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The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution
Surface & Coatings Technology 204 (2010) 3546–3551
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution V. Azar, B. Hashemi, Mahboobeh Rezaee Yazdi ⁎ Department of Material Science and Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 9 November 2009 Accepted in revised form 7 April 2010 Available online 18 April 2010 Keywords: 316L stainless steel Shot peening Corrosion Fatigue Passive layer Cyclic polarization
a b s t r a c t Stainless steel 316L is one of the most common biomaterials utilized for producing orthopedic implants. But it has low resistance to fatigue and wear. Therefore surface treatments such as shot peening are used to modify the surface properties. In the present research, the influence of shot peening treatment on hardness, fatigue and corrosion behavior of 316L stainless steel in Ringer's solution was investigated. For this purpose, the steel specimens were shot peened for 5, 10, 15, 20 and 25 min. Hardness, fatigue and electrochemical tests were performed on each specimen before and after shot peening treatment. The open circuit potential (OCP) of the specimens, after 2 h of equilibrium time, was measured in Ringer's solution for 300 s. The cyclic potentiodynamic polarization tests were performed with 5 mV/s scan rate. According to the results, the shot peening treatment increases the surface hardness and fatigue resistance. In addition, this treatment decreases the break-down potential of the passive layer and increases the corrosion current density in shot peened specimens up to 10 min, which shows a reduction in resistance to pitting corrosion. However, the break-down potential of the passive layer begins to increase and the corrosion current density decreases at upper times. This trend continues such that even the conditions of resistance to pitting corrosion improve in comparison with un-shot peened specimens at longer times of shot peening. The morphology of the fractured surfaces of samples was investigated by scanning electron microscopy (SEM). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steel (especially medical grade of 316L stainless steel) is utilized as an implant material to make devices like artificial joints, bone plates, stents and prosthesis. [1–4]. Corrosion is one of the most significant phenomena which happens for the alloys or metals used as implants in the body [5,6]. Because of the high concentration of chloride ions (Cl−) and temperature range of the body (36.7–37.2 °C), the human body fluid is considered as a severely corrosive environment [3]. The metals and alloys used as surgical implants achieve passivity by the presence of a protective surface passive or oxide film. This film inhibits or retards corrosion and keeps current flow and the release of corrosion products at a very low level [4]. The dynamic features of oxide film are the key factors of high corrosion resistance of stainless steels in corrosive environments. In addition, the presence of Molybdenum in 316L stainless steel increases the resistance to pitting corrosion in saline environments [7]. The medical grade of 316L stainless steel, which is used as an orthopedic implant, is not heat treatable therefore it possesses relatively low yield strength and fatigue and wear resistance [8,9].
⁎ Corresponding author. Tel.: + 98 9177364415; fax: + 98 711 2307293. E-mail address: [email protected] (M. Rezaee Yazdi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.015
Despite these problems, stainless steel implants are still currently used due to a combination of corrosion resistance, mechanical strength, ductility, toughness and easy fabrication at low cost [7]. Survey of failed stainless implants indicates that majority of failure in 316L SS implants is because of pitting or crevice corrosion, fatigue and wear [5,8,9]. The surface condition of an alloy has the most effect on these phenomena and a very limited number of surface modification techniques can be applied to austenitic stainless steels without causing any loss of their advantageous properties [10,11]. Roland et al. have shown that fatigue limit and yield stress are improved by surface mechanical attrition treatment due to surface nanocrystallization [12]. According to Wang and Yu better corrosion resistance to pitting in chloride solutions achieved through surface nanocrystallization induced by high energy shot peening of 1Cr18Ni9Ti stainless steel [13]. Onizawa et al. reported that shot peening increases corrosion and fatigue resistance of high nitrogen austenitic stainless steel (RS561) [14]. A fine particle peening treatment prior to gas nitriding on 316L stainless steel exhibited higher fatigue resistance [15]. Shot peening is an effective method of surface treatment for the introduction of residual compressive stresses in the surface and subsurface layers and improving the fatigue strength. Surface modifications produced by the shot peening treatment are (a) roughening of the surface (b) an increased, near surface, strain hardening and (c) the development of a characteristic profile of residual stress.
