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Fatigue behavior of notched steel specimens with nanocrystallized surface obtained by severe shot peening
Materials and Design 45 (2013) 497–503
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Fatigue behavior of notched steel specimens with nanocrystallized surface obtained by severe shot peening S. Bagherifard a,⇑, I. Fernandez-Pariente b, R. Ghelichi a, M. Guagliano a a b
Politecnico di Milano, Department of Mechanical Engineering, via G. La Masa, 1 Milano, Italy University of Oviedo, Department of Material Science and Metallurgical Engineering, Gijón, Spain
a r t i c l e
i n f o
Article history: Received 18 July 2012 Accepted 13 September 2012 Available online 26 September 2012
a b s t r a c t Among severe plastic deformation methods that result in surface nanocrystallization, shot peening has proved to be a promising technique. Application of severe air blast shot peening results in surface nanocrystallization, affects a thick layer of material with high compressive residual stresses but at the same time produces rather high surface roughness. In this study notched specimens with a stress concentration factor common in many structural components have been subjected to severe shot peening process. The mentioned treatment uses peening parameters essentially different from conventional ones. Roughness and X-ray diffraction residual stress measurements as well as microscopy observations have been carried out on the treated specimens. Room temperature rotating bending fatigue tests are performed to evaluate the effect of the treatment on specimens’ fatigue strength. Fracture surfaces have been then observed by scanning electron microscopy. The results indicate a very significant fatigue strength improvement for severely shot peened specimens in spite of their very high surface roughness. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Shot peening (SP) is a mechanical surface treatment generally applied to improve fatigue behavior of metallic components. During the process propelled stream of thousands of small spheres impact the surface of the work piece. The process is aimed to create compressive residual stresses on the surface and a thin layer underneath it and also to work harden the near surface layer of material. These effects are very useful in order to totally prevent or greatly delay the failure of the part [1–3]. Recent advancement of SP has revealed that particular peening processes, different from conventional air blast shot peening (ABSP), can be aimed at achieving ultrafine grained materials on the surface of treated parts [4]. It is well-known that fatigue properties of materials are highly sensitive to the grain size. A small grain size can enhance the fatigue crack initiation threshold and coarse grains may deflect the propagation paths of fatigue cracks by grain boundaries, thus introducing crack closure and decreasing the rate of crack growth [5]. Since most fatigue cracks initiate from the surface and propagate to the interior, a component with a nanocrystallized (NC) surface layer and coarse grained interior is expected to show improved fatigue properties [6]. Especially in case of surface nanocrystallization shot peening based processes,
⇑ Corresponding author. Tel.: +39 02 23998667; fax: +39 02 23998263. E-mail addresses: [email protected] (S. Bagherifard), inesfp@ uniovi.es (I. Fernandez-Pariente), [email protected] (R. Ghelichi), [email protected] (M. Guagliano). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.025
the induced residual compressive stresses can also effectively delay the propagation of fatigue cracks [7–11]. This study is performed to continue an authors’ preceding research that focused on application of severe shot peening (SSP) on fatigue strength of smooth steel specimens [12]. The developed SSP process is performed using a standard ABSP device applying a combination of severe peening parameters to multiply the kinetic energy of the conventional shot peening (CSP). In the previous study, generation of NC surface layer by the proposed peening parameters was experimentally confirmed through transmission electron microscopy (TEM) observation and grain size measurements [12]. The results of the roughness measurements indicated considerable increase in surface roughness parameters of SSP specimens that obviously had notable deteriorating influence on fatigue strength. Notwithstanding the high surface roughness, the smooth surface NC specimens showed a better fatigue strength with respect to the not peened (NP) series. Different approaches were applied to decrease the deteriorating effect of high surface roughness on fatigue strength of smooth specimens. Better results were obtained through repeening by smaller and harder shots without removing NC layer, even if roughness parameters did not vary appreciably after repeening. Along these lines, despite the considerably high surface roughness, a fatigue strength improvement of 10% in comparison with NP series was obtained for smooth specimens. However, SP is well-recognized to be more efficient in increasing fatigue life of notched components with high stress gradient compared to smooth ones [13–15]. In notched specimens, the high
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stress gradient just under the notch, results in considerable decrease in stress level while the crack is still very small, thus decreasing crack growth rate. Residual stresses have a minor effect on the resistance to the development of microscopic cracks. On the other hand they have a very strong effect on the resistance to the propagation of small cracks and may arrest the crack growth. The effect of surface work hardening shall be also taken into consideration. Very few studies have been performed on fatigue behavior of notched surface NC material obtained by mechanical treatment. Adachi [16] performed rotating bending fatigue tests on case-hardened gear steel with a stress concentration factor of Kt = 3, shot peened by extremely high intensity conditions. Microstructural observation of the specimens revealed the presence of a near surface white etching layer. Although Adachi at that time did not comment the presence of ultrafine grains on the surface of treated specimens, based on recent theories this white etching layer could have been the sign of surface nanocrystallization [17]. The results indicated 200% improvement in fatigue life with respect to NP specimens [16]. Wen et al. [18] studied the fatigue strength of pure titanium treated by high energy shot peening (HESP), through rotating bending fatigue tests performed on notched specimen (Kt = 1.27). They mentioned that surface damage, roughness, and edge of specimen, reduced the fatigue limit and in some cases even caused the function of NC layer to be counteracted. Repeening using low energy small shots was applied to decrease the surface roughness up to 40%. Comparing with the fatigue limit of NP specimens, the results were increased up to 9% by CSP, up to 34% by HESP and up to 52% after repeening [18]. To the authors’ best knowledge these are the only studies performed on fatigue strength of notched surface NC specimens available in the literature. Nevertheless, a feature of the available data which bears discussion is the fact that the experiments have been performed on different materials that are surface NC through various processes and are tested using different set ups, specimen geometries and consequently produce disparate and essentially different results. This highlights an important challenge in assessing the mechanical behavior of surface NC metals; as indicated also by Padilla and Boyce [6] variations in microstructure and test method can have significant effects on the results. In this study, in order to investigate the effect of the proposed SSP process on notched components’ fatigue life, two series of notched specimens including NP and SSP series, were fatigue tested. Treated specimens have been characterized using X-ray diffraction (XRD) and roughness measurements as well as scanning electron microscopy (SEM) and TEM observations. SEM fractography observations have been performed on the fracture surface of broken specimens. In addition, to underline the notable effect of SSP process, the obtained results have been compared to the fatigue strength improvement of specimens with the same geometry but a slightly different material composition, treated with usual peening parameters. The final results are then critically discussed. 2. Material and experimental procedure Low alloy steel (39NiCrMo3, UNI EN 10083) notched specimens are subjected to severe ABSP. The nominal chemical composition of
Table 2 Aspects of the SSP treatment. Treatment
Shot type and diameter (mm)
Almen intensity (0.0001 in.)
