Mechanical and Tribological Properties of Shot-Peened SAE 1070 ...

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However, the shot- peened steel strips at shot peening pressures less than 345 kPa did not exhibit better wear resistance than the as-received steel strip, ...
Tribology Transactions

ISSN: 1040-2004 (Print) 1547-397X (Online) Journal homepage: http://www.tandfonline.com/loi/utrb20

Mechanical and Tribological Properties of ShotPeened SAE 1070 Steel N. W. Khun, P. Q. Trung & D. L. Butler To cite this article: N. W. Khun, P. Q. Trung & D. L. Butler (2016): Mechanical and Tribological Properties of Shot-Peened SAE 1070 Steel, Tribology Transactions, DOI: 10.1080/10402004.2015.1121313 To link to this article: http://dx.doi.org/10.1080/10402004.2015.1121313

Accepted author version posted online: 18 Feb 2016. Published online: 06 Jul 2016. Submit your article to this journal

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Date: 06 August 2016, At: 20:56

TRIBOLOGY TRANSACTIONS http://dx.doi.org/10.1080/10402004.2015.1121313

Mechanical and Tribological Properties of Shot-Peened SAE 1070 Steel N. W. Khuna, P. Q. Trunga,b, and D. L. Butlera,b

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a School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore; bAdvanced Remanufacturing and Technology Centre, Singapore

ABSTRACT

ARTICLE HISTORY

The effects of shot peening pressure on the mechanical and tribological properties of shot-peened SAE 1070 steel strips were systematically investigated. The surface hardness of the shot-peened steel strips significantly increased with increased shot peening pressure due to the promoted cold work-hardening effect. The tribological results showed that the increased surface roughness of the shot-peened steel strips associated with increased shot peening pressure resulted in their increased friction by enhancing mechanical interlocking between two rubbing surfaces. The wear of the shot-peened steel strips decreased with increased shot peening pressure via their increased surface hardness. However, the shotpeened steel strips at shot peening pressures less than 345 kPa did not exhibit better wear resistance than the as-received steel strip, indicating that a certain intensity of shot peening was required to improve the wear resistance of the shot-peened steel strips. It could be concluded that the mechanical and tribological properties of the shot-peened steel strips were significantly influenced by the shot peening pressure.

Received 18 June 2015 Accepted 11 December 2015

Introduction The major failures of engineering materials such as fatigue fracture, fretting fatigue, wear, corrosion, etc., are very sensitive to the structures and properties of the material surfaces. In most cases, material failures originate from the surfaces, especially surface defects such as fatigue cracks (Ke, et al. (1); Khellouki, et al. (2); Lang, et al. (3). Therefore, surface treatments are generally applied to metallic materials to improve service life and efficiency. Generally, surface treatments are studied within the branches of mechanical and thermal surface treatments. Mechanical surface treatments cover a wide range of processes such as shot peening, laser shock peening, deep drawing, burnishing, sand blasting, and brush cleaning processes (Ke, et al. (1); Khellouki, et al. (2); Lang, et al. (3). Shot peening is a well-established mechanical surface treatment enhancing the resistance of metallic components that are exposed to cyclic loading, wear, and corrosion under applied stress (Miao and Demers (4); Luong and Hill (5); Hatamleh (6). In addition, shot peening is widely applied on gears, cams, camshafts, crankshafts, clutch springs, coil springs, connecting rods, gearwheels, leaf and suspension springs, rock drills, turbine blades, and so on (Miao and Demers (4); Luong and Hill (5); Hatamleh (6). During the shot peening process, a large amount of shot peening media with a high velocity impacts metal components so that it induces a compressive residual stress field in near-surface layers, which in turn impedes initiation and propagation of microcracks. Shot peening basically modifies the surface and subsurface parameters of metal components such as surface texture or surface topography, surface hardness or