V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
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Table 1 Chemical composition of 316L stainless steel. Element
Fe
C
Ni
Cr
Mo
Mn
P
S
Cu
Si
Etc.
wt.%
67.603
0.023
10.547
16.934
2.033
1.624
0.031
0.015
0.345
0.449
0.396
Table 2 Chemical composition of Ringer's solution.
2. Materials and methods
Compounds
NaCl
KCl
CaCl2·2H2O
pH
Concentration (g/l)
8.6
0.3
0.33
7.4
Considering fatigue damage, surface roughening will accelerate the nucleation and early propagation of cracks, strain hardening will retard the propagation of cracks by increasing the resistance to plastic deformation and residual stress profile will provide a corresponding crack closure stress that will reduce the driving force for crack propagation [16]. It also in some cases due to introduction of a large amount of defects and/or interfaces into the surface layers transform the microstructure of surface to nano sized crystals [17]. The formation of a passive film becomes complex when the surface has been altered by shot peening in comparison to polished surface [18]. The main purpose of this research is to investigate the influence of shot peening surface treatment on resistance to fatigue and pitting corrosion of 316L stainless steel in Ringer's solution which is a simulated human body fluid.
Fig. 1. Effect of shot peening times on the fatigue resistance.
The 316L stainless steel specimens used in this research were taken from a rod and rolled plate 18 mm in diameter and 3 mm in thickness respectively. The composition of the steel is presented in Table 1. Some specimens with 7 × 7 cm2 size were cut from the plate and grinded and polished by SiC papers (in order to perform the shot peening treatment). Fatigue test samples were prepared according to the ASTM E466-96 standard [19]. The shot peening treatment was done on fatigue samples and just on one side of the steel plates by steel balls with 1–2 mm diameter and 40–45 HRC hardness. Time is the variable parameter in shot peening treatment which was done in 5, 10, 15, 20 and 25 min. The hardness and roughness of surface were measured after shot peening. To perform the corrosion experiments, some specimens with 1.5 × 1 cm2 size were prepared from the shot peened and un-shot peened plates. The fatigue tests were performed by the rotating–bending method and a rotational speed of 3000 rpm. The specimens were tested under a constant load (35 kg force) and the fracture time or the number of cycles needed for fracture of each specimen was measured (each test was repeated 3 times and an average value reported). Furthermore, Vickers microhardness tests were performed on the surfaces and cross sections of specimens by using a Leitz model with a 25 g force in 15 s (time of loading), according to the ASTM E384-89 standard [20]. The microhardness test was done for at least 10 points of each specimen and the average value was reported for each one. The corrosion tests were accomplished in a Ringer's solution as a corrosive environment, using direct current method with an electrochemical system Autolab Type III model made by Ecochemie Company. On the other hand, result recording and data analysis were done using GPES software. Table 2 represents the chemical composition of the Ringer's solution used here. All the potentials were measured with respect to a reference electrode (Ag/AgCl[3MKCl]) and a platinum rod was used as the auxiliary electrode. The specimens were mounted and their effective surfaces which would be exposed to Ringer's solution were exactly measured.
Fig. 2. Scanning electron micrograph of fracture surface of the specimens after fatigue test, (a) for un-shot peened specimen and (b) for 10 min shot peened specimen.
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V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
Fig. 5. The effect of shot peening treatment on surface roughness.
Fig. 3. Surface hardness of shot peened specimens varying with shot peening time.
