Surface modification of 17-4 PH stainless steel by laser peening without protective coating process

Surface & Coatings Technology 278 (2015) 138–145

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Surface modification of 17-4 PH stainless steel by laser peening without protective coating process D. Karthik, S. Kalainathan ⁎, S. Swaroop School of Advanced Sciences, VIT University, Vellore, 632 014 Tamilnadu, India

a r t i c l e

i n f o

Article history: Received 26 April 2015 Revised 6 August 2015 Accepted in revised form 7 August 2015 Available online 12 August 2015 Keywords: Stainless steel Laser peening without protective coating (LPwC) Residual stress Micro-hardness Surface roughness and topography

a b s t r a c t The objective of the present study was to examine the influence of laser peening without protective coating (LPwC) process on surface mechanical properties of precipitation hardened stainless steel 17-4 PH. Pulse densities, 2500 and 6250 pulses cm−2, using a constant laser power density of 5.97 GW cm−2 were chosen and their effect on residual stresses, micro-structure, micro-hardness, surface roughness and topography were investigated. Higher pulse density induced large compressive residual stresses and influenced up to depth of 600 μm. It was confirmed from resulted larger lattice strain in LPwC processed 17-4 PH steel with higher pulse density. No phase transformation and microstructural change were observed, which represented pure mechanical effect of LPwC. Hardness was increased with depth of hardened layer up to 600 μm for higher pulse density. However, the surface roughness was increased by the process, and resulted in more pronounced peening pattern. Surface topography revealed columnar structure after LPwC process. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The 17-4 PH is precipitation hardened, low carbon martensitic stainless steel, widely used in many industrial applications including oil field valve parts, chemical process equipment, aircraft fittings, fasteners, pump shafts, nuclear reactor components, gears, paper mill equipment, missile fittings, and jet engine parts because of its variable strength through heat treatment and moderate corrosion resistance [1–3]. Nevertheless, Arisoy et al. [4] report on stress corrosion cracking (SCC) failure of 17-4 PH steel sail boat propeller shaft exposed to fluctuating stress and in marine environment. Further, Mazur et al. [5] report on failure of steam turbine blades under high cycle fatigue regime of 17-4 PH steel. Since these failures initiate at surface, need of a proper surface treatment is important. Examples of such surface treatments reported in literature are Shot Peening (SP) and Low Plasticity Burnishing (LPB). Variations in the above techniques such as dual SP (i.e. peening SP treated surface again with lower diameter ceramic balls to reduce the previously introduced surface roughness) are also reported. Zhang et al. [6–9] investigated influence dual SP on laser hardened on 17-4 PH steel and reported improved hardness, large near surface compressive residual stresses, reduction in retained austenite and micro-strain and fatigue life improvement. Prevey et al. [10–12] in their studies showed improved high cycle fatigue life; reduction in foreign object damage and improved corrosion fatigue properties by LPB treatment. ⁎ Corresponding author. E-mail addresses: [email protected] (D. Karthik), [email protected] (S. Kalainathan), [email protected] (S. Swaroop).

http://dx.doi.org/10.1016/j.surfcoat.2015.08.012 0257-8972/© 2015 Elsevier B.V. All rights reserved.

However, these surface treatments have certain disadvantages. For example, there was introduced surface roughness with microdefects and lower depth of compressive residual stresses after dual SP [6–9]. Moreover, it is important to note that further removal of roughened layer introduced by SP may partially or completely undermine the effect of induced compressive residual stress. These factors indicate the need for a more efficient surface modification method such as Laser Shock Peening (LSP) for 17-4 PH steel. LSP is a promising surface treatment investigated first time practically by Fairand and Clauer [13] in 1978. LSP involves an interaction of short duration laser pulses with material surface, creating of plasma which is further confined by a medium to enhance the developed shock wave pressure (several Giga Pascals) and propagation of the waves into the material resulting in plastic deformation when this pressure exceeds material's Hugoniot Elastic Limit [14–18]. LSP is superior to SP in developing large, deep compressive residual stresses with lower surface roughness [15]. There are two common methods of LSP in practice. First, method involves LSP using protective coating like black paint or Al foils or Vinyl tape on the surface to be treated [19] and the second uses no such coatings, called LSP without protective coating (LSPwC or LPwC), developed first time by Mukai et al. in 1995 [20]. Advantages of LPwC over LSP with coating include ease in delivering laser pulses through optical fibers (due to low pulse energy), possibility to under water components of nuclear power plants [21]. Ultrahigh strain rate plastic deformation mechanism associated with LSP resulted in grain refinement ranging from 50 to 200 nm and was reported for various alloys such as LY2 Al [22], ANSI 304 [23], NiTi shape memory alloy [24], AISI 4140 [25], AA6082-T651 [26]. Grain refinement usually occurs through processes involving formation of