Shot peening; SAE 1070 steel; hardness; friction; wear

subsurface dislocation, and microstructure modification and residual stress condition. However, shot peening treatment is complex, with different process parameters (Miao and Demers (4); Luong and Hill (5); Hatamleh (6). Many different process parameters need to be reconciled to make shot peening treatment most successful, efficient, and reliable. Shot peening is generally considered simple to handle for the majority of steel parts made of less sophisticated steel grades. For higher performance metallic materials such as high-strength steels and titanium, aluminum, and magnesium alloys, mechanical surface treatment with shot peening needs to be understood in more detail (Miao and Demers (4); Luong and Hill (5); Hatamleh (6). Mitrovic, et al. (7) investigated the tribological properties of shot-peened 36CrNiMo4 and 36NiCrMo16 steels and found that their wear resistance was improved after shot peening. Singh and Mondal (8) reported that shot peening could further reduce wear of quenched and tempered SAE 6150 steel. It was reported (Ohba, et al. (9) that rolling contact fatigue wear of tempered ductile iron under line contact condition was improved by shot peening. Allison, et al. (10) performed rolling contact fatigue tests of AISI M50 bearing balls and discovered that the balls with residual compressive stresses showed lower rolling contact fatigue wear than those without such residual stresses. Girish, et al. (11) proved that shot peening gave rise to less wear-induced surface deterioration of shot-peened EN24 steel spur gears than that of untreated ones. Kuruschov (12) and Richardson (13) reported that improved hardness of metals by cold work-hardening had little effect on wear resistance. However, data on the tribological properties of shot-peened steels are

CONTACT N. W. Khun [email protected] Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/utrb. Review led by Daniel Nelias © 2016 Society of Tribologists and Lubrication Engineers

KEYWORDS

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Table 1. Chemical composition of SAE 1070 steel. C

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»0.73

Si

Mn

S

P

»0.18

»0.65

»0.002

»0.011

not satisfactorily available. Among steels, SAE 1070 steel is widely used in automotive, agricultural, and aerospace industries due to its high strength, durability, and flexibility. Therefore, an understanding of a correlation between shot peening effects of SAE 1070 steel and its mechanical and tribological properties is important for successful industrial applications. In this study, SAE 1070 steel strips were shot peened at different shot peening pressures of 207–552 kPa to investigate their mechanical and tribological properties with respect to shot peening pressure. The surface hardness of the shot-peened steel strips was measured using a Vickers hardness test and their friction and wear were investigated using a ball-on-disc microtribological test.

Experimental details In this study, SAE 1070 steel (Peening Accessories) was selected as the material for all shot peening experiments. The nominal chemical composition of the SAE 1070 steel is shown in Table 1. Prior to the shot peening process, shot peening intensities at different shot peening pressures of 207–552 kPa were identified by employing standard Almen test strip A (SAE 1070) with a size of 76.1 mm £ 18.95 mm £ 1.29 mm, Almen gauge (Alemn Gages, Peening Accessories), and holding fixture. After shot peening with various cycles T, arc heights of SAE 1070 steel strips were plotted with respect to exposure time using PA2 software to obtain a saturation curve and determine the shot peening intensity. All of the steel strips were shot peened for 120 s because the saturation point was achieved before 120 s. Shot peening was carried out using standard S230 steel shots with a mean diameter of about 600 mm at an impact angle of 90 , rotating table speed of 1 /s, and horizontal nozzle speed of 150 mm/s under different shot peening pressures. Air blast shot peening equipment (Abrasive Engineering Pte. Ltd.) was used for shot peening. The resulting shot peening intensities are presented in Table 2. Visual inspection was employed to determine the coverage of shot-peened steel strips. It showed 100% coverage of the shot-peened surfaces for all shot peening pressures. The surface roughness of the samples was measured using surface profilometry (Talyscan 150) with a diamond stylus of 4 mm diameter. Three-dimensional surface roughness parameters such as root mean squared surface roughness (Sq), maximum peak height (Sp), maximum valley depth (Sv), and