Coverage (%)
SSP
S230 (steel, / = 0.58)
6–7 C
1500
this steel is shown in Tables 1 and 2 shows the applied SP parameters. To study the state of residual stresses, XRD analysis was performed on all series of specimens using an AST X-Stress 3000 Xray diffractometer (radiation Cr Ka, irradiated area 1 mm2, sin2 w method, diffraction angles (2h) scanned between 45° and 45°). Measurements were carried out in depth step by step removing a very thin layer of material using an electro-polishing device in order to obtain the in-depth trend of residual stresses. A solution of acetic acid (94%) and perchloric acid (6%) was used for electro-polishing. Microstructure observations were performed by a Zeiss EVO50 SEM with thermionic source. Specimens for SEM observations were etched by 2% Nital. TEM observations were carried out using a Philips CM12 microscope operating at 120 kV. To perform TEM observations very thin pieces of specimens were first cut by electrical discharge machine, then were mechanically polished from the untreated side and finally the last step of thinning was performed by means of ion milling with proper incident angles. Microscopy observations and roughness measurements were also performed on treated surfaces. Room temperature rotating bending fatigue tests were executed to investigate the effect of the applied process on fatigue strength. In order to highlight the significant improvement due to SSP treatment with respect to conventional SP parameters, the results of a preceding study performed by the authors [19] on another batch of specimens with the same geometry and material with a slightly different chemical composition (40NiCrMo7, UNI 7845) treated by three different conventional shot peening (CSP) set of parameters, are also provided in this paper. The material nominal chemical composition, the CSP parameters as well as a comparison of mechanical properties of the two low alloy steels are respectively presented in Tables 3–5. 3. Results and discussion 3.1. Analysis of residual stress profile XRD measurements demonstrate trivial values of residual stresses for NP specimens as expected, apart from vey local high tensile surface stresses in case of NP 40NiCrMo7 that is potentially due to the machining process. The results imply that increasing the applied Almen intensity that is to raise the kinetic energy of impacts, for all the three CSP series results in increase in on-surface and maximum values of residual stresses. Regarding SSP treated specimen, a considerable depth of material is characterized by significant compressive residual stresses. This is verified by comparing the in-depth residual stress trend of other series including NP, CSP1, CSP2 and CSP3 with SSP specimens as shown in Fig. 1. The SSP specimens show lower on-surface stress and maximum stress values with respect to other series, whereas the thickness of layer affected by considerable compressive residual stresses is almost 5 times that of other specimens.
Table 1 Nominal chemical composition of 39NiCrMo3 steel in mass density. C (wt.%)
Si (wt.%) max
Mn (wt.%)
P wt(wt.%) max
S (wt.%) max
Cr (wt.%)
Mo (wt.%)
Ni (wt.%)
0.35–0.43 ±0.02
0.40 ±0.03
0.5–0.8 ±0.04
0.025 +0.005
0.035 ±0.005
0.60–1.00 ±0.05
0.15–0.25 ±0.03
0.70–1.00 ±0.05
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S. Bagherifard et al. / Materials and Design 45 (2013) 497–503 Table 3 Nominal chemical composition of 40NiCrMo7 steel in mass density. C (wt.%)
Si (wt.%) max
Mn (wt.%)
P wt(wt.%) max
S (wt.%) max
Cr (wt.%)
Mo (wt.%)
Ni (wt.%)
0.37–0.44 ±0.02
0.15–0.40 ±0.03
0.50–0.80 ±0.04
0.035 +0.005
0.035 ±0.005
0.60–0.90 ±0.05
0.20–0.30 ±0.03
1.60–1.90 ±0.05
3.2. Estimation of work-hardened layer thickness
Table 4 Aspects of the conventional shot peening treatment. Treatment
Shot type and diameter (mm)
Almen Intensity (0.0001 in.)
Coverage (%)
CSP1 CSP2 CSP3
Z100 (ceramic, / = 0.1) S110 (steel, / = 0.3) S170 (steel, / = 0.43)
10–12 N 4–6 A 10–12 A
100 100 100
Table 5 Nominal mechanical properties of 39NiCrMo3 and 40NiCrMo7 steel. Material type
E (GPa)
Yield stress (MPa)
Ultimate stress (MPa)
A (%)
39NiCrMo3 40NiCrMo7
210 203
734 1170
908 1292
14.8 14.9
Another parameter measured by XRD is FWHM. This parameter, the distribution of which is shown in Fig. 2, represents the full width of the diffraction peak at half of the maximum intensity. It can be assumed as an index of hardening of the material [12]. As it is observed in Fig. 2 the on-surface value of FWHM is growing with increasing the Almen intensity of the peening process i.e. increasing the kinetic energy of the impacts. This issue has been validated also numerically in authors’ previous study [20]. It is to be noted that the thickness of the work-hardened layer can be estimated as the thickness of the layer which shows considerably increased FWHM values in comparison with the core material. As the results demonstrate this thickness is slightly increasing by enhancing the Almen intensity of the process and is significantly increased for SSP specimens compared to all other series.