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Fig. 1. Laser peening without coating setup and sweep directions.

increased dislocation lines (large dislocation density), dislocation tangles, dense dislocation walls and cells, and recrystallization as in the case of AA6082-T651 [26]. Further, increased passivity, polarization resistance and reduction in number of pits, corrosion current and SCC resistance and corrosion resistance improvement were observed in different alloys with significant grain refinement in few cases [27–33]. Furthermore, increased fatigue life, reduced fatigue crack growth, increased fracture toughness and wear resistance in various other alloys were reported [34–40]. Objective of present study was to improve the surface mechanical properties of 17-4 PH steel by LPwC process and to investigate resulting residual stress distribution and its dependence on lattice strain, microstructure, surface roughness and topography and micro-hardness. 2. Material and methods

dichroic mirror and focused on specimen surface by a bi-convex lens of focal length 300 mm. Spot diameter of 0.8 mm at specimen was maintained by precisely adjusting motorized vertical translation stage (Standa, Lithuania). As indicated in Fig. 1, LPwC sweep in longitudinal (L) and transverse (T) directions of specimen surface over 1 cm × 1 cm area was carried out using a 2D motorized translation stage (SVP lasers, India). A square tray (20 × 20 cm2 and 10 cm height) having 20 mm height stage of 10 cm2 area was used to hold the specimen. A water jet set up was employed and controlled to maintain bubble free, uniform confinement layer with tap water of thickness nearly 1–2 mm on the surface of the specimen to be treated, and to confine plasma developed during laser–target interaction and also to replace laser ablated particles to insure a pure laser–matter interaction. Taking shock impedance of target (Zsteel = 3.96 × 106 g cm−2) [35] and that of confining medium (Zwater = 0.165 × 106 g cm−2) [16], peak pressure of shock wave for power density, I0 = 5.97 GW cm−2 was estimated from [17],

2.1. Material preparation  Specimens of dimension 30 mm × 30 mm × 3 mm were prepared by Electric Discharge Machining (EDM) from as received 17-4 PH steel. The tensile strength, yield strength and elongation of 17-4 PH steel are respectively 620 MPa, 450 MPa and 25% [41]. Chemical composition of 17-4 PH steel estimated using Spark Analyser (Thermo Electron, USA) was 0.048 C, 0.017 P, 0.002 S, 15.24 Cr, 4.66 Ni, 3.16 Cu, 0.45 Mn, 0.5 Si, 0.35 Nb + Ta and 74.84 Fe (all in wt.%). Before LPwC treatment, all specimens were solution annealed at 1311 K for 1 h, water quenched and tempered at 1090 K for 0.5 h and 843 K for 3 h and furnace cooled to have good mechanical strength [6–9] and mechanically polished. 2.2. Laser peening without protective coating process LPwC treatment was carried out using Q-switched Nd:YAG laser (Litron, UK), operated at fundamental wavelength of 1064 nm, energy of 300 mJ, repetition rate of 10 Hz and pulse duration of 10 ns. The profile of the laser beam was Gaussian with beam divergence of output ≤ 0.5 mrad (full angle measured at FWHM points) and pointing stability of less than ± 70 μrad. Laser pulses were reflected using a

P ðGPaÞ ¼ 0:01

zI 0 α 2α þ 3

1=2 ð1Þ

where, α = 0.1 is the efficiency of laser matter interaction and Z = 2 / (1 / Zsteel + 1 / Zwater) is shock impedance between Zsteel and Zwater. The peak pressure of shock wave in present case was 8.41 GPa. The scanning velocity in L and T directions was varied in such a way to achieve pulse densities of 2500 and 6250 pulses cm−2 with overlapping rate 90% in L and 40% in T directions. Table 1 Parameters used in residual stress measurement. Characteristic X-ray X-ray tube voltage X-ray tube current Diffractive plane Diffraction angle Range of sin2ψ X-ray irradiated area

Cu Kα1 45 kV 40 mA (2 1 1) 82.3° 0–0.5 2 mm2

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Fig. 2. Residual stresses distribution in depth direction.