maximum height of the profile (St) were obtained as the average of three measurements per sample. The maximum arc height of the samples with a size of 76.1 mm £ 18.95 mm £ 1.29 mm was measured using surface profilometry. A Vickers hardness tester was used to measure the hardness of the samples. The total normal load was changed from 100 g (0.98 N) to 300 g (2.94 N) and the average hardness value was taken from five indentation measurements on each sample under each normal load. For the cross-sectional observation, the samples were ground using 320-grit papers followed by chemical–mechanical polishing with DraPo solution containing 9-mm diamond particles and oxide polishing suspension (OP-S) suspension solution containing 0.04-mm colloidal silica particles. Then, the polished samples were etched with 4% Nital and dried with compressed air. The tribological properties of the samples were investigated using a ball-on-disc microtribometer (CSM High Temperature) by sliding against alumina (Al2O3) and 100Cr6 steel balls of 6 mm in diameter in a circular path of 1.5 mm in radius for 40,000 laps at a sliding speed of 5 cm/s under different normal loads at laboratory temperature (»22–24 C). Three wear tests per sample were carried out to get an average friction coefficient. The average wear volume was calculated by measuring the width and depth of wear tracks using surface profilometry.

Results and discussion Figure 1 shows an overview of the spherical-shaped S230 steel shots, from which it is found that the shots generally have smooth surfaces. Figure 2 shows the maximum arc height of the shot-peened SAE 1070 steel strips as a function of shot peening pressure. As shown in Fig. 2, the maximum arc height of the shot-peened SAE 1070 steel strips significantly increases from 0.94 to 1.44 mm with increased shot peening pressure from 207 to 552 kPa although the as-received SAE 1070 steel strip does not have any measurable arc height. The measured arc height of the SAE 1070 steel strips is attributed to their convex deformation, which means that their topmost layers have been

Table 2. Shot peening intensities measured at different shot peening pressures. Pressure (kPa) 207 276 345 414 483 552

Intensity 14.6 A 17.2 A 19.4 A 22.9 A 23.3 A 25.1 A

Figure 1. SEM micrograph showing an overview of S230 steel shots.

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Figure 2. Maximum arc heights of shot-peened SAE 1070 steel strips at different shot peening pressures.

plastically elongated by imparting with shot peening media with sufficient energy. Therefore, the increased maximum arc height of the shot-peened SAE 1070 steel strips with increased shot peening pressure can be related to their increased convex deformation due to the increased impact energy of shot peening media (Baiker (14). A comparison of Figs. 3a, 3b, and 3c shows that dimples formed by shot peening can be apparently found on the cross sections of the shot-peened SAE 1070 steel strips with larger dimples for the higher shot peening pressure. Figure 4 presents the hardnesses of the shot-peened SAE 1070 steel strips measured under different applied loads as a function of shot peening pressure. The hardnesses of the asrecieved SAE 1070 steel strip measured under applied loads of 100, 200, and 300 g are 571.6 § 10.3, 551.4 § 16.8, and 541.9 § 5.6 Hv, respectively, which indicates that the hardness of the as-received steel strip decreases with increased applied load due to the indentation size effect (Milman, et al. (15); Gerberich, et al. (16). Increasing the shot peening pressure from 207 to 552 kPa significantly increases the hardnesses of the shotpeened SAE 1070 steel strips measured under the applied loads of 100, 200, and 300 g from about 594.1, 567.3, and 552 Hv to about 690.1, 631.8, and 612.7 Hv, respectively. The decreased hardnesses of the shot-peened steel strips with increased applied load are also indicative of the indentation size effect. Different hardnessses between the surface and subsurface caused by shot peening pronounce the indentation size effect, which is confirmed by a large variation in the hardness of the shot-peened SAE 1070 steel strip at the highest shot peening pressure of 552 kPa with applied load as shown in Fig. 4 (Milman, et al. (15); Gerberich, et al. (16). Nevertheless, the hardness of the shot-peened SAE 1070 steel strips significantly increases for all applied loads with increased shot peening pressure as a result of their increased surface hardness because the hardnesses of the shot-peened SAE 1070 steel strips are apparently higher than those of the as-received SAE 1070 steel strip probably due to the cold work-hardening process caused by shot peening (Mahagaonkar, et al. (17); Tosha, et al. (18); Zupanc and Grum (19); Chang, et al. (20); Zhan, et al. (21);

Figure 3. Optical images showing cross sections of (a) as-received SAE 1070 steel strip and shot-peened SAE 1070 steel strips at shot peening pressures of (b) 207 and (c) 552 kPa.

