo T, Oba R, Oshima R, Yamabe M. The suitable region of high-speed submerged water-jets for cutting and peening. JSME Int J: (Ser. B) 1995;38:31–8. [14] Brennen CE. In: Cavitation and bubble dynamics. Oxford University Press; 1995. p. 78–85. [15] SAE J442. Test strip, holder and gage shot peening, 1979;1–2. [16] Okada T, Suzuki T, Hattori S. Pressure distribution on material surface due to cavitation bubble collapses. In: Proceedings of the 9th Symposium on Cavitation. 1997. p. 30–1 [In Japanese]. [17] Little RE. Estimating the median fatigue limit for very small up and down quantal response tests and for S-N data with runouts. Probabilistic aspects of fatigue, ASTM STP 511. American Society for Testing and Materials; 1972. p. 29–42.
Cavitation shotless peening for improvement of fatigue strength of carbonized steel Dan Odhiambo, Hitoshi Soyama ∗ Tohoku University, Graduate School of Engineering, Department of Mechanical Engineering, Aoba-yama 01, Aoba-ku, Sendai 980-8579, Japan
Abstract Cavitation shotless peening (CSP) is a surface enhancement technique which makes use of cavitation impact to induce compressive residual stress on the metallic materials, thereby increasing the fatigue life of components. The technique is similar to shot peening except that shots are not used and that is why we refer to it as shotless peening. In case of CSP a submerged high-speed water jet with cavitation, i.e. a cavitating jet is used. To explore the potentials of CSP as a means of improving fatigue strength, carbonized chrome-molybdenum alloy steel (JIS SCM415) has been analyzed in the non-peened, shot-peened and CSP conditions with respect to processing times, residual stress and cyclic-stress curves. The residual stress was measured by an X-ray diffraction method. Experimental results confirmed that the rotating beam fatigue strength of a CSP specimen was stronger than non-peened and shotpeened specimens by 79 and 27 MPa, respectively. 2003 Elsevier Ltd. All rights reserved. Keywords: Peening; Residual stress; Fatigue strength; Cavitation; Processing time
1. Introduction Cavitation impacts have been intensely investigated using hydraulic machinery. Experimental interest stems from the time it was discovered that cavitation was a cause of erosion in ship propellers [1]. Most studies on cavitation have focused on the damage mechanism behind the erosion problem [2–6]. In this study, we discuss the turn-around point of cavitation impacts and focus on the positive engineering applications. Cavitation shotless peening is a new technology for surface enhancement of metallic materials. In CSP, cavitation impact is induced by a submerged high-speed water jet with cavitation, i.e. a cavitating jet. The collapsing cavitation bubble causes plastic deformation on the surface, thereby creating a compressive layer which impedes fatigue crack initiation, as well as improves the strength of the material. The cavitating jet intensity and the occurring region are controlled with parameters such as upstream pressure and nozzle size. Soyama et al. [7– 9] discovered the optimum cavitating jet conditions for
Corresponding author. Tel.: +81-22-217-6891; fax: +81-22-2176893. E-mail address: [email protected] (H. Soyama). ∗
0142-1123/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0142-1123(03)00121-X
introducing compressive residual stress in metallic materials. The authors have already proved that the surface finish of CSP specimen are known to be of a better quality finish than shot-peened specimen due to the fact that there are no solid body collisions [10–12]. This paper is in two parts: First, we illustrate the effects of processing times on the residual stress and effect of residual stress changing with depth by comparing the non-peened, shot-peened and cavitation shotless peened carbonized chrome-molybdenum alloy steel (JIS SCM415) specimens. Secondly, the paper demonstrates the improvement of fatigue strength using CSP. Note that this is the first evidence to compare CSP with shot peening.
2. Experimental facilities and procedures Fig. 1 shows the experimental set-up of the cavitating jet apparatus for CSP. The test chamber was filled with tap water. The specimen was mounted horizontally along the motor axis for both transverse and rotational movements. The cavitating jet would then impinge the specimen at 90° to its axis, while the motor gave a scanning speed of 1 mm/s. A water jet pressurized by a plunger pump was passed through a nozzle having a throat diam-
1218
D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
n t⫽ . v
Fig. 1.
Cavitating jet apparatus for CSP.
eter of 1.8 mm. The upstream and downstream pressures at the nozzle were measured by the pressure transducers. The cavitating conditions for the test specimen were as shown in Table 1. The main parameter for cavitating jet was the cavitation number, which is defined as a measure of the resistance of the flow to cavitation [13,14]. In nozzles and orifices, the flow velocity depends on the pressure difference between upstream and downstream pressures. Hence the cavitating number s is given by: s⫽
p2⫺pv p1⫺p2
(1)
where p1, p2, and pv are upstream, downstream and vapour pressures, respectively. Since p 1 ⬎ p 2 ⬎ pv, Eq. (1) can be rewritten as: s⫽
p2 . p1
(2)
The maximum intensity of the cavitation impacts induced by the cavitating jet occurs at s = 0.014, with constant pressure difference, ⌬p = p1⫺p2 [4]. Therefore the cavitating condition with s = 0.014 was chosen. The standoff distance s, a variable of major concern, is defined as the distance between the upstream corner of the nozzle throat and the surface of the specimen under test. Hence, the optimum standoff distance sopt was determined qualitatively by the erosion test changing with standoff distance. The residual stress was measured at different exposure time per unit length t to determine the optimum scanning speed vopt. Thus, the exposure time per unit length is expressed as the ratio of number of scans n to scanning speed v.