Residual stresses (MPa)
3.3. Roughness measurements NP 40NiCrMo7 CSP2
600 400
NP 39NiCrMo3 CSP3
CSP1 SSP
200 0 -200
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
-400 -600
Depth (mm)
-800
Fig. 1. Distribution of residual stresses obtained by XRD for different shot peened series.
Table 6 shows the surface roughness parameters of the as-received NP, CSP and SSP series. The parameters are based on the definition of ISO 4287 [21]. Increasing roughness is well-recognized as a side effect of shot peening process. The results indicate that surface roughness values rise by increasing the Almen intensity. This observation is verified also by numerical simulations [22]. In the case of specimens treated with severe parameters the obtained surface is noticeably rougher than that of the specimens treated by conventional parameters. It shall be mentioned that roughness has been measured on three specimens for each series and the average values are presented in Table 6. 3.4. Microscopy observations
5
NP 40NiCrMo7 CSP2
FWHM (º)
4
NP 39NiCrMo3 CSP3
CSP1 SSP
3 2 1 0
0
0.1
0.2
0.3
0.4
0.5
Depth (mm) Fig. 2. FWHM profile obtained by XRD for different shot peened series.
Fig. 3 represents the SEM observations of NP and SSP 39NiCrMo3 specimens. From the overall view of the cross sections of SSP specimen, a distinct region separated with sharp boundaries from the underlying layer is easily recognized on the top surface (see Fig. 3b). This layer that represents a very dense structure near the surface as stated by Saitoh et al. [17] is considered to be the fine grained layer. Beneath this region, the severely deformed work hardened area can be seen, while these two regions are separated by sharp boundaries. Fig. 4 shows TEM bright field image and the corresponding selected area diffraction (SAD) pattern obtained at impacted surface of SSP specimen. The bright field image represents irregularly shaped grains the average size of which is measured to be
Table 6 Surface roughness parameters of shot peened specimen.
a b
Treatment (lm)
LT (mm)
LM (mm)
Ra (lm)
Rq (lm)
Rz (lm)
Rt (lm)
NPa NPb CSP1b CSP2b CSP3b SSPa
5.60 5.60 5.60 5.60 5.60 5.60
4.00 4.00 4.00 4.00 4.00 4.00
0.57 1.37 1.49 1.73 2.35 7.53
0.79 1.64 1.88 2.11 2.85 8.98
3.45 5.84 8.50 9.28 12.07 33.90
4.94 6.21 9.80 12.36 14.32 44.34
39CrMoNi3. 40CrMoNi7.
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Fig. 3. Cross-sectional SEM observation (500) of a. NP specimen b. SSP specimen.
Table 7 Fatigue test results (run out = 3 million cycles). Specimen series
CSP1
CSP2
CSP3
SSP
Fatigue limit/fatigue limit of NP series
1.12
1.26
1.33
2.46
Fig. 4. Impacted surface TEM image of the SSP specimen; insert is the correspondent SAD pattern.
50 nm. This mean size is close to the average grain size obtained from in situ high energy synchrotron XRD device which was 56 nm. As it is shown in the upper left corner of Fig. 4, SAD pattern is composed of partially continuous diffraction rings, which confirm that the as-received large crystalline grains have been broken down to nanograins at this region.
Fig. 6. The S–N diagram obtained from fatigue tests.
3.5. Fatigue tests Rotating bending fatigue tests (stress ratio R = 1) have been carried out at room temperature on the as received NP, and
differently shot peened specimens the geometry of which is presented in Fig. 5. The stress concentration factor of the notch for all series is Kt = 2 that is common in many machine elements such
Fig. 5. Fatigue specimen geometry.
S. Bagherifard et al. / Materials and Design 45 (2013) 497–503
501
(a) NP (40NiCrMo3)
(b) CSP1
(c) CSP2
(d) CSP3 Fig. 7. Fracture surface SEM observation of differently treated notched specimens.
as shafts and springs. The tests have been performed following staircase [23] procedure and the fatigue strength corresponding to a fatigue life of 3 million cycles has been calculated through
Hodge–Rosenblatt approach [24]. Since there has been a slight difference in the chemical composition of the considered materials, the obtained results in terms of fatigue limit normalized by fatigue
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(e) SSP Fig. 7. (continued)
limit of NP series of the corresponding material are presented and compared for all series in Table 7. The fatigue test data has been elaborated based on the ASTM standard E739-10 to obtain the S–N diagram for different cases with a failure probability of 50% in bilogarithmic scale, as presented in Fig. 6. As can be observed in Fig. 6 and Table 7, shot peening has improved the fatigue strength with respect to the not peened series in case of both materials. Besides the results of all series clearly indicate growing life improvement trend as the Almen intensity is increased. Indeed among CSP specimens, increasing the Almen intensity from 10–12 N to 4–6 A and eventually to 10–12 A has resulted respectively in 12%, 26% and 33% fatigue life improvement compared to NP series with the same material properties. These results can be attributed to the fact that increasing the Almen intensity causes more extensive compressive residual stress distribution and deeper work-hardened near surface layer. An even more significant improvement is observed in case of SSP specimens. This noteworthy improvement, apart from extensive distribution of residual stresses and work hardening, is also attributed to the presence of the NC layer on the surface of treated specimens as has been previously verified and reported by the authors’ [12,20,25]. The results are aligned with the few data available in the literature about the fatigue strength improvement of surface NC notched specimens, that although are quite disperse in terms of material, specimen geometry, surface NC process and test type, have all indicated of fatigue life improvement through surface nanocrystallization [16–18]. Compared with the surface NC smooth specimens [12], the considerable difference fatigue strength improvement (246% versus 10%) can be attributed to the presence of high stress gradient on the surface of notched specimens. This stress gradient and the different distribution of applied stresses and residual stresses below the surface of the specimen result in crack closure and correspondingly increase the fatigue strength of the treated specimen. 3.6. Fractography observations Study of fatigue crack initiation processes in NC metals has relied mainly on post mortem microscale observations to draw conclusions about the relevant mechanisms. In NC metals, both internal and surface defects have been noted as nucleation sites for fatigue cracks [6,26], while by contrast subsurface initiation is much less common in coarse grained materials. Fracture surface of all tested series of specimens have been observed by SEM. Some examples are presented in Fig. 7. The SEM observations reveal that fatigue fracture mechanism in case of all
applied treatments has started from surface defects, although in case of SSP treated specimens in presence of the NC layer, it was expected to initiate from sub-surface layers. This observation can be contributed to the high density of surface defects and presence of many overlapping dimples and craters on the SSP specimens’ surface. In all series the plane of the fatigue zone is developed perpendicular to the plane of maximum applied stress and the overload (final fracture) zone is observed to be macroscopically brittle. Fig. 7a–d that represent NP and CSP specimens, demonstrate one or two surface crack origins; whereas fracture surface observation of SSP specimens (Fig. 7e), presents numerous crack initiation points. In Fig. 7e, the presence of ratchet marks indicating the boundary between adjacent failure planes implies multiple origins and relatively high stress concentration. This issue highlights the dominating effect of high surface roughness on fatigue resistance of the treated specimens.