2.3. LPwC induced residual stress Followed by LPwC, residual stresses were estimated from 50 μm depth of treated surface (in order to avoid ablation introduced by thermal effect of LPwC) using electro polishing with 80% methanol and 20% perchloric acid solution at 18 V, 130–150 s. For further evaluation (up to 600 μm) successive layers were removed. Principal residual stresses along L and T directions were obtained for (2 1 1) plane using X'pert Pro system (PANalytical, Netherlands) with standard XRD sin2Ψ method [42]. Parameters used in residual stress measurement are given in Table 1. 2.4. X-ray diffraction and microstructure High resolution XRD instrument (Brucker, USA) was employed on each specimen (after removing a layer 50 μm thickness) for Bragg angles ranging from 35° to 120° with a step size of 0.02°. From recorded Bragg reflections, lattice strain was evaluated quantitatively from peak shift and Williamson–Hall plot. Microstructures of these specimens

were analysed by a scanning electron microscope (SEM) (ZEISS EVO 18, Germany) at a depth of 50 μm. 2.5. Micro-hardness Unpeened and LPwC treated specimens were cut by EDM and their micro-hardness from peened surface in steps of 100 μm up to 2000 μm was measured using Vickers micro-hardness tester (Mitutoyo, Japan) with a load of 200 g and holding time of 10 s. 2.6. Surface roughness and topography Profile roughness parameters on LPwC specimens at untreated and treated regions along L, T directions were measured with surface profilometer (MarSurf, Germany), using roughness filter cut off 0.8 mm by moving profile to 5.6 mm in each measurement. Further, macroimages using Tesa Visio 300 DCC (Tesa group, Switzerland) with resolution of 0.001 mm and photographs of these specimens were recorded to investigate surface features. Furthermore, 3D surface topography were

Fig. 3. XRD pattern and W–H plot of unpeened and LPwC specimens recorded at a depth of 50 μm. (a) XRD patterns (b) Peak shifts in (2 1 1) plane and (c) W–H plot.

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3. Results and discussion

Table 2 Lattice strain (ε) evaluated from W–H plot. Specimen

Lattice strain ε (%)

Unpeened 2500 pulses cm−2 6250 pulses cm−2

0.14 0.54 0.69

recorded using an atomic force microscope (AFM) (Nanosurf easyScan 2, Switzerland) equipped with cantilever of length 450 μm, width of 45 μm, thickness of 1.5 μm, tip height of 12 μm and spring constant of 0.15 Nm−1 and vertical resolution of 0.2 nm.

Fig. 4. SEM microstructure of (a) unpeened, (b) 2500 pulses cm−2 and (c) 6250 pulses cm−2 specimens.

3.1. LPwC induced residual stress Depth wise residual stresses (RS) distribution is shown in Fig. 2. Unpeened specimen exhibits large tensile RS throughout measured depths. In contrast to Wang et al. [6–9] studies on 17-4 PH steel, where nearly zero RS throughout the depth (nearly up to 700 μm in 10 mm thick specimen) after step of heat treatments identical to present study and subsequent electro-polishing was reported, existing large tensile RS in untreated specimen could be the reason no such layer removal involved. However, the objective in the present study was to examine RS distribution irrespective of the initial stress state and considering the service conditions in industry where the components to be peened are seldom in stress free state. It is further supported by an earlier study by Sano et al. [41] where beneficial effect of LPwC with large initial tensile RS state was reported. In LPwC processed specimen with 2500 pulses cm−2, lower tensile at 50 μm and compressive at 100 μm and nearly zero residual stresses in further depths were observed. It implies that no appreciable plastic deformation occurred due to initial stress state, high strength of unpeened specimen and lower pulse density used. This result is consistent with Altenberger et al. [37] report, where, high tensile RS after LSP was observed due to larger strength of beta titanium alloy (Timetal LCB). On the other hand, for 6250 pulses cm−2, pure compressive stresses showing higher value in T direction (− 521 ± 6 MPa) compared to L direction (−293 ± 4 MPa) at 50 μm depth were induced. It was gradually decreased further and approached zero at a depth of 600 μm. Gomez-Rosas et al. [14] reported similar trend where, high compressive RS within 100 μm depth from treated surface and gradual decrease thereafter were observed. Generally, depth of RS depends on laser spot diameter and pulse density. For instance, Trdan et al. [26] and Peyre et al. [38] in their studies reported larger compressive RS near treated surface with smaller plastically affected depth for smaller laser beam spot diameter. In another study on aluminium alloy [34], it was reported that depth of compressive extended up to 1.4 mm with spot diameter of 1.5 mm. Hence, it can be concluded that smaller spot diameter (0.8 mm) used in the present study could be the reason of this smaller plastically affected depth (600 μm). Another factor to be noted is anisotropy in RS distribution which is more for higher pulse density in our study. This can be addressed by type of LPwC scanning (zig-zag) involved. For instance, Correa et al. [46] suggested a random LSP/LPwC scan type model to reduce RS anisotropy. Therefore,

Fig. 5. Micro-hardness profile of unpeened and LPwC treated specimens.