The open circuit potential of the specimens, after 2 h of equilibrium time, was measured for 300 s. Also, the cyclic potentiodynamic anodic polarization was started from a potential 300 mV lower than the open circuit potential and performed in anodic direct (positive potential) with 5 mV/s scan rate. Then, scan direct was reversed in current density of 10 mA/cm2 and continued in cathodic direct to form a hysteresis loop. The morphologies of surfaces after different shot peening times were observed using a scanning electron microscope (SEM), Cambridge Stereoscan model. 3. Results and discussion The results of fatigue tests on 10 and 20 min shot peened and also the initial specimens are presented in Fig. 1. This figure shows that shot peening process increases the resistance to fatigue of specimens, such that 20 min of shot peening can increase it up to 6 times the initial value. Shot peening introducing compressive residual stresses, work hardening and grain refinement in the surface layers as has been reported in other researches [16,17,21] postpones the initiation and growth of surface cracks. The higher time of treatment produces the higher compressive residual stress area and therefore the higher fatigue strength. The scanning electron micrograph of the fracture surface of specimens represents the fracture conditions (Fig. 2). According to the experiments, for un-shot peened specimen, a fibrous fracture is observed at the borders of the fracture surface and a brittle fracture at
the center. However, for the 10 min shot peened specimen, the presence of a superficial layer which is affected by shot peening is clearly observed, and also the brittle fracture zone (the lighter zone) is moved from center to a position beneath this layer, which shows that the crack initiation has been done from the side upon this layer, and after its propagation, brittle fracture has occurred. Hardness of specimen's surface is shown in Fig. 3. Hardness increases with increasing shot peening time. It is due to the increase of density of dislocations and twin boundaries that the work hardening of surface layers increase. With increasing of shot peening time work hardening increases and plastic deformation becomes difficult. Therefore the high density of dislocations decelerates or prevents the motion of dislocations which causes hardness to increase. However, hardness increase does not have a linear relation with the density of dislocations [22]. Increasing the hardness of surface could improve the wear resistance of material. Furthermore, a microhardness test was performed on the cross section of the specimens 5 and 25 min shot peened in order to determine the hardness of layers and depth of hardening. The
Fig. 4. Vickers hardness, depth profile for different time shot peened specimens.
Table 3 Surface roughness of specimens after different shot peening times. Shot peening time (min) Surface roughness (µm)
0 0.13
5 3.88
10 6.25
15 5.68
20 4.63
25 3.55
Fig. 6. Image analysis of shot peened surfaces of specimens at different shot peening times; (a) size distribution of dimples and (b) number of dimples at different shot peening times.
V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
hardness values versus distance from the surface of specimen are presented in Fig. 4. According to this figure, the hardness of 25 min shot peened specimens is higher than 5 min shot peened specimens and hardness decreases with a moderate slope until the depth of 400 and 200 µm respectively with respect to the surface of two specimens. After these depths, no considerable variations are observed and constant behaviors are shown for hardness. This hardness increase in specimen surface and its gradual decrease (as discussed ago) shows the presences of compressive residual stresses and work hardening which is of course decreased and disappeared with distancing from the surface. Therefore, the region where hardness is more than the hardness in the depth of the specimen could be considered as the
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region affected by shot peening and thickness of this layer is proportional to shoot peening time. Thus, fatigue resistance increases with shot peening time. Surface roughness tests were performed on the surface of shot peened and initial (as received) plates. The surface roughness and homogeneity of surface are very effective in passive layer formation and so in corrosion resistance of material. The results are presented in Table 3. As shown in Table 3 and Fig. 5, surface roughness is increased until 10 min, but it is decreased after this time with increasing the shot peening time although it is higher than surface roughness of as received specimen yet. Indeed, at first due to local deformation of
Fig. 7. Scanning electron microscope (SEM) micrographs from specimen's surfaces at different shot peening times.
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V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