4. Conclusions Steel specimens have been treated by severe shot peening processes aimed at surface nanocrystallization. Properties of treated specimens have been investigated through various experimental tests. The obtained results have been compared to the results of a study [19] performed by the authors on another batch of specimens with the same geometry and slightly different chemical composition (40NiCrMo7, UNI 7845) subjected to three different peening processes varying the Almen intensity of the treatment from light treatment to higher intensities used conventionally in the industry for these classes of materials. On the basis of the obtained results the following conclusions can be drawn: X-ray diffraction residual stress measurements confirm that increasing Almen intensity affects a thicker layer of near surface material by compressive residual stresses. The SSP process leads to a significant increase in the depth affected by residual stresses. Rising the process Almen intensity produces a thicker workhardened layer near the surface of treated specimens. In case of SSP treated series a considerable enhancement of the workhardened layer thickness is observed in comparison with the CSP series. Roughness measurements indicate surface roughness parameters’ rise by increasing Almen intensity. Due to very high kinetic energy of impact in SSP treatment, a considerable increase in surface roughness parameters is observed in comparison with all other CSP series.
S. Bagherifard et al. / Materials and Design 45 (2013) 497–503
Scanning electron microscopy and transmission electron microscoy observations confirmed the presence of nanosized grains on the surface of the specimens treated by severe shot peening. Fatigue test results indicate that, by increasing the Almen intensity in the studied range the fatigue life is improved. It is worth noting that notwithstanding the high surface roughness, the SSP specimen has a considerably higher fatigue limit with respect to the other specimens. The remarkable fatigue life enhancement of 246% obtained through application of SSP treatment on notched specimens is very promising and way higher than the improvement obtained from conventional treatments commonly performed on this class of material. Work is still in progress in order to optimize the combination of peening parameters for SSP treatment to reduce the final surface roughness and consequently take more benefit from the presence of NC layer. Acknowledgements This work was financially supported by Ministry of Science and Innovation of Spain under Grant MAT2009-12308. The authors would also like to thank Dr. M. Bandini, for his support and performing shot peening in Peen Service srl. References [1] Almen JO, Black PH. Residual stresses and fatigue in metals. McGraw-Hill Publ.; 1963. [2] Marsh KJ. Shot Peening: techniques and applications. London: EMAS; 1993. [3] Schulze V. Modern Mechanical surface treatment, states, stability, effects. WILEY-VCH; 2006. [4] Bagheri S, Guagliano M. Review of shot peening processes to obtain nanocrystalline surfaces in metal alloys. Surf Eng 2009;25:3–14. [5] Hanlon T, Kwon YN, Suresh S. Grain size effects on the fatigue response of nanocrystalline metals. Scripta Mater 2003;49:675–80. [6] Padilla HA, Boyce BL. A review of fatigue behavior in nanocrystalline metals. Exp Mech 2010;50:5–23.