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Figure 4. Hardnesses of shot-peened SAE 1070 steel strips at different shot peening pressures measured under different applied loads.

Manfridini, et al. (22). In Fig. 4, the largest increment in the hardness of the shot-peened SAE 1070 steel strips measured under the lowest applied load of 100 g with increased shoot peening pressure clearly confirms that shot peening causes significantly different hardnesses between the surfaces and subsurfaces of the shot-peened steel strips. Figure 5a shows the surface morphology and topography of the as-received SAE 1070 steel strip on which a relatively smooth surface is found. Apparent dimples formed by the shot peening can be found on the surface morphologies and topographies of the shot-peened steel strips as shown in Figs. 5b–5d, and their dimples become deeper with higher shot peening pressure due to the higher intensity of the shot peening (Table 2). Figure 6 presents the Sq, Sp, Sv, and St values of the shotpeened SAE 1070 steel strips as a function of shot peening pressure. The Sq, Sp, Sv, and St values of the as-received SAE 1070 steel strip are 0.08 § 0.02 mm, 0.47 § 0.18 mm, 0.72 § 0.3 mm, and 1.18 § 0.52 mm, respectively. The Sq, Sp, Sv, and St values of the shot-peened SAE 1070 steel strips significantly increase from about 2.62, 9.16, 8.41, and 7.53 mm to about 4.16, 15.92, 10.99, and 26.93 mm, respectively, with increased shot peening pressure from 207 to 552 kPa. It is clear that the Sq, Sp, Sv, and St values of the shot-peened steel strips are significantly larger than those of the as-received steel strip. This indicates that the shot peening apparently roughens the surfaces of the steel strips, which is in agreement with the rougher surfaces of the shot-peened steel strips with dimples than that of the asreceived steel strip (Fig. 5). The increased Sq value of the shotpeened steel strips with increased shot peening pressure (Fig. 6) is an indication of their increased surface roughness, which is confirmed by the apparently deeper dimples on the surfaces of the shot-peened steel strips at the higher shot peening pressures. The larger Sp, Sv, and St values of the shot-peened steel strips at the higher shot peening pressures (Fig. 6) can be correlated to the significantly higher peaks and deeper valleys on their surfaces formed by the deeper dimples (Fig. 5). Figure 7 shows the EDX spectra of the as-received and shotpeened SAE 1070 steel strips. The Fe, Mn, O, and C peaks are mainly detected in all of the EDX spectra of the SAE 1070 steel strips. The surface O content of the steel strips significantly increases from 7 to 16.3 at% with increased shot peening