surface by peening, the surface roughness and surface heterogeneity (difference in dislocation density) increase with time but after 10 min due to over peening effect and surface erosion of surface the surface roughness decreases and the surface homogeneity increases. In addition, at upper shot peening times, the surface undergoes severe plastic deformation and work hardening because 316L stainless steel has a high work hardening capacity. Therefore, the effect of impacts on the surface decreases and the following shot peening in a suitable time contributes to the reduction of heterogeneity and surface roughness. This behavior has been reported in other research and depends on material properties [15,16,23]. The image analysis results and scanning electron microscope photographs taken from the surface of specimens in different shot peening times are presented in Figs. 6 and 7, respectively. According to Fig. 6 as shot peening time increases size of dimples decreases and number of dimples per unit area increases while dimples approach together (Fig. 7). Therefore, uniformity and homogeneity of the surface increase as more shot peening time is used. However, although increasing the shot peening time up to a suitable value decreases the heterogeneity and roughness of the surface shot peening causes heterogeneity on the surface, with respect to un-shot peened specimens. The cyclic polarization curves of specimens are shown in Fig. 8. As it is seen there are some hysteresis loops for all the specimens which represent their susceptibility to pitting corrosion. Table 4 reports the break-down potential of the passive layer and corrosion current density of specimens which are extracted from polarization curves. According to Table 4 and the cyclic polarization curves of specimens, at low shot peening time treatment, the break-down potential of the passive layer decreases and corrosion current density increases with respect to un-shot peened specimens. It shows a decrease in resistance to pitting corrosion after shot peening treatment. However, the value of break-down potential of the passive layer increases and the corrosion current density decreases at upper times with respect to lower times. So that at 25 min of shot peening the break-down potential approaches to the values for initial specimens. Also, corrosion current density has been improved in this specimen and it has become even less than the initial specimen. Therefore, it can be concluded that shot peening treatment for a long enough time (25 min in this case) causes a reduction in the corrosion rate of the specimens. Comparing all the curves, it is observable that the corrosion rate in presence of the passive layer (corrosion current density in the passive region) has been changed with shot peening. The location of the
Table 4 Break-down potential of passive layer and corrosion current density of specimens. Specimen
A1
C1
D1
E1
F1
G1
Shot peening time (min) Break-down potential of passive layer (mV) Corrosion current density (A/cm2)
0 573
5 396
10 381
15 480
20 488
25 523
3.8 × 10−7
4.5 × 10−7
14 × 10−7
13 × 10−7
6× 10−7
3× 10−7
passive region changes after performing shot peening treatment. At low shot peening times, the corrosion rate increases in the passive state. However, after 20 and 25 min, the corrosion rate has been decreased (the curves have been shifted considerably to lower currents i.e. to the left). Such behavior may be because of the surface roughness, the surface heterogeneities and compressive stress level of specimens after shot peening. Due to this treatment, roughness and heterogeneity in the surface increase with time of shot peening up to about 15 min after that a reduction in surface roughness and inhomogeneities was observed for the specimens with higher shot peening time. Also the compressive stress level in specimens increases with time of shot peening. Therefore with increase in roughness and heterogeneity of the surface the preferred locations for initiation of pits increase and pitting corrosion rises. At higher time of shot peening roughness and heterogeneity decrease while depth and level of compressive stresses increase then an effective passive layer could be formed on the surface and decreases pitting corrosion. In some research such behavior has been seen [16,18,23] and also it has been reported that compressive residual stresses and grain refinement due to severe plastic deformations in surface layers may be effective in the decrease of corrosion rate [24]. 4. Conclusion It can be concluded from this research that shot peening increases fatigue resistance and hardness. But this treatment at first deteriorates the resistance to pitting corrosion of 316L stainless steel in Ringer's solution at low shot peening time. However, increasing the shot peening time causes the break-down potential of the passive layer to increase and corrosion current density and corrosion in the passive region to decrease, what finally makes an improvement in the resistance to pitting corrosion. A combination of a suitable time of shot peening and passivation authored a great improvement in the
Fig. 8. Cyclic polarization curves for different shot peening times in Ringer's solution.
V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
break-down potential of passive layer, corrosion current density and corrosion rate in passive region, in comparison with nontreated specimens. References [1] J.A. Disegi, L. Eschbach, Injury 31 (2000) D2. [2] H. Alexander, J.B. Cooper, L. Hench, in: B.D. Ratner (Ed.), Biomaterials, An Introduction to Materials in Medicine, Academic Press Limited, 1996, p. 33. [3] H. Yang, K. Yang, B. Zhang, Mater. Lett. 61 (2007) 1154. [4] U. Kamachi Mudali, T.M. Sridhar, Baldev Raj, Sadhana 28 (2003) 601. [5] J.B. Park, R.S. Lakes, BIOMATERIALS, An Introduction, Plenum Publishing, 1992. [6] J.D. Redamond, K.H. Miska, Chem. Eng. (1982) 79. [7] M. Terada, R.A. Antunes, A.F. Padilha, Mater. Res. 9 (3) (2006) 281. [8] J.B. Park, J.D. Bronzino, Biomaterials: Principles and Applications, Plenum Press, New York, 2002. [9] S.H. Teoh, Int. J. Fatigue 22 (2002) 825. [10] T. Kastilink, Surface Engineering, ASM Handbook, vol. 5, 1994, p. 278.