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[7] Martin U, Altenberger I, Scholtes B, Kremmer K, Oettel H. Cyclic deformation and near surface microstructures of normalized shot peened steel SAE 1045. Mater Sci Eng A 1998;246:69–80. [8] Altenberger I, Scholtes B, Martin U, Oettel H. Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304. Mater Sci Eng A 1999;264:1–16. [9] Mano H, Kondo S, Matsumuro A. Microstructured surface layer induced by shot peening and its effect on fatigue strength. J Japan Inst Metals 2006;70:415–9. [10] Abdullah A, Malaki M, Eskandari A. Strength enhancement of the welded structures by ultrasonic peening. Mater Des 2012;38:7–18. [11] Franchim AS, de Campos VS, Travessa DN, De Moura Neto C. Analytical modelling for residual stresses produced by shot peening. Mater Des 2009;30:1556–60. [12] Bagherifard S, Guagliano M. Fatigue behaviour of a low-alloy steel with a nanostructured surface obtained by severe shot peening. Eng Fract Mech 2012;81:56–68. [13] Fuchs HO. The effect of self stresses on high cycle fatigue. J Test Eval 1982;10:168–73. [14] Harris WJ. Metallic fatigue. Pergamon New; 1961. [15] Fuchs HO, Stephens RI. Metal fatigue in engineering. John Wiley & Sons; 1980. [16] Adachi A. Fatigue strength of gear steels shot peened in extremely high intensity conditions. In: Proceeding of 4th international conference on shot peening, Tokyo, Japan; 1990: 363–372. [17] Saitoh H, Ochi T, Kubota M. Formation of surface nanocrystalline structure in steels by air blast shot peening. In: Proceedings of the 10th international conference on shot peening, Tokyo, Japan; 2008: 488-493. [18] Wen AL, Ren RM, Wang S, Yang JY. Effect of surface nanocrystallization method on fatigue strength of TA2. Mater Sci Forum 2009;620–622:545–9. [19] Bagherifard S, Fernandez Pariente I, Guagliano M, Bandini M. Fatigue analysis of shot peened smooth and notched steel specimens. In: Proceeding of 11th international conference on shot peening, South Bend, Indiana, USA; 2011. [20] Bagherifard S, Ghelichi R, Guagliano M. A numerical model of severe shot peening (SSP) to predict the generation of a nanostructured surface layer of material. Surf Coat Technol 2010;204:4081–90. [21] ISO Standard 4278. Geometrical product specifications (GPS) – surface texture: profile method – terms, definitions and surface texture parameters. 1st ed.; 1997. [22] Bagherifard S, Ghelichi R, Guagliano M. Numerical and experimental analysis of surface roughness generated by shot peening. Appl Surf Sci 2012;258– 18:6831–40. [23] Dixon W, Massey F. Introduction to statistical analysis. McGraw-Hill; 1969. [24] Brownlee KA, Hodges JL, Rossenblatt JRM. The up-and-down method with small samples. J Amer Statist Assoc 1953;48:262–77. [25] Bagherifard S, Fernandez Pariente I, Ghelichi R, Guagliano M. Fatigue properties of nanocrystallized surfaces obtained by high energy shot peening. Procedia Eng 2010;2:1683–90. [26] Jiang XP, Wang XY, Li JX, Li DY, Man CS, Shepard MJ, et al. Enhancement of fatigue and corrosion properties of pure Ti by sandblasting. Mater Sci Eng A 2006;429:30–5.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Fatigue behavior of notched steel specimens with nanocrystallized surface obtained by severe shot peening S. Bagherifard a,⇑, I. Fernandez-Pariente b, R. Ghelichi a, M. Guagliano a a b
Politecnico di Milano, Department of Mechanical Engineering, via G. La Masa, 1 Milano, Italy University of Oviedo, Department of Material Science and Metallurgical Engineering, Gijón, Spain
a r t i c l e
i n f o
Article history: Received 18 July 2012 Accepted 13 September 2012 Available online 26 September 2012
a b s t r a c t Among severe plastic deformation methods that result in surface nanocrystallization, shot peening has proved to be a promising technique. Application of severe air blast shot peening results in surface nanocrystallization, affects a thick layer of material with high compressive residual stresses but at the same time produces rather high surface roughness. In this study notched specimens with a stress concentration factor common in many structural components have been subjected to severe shot peening process. The mentioned treatment uses peening parameters essentially different from conventional ones. Roughness and X-ray diffraction residual stress measurements as well as microscopy observations have been carried out on the treated specimens. Room temperature rotating bending fatigue tests are performed to evaluate the effect of the treatment on specimens’ fatigue strength. Fracture surfaces have been then observed by scanning electron microscopy. The results indicate a very significant fatigue strength improvement for severely shot peened specimens in spite of their very high surface roughness. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Shot peening (SP) is a mechanical surface treatment generally applied to improve fatigue behavior of metallic components. During the process propelled stream of thousands of small spheres impact the surface of the work piece. The process is aimed to create compressive residual stresses on the surface and a thin layer underneath it and also to work harden the near surface layer of material. These effects are very useful in order to totally prevent or greatly delay the failure of the part [1–3]. Recent advancement of SP has revealed that particular peening processes, different from conventional air blast shot peening (ABSP), can be aimed at achieving ultrafine grained materials on the surface of treated parts [4]. It is well-known that fatigue properties of materials are highly sensitive to the grain size. A small grain size can enhance the fatigue crack initiation threshold and coarse grains may deflect the propagation paths of fatigue cracks by grain boundaries, thus introducing crack closure and decreasing the rate of crack growth [5]. Since most fatigue cracks initiate from the surface and propagate to the interior, a component with a nanocrystallized (NC) surface layer and coarse grained interior is expected to show improved fatigue properties [6]. Especially in case of surface nanocrystallization shot peening based processes,
⇑ Corresponding author. Tel.: +39 02 23998667; fax: +39 02 23998263. E-mail addresses: [email protected] (S. Bagherifard), inesfp@ uniovi.es (I. Fernandez-Pariente), [email protected] (R. Ghelichi), [email protected] (M. Guagliano). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.025
the induced residual compressive stresses can also effectively delay the propagation of fatigue cracks [7–11]. This study is performed to continue an authors’ preceding research that focused on application of severe shot peening (SSP) on fatigue strength of smooth steel specimens [12]. The developed SSP process is performed using a standard ABSP device applying a combination of severe peening parameters to multiply the kinetic energy of the conventional shot peening (CSP). In the previous study, generation of NC surface layer by the proposed peening parameters was experimentally confirmed through transmission electron microscopy (TEM) observation and grain size measurements [12]. The results of the roughness measurements indicated considerable increase in surface roughness parameters of SSP specimens that obviously had notable deteriorating influence on fatigue strength. Notwithstanding the high surface roughness, the smooth surface NC specimens showed a better fatigue strength with respect to the not peened (NP) series. Different approaches were applied to decrease the deteriorating effect of high surface roughness on fatigue strength of smooth specimens. Better results were obtained through repeening by smaller and harder shots without removing NC layer, even if roughness parameters did not vary appreciably after repeening. Along these lines, despite the considerably high surface roughness, a fatigue strength improvement of 10% in comparison with NP series was obtained for smooth specimens. However, SP is well-recognized to be more efficient in increasing fatigue life of notched components with high stress gradient compared to smooth ones [13–15]. In notched specimens, the high