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Table 3 Profile roughness parameters. Specimen

Unpeened 2500 pulses cm−2 6250 pulses cm−2

Ra (μm)

Rz (μm)

Increment in Ra (%)

In L

In T

In L

In T

In L

In T

0.0684 1.1052 1.0665

0.0684 1.1153 1.1328

0.8481 4.9263 5.2753

0.8444 5.7830 6.1600

– 15.15 14.59

– 15.30 15.56

Peyre et al. [15] report on 316L, where, due larger number impacts compared to LSP, phase transformation resulted drastically after SP process. Further, larger peak shift in (2 1 1) plane (preferred as it was used for residual stress analysis) as indicated in Fig. 3b, was occurred for higher pulse density. From this, lattice strain (ε) quantitatively evaluated through lattice spacing of LPwC specimens (d) and unpeened specimen (d0) [43],

Ra — Average roughness; Rz — mean roughness depth and L, T — LPwC sweep directions.

ε¼ observed anisotropy in this study can be explained by involved zig-zag type LPwC scan.

d−d0 : d0

ð2Þ

The results show large lattice strain (0.41) for higher pulse density than lower pulse density (0.39). On the other hand, ε was evaluated from Williamson–Hall plot using the relation [44],

3.2. X-ray diffraction and microstructure X-ray diffraction analysis in order to investigate the effect of LPwC induced residual stress on phase transformation; lattice strain through peak shift and Williamson–Hall plot calculations was carried out. Prominent martensite peaks with two less intense austenite peaks were observed from Bragg reflections for unpeened and LPwC specimens (Fig. 3a). It is clear from Fig. 3a that no phase transformation induced after LPwC process due to lesser number impacts and high strengthened phase of unpeened sample in the present study. It is consistent with

β cosθ ¼

kλ þ ε sinθ L

ð3Þ

where, β -peak broadening, k-shape constant (taken to be unity), L-Crystallite size and λ-wavelength of X-ray used, also indicated similar result. It is clear from summarized results in Table 2 that large lattice strain (ε) was induced for higher pulse density. Further, Fig. 3c indicates less peak broadening non-linearity after LPwC process compared to unpeened specimen, the same trend was observed in

Fig. 6. Interface between unpeened and peened regions of (a) 2500 and (b) 6250 pulses cm−2. Peened surface of (c) 2500 and (d) 6250 pulses cm−2 (at 1×). Photograph of peened surface with (e) 2500 and (f) 6250 pulses cm−2.

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one of the previous studies [19]. These results suggest large compressive residual stresses for higher pulse density, which is well proved in Section 3.1. In accordance with Lu et al. [22] report of single LSP on LY2 aluminium alloy, resulted plastic deformation (PD) layer of 372 μm is divided into severe plastic deformation (SPD) layer up to 239 μm and minor plastic deformation layer up to 133 μm. Keeping this in mind and based on high compressive residual stresses, depth of 50 μm was chosen for microstructure analysis. The recorded SEM microstructures as shown in Fig. 4, indicate fully martensite phase of needle like structure with minimal ferrite contents in both unpeened and LPwC specimens. It implies that no microstructural changes including grain size refinement and phase transformation occurred and are consistent with XRD results observed from Fig. 3a.