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[11] P. Villochaise, J. Mendez, in: T.S. Sudarshan, M. Jeandin (Eds.), Surface Modifications Technologies, IV, The Minérals, Métals and Matériels society, War-rendale, 1991, p. 335. [12] T. Roland, D. Retraint, K. Lu, Scripta Mater. 54 (2006) 1949. [13] T. Wang, J. Yu, Surf. Coat. Technol. 200 (2006) 4777. [14] A. Onizawa, M.A. Islam, Y. Tomota, ARPN J. Eng. Appl. Sci. 1 (1) (2006) 12. [15] S. Kikuchi, J. Komotori, Int. J. Fatigue 32 (2010) 403. [16] S.B. Mahagaonkar, P.K. Brahmankar, C.Y. Seemikeri, Int. J. Fatigue 31 (2009) 693. [17] S. Bagheri, M. Guagliano, J. Surf. Eng. 25 (2009) 3. [18] H. Yun-wei, D. Bo, Z. Cheng, J. Yi-ming, J. Iron Steel Res. Int. 16 (2) (2009) 68. [19] Standard practice for cleaning, descaling and passivation of stainless steels parts, equipment and systems, A380, Annual Book of ASTM Standards, American Society for Testing and Materials, 1999. [20] Standard test method for microindentation hardness of materials, E384–99, Annual Book of ASTM Standards, American Society for Testing and Materials, 1999. [21] E.R. Rios, A. Walley, M.T. Milan, G. Hammersley, Int. J. Fatigue 17 (7) (1995) 493. [22] F.J. Humphreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Elsevier Science Ltd, 1995, p. 17. [23] P. Peyre, X. Scherpereel, L. Berthe, C. Carboni, Mater. Sci. Eng. A280 (2000) 294. [24] X.Y. Wang, D.Y. Li, Electrochim. Acta 47 (2002) 3939.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution V. Azar, B. Hashemi, Mahboobeh Rezaee Yazdi ⁎ Department of Material Science and Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 9 November 2009 Accepted in revised form 7 April 2010 Available online 18 April 2010 Keywords: 316L stainless steel Shot peening Corrosion Fatigue Passive layer Cyclic polarization
a b s t r a c t Stainless steel 316L is one of the most common biomaterials utilized for producing orthopedic implants. But it has low resistance to fatigue and wear. Therefore surface treatments such as shot peening are used to modify the surface properties. In the present research, the influence of shot peening treatment on hardness, fatigue and corrosion behavior of 316L stainless steel in Ringer's solution was investigated. For this purpose, the steel specimens were shot peened for 5, 10, 15, 20 and 25 min. Hardness, fatigue and electrochemical tests were performed on each specimen before and after shot peening treatment. The open circuit potential (OCP) of the specimens, after 2 h of equilibrium time, was measured in Ringer's solution for 300 s. The cyclic potentiodynamic polarization tests were performed with 5 mV/s scan rate. According to the results, the shot peening treatment increases the surface hardness and fatigue resistance. In addition, this treatment decreases the break-down potential of the passive layer and increases the corrosion current density in shot peened specimens up to 10 min, which shows a reduction in resistance to pitting corrosion. However, the break-down potential of the passive layer begins to increase and the corrosion current density decreases at upper times. This trend continues such that even the conditions of resistance to pitting corrosion improve in comparison with un-shot peened specimens at longer times of shot peening. The morphology of the fractured surfaces of samples was investigated by scanning electron microscopy (SEM). © 2010 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steel (especially medical grade of 316L stainless steel) is utilized as an implant material to make devices like artificial joints, bone plates, stents and prosthesis. [1–4]. Corrosion is one of the most significant phenomena which happens for the alloys or metals used as implants in the body [5,6]. Because of the high concentration of chloride ions (Cl−) and temperature range of the body (36.7–37.2 °C), the human body fluid is considered as a severely corrosive environment [3]. The metals and alloys used as surgical implants achieve passivity by the presence of a protective surface passive or oxide film. This film inhibits or retards corrosion and keeps current flow and the release of corrosion products at a very low level [4]. The dynamic features of oxide film are the key factors of high corrosion resistance of stainless steels in corrosive environments. In addition, the presence of Molybdenum in 316L stainless steel increases the resistance to pitting corrosion in saline environments [7]. The medical grade of 316L stainless steel, which is used as an orthopedic implant, is not heat treatable therefore it possesses relatively low yield strength and fatigue and wear resistance [8,9].