pressure from 0 to 552 kPa. The possible reason is that the shot peening activates the surface of the steel strip so that the shotpeened steel strip at the higher shot peening pressure has higher surface activity with oxygen for its higher surface O content (Neumann (23); Lacroix, et al. (24); Livingston and Swingley (25); Cioffi, et al. (26). In addition, deeper dimples on the surface of the shot-peened steel strip at the higher shot peening pressure can contribute to its higher surface O content due to the larger surface area to react with oxygen. Figure 8 presents the wear volumes of the shot-peened SAE 1070 steel strips tested against ceramic and steel balls under different normal loads. The wear volumes were obtained after 40,000 laps. The wear volumes of the as-received SAE 1070 steel strips tested against the ceramic ball under 1 and 5 N are 8.5 § 0.5 £ 10¡3 mm3 and 26.9 § 3.1 £ 10¡3 mm3, respectively, indicating that the increased normal load increases the wear of the as-received steel strip (Khun, et al. (27)–(29); Tian, et al. (30); Blau (31); Bhushan (32). The wear volume of the asreceived steel stip tested against the steel ball under 5 N is 23.4 § 2.1 £ 10¡3 mm3. This shows that the as-received steel strip has lower wear for the steel ball than for the ceramic ball under the same normal load because the higher wear resistance of the ceramic ball than that of the steel ball generates the higher surface wear of the steel strip. In Fig. 8, increasing the shot peening pressure from 207 to 552 kPa decreases the wear volumes of the shot-peened SAE 1070 steel strips tested against the ceramic ball under 1 and 5 N from about 9.1 £ 10¡3 mm3 and 28.5 £ 10¡3 mm3 to about 5.4 £ 10¡3 mm3 and 23.9 £ 10¡3 mm3, respectively. It can be seen that the increased normal load significantly increases the wear of the shot-peened steel strips (Khun, et al. (27). However, increasing the shot peening pressure decreases the wear of the shot-peened steel strips due to the increased wear resistance of the steel stirps associated with their increased surface hardness (Neumann (23); Ganesh, et al. (33); Schmidt (34). This is further confirmed by the decreased wear volume of the shotpeened steel strips tested against the steel ball under 5 N from about 25.2 £ 10¡3 mm3 to about 14 £ 10¡3 mm3 with increased shot peening pressure from 207 to 552 kPa. The shot-peened steel strips at relativey lower shot peening presures exhibit slightly higher wear for all of the counterballs and normal loads than the as-received steel strip. It is therefore supposed that shot peening at lower shot peening pressures than 345 kPa does not significantly improve the resistance of the steel strips to abrasive wear caused by the repeated sliding of the counterball. Under this condition, the apparently increased surface roughness of the shot-peened steel strips promotes vibration of the sliding system and gives rise to the higher wear of the shot-peened steel strips than that of the as-received steel strip (Khun and Liu (35); Khun, et al. (36). However, shot peening at higher shot peening pressures than 345 kPa results in lower wear of the shot-peened steel strips via their sufficiently higher surface hardness, although their surface rougness is further increased by increasing the shot peening pressure. The shot-peened steel strips exhibit a larger decrement in their wear volumes with increased shot peening pressure for the steel ball than for the ceramic ball because the much lower wear resistance of the steel ball than that of the

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Figure 5. Surface morphologies and topographies of (a) as-received SAE 1070 steel strip and shot-peened SAE 1070 steel strips at shot peening pressures of (b) 207, (c) 414, and (d) 552 kPa.

ceramic ball significantly lowers the removal of surface materials of the shot-peened steel strips, especially at the higher shot peening pressures. Figure 9 presents the friction coefficients of the shot-peened SAE 1070 steel strips tested against the ceramic and steel balls under different normal loads. The friction coefficients of the

as-received steel strip tested against the ceramic ball under normal loads of 1 and 5 N are 0.56 § 0.02 and 0.51 § 0.01, respectively. Normally, an effective interfacial shear strength between two contacting surfaces can give rise to a high friction (Khun, et al. (27)–(29); Tian, et al. (30); Mate (37); Ronkainen, et al. (38). The sliding of the ceramic ball generates abrasive wear of

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Figure 6. Sq, Sp, Sv, and St values of shot-peened SAE 1070 steel strips at different shot peening pressures.

the steel strip, which in turn roughens rubbing surfaces so that the roughened rubbing surfaces lessen the real contact area between them and decrease the friction by reducing the interfacial shear strength between them (Khun, et al. (27)–(29); Tian, et al. (30); Blau (31); Bhushan (32). At the same time, generated wear debris is released into the interface between rubbing surfaces to decrease the friction because the wear debris reduces the interfacial shear strength between two rubbing surfaces by decreasing the direct contact between them and freely rolling

or sliding under a lateral force (Khun, et al. (27)–(29); Tian, et al. (30); Blau (31); Bhushan (32). It can therefore be seen that the increased wear of the as-received steel strip with increased normal load decreases its friction by promoting surface roughening and increasing the amount of wear debris. The friction coefficient of the as-received steel strip tested against the steel ball under the normal load of 5 N is 0.56 § 0.02, which is higher than that of the same strip tested against the ceramic ball under the same normal load. The reason is that the much lower elastic modulus of the steel ball than that of the ceramic ball causes a significantly larger contact between the steel ball and steel strip during sliding for the higher friction because a larger contact between two rubbing surfaces generates higher friction (Khun, et al. (27)–(29); Tian, et al. (30). In Fig. 9, increasing the shot peening pressure from 207 to 552 kPa slightly increases the friction coefficients of the shotpeened steel strips tested against the ceramic ball under normal loads of 1 and 5 N from about 0.56 and 0.47 to about 0.65 and 0.5, respectively, and their friction coefficient tested against the steel ball under 5 N also consistently increases from 0.51 to 0.54. Because a rougher surface can give a higher friction via mechanical interlocking between two rubbing surfaces, the increased friction of the shot-peened steel strips with increased shot peening pressure for all testing conditions (Fig. 9) can be correlated to their significantly increased surface roughness (Fig. 6; Barrett, et al. (39); Svahn, et al. (40); Menezes, et al. (41).