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It further demonstrates the pure mechanical effect of LPwC process besides of practically negligible thermal damage [26,48]. 3.3. Micro-hardness Fig. 5 shows measured micro-hardness profile in depth direction. Average hardness of unpeened specimen was 295 HV. Hardness at LPwC treated surface, regardless of pulse density, exhibit lower value i.e. 244 and 270 HV for 2500 and 6250 pulses cm− 2 respectively. Large hardness of 341 HV at depth of 200 μm was noticed for higher pulse density compared lower pulse density (320 HV at 300 μm) and was gradually decreased in further depths. Higher pulse density induced greater depth of hardened layer (600 μm) than lower pulse density (400 μm). Further, it is worth noting that depth of hardened layer (600 μm) developed in this study is much larger than some of the previous SP studies on 17-4 PH [5,7]. The lower hardness value resulted at LPwC treated surface is due to increased thermal effect and results of melting which is greater for high pulse density. On the other hand, decrease in hardness after certain depth is because of reduction and dissipation of shock wave intensity created from confined plasma as it propagates deeper into the material; and depth of hardened layer for high pulse density increased due to better strain hardening occurred [19,33,49]. Hence, it can be concluded that higher hardness and LPwC induced residual stresses resulted for higher pulse density with comparable influenced depth. Normally the level of work hardening is much lower in LSP/LPwC process compared to SP treatment [15,38]. With the aim of studying it in the present case, averaged FWHM of X-ray diffraction peaks from Fig. 3a, was included. The results indicate greater value in unpeened specimen (3.73°) compared to LPwC specimens (2.52° and 2.30° respectively for 2500 and 6250 pulse cm−2) and are due to lesser number of LPwC impacts used [15,38]. 3.4. Surface roughness and topography

Fig. 7. 3D surface topography of (a) unpeened, (b) 2500 pulses cm−2 and (c) 6250 pulses cm −2 specimens. (Note: Line fit scale of (c) is different).

The profile roughness parameters, average roughness (Ra) and mean roughness depth (Rz) measured at different regions along L, T directions and percentage of increment in Ra are shown in Table 3. Unpeened specimen shows relatively smaller roughness both in L and T directions: Ra ~ 0.068 μm and Rz ~ 0.084 μm. After LPwC process, Rz increased drastically to 4.9 and 5.2 μm in L and 5.7 and 6.1 μm in L directions respectively for 2500 and 6250 pulses cm−2. Dai et al. [47] suggested smaller initial roughness to decrease final roughness after LPwC. Trdan et al. [26] showed increased roughness for higher pulse density and high overlapping rates. In another study, Trdan et al. [45] reported greater crater area, depth and wave period for higher pulse density. Many others working on LPwC reported the same [33,48,49]. Hence, it can be concluded that higher pulse density increased Ra and Rz in present study, while, different overlapping rates maintained (90% in L and 40% in T directions) did not have any significant effect on roughness. This shows instability of roughness for LPwC process parameters and can be inferred from others studies too [26,45]. Fig. 6 shows macroimages and photographs of LPwC induced pattern at surface. More pronounced pattern with wider valleys in T directions can be seen for lower pulse density (Fig. 6c) compared to higher pulse density (Fig. 6d). Irrespective of greater roughness introduced in higher pulse density, the pattern revealed could be the reason of lower overlapping rate (40%) maintained in T directions. Further, Bugayev et al. [50] from their LPwC studies reported similar columnar structure as revealed in this study (Fig. 6). AFM 3D topography of untreated and LPwC specimens is presented in Fig. 7. Unpeened surface (Fig. 7a) reveals no or minimal peaks and valleys with average height of selected area (Sa) of 30 nm and maximum peak height of selected area (Sp), maximum valley depth of selected area (Sv) respectively 348 and −233 nm. By contrast, LPwC processed surface in Fig. 7a and b shows increased peak height and valley depth due to pulse density and different overlapping rates and are Sa (415 nm vs. 406 nm), Sp (2631 nm vs. 1602 nm) and Sp (− 2036 nm vs.