(3)
In this paper, the processing times of 2, 20, and 100 s/mm were used to deduce the exposure effects. The tested material was carbonized chrome-molybdenum alloy steel (JIS SCM415) with a standoff distance of 55 mm. The shape of specimen was designed according to Japanese Industrial Standards (JIS Z2274), as shown in Fig. 2. The chemical composition of experimental specimen is as shown in Table 1. The test specimen was first heat-treated at 1193 K for 3.5 h and then at 1123 K for 30 min, and oil-quench hardened. It was annealed at 443 K for 2 h. This gave a Rockwell hardness HRC of 58– 62 and hardness layer of 0.45–0.8 mm. For the determination of compressive residual stress changing with depth, a blind slot of 3 × 5 mm on test specimen was removed by electrochemical polishing. The fatigue strength was evaluated by a rotating bending fatigue test. In order to evaluate the peening effect, the residual stress sR in the surface was measured by a side inclination method using X-ray diffraction. The stress was measured parallel to the longitudinal direction of the specimen, since the direction of the applied maximum bending stress during the rotating bending fatigue test was in this direction. A Cr tube operated at 30 kV and 8 mA was used for producing Ka1 X-rays. The angle of the solar slit was 1° and the slit of 3 × 5 mm. X-rays were counted for 2 s at each step using a scintillation counter at angles of y = 11.8, 21.8, 26.8, 31.8, 41.8 and 51.8°. Here, y is the angle between the normal to the specimen surface and the normal to the diffractive face. The lattice plane hkl was (211). The diffractive angle 2q without strain was 156.4° and the stress factor of the Xray diffraction method was –318.0 MPa/deg. The diffractive angle was determined by the half value method. Residual stress was calculated using the sin2q⫺sin2y method, i.e. the gradient of the line from six points on the diagram using the least square method. For the purpose of comparing the peening effect on shot peened and CSP specimens, an A-type Almen strip, 76 × 19 × 1.3 mm thick was scanned at a speed of 20 mm/min. Then the arc height was measured using an
Table 1 Composition of JIS SCM415 by weight % C
Si
Mn
P
S
Cu
Ni
Cr
Mo
0.16
0.18
0.64
0.012
0.018
0.019
0.11
0.99
0.15
Fig. 2. Geometry of test specimen for rotating bending fatigue test.
D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
1219
Table 2 Cavitation shotless peening condition for JIS SCM415 Upstream pressure (MPa)
Downstream pressure (MPa)
Cavitation number
Nozzle diameter (mm)
Standoff distance (mm)
30
0.42
0.014
1.8
55
Table 3 Shot peening condition for JIS SCM415 Project amount (kg/min)
Shot pressure (MPa)
Peening Standoff time (sec) distance (mm)
Arc height (mmA)
Coverage (%)
13.5
0.3
18
0.55
300
200
Almen gauge according to the required standard [15]. The shot peening material was casting steel of diameter 0.6 mm (SAE S280). Tables 2 and 3 show the CSP and shot peening conditions, respectively, for the test specimen.
3. Results In order to determine the appropriate processing time, the residual stress on the surface was measured changing with processing time. The initial specimen in the nonpeened condition had a tensile residual stress of 240 MPa on the surface. When the specimen was subjected to CSP condition at different exposure times, a gradual receding plot was obtained as shown in Fig. 3. There was an increase of residual stress up to ⫺560 MPa. After a processing time of 10 s/mm, the residual stress on the surface reached the saturation level.
Fig. 4.
Surface roughness vs processing time.
Fig. 4 shows the plots of the surface roughness Ra vs. the processing time of the test specimen. The values of the surface roughness for the tested specimen are shown in Table 4. The surface roughness of peened specimen by CSP decreased with processing time up to 5 s/mm. Consequently, there was a gradual increment of surface roughness value up to processing times of 20 s/mm. In the limits of 20–60 s/mm, the surface roughness value was uniform. After a processing time of 60 s/mm, the surface roughness increased sharply. Fig. 5 shows the surface micrographs for the non-peened, shot peened and CSP specimens treated at different exposure times at a magnification of 50×. The surface roughness value of the specimen treated by CSP at low exposure times was the same as the non-peened specimen. This is because CSP only caused plastic deformation on the material surface and not morphological damage. From Table 4 and Fig. 5, it is shown that shot-peened specimen had the roughest and most uneven surface finish. Fig. 6 shows the compressive residual stress changing with depth z. At depth z = 0 µm, i.e. on the specimens surface, the residual stress for CSP treated specimens were ⫺359, ⫺560 and ⫺566 MPa for processing times of 2, 20 and 100 s/mm, respectively. The shot peened specimen had a surface residual stress of ⫺386 MPa. Thus, the skin effect was better in CSP specimens than in Table 4 Surface roughness values for tested specimen
Fig. 3.