Figure 7. EDX spectra of (a) as-received SAE 1070 steel strip and shot-peened SAE 1070 steel strips at shot peening pressures of (b) 207, (c) 414, and (d) 552 kPa measured at locations A in Fig. 5a, B in Fig. 5b, C in Fig. 5c, and D in Fig. 5d, respectively.

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Figure 8. Wear volumes of shot-peened SAE 1070 steel strips at different shot peening pressures tested against 6-mm-diameter ceramic and steel balls in a circular path with a 1.5-mm radius for 40,000 laps at a sliding speed of 5 cm/s under different normal loads.

The shot-peened steel strips tested against the ceramic ball consistently exhibit lower friction coefficients for all of the shot peening pressures under 5 N compared to under 1 N because the higher wear of the shot-peened steel strips associated with the higher normal load (Fig. 8) gives rise to greater roughening of rubbing surfaces and production of more wear debris (Khun, et al. (27)–(29); Tian, et al. (30); Blau (31); Bhushan (32). In addition, the shot-peened steel strips at different shot peening pressures tested under 5 N consistently have higher friction coefficients for the steel ball than for the ceramic ball due to their significantly larger contact with the steel balls. The friction coefficients of the shot-peened steel strips tested against the ceramic ball under 1 N are higher than that of the as-received steel strip tested under the same conditions although their friction coefficients tested against the ceramic and steel balls under 5 N are lower. A possible reason is that the lower normal load gives rise to the more significant influence of surface roughness on the friction so that the shot-peened steel strips tested under the lower normal load of 1 N exhibit higher friction than the as-received steel strip.

Figure 9. Friction coefficients of shot-peened SAE 1070 steel strips at different shot peening pressures tested against 6-mm-diameter ceramic and steel balls in a circular path with a 1.5-mm radius for 40,000 laps at a sliding speed of 5 cm/s under different normal loads.

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It can be deduced that the as-received SAE 1070 steel strip tested against the ceramic ball has decreased friction with increased normal load due to the promoted surface roughening and the increased amount of wear debris. In addition, the as-received SAE 1070 steel strip exhibits higher friction for the steel ball than for the ceramic ball as a result of its larger contact with the steel ball. The increased shot peening pressure increases the friction of the shot-peened SAE 1070 steel strips tested against the ceramic and steel balls under different normal loads via the increased mechanical interlocking between two rubbing surfaces. The shot-peened SAE 1070 steel strips tested against the ceramic ball consistently exhibit a decrease in friction with increased normal load, whereas they have higher friction for the steel ball than for the ceramic ball. Figure 10a shows the friction coefficients of the shot-peened steel strips at different shot peening pressures tested against the ceramic ball under 1 N as a function of the number of laps. Increasing the shot peening pressure significantly increases the friction of the shot-peened steel strips throughout the wear test, and a large variation in their friction coefficients with respect to shot peening pressure (Fig. 10a) is found as a result of their apparently increased surface roughness (Figs. 5 and 6). Such a large variation in friction with respect to shot peening pressure is not found for the shot-peened steel strips tested against the ceramic and steel balls under 5 N, as shown in Figs. 10b and 10c, probably due to the depressed effect of their surface roughness with incresed normal load. In Fig. 10b, the shot-peened steel strips at different shot peening pressures exhibit stable friction during the entire sliding against the ceramic ball under 5 N, resulting from their stable wear. However, the friction of the shot-peened steel strips tested against the steel ball under 5 N increases for all shot peening pressures with prolonged sliding as shown in Fig. 10c. This clearly indicates that the prolonged sliding increases the wear of the shot-peened steel strips against the steel ball under 5 N due to the much lower wear resistance of the steel ball than that of the ceramic ball. Figure 11 shows the wear morphologies of the as-received and shot-peened SAE 1070 steel strips. As shown in Figs. 11a and 11b, the as-received steel strip tested against the ceramic ball under the higher normal load of 5 N has a larger wear track on the surface due to its higher wear. In addition, abrasive wear caused by repeated sliding of the ceramic ball apparently generates abrasive lines on the wear tracks of the asreceived steel strip tested under the both normal loads with more severe abrasive wear for the higher normal load (Khun, et al. (27–29); Tian, et al. (30). Comparison of Figs. 11b and 11c shows that the as-received steel strip tested under 5 N exhibits higher wear for the ceramic ball than for the steel ball due to the much higher wear resistance of the ceramic ball than that of the steel ball. The formation of tribolayers is apparently found on the wear track of the as-received steel strip tested against the steel ball because the repeated sliding of the steel ball generates wear debris and compacts it to form tribolayers ((Khun, et al. (27–29); Tian, et al. (30); Blau (31); Kasai, et al. (42); Stark, et al. (43); Chen, et al. (44). Such tribolayers are not found on the wear track of the as-received steel strip tested against the ceramic ball because the higher wear of the steel strip does not allow the formation of tribolayers during the sliding.