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− 1462 nm) respectively 6250 and 2500 pulses cm−2. On comparison with Trdan et al. [30,45] concerning LPwC, important point to be noted in present study is that the roughness is more for higher pulse density, and large crater depths and areas may be resulted in T directions because of lower overlapping rate maintained. Furthermore, columnar peened structure as reported by Bugayev et al. [50] shown in Fig. 7b and c is due to LPwC zig-zag type scan with aforesaid overlapping rates. However, in comparison to one of the previous studies using dual SP treatment on 17-4 PH steel [7], and report of LPwC on titanium and aluminium alloys [51], the final roughness in the present study is much smaller (~1 μm). 4. Conclusion The 17-4 PH steel was LPwC processed with pulse densities, 2500 and 6250 pulses cm−2. Following conclusions can be drawn. Higher pulse density resulted large compressive residual stresses with influenced depth of 600 μm. However, residual stress anisotropy observed as a result of zig-zag type LPwC scanning used. Induced large lattice strain for higher pulse density found to be correlated with LPwC induced residual stresses. The XRD and microstructure results showed no phase transformation or grain refinement after LPwC process and reveals its pure mechanical effect. Larger hardness was resulted for higher pulse density with larger depth of hardened layer (600 μm). The roughness was increased by the process, resulted in more pronounced peening pattern. Surface topography revealed columnar structure after LPwC process. Acknowledgement The authors would like to thank VIT University, Vellore, India for providing facility to carry out this work and for financial support to file a part of this work as an Indian patent under the title “Method for improving surface mechanical properties in 17-4 PH stainless steel using laser peening” (Application no.: 1364/CHE/2015). The Indian Institute of Technology, Bombay (IIT-B) is appreciated for the residual stress measurement. References [1] C.N. Hsiao, C.S. Chiou, J.R. Yang, Aging reactions in a 17-4 PH stainless steel, Mater. Chem. Phys. 74 (2002) 134–142, http://dx.doi.org/10.1016/S0254-0584 (01)00460-6. [2] D. Nakhaie, M.H. Moayed, Pitting corrosion of cold rolled solution treated 17-4 PH stainless steel, Corros. Sci. 80 (2014) 290–298, http://dx.doi.org/10.1016/j.corsci. 2013.11.039. [3] H. Mirzadeh, A. Najafizadeh, Aging kinetics of 17-4 PH stainless steel, Mater. Chem. Phys. 116 (2009) 119–124, http://dx.doi.org/10.1016/j.matchemphys.2009.02.049. [4] C.F. Arisoy, G. Başman, M.K. Şeşen, Failure of a 17-4 PH stainless steel sailboat propeller shaft, Eng. Fail. Anal. 10 (2003) 711–717, http://dx.doi.org/10.1016/S13506307(03)00041-4. [5] Z. Mazur, R. Garcia-Illescas, J. Aguirre-Romano, N. Perez-Rodriguez, Steam turbine blade failure analysis, Eng. Fail. Anal. 15 (2008) 129–141, http://dx.doi.org/10. 1016/j.engfailanal.2006.11.018. [6] Z. Wang, C. Jiang, X. Gan, Y. Chen, Effect of shot peening on the microstructure of laser hardened 17-4PH, Appl. Surf. Sci. 257 (2010) 1154–1160, http://dx.doi.org/ 10.1016/j.apsusc.2010.07.015. [7] Z. Wang, C. Jiang, X. Gan, Y. Chen, V. Ji, Influence of shot peening on the fatigue life of laser hardened 17-4PH steel, Int. J. Fatigue 33 (2011) 549–556, http://dx.doi.org/10. 1016/j.ijfatigue.2010.10.010. [8] Z. Wang, Y. Chen, C. Jiang, Thermal relaxation behavior of residual stress in laser hardened 17-4PH steel after shot peening treatment, Appl. Surf. Sci. 257 (2011) 9830–9835, http://dx.doi.org/10.1016/j.apsusc.2011.06.032. [9] Z. Wang, W. Luan, J. Huang, C. Jiang, XRD investigation of microstructure strengthening mechanism of shot peening on laser hardened 17-4PH, Mater. Sci. Eng. A 528 (2011) 6417–6425, http://dx.doi.org/10.1016/j.msea.2011.03.098. [10] P.S. Prevéy, R.A. Ravindranath, M. Shepard, T. Gabb, Case studies of fatigue life improvement using low plasticity burnishing in gas turbine engine applications, J. Eng. Gas Turbines Power 128 (2006) 865, http://dx.doi.org/10.1115/1.1807414. [11] P.S. Prevéy, N. Jayaraman, R. Ravindranath, Fatigue life extension of steam turbine alloys using low plasticity burnishing (LPB), Proceedings of ASME Turbo Expo 2010, pp. 2277–2287, http://dx.doi.org/10.1115/GT2010-2295. [12] P.S. Prevéy, R. Ravindranath, Low plasticity burnishing (LPB) treatment to mitigate FOD and corrosion fatigue damage in 17-4PH stainless steel, Proceedings of the Tri-Service Corrosion Conference 2003, pp. 1–11.