Effect of processing time on residual stress.
Specimen
Roughness value Ra (µm)
Non-peened Shot peened CSP t = 2s / mm CSP t = 20 CSP t = 100
0.25 1.26 0.23 0.25 0.49
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Fig. 5. Micrographs of tested specimen.
Fig. 6.
Residual stress vs. depth from the surface.
shot-peened specimen. In CSP, the compressive residual stress is maximum on the surface due to the pressure distribution of cavitation impacts. One of the reasons is that the pressure distribution of individual cavitation impact is cone-shaped with maximum intensity at the cone centre. We have also shown that the cavitation bubbles consist of multiple continuous shots; large shots responsible for high compressive residual stress and vice versa [12]. In the case of the shot peened specimen, the maximum residual stress was attained at a depth z = 75 µm. This is due to the fact that the maximum residual stress occurs below the center of the arc radius of each dimple. Okada et al. [16] have shown that the spherical indenter of shot peening produces Hertzian stress distribution. In order to determine the fatigue strength, the fatigue limit was considered to be at 107 cycles for each
exposure time as shown in Fig. 7. The specimens which failed as a result of inclusions are shaded black, while those which failed due to surface defects are unshaded. Applying Little’s method on estimating the median fatigue limit [17], CSP specimen treated at 2, 20 and 100 s/mm had an increase in fatigue limit by 79, 88 and 99 MPa, respectively in comparison with non-peened specimen. This accounted for more than 11% increase in fatigue limits. Similarly, the fatigue limit was a function of exposure time, i.e. longer exposure time had higher fatigue limit. The increase in fatigue limit was due to delay in crack initiation and retardation of fatigue crack growth by the residual compressive stress induced as a result of cavitation bubble collapse. The shot peened specimen gave an increment of 52 MPa (7%) in fatigue limit compared to the non-peened specimen despite the fact that it had a higher maximum compressive residual stress of 1189 MPa.
Fig. 7.
Cyclic-stress curves.
D. Odhiambo, H. Soyama / International Journal of Fatigue 25 (2003) 1217–1222
Fig. 8 shows the fractographs of the non-peened, shot peened and CSP specimens. Fracture in all specimens were examined optically to ascertain the fatigue origin locations relative to the processed surface. The fatigue failures occurred near or at the centre of the specimens’ gauge length. Fatigue cracks are known to nucleate when the applied stress is higher than the fatigue limit. The presence of the surface flaw either as intrusions marked ‘I’ or as extrusions marked ‘E’ (see Fig. 8) provided sites for crack initiation. From the fractographs, it can be seen that the fatigue crack on the non-peened specimen were initiated at the subsurface. Macropersistent
Fig. 8.
1221
slip bands (PSB) were visible in the protruded form (see Fig. 8a,b) or as a snapping intrusion, marked ‘C’ (see Fig. 8g). Also, local striations occurred in the annular regions marked ‘S’ on Fig. 8. The shot peened specimen had its crack originating on the surface leading to the intrusion ‘I’ (see Fig. 8d). This is due to the surface unevenness and ‘stress-raising’ regions caused by the shot dimples. The CSP specimens had crack initiated from the sub-surface as shown in Fig. 8e–h. The shot peened specimen had shorter fatigue limit in comparison to CSP specimens due to the surface roughness and low residual stress on the surface.
Fractographs of tested specimen.
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4. Conclusions In the present study, a high-speed submerged water jet, cavitating jet, was used to produce cavitation bubbles. Upon collapse, the bubbles produced shock waves and micro-jet which impinged the carbonized chrome-molybdenum steel alloy (JIS SCM415) specimen to introduce compressive residual stress on the surface. The key points would be summarized as follows: 1. There was a rapid increase, as a function of time, in compressive residual stress up to the saturation level. Thus, longer processing times had profound effect on the near-surface microstructure of the material. 2. The rotating beam fatigue strength of carbonized chrome-molybdenum steel alloy (JIS SCM415) increased by 7 and 11% for shot peened and CSP specimens, respectively, in comparison with nonpeened specimen. The high increase in fatigue strength for the CSP specimen was attributed by good surface finish, thereby impeding the initiation and development of surface cracks. Shot peened specimen had lower fatigue strength in comparison to CSP due to the microcracks developed during peening and could only retard crack development. 3. CSP can introduce compressive residual stress on the material thereby prolonging its fatigue life. However, the intensity of residual stress is not the only parameter governing the fatigue strength, but surface finish plays a more significant role in extending the fatigue life of components as evidenced in the comparison of shot peened and CSP specimens.
Acknowledgements This work was partly supported by the Japan Society for the Promotion of Science under the Grant-in-aid for Scientific Research (B) (2) 13555022 and 14350049.
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