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stress gradient just under the notch, results in considerable decrease in stress level while the crack is still very small, thus decreasing crack growth rate. Residual stresses have a minor effect on the resistance to the development of microscopic cracks. On the other hand they have a very strong effect on the resistance to the propagation of small cracks and may arrest the crack growth. The effect of surface work hardening shall be also taken into consideration. Very few studies have been performed on fatigue behavior of notched surface NC material obtained by mechanical treatment. Adachi [16] performed rotating bending fatigue tests on case-hardened gear steel with a stress concentration factor of Kt = 3, shot peened by extremely high intensity conditions. Microstructural observation of the specimens revealed the presence of a near surface white etching layer. Although Adachi at that time did not comment the presence of ultrafine grains on the surface of treated specimens, based on recent theories this white etching layer could have been the sign of surface nanocrystallization [17]. The results indicated 200% improvement in fatigue life with respect to NP specimens [16]. Wen et al. [18] studied the fatigue strength of pure titanium treated by high energy shot peening (HESP), through rotating bending fatigue tests performed on notched specimen (Kt = 1.27). They mentioned that surface damage, roughness, and edge of specimen, reduced the fatigue limit and in some cases even caused the function of NC layer to be counteracted. Repeening using low energy small shots was applied to decrease the surface roughness up to 40%. Comparing with the fatigue limit of NP specimens, the results were increased up to 9% by CSP, up to 34% by HESP and up to 52% after repeening [18]. To the authors’ best knowledge these are the only studies performed on fatigue strength of notched surface NC specimens available in the literature. Nevertheless, a feature of the available data which bears discussion is the fact that the experiments have been performed on different materials that are surface NC through various processes and are tested using different set ups, specimen geometries and consequently produce disparate and essentially different results. This highlights an important challenge in assessing the mechanical behavior of surface NC metals; as indicated also by Padilla and Boyce [6] variations in microstructure and test method can have significant effects on the results. In this study, in order to investigate the effect of the proposed SSP process on notched components’ fatigue life, two series of notched specimens including NP and SSP series, were fatigue tested. Treated specimens have been characterized using X-ray diffraction (XRD) and roughness measurements as well as scanning electron microscopy (SEM) and TEM observations. SEM fractography observations have been performed on the fracture surface of broken specimens. In addition, to underline the notable effect of SSP process, the obtained results have been compared to the fatigue strength improvement of specimens with the same geometry but a slightly different material composition, treated with usual peening parameters. The final results are then critically discussed. 2. Material and experimental procedure Low alloy steel (39NiCrMo3, UNI EN 10083) notched specimens are subjected to severe ABSP. The nominal chemical composition of
Table 2 Aspects of the SSP treatment. Treatment
Shot type and diameter (mm)
Almen intensity (0.0001 in.)
Coverage (%)
SSP
S230 (steel, / = 0.58)
6–7 C
1500
this steel is shown in Tables 1 and 2 shows the applied SP parameters. To study the state of residual stresses, XRD analysis was performed on all series of specimens using an AST X-Stress 3000 Xray diffractometer (radiation Cr Ka, irradiated area 1 mm2, sin2 w method, diffraction angles (2h) scanned between 45° and 45°). Measurements were carried out in depth step by step removing a very thin layer of material using an electro-polishing device in order to obtain the in-depth trend of residual stresses. A solution of acetic acid (94%) and perchloric acid (6%) was used for electro-polishing. Microstructure observations were performed by a Zeiss EVO50 SEM with thermionic source. Specimens for SEM observations were etched by 2% Nital. TEM observations were carried out using a Philips CM12 microscope operating at 120 kV. To perform TEM observations very thin pieces of specimens were first cut by electrical discharge machine, then were mechanically polished from the untreated side and finally the last step of thinning was performed by means of ion milling with proper incident angles. Microscopy observations and roughness measurements were also performed on treated surfaces. Room temperature rotating bending fatigue tests were executed to investigate the effect of the applied process on fatigue strength. In order to highlight the significant improvement due to SSP treatment with respect to conventional SP parameters, the results of a preceding study performed by the authors [19] on another batch of specimens with the same geometry and material with a slightly different chemical composition (40NiCrMo7, UNI 7845) treated by three different conventional shot peening (CSP) set of parameters, are also provided in this paper. The material nominal chemical composition, the CSP parameters as well as a comparison of mechanical properties of the two low alloy steels are respectively presented in Tables 3–5. 3. Results and discussion 3.1. Analysis of residual stress profile XRD measurements demonstrate trivial values of residual stresses for NP specimens as expected, apart from vey local high tensile surface stresses in case of NP 40NiCrMo7 that is potentially due to the machining process. The results imply that increasing the applied Almen intensity that is to raise the kinetic energy of impacts, for all the three CSP series results in increase in on-surface and maximum values of residual stresses. Regarding SSP treated specimen, a considerable depth of material is characterized by significant compressive residual stresses. This is verified by comparing the in-depth residual stress trend of other series including NP, CSP1, CSP2 and CSP3 with SSP specimens as shown in Fig. 1. The SSP specimens show lower on-surface stress and maximum stress values with respect to other series, whereas the thickness of layer affected by considerable compressive residual stresses is almost 5 times that of other specimens.
Table 1 Nominal chemical composition of 39NiCrMo3 steel in mass density. C (wt.%)
Si (wt.%) max
Mn (wt.%)
P wt(wt.%) max
S (wt.%) max
Cr (wt.%)
Mo (wt.%)
Ni (wt.%)
0.35–0.43 ±0.02
0.40 ±0.03
0.5–0.8 ±0.04
0.025 +0.005
0.035 ±0.005
0.60–1.00 ±0.05
0.15–0.25 ±0.03
0.70–1.00 ±0.05
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S. Bagherifard et al. / Materials and Design 45 (2013) 497–503 Table 3 Nominal chemical composition of 40NiCrMo7 steel in mass density. C (wt.%)
Si (wt.%) max
Mn (wt.%)
P wt(wt.%) max
S (wt.%) max
Cr (wt.%)
Mo (wt.%)
Ni (wt.%)
0.37–0.44 ±0.02
0.15–0.40 ±0.03
0.50–0.80 ±0.04
0.035 +0.005
0.035 ±0.005
0.60–0.90 ±0.05
0.20–0.30 ±0.03
1.60–1.90 ±0.05
3.2. Estimation of work-hardened layer thickness
Table 4 Aspects of the conventional shot peening treatment. Treatment
Shot type and diameter (mm)
Almen Intensity (0.0001 in.)