[13] B.P. Fairand, A.H. Clauer, Laser generated stress waves: their characteristics and their effects to materials, Proceedings of Laser–Solid Interactions and Laser Processing, 1978. [14] G. Gomez-Rosas, C. Rubio-Gonzalez, J.L. Ocaña, C. Molpeceres, J.A. Porro, M. Morales, et al., Laser shock processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths, Appl. Surf. Sci. 256 (2010) 5828–5831, http://dx.doi.org/10.1016/j.apsusc. 2010.03.043. [15] P. Peyre, X. Scherpereel, L. Berthe, C. Carboni, R. Fabbro, G. Be, Surface modifications induced in 316 L steel by laser peening and shot-peening. Influence on pitting corrosion resistance, Mater. Sci. Eng. A 280 (2000) 294–302, http://dx.doi.org/10.1016/S09215093(99)00698-X. [16] L. Berthe, R. Fabbro, P. Peyre, L. Tollier, E. Bartnicki, Shock waves from a water-confined laser-generated plasma, J. Appl. Phys. 82 (1997) 2826, http://dx.doi.org/10.1063/1. 366113. [17] R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, Physical study of laser-produced plasma in confined geometry, J. Appl. Phys. 68 (1990) 775, http://dx.doi.org/10.1063/1. 346783. [18] P. Peyre, L. Berthe, V. Vignal, I. Popa, T. Baudin, Analysis of laser shock waves and resulting surface deformations in an Al–Cu–Li aluminum alloy, J. Phys. D. Appl. Phys. 45 (2012) 335304, http://dx.doi.org/10.1088/0022-3727/45/33/335304. [19] S. Sathyajith, S. Kalainathan, Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser, Opt. Lasers Eng. 50 (2012) 345–348, http://dx.doi.org/10.1016/j.optlaseng.2011.11.002. [20] N. Mukai, N. Aoki, M. Obata, A. Ito, Y. Sano, C. Konagai, Proceedings of the third JSME/ ASME International Conference on Nuclear Engineering (ICONE-3), Kyoto 1995, p. 1489. [21] Y. Sano, Laser peening without coating as a surface enhancement technology, J. Laser Micro/Nanoeng. 1 (2006) 161–166, http://dx.doi.org/10.2961/jlmn.2006.03.0002. [22] J.Z. Lu, K.Y. Luo, Y.K. Zhang, C.Y. Cui, G.F. Sun, J.Z. Zhou, et al., Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts, Acta Mater. 58 (2010) 3984–3994, http://dx.doi.org/10.1016/j. actamat.2010.03.026. [23] J.Z. Lu, K.Y. Luo, Y.K. Zhang, G.F. Sun, Y.Y. Gu, J.Z. Zhou, et al., Grain refinement mechanism of multiple laser shock processing impacts on ANSI 304 stainless steel, Acta Mater. 58 (2010) 5354–5362, http://dx.doi.org/10.1016/j.actamat.2010.06.010. [24] C. Ye, S. Suslov, X. Fei, G.J. Cheng, Bimodal nanocrystallization of NiTi shape memory alloy by laser shock peening and post-deformation annealing, Acta Mater. 59 (2011) 7219–7227, http://dx.doi.org/10.1016/j.actamat.2011.07.070. [25] C. Ye, S. Suslov, B.J. Kim, E.A. Stach, G.J. Cheng, Fatigue performance improvement in AISI 4140 steel by dynamic strain aging and dynamic precipitation during warm laser shock peening, Acta Mater. 59 (2011) 1014–1025, http://dx.doi.org/10.1016/ j.actamat.2010.10.032. [26] U. Trdan, M. Skarba, J. Grum, Laser shock peening effect on the dislocation transitions and grain refinement of Al–Mg–Si alloy, Mater. Charact. 97 (2014) 57–68, http://dx. doi.org/10.1016/j.matchar.2014.08.020. [27] J.Z. Lu, H. Qi, K.Y. Luo, M. Luo, X.N. Cheng, Corrosion behaviour of AISI 304 stainless steel subjected to massive laser shock peening impacts with different pulse energies, Corros. Sci. 80 (2014) 53–59, http://dx.doi.org/10.1016/j.corsci.2013.11.003. [28] J.Z.Z. Lu, K.Y.Y. Luo, D.K.K. Yang, X.N.N. Cheng, J.L.L. Hu, F.Z.Z. Dai, et al., Effects of laser peening on stress corrosion cracking (SCC) of ANSI 304 austenitic stainless steel, Corros. Sci. 60 (2012) 145–152, http://dx.doi.org/10.1016/j.corsci.2012.03.044. [29] U. Trdan, J. Grum, Evaluation of corrosion resistance of AA6082-T651 aluminium alloy after laser shock peening by means of cyclic polarisation and ElS methods, Corros. Sci. 59 (2012) 324–333, http://dx.doi.org/10.1016/j.corsci.2012.03.019. [30] U. Trdan, J. Grum, SEM/EDS characterization of laser shock peening effect on localized corrosion of Al alloy in a near natural chloride environment, Corros. Sci. 82 (2014) 1–11, http://dx.doi.org/10.1016/j.corsci.2014.01.032. [31] A. Telang, A.S. Gill, S. Teysseyre, S.R. Mannava, D. Qian, V.K. Vasudevan, Effects of laser shock peening on SCC behavior of Alloy 600 in tetrathionate solution, Corros. Sci. 90 (2015) 434–444, http://dx.doi.org/10.1016/j.corsci.2014.10.045. [32] L. Zhang, Y.K. Zhang, J.Z. Lu, F.Z. Dai, A.X. Feng, K.Y. Luo, et al., Effects of laser shock processing on electrochemical corrosion resistance of ANSI 304 stainless steel weldments after cavitation erosion, Corros. Sci. 66 (2013) 5–13, http://dx.doi.org/10. 1016/j.corsci.2012.08.034. [33] S. Kalainathan, S. Sathyajith, S. Swaroop, Effect of laser shot peening without coating on the surface properties and corrosion behavior of 316 L steel, Opt. Lasers Eng. 50 (2012) 1740–1745, http://dx.doi.org/10.1016/j.optlaseng.2012.07.007. [34] C. Rubio-González, J.L. Ocaña, G. Gomez-Rosas, C. Molpeceres, M. Paredes, A. Banderas, et al., Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy, Mater. Sci. Eng. A 386 (2004) 291–295, http://dx.doi.org/10.1016/j.msea.2004.07.025. [35] Y. Liao, S. Suslov, C. Ye, G.J. Cheng, The mechanisms of thermal engineered laser shock peening for enhanced fatigue performance, Acta Mater. 60 (2012) 4997–5009, http://dx.doi.org/10.1016/j.actamat.2012.06.024. [36] Y. Sano, K. Masaki, T. Gushi, T. Sano, Improvement in fatigue performance of friction stir welded A6061-T6 aluminum alloy by laser peening without coating, Mater. Des. 36 (2012) 809–814, http://dx.doi.org/10.1016/j.matdes.2011.10.053. [37] I. Altenberger, Y. Sano, M.A. Cherif, I. Nikitin, B. Scholtes, Residual stress state and fatigue behaviour of laser shock peened titanium alloys, Mater. Sci. Forum 524–525 (2006) 129–134, http://dx.doi.org/10.4028/www.scientific.net/MSF.524-525.129. [38] P. Peyre, et al., Laser-shock processing of aluminium-coated 55C1 steel in waterconfinement regime, characterization and application to high-cycle fatigue, Behaviour 3 (1998) 1421–1429. [39] U. Sánchez-Santana, C. Rubio-González, G. Gomez-Rosas, J.L. Ocaña, C. Molpeceres, J. Porro, et al., Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing, Wear 260 (2006) 847–854, http://dx.doi.org/10.1016/j.wear.2005.04.014.