⁎ Corresponding author. Tel.: + 98 9177364415; fax: + 98 711 2307293. E-mail address: [email protected] (M. Rezaee Yazdi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.015
Despite these problems, stainless steel implants are still currently used due to a combination of corrosion resistance, mechanical strength, ductility, toughness and easy fabrication at low cost [7]. Survey of failed stainless implants indicates that majority of failure in 316L SS implants is because of pitting or crevice corrosion, fatigue and wear [5,8,9]. The surface condition of an alloy has the most effect on these phenomena and a very limited number of surface modification techniques can be applied to austenitic stainless steels without causing any loss of their advantageous properties [10,11]. Roland et al. have shown that fatigue limit and yield stress are improved by surface mechanical attrition treatment due to surface nanocrystallization [12]. According to Wang and Yu better corrosion resistance to pitting in chloride solutions achieved through surface nanocrystallization induced by high energy shot peening of 1Cr18Ni9Ti stainless steel [13]. Onizawa et al. reported that shot peening increases corrosion and fatigue resistance of high nitrogen austenitic stainless steel (RS561) [14]. A fine particle peening treatment prior to gas nitriding on 316L stainless steel exhibited higher fatigue resistance [15]. Shot peening is an effective method of surface treatment for the introduction of residual compressive stresses in the surface and subsurface layers and improving the fatigue strength. Surface modifications produced by the shot peening treatment are (a) roughening of the surface (b) an increased, near surface, strain hardening and (c) the development of a characteristic profile of residual stress.
V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
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Table 1 Chemical composition of 316L stainless steel. Element
Fe
C
Ni
Cr
Mo
Mn
P
S
Cu
Si
Etc.
wt.%
67.603
0.023
10.547
16.934
2.033
1.624
0.031
0.015
0.345
0.449
0.396
Table 2 Chemical composition of Ringer's solution.
2. Materials and methods
Compounds
NaCl
KCl
CaCl2·2H2O
pH
Concentration (g/l)
8.6
0.3
0.33
7.4
Considering fatigue damage, surface roughening will accelerate the nucleation and early propagation of cracks, strain hardening will retard the propagation of cracks by increasing the resistance to plastic deformation and residual stress profile will provide a corresponding crack closure stress that will reduce the driving force for crack propagation [16]. It also in some cases due to introduction of a large amount of defects and/or interfaces into the surface layers transform the microstructure of surface to nano sized crystals [17]. The formation of a passive film becomes complex when the surface has been altered by shot peening in comparison to polished surface [18]. The main purpose of this research is to investigate the influence of shot peening surface treatment on resistance to fatigue and pitting corrosion of 316L stainless steel in Ringer's solution which is a simulated human body fluid.
Fig. 1. Effect of shot peening times on the fatigue resistance.
The 316L stainless steel specimens used in this research were taken from a rod and rolled plate 18 mm in diameter and 3 mm in thickness respectively. The composition of the steel is presented in Table 1. Some specimens with 7 × 7 cm2 size were cut from the plate and grinded and polished by SiC papers (in order to perform the shot peening treatment). Fatigue test samples were prepared according to the ASTM E466-96 standard [19]. The shot peening treatment was done on fatigue samples and just on one side of the steel plates by steel balls with 1–2 mm diameter and 40–45 HRC hardness. Time is the variable parameter in shot peening treatment which was done in 5, 10, 15, 20 and 25 min. The hardness and roughness of surface were measured after shot peening. To perform the corrosion experiments, some specimens with 1.5 × 1 cm2 size were prepared from the shot peened and un-shot peened plates. The fatigue tests were performed by the rotating–bending method and a rotational speed of 3000 rpm. The specimens were tested under a constant load (35 kg force) and the fracture time or the number of cycles needed for fracture of each specimen was measured (each test was repeated 3 times and an average value reported). Furthermore, Vickers microhardness tests were performed on the surfaces and cross sections of specimens by using a Leitz model with a 25 g force in 15 s (time of loading), according to the ASTM E384-89 standard [20]. The microhardness test was done for at least 10 points of each specimen and the average value was reported for each one. The corrosion tests were accomplished in a Ringer's solution as a corrosive environment, using direct current method with an electrochemical system Autolab Type III model made by Ecochemie Company. On the other hand, result recording and data analysis were done using GPES software. Table 2 represents the chemical composition of the Ringer's solution used here. All the potentials were measured with respect to a reference electrode (Ag/AgCl[3MKCl]) and a platinum rod was used as the auxiliary electrode. The specimens were mounted and their effective surfaces which would be exposed to Ringer's solution were exactly measured.