Coverage (%)
CSP1 CSP2 CSP3
Z100 (ceramic, / = 0.1) S110 (steel, / = 0.3) S170 (steel, / = 0.43)
10–12 N 4–6 A 10–12 A
100 100 100
Table 5 Nominal mechanical properties of 39NiCrMo3 and 40NiCrMo7 steel. Material type
E (GPa)
Yield stress (MPa)
Ultimate stress (MPa)
A (%)
39NiCrMo3 40NiCrMo7
210 203
734 1170
908 1292
14.8 14.9
Another parameter measured by XRD is FWHM. This parameter, the distribution of which is shown in Fig. 2, represents the full width of the diffraction peak at half of the maximum intensity. It can be assumed as an index of hardening of the material [12]. As it is observed in Fig. 2 the on-surface value of FWHM is growing with increasing the Almen intensity of the peening process i.e. increasing the kinetic energy of the impacts. This issue has been validated also numerically in authors’ previous study [20]. It is to be noted that the thickness of the work-hardened layer can be estimated as the thickness of the layer which shows considerably increased FWHM values in comparison with the core material. As the results demonstrate this thickness is slightly increasing by enhancing the Almen intensity of the process and is significantly increased for SSP specimens compared to all other series.
Residual stresses (MPa)
3.3. Roughness measurements NP 40NiCrMo7 CSP2
600 400
NP 39NiCrMo3 CSP3
CSP1 SSP
200 0 -200
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
-400 -600
Depth (mm)
-800
Fig. 1. Distribution of residual stresses obtained by XRD for different shot peened series.
Table 6 shows the surface roughness parameters of the as-received NP, CSP and SSP series. The parameters are based on the definition of ISO 4287 [21]. Increasing roughness is well-recognized as a side effect of shot peening process. The results indicate that surface roughness values rise by increasing the Almen intensity. This observation is verified also by numerical simulations [22]. In the case of specimens treated with severe parameters the obtained surface is noticeably rougher than that of the specimens treated by conventional parameters. It shall be mentioned that roughness has been measured on three specimens for each series and the average values are presented in Table 6. 3.4. Microscopy observations
5
NP 40NiCrMo7 CSP2
FWHM (º)
4
NP 39NiCrMo3 CSP3
CSP1 SSP
3 2 1 0
0
0.1
0.2
0.3
0.4
0.5
Depth (mm) Fig. 2. FWHM profile obtained by XRD for different shot peened series.
Fig. 3 represents the SEM observations of NP and SSP 39NiCrMo3 specimens. From the overall view of the cross sections of SSP specimen, a distinct region separated with sharp boundaries from the underlying layer is easily recognized on the top surface (see Fig. 3b). This layer that represents a very dense structure near the surface as stated by Saitoh et al. [17] is considered to be the fine grained layer. Beneath this region, the severely deformed work hardened area can be seen, while these two regions are separated by sharp boundaries. Fig. 4 shows TEM bright field image and the corresponding selected area diffraction (SAD) pattern obtained at impacted surface of SSP specimen. The bright field image represents irregularly shaped grains the average size of which is measured to be
Table 6 Surface roughness parameters of shot peened specimen.
a b
Treatment (lm)
LT (mm)
LM (mm)
Ra (lm)
Rq (lm)
Rz (lm)
Rt (lm)
NPa NPb CSP1b CSP2b CSP3b SSPa
5.60 5.60 5.60 5.60 5.60 5.60
4.00 4.00 4.00 4.00 4.00 4.00
0.57 1.37 1.49 1.73 2.35 7.53
0.79 1.64 1.88 2.11 2.85 8.98
3.45 5.84 8.50 9.28 12.07 33.90
4.94 6.21 9.80 12.36 14.32 44.34
39CrMoNi3. 40CrMoNi7.
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Fig. 3. Cross-sectional SEM observation (500) of a. NP specimen b. SSP specimen.
Table 7 Fatigue test results (run out = 3 million cycles). Specimen series
CSP1
CSP2
CSP3
SSP
Fatigue limit/fatigue limit of NP series
1.12
1.26
1.33
2.46
Fig. 4. Impacted surface TEM image of the SSP specimen; insert is the correspondent SAD pattern.
50 nm. This mean size is close to the average grain size obtained from in situ high energy synchrotron XRD device which was 56 nm. As it is shown in the upper left corner of Fig. 4, SAD pattern is composed of partially continuous diffraction rings, which confirm that the as-received large crystalline grains have been broken down to nanograins at this region.
Fig. 6. The S–N diagram obtained from fatigue tests.
3.5. Fatigue tests Rotating bending fatigue tests (stress ratio R = 1) have been carried out at room temperature on the as received NP, and
differently shot peened specimens the geometry of which is presented in Fig. 5. The stress concentration factor of the notch for all series is Kt = 2 that is common in many machine elements such
Fig. 5. Fatigue specimen geometry.