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Figure 10. Friction coefficients of shot-peened SAE 1070 steel strips at different shot peening pressures tested against different balls with a 6-mm diameter in a circular path with a 1.5-mm radius for 40,000 laps at a sliding speed of 5 cm/s under different normal loads: (a) ceramic ball under 1 N, (b) ceramic ball under 5 N, and (c) steel ball under 5 N, as a function of the number of laps.

The shot-peened steel strip at 207 kPa tested against the ceramic ball under both normal loads of 1 and 5 N (Figs. 11d and 11e) has larger wear tracks compared to those of the asreceived steel strip tested under the same conditions (Figs. 11a

and 11b), which confirms that the shot-peened steel strips at relatively low shot peening pressures have higher wear than the as-received steel strip. In addition, the shot-peened steel strip consistently exhibits a larger wear track for the higher normal load of 5 N as found in Figs. 11d and 11e. Comparison of Figs. 11e and 11f shows that the the higher wear of the shotpeened steel strip at 207 kPa tested against the ceramic ball under 5 N also does not allow the formation of tribolayers on its wear track (Fig. 11e) although the tribolayers are apparently found on the wear track of the same one tested against the steel ball under the same normal load as shown in Fig. 11f. The wear tracks of the shot-peened steel strip at 552 kPa tested against the ceramic ball under both normal loads of 1 and 5 N (Figs. 11g and 11h) are apparently smaller than those of the one at 207 kPa (Figs. 11d and 11e), which is indicative of the higher surface wear resistance of the shot-peened steel strip at higher shot peening pressure. The wear track of the shotpeened steel strip at 552 kPa tested against the steel ball under 5 N (Fig. 11i) is also consistently smaller than that of the one at 207 kPa (Fig. 11f). Comparison of Figs. 11e, 11f, 11h, and 11i shows that sliding of the steel ball generates a significantly smaller wear track on the surface of the shot-peened steel strip at 552 kPa under 5 N than that of the ceramic ball because the much lower wear resistance of the steel ball and the improved surface wear resistance of the shot-peened steel strip probably result in preferential wear of the steel ball during sliding. Figure 12 illustrates the EDX spectra measured on the wear tracks of the as-received and shot-peened SAE 1070 steel strips. The EDX spectra measured on the wear tracks of the steel strips tested against the ceramic ball under normal loads of 1 and 5 N (Fig. 12) have peaks of Fe, Mn, O, and C elements similar to those of the untested steel strips (Fig. 7). However, the EDX spectra measured on the wear tracks of the steel strips tested against the steel ball under 5 N have additional Cr peaks that result from the wear of the steel ball. As shown in Fig. 12, the as-received and shot-peened steel strips tested against the steel ball under 5 N have stronger O peaks on their EDX spectra than the ones tested against the ceramic ball under the same normal load, although they do not show any significant difference in their O peaks in the tests against the ceramic ball under 1 and 5 N. A possible reason is that the rubbing of the steel ball under 5 N enhances the oxidization process via generation of higher frictional heat than that of the ceramic ball, which is indicated by the higher friction of the steel strips tested against the steel ball than that of the ones tested against the ceramic ball (Fig. 8; Khun, et al. (27– 29). In addition, the EDX spectra of the shot-peened steel strips at higher shot peening pressures exhibit stronger O peaks due to the pronounced oxidation process during sliding, especially against the ceramic ball, associated with their higher friction. It can be deduced that the rubbing of the steel ball causes a pronounced oxidation process via higher frictional heating than that of the ceramic ball, and the shot-peened steel strips at higher shot peening pressure also have a pronounced oxidation process during the sliding by generating higher friction. Figures 13a and 13b show the wear morphologies of the ceramic balls rubbed on the shot-peened steel strips at a shot peening pressure of 552 kPa under normal loads of 1