Fig. 2. Scanning electron micrograph of fracture surface of the specimens after fatigue test, (a) for un-shot peened specimen and (b) for 10 min shot peened specimen.
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V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
Fig. 5. The effect of shot peening treatment on surface roughness.
Fig. 3. Surface hardness of shot peened specimens varying with shot peening time.
The open circuit potential of the specimens, after 2 h of equilibrium time, was measured for 300 s. Also, the cyclic potentiodynamic anodic polarization was started from a potential 300 mV lower than the open circuit potential and performed in anodic direct (positive potential) with 5 mV/s scan rate. Then, scan direct was reversed in current density of 10 mA/cm2 and continued in cathodic direct to form a hysteresis loop. The morphologies of surfaces after different shot peening times were observed using a scanning electron microscope (SEM), Cambridge Stereoscan model. 3. Results and discussion The results of fatigue tests on 10 and 20 min shot peened and also the initial specimens are presented in Fig. 1. This figure shows that shot peening process increases the resistance to fatigue of specimens, such that 20 min of shot peening can increase it up to 6 times the initial value. Shot peening introducing compressive residual stresses, work hardening and grain refinement in the surface layers as has been reported in other researches [16,17,21] postpones the initiation and growth of surface cracks. The higher time of treatment produces the higher compressive residual stress area and therefore the higher fatigue strength. The scanning electron micrograph of the fracture surface of specimens represents the fracture conditions (Fig. 2). According to the experiments, for un-shot peened specimen, a fibrous fracture is observed at the borders of the fracture surface and a brittle fracture at
the center. However, for the 10 min shot peened specimen, the presence of a superficial layer which is affected by shot peening is clearly observed, and also the brittle fracture zone (the lighter zone) is moved from center to a position beneath this layer, which shows that the crack initiation has been done from the side upon this layer, and after its propagation, brittle fracture has occurred. Hardness of specimen's surface is shown in Fig. 3. Hardness increases with increasing shot peening time. It is due to the increase of density of dislocations and twin boundaries that the work hardening of surface layers increase. With increasing of shot peening time work hardening increases and plastic deformation becomes difficult. Therefore the high density of dislocations decelerates or prevents the motion of dislocations which causes hardness to increase. However, hardness increase does not have a linear relation with the density of dislocations [22]. Increasing the hardness of surface could improve the wear resistance of material. Furthermore, a microhardness test was performed on the cross section of the specimens 5 and 25 min shot peened in order to determine the hardness of layers and depth of hardening. The
Fig. 4. Vickers hardness, depth profile for different time shot peened specimens.
Table 3 Surface roughness of specimens after different shot peening times. Shot peening time (min) Surface roughness (µm)
0 0.13
5 3.88
10 6.25
15 5.68
20 4.63
25 3.55
Fig. 6. Image analysis of shot peened surfaces of specimens at different shot peening times; (a) size distribution of dimples and (b) number of dimples at different shot peening times.
V. Azar et al. / Surface & Coatings Technology 204 (2010) 3546–3551
hardness values versus distance from the surface of specimen are presented in Fig. 4. According to this figure, the hardness of 25 min shot peened specimens is higher than 5 min shot peened specimens and hardness decreases with a moderate slope until the depth of 400 and 200 µm respectively with respect to the surface of two specimens. After these depths, no considerable variations are observed and constant behaviors are shown for hardness. This hardness increase in specimen surface and its gradual decrease (as discussed ago) shows the presences of compressive residual stresses and work hardening which is of course decreased and disappeared with distancing from the surface. Therefore, the region where hardness is more than the hardness in the depth of the specimen could be considered as the
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region affected by shot peening and thickness of this layer is proportional to shoot peening time. Thus, fatigue resistance increases with shot peening time. Surface roughness tests were performed on the surface of shot peened and initial (as received) plates. The surface roughness and homogeneity of surface are very effective in passive layer formation and so in corrosion resistance of material. The results are presented in Table 3. As shown in Table 3 and Fig. 5, surface roughness is increased until 10 min, but it is decreased after this time with increasing the shot peening time although it is higher than surface roughness of as received specimen yet. Indeed, at first due to local deformation of
Fig. 7. Scanning electron microscope (SEM) micrographs from specimen's surfaces at different shot peening times.