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501
(a) NP (40NiCrMo3)
(b) CSP1
(c) CSP2
(d) CSP3 Fig. 7. Fracture surface SEM observation of differently treated notched specimens.
as shafts and springs. The tests have been performed following staircase [23] procedure and the fatigue strength corresponding to a fatigue life of 3 million cycles has been calculated through
Hodge–Rosenblatt approach [24]. Since there has been a slight difference in the chemical composition of the considered materials, the obtained results in terms of fatigue limit normalized by fatigue
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(e) SSP Fig. 7. (continued)
limit of NP series of the corresponding material are presented and compared for all series in Table 7. The fatigue test data has been elaborated based on the ASTM standard E739-10 to obtain the S–N diagram for different cases with a failure probability of 50% in bilogarithmic scale, as presented in Fig. 6. As can be observed in Fig. 6 and Table 7, shot peening has improved the fatigue strength with respect to the not peened series in case of both materials. Besides the results of all series clearly indicate growing life improvement trend as the Almen intensity is increased. Indeed among CSP specimens, increasing the Almen intensity from 10–12 N to 4–6 A and eventually to 10–12 A has resulted respectively in 12%, 26% and 33% fatigue life improvement compared to NP series with the same material properties. These results can be attributed to the fact that increasing the Almen intensity causes more extensive compressive residual stress distribution and deeper work-hardened near surface layer. An even more significant improvement is observed in case of SSP specimens. This noteworthy improvement, apart from extensive distribution of residual stresses and work hardening, is also attributed to the presence of the NC layer on the surface of treated specimens as has been previously verified and reported by the authors’ [12,20,25]. The results are aligned with the few data available in the literature about the fatigue strength improvement of surface NC notched specimens, that although are quite disperse in terms of material, specimen geometry, surface NC process and test type, have all indicated of fatigue life improvement through surface nanocrystallization [16–18]. Compared with the surface NC smooth specimens [12], the considerable difference fatigue strength improvement (246% versus 10%) can be attributed to the presence of high stress gradient on the surface of notched specimens. This stress gradient and the different distribution of applied stresses and residual stresses below the surface of the specimen result in crack closure and correspondingly increase the fatigue strength of the treated specimen. 3.6. Fractography observations Study of fatigue crack initiation processes in NC metals has relied mainly on post mortem microscale observations to draw conclusions about the relevant mechanisms. In NC metals, both internal and surface defects have been noted as nucleation sites for fatigue cracks [6,26], while by contrast subsurface initiation is much less common in coarse grained materials. Fracture surface of all tested series of specimens have been observed by SEM. Some examples are presented in Fig. 7. The SEM observations reveal that fatigue fracture mechanism in case of all
applied treatments has started from surface defects, although in case of SSP treated specimens in presence of the NC layer, it was expected to initiate from sub-surface layers. This observation can be contributed to the high density of surface defects and presence of many overlapping dimples and craters on the SSP specimens’ surface. In all series the plane of the fatigue zone is developed perpendicular to the plane of maximum applied stress and the overload (final fracture) zone is observed to be macroscopically brittle. Fig. 7a–d that represent NP and CSP specimens, demonstrate one or two surface crack origins; whereas fracture surface observation of SSP specimens (Fig. 7e), presents numerous crack initiation points. In Fig. 7e, the presence of ratchet marks indicating the boundary between adjacent failure planes implies multiple origins and relatively high stress concentration. This issue highlights the dominating effect of high surface roughness on fatigue resistance of the treated specimens.
4. Conclusions Steel specimens have been treated by severe shot peening processes aimed at surface nanocrystallization. Properties of treated specimens have been investigated through various experimental tests. The obtained results have been compared to the results of a study [19] performed by the authors on another batch of specimens with the same geometry and slightly different chemical composition (40NiCrMo7, UNI 7845) subjected to three different peening processes varying the Almen intensity of the treatment from light treatment to higher intensities used conventionally in the industry for these classes of materials. On the basis of the obtained results the following conclusions can be drawn: X-ray diffraction residual stress measurements confirm that increasing Almen intensity affects a thicker layer of near surface material by compressive residual stresses. The SSP process leads to a significant increase in the depth affected by residual stresses. Rising the process Almen intensity produces a thicker workhardened layer near the surface of treated specimens. In case of SSP treated series a considerable enhancement of the workhardened layer thickness is observed in comparison with the CSP series. Roughness measurements indicate surface roughness parameters’ rise by increasing Almen intensity. Due to very high kinetic energy of impact in SSP treatment, a considerable increase in surface roughness parameters is observed in comparison with all other CSP series.
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Scanning electron microscopy and transmission electron microscoy observations confirmed the presence of nanosized grains on the surface of the specimens treated by severe shot peening. Fatigue test results indicate that, by increasing the Almen intensity in the studied range the fatigue life is improved. It is worth noting that notwithstanding the high surface roughness, the SSP specimen has a considerably higher fatigue limit with respect to the other specimens. The remarkable fatigue life enhancement of 246% obtained through application of SSP treatment on notched specimens is very promising and way higher than the improvement obtained from conventional treatments commonly performed on this class of material. Work is still in progress in order to optimize the combination of peening parameters for SSP treatment to reduce the final surface roughness and consequently take more benefit from the presence of NC layer. Acknowledgements This work was financially supported by Ministry of Science and Innovation of Spain under Grant MAT2009-12308. The authors would also like to thank Dr. M. Bandini, for his support and performing shot peening in Peen Service srl. References [1] Almen JO, Black PH. Residual stresses and fatigue in metals. McGraw-Hill Publ.; 1963. [2] Marsh KJ. Shot Peening: techniques and applications. London: EMAS; 1993. [3] Schulze V. Modern Mechanical surface treatment, states, stability, effects. WILEY-VCH; 2006. [4] Bagheri S, Guagliano M. Review of shot peening processes to obtain nanocrystalline surfaces in metal alloys. Surf Eng 2009;25:3–14. [5] Hanlon T, Kwon YN, Suresh S. Grain size effects on the fatigue response of nanocrystalline metals. Scripta Mater 2003;49:675–80. [6] Padilla HA, Boyce BL. A review of fatigue behavior in nanocrystalline metals. Exp Mech 2010;50:5–23.
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