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Figure 11. Wear morphologies and topographies of (a)–(c) as-received SAE 1070 steel strip and shot-peened SAE 1070 steel strips at (d)–(f) 207 kPa and (g)–(i) 552 kPa tested against (a), (b), (d), (e), (g), and (h) ceramic and (c), (f), and (i) steel balls with a 6-mm diameter in a circular path with a 1.5-mm radius for 40,000 laps at a sliding speed of 5 cm/s under normal loads of (a), (d), and (g) 1 and (b), (c), (e), (f), (h), and (i) 5 N.

and 5 N, respectively, on which a larger wear scar is found for the higher normal load as a result of the higher surface wear of the ceramic ball. Comparison of Figs. 13b and 13c shows that the steel ball has a much larger wear scar than

the ceramic ball although they were slid on the same shotpeened steel strip at 552 kPa under the same normal load of 5 N. This indicates that the much lower wear resistance of the steel ball is responsible for its much higher surface

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Figure 12. EDX spectra of of (a)–(c) as-received SAE 1070 steel strip and shot-peened SAE 1070 steel strips at (d)–(f) 207 kPa and (g)–(i) 552 kPa measured at locations A in Fig. 11a, B in Fig. 11b, C in Fig. 11c, D in Fig. 11d, F in Fig. 11f, G in Fig. 11g, H in Fig. 11h, and I in Fig. 11i.

Figure 13. Wear morphologies of (a), (b) ceramic and (c) steel balls rubbed on shot-peened SAE 1070 steel strips at a shot peening pressure of 552 kPa for 40,000 laps at a sliding speed of 5 cm/s under normal loads of (a) 1 N and (b), (c) 5 N.

TRIBOLOGY TRANSACTIONS

wear. In addition, abrasive lines on the worn surface of the steel ball are indicative of its abrasive wear (Khun, et al. (27–29); Tian, et al. (30). The optical images clearly show that the rubbing of the counterball on the shot-peened steel strip generates significant wear of both rubbing surfaces.

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Conclusions In this study, SAE 1070 steel strips were shot peened at different shot peening pressures of 207–552 kPa. Their mechanical and tribological properties were systematically investigated with respect to shot peening pressure with the following conclusions:  The increased shot peening pressure increased the maximum arc height of the shot-peened steel strips due to the increased impact energy of the shot peening media.  The increased shot peening pressure increased the surface hardness of the shot-peened steel strips as a result of the increased cold work-hardening effect.  The higher shot peening pressure resulted in the higher surface roughness of the shot-peened steel strips by forming deeper dimples on the surfaces.  The increased surface roughness of the shot-peened steel strips was responsible for their increased friction coefficients measured against the ceramic and steel balls under different normal loads with increased shot peening pressure.  The increased shot peening pressure gave rise to the decreased wear of the shot-peened steel strips due to their increased abrasive wear resistance associated with their increased surface hardening. However, the wear resistance of the shot-peened steel strips was lowered for lower shot peening pressures than 345 kPa because their surface hardening was almost negible, whereas their surfaces became rougher.  It could be concluded that the shot peening pressure had a significant influence on the mechanical and tribological properties of the shot-peened steel strips.

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