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surface by peening, the surface roughness and surface heterogeneity (difference in dislocation density) increase with time but after 10 min due to over peening effect and surface erosion of surface the surface roughness decreases and the surface homogeneity increases. In addition, at upper shot peening times, the surface undergoes severe plastic deformation and work hardening because 316L stainless steel has a high work hardening capacity. Therefore, the effect of impacts on the surface decreases and the following shot peening in a suitable time contributes to the reduction of heterogeneity and surface roughness. This behavior has been reported in other research and depends on material properties [15,16,23]. The image analysis results and scanning electron microscope photographs taken from the surface of specimens in different shot peening times are presented in Figs. 6 and 7, respectively. According to Fig. 6 as shot peening time increases size of dimples decreases and number of dimples per unit area increases while dimples approach together (Fig. 7). Therefore, uniformity and homogeneity of the surface increase as more shot peening time is used. However, although increasing the shot peening time up to a suitable value decreases the heterogeneity and roughness of the surface shot peening causes heterogeneity on the surface, with respect to un-shot peened specimens. The cyclic polarization curves of specimens are shown in Fig. 8. As it is seen there are some hysteresis loops for all the specimens which represent their susceptibility to pitting corrosion. Table 4 reports the break-down potential of the passive layer and corrosion current density of specimens which are extracted from polarization curves. According to Table 4 and the cyclic polarization curves of specimens, at low shot peening time treatment, the break-down potential of the passive layer decreases and corrosion current density increases with respect to un-shot peened specimens. It shows a decrease in resistance to pitting corrosion after shot peening treatment. However, the value of break-down potential of the passive layer increases and the corrosion current density decreases at upper times with respect to lower times. So that at 25 min of shot peening the break-down potential approaches to the values for initial specimens. Also, corrosion current density has been improved in this specimen and it has become even less than the initial specimen. Therefore, it can be concluded that shot peening treatment for a long enough time (25 min in this case) causes a reduction in the corrosion rate of the specimens. Comparing all the curves, it is observable that the corrosion rate in presence of the passive layer (corrosion current density in the passive region) has been changed with shot peening. The location of the
Table 4 Break-down potential of passive layer and corrosion current density of specimens. Specimen
A1
C1
D1
E1
F1
G1
Shot peening time (min) Break-down potential of passive layer (mV) Corrosion current density (A/cm2)
0 573
5 396
10 381
15 480
20 488
25 523
3.8 × 10−7
4.5 × 10−7
14 × 10−7
13 × 10−7
6× 10−7
3× 10−7
passive region changes after performing shot peening treatment. At low shot peening times, the corrosion rate increases in the passive state. However, after 20 and 25 min, the corrosion rate has been decreased (the curves have been shifted considerably to lower currents i.e. to the left). Such behavior may be because of the surface roughness, the surface heterogeneities and compressive stress level of specimens after shot peening. Due to this treatment, roughness and heterogeneity in the surface increase with time of shot peening up to about 15 min after that a reduction in surface roughness and inhomogeneities was observed for the specimens with higher shot peening time. Also the compressive stress level in specimens increases with time of shot peening. Therefore with increase in roughness and heterogeneity of the surface the preferred locations for initiation of pits increase and pitting corrosion rises. At higher time of shot peening roughness and heterogeneity decrease while depth and level of compressive stresses increase then an effective passive layer could be formed on the surface and decreases pitting corrosion. In some research such behavior has been seen [16,18,23] and also it has been reported that compressive residual stresses and grain refinement due to severe plastic deformations in surface layers may be effective in the decrease of corrosion rate [24]. 4. Conclusion It can be concluded from this research that shot peening increases fatigue resistance and hardness. But this treatment at first deteriorates the resistance to pitting corrosion of 316L stainless steel in Ringer's solution at low shot peening time. However, increasing the shot peening time causes the break-down potential of the passive layer to increase and corrosion current density and corrosion in the passive region to decrease, what finally makes an improvement in the resistance to pitting corrosion. A combination of a suitable time of shot peening and passivation authored a great improvement in the
Fig. 8. Cyclic polarization curves for different shot peening times in Ringer's solution.
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