D. Karthik et al. / Surface & Coatings Technology 278 (2015) 138–145 [40] D. Kumar, S. Nadeem Akhtar, A. Kumar Patel, J. Ramkumar, K. Balani, Tribological performance of laser peened Ti–6Al–4 V, Wear 322–323 (2015) 203–217, http:// dx.doi.org/10.1016/j.wear.2014.11.016. [41] Standard Specification for Chromium and Chromium–Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications, ASTM International, West Conshohocken, PA, 2014http://dx.doi.org/10.1520/A0240_A0240M. [42] H. Hughes, X-ray techniques for residual stress measurement, Strain 3 (1967) 26–31, http://dx.doi.org/10.1111/j.1475-1305.1967.tb00885.x. [43] M.E. Fitzpatrick, A. Lodini, Analysis of Residual Stress by Diffraction using Neutron and Synchrotron Radiation, Second ed. Taylor & Francis, New York, 2003. [44] C. Suryanarayana, M. Grant Norton, X-Ray Diffraction — A Practical Approach, First ed. Springer, New York, 1998. [45] U. Trdan, J.A. Porro, J.L. Ocaña, J. Grum, Laser shock peening without absorbent coating (LSPwC) effect on 3D surface topography and mechanical properties of 6082-T651 Al alloy, Surf. Coat. Technol. 208 (2012) 109–116, http://dx.doi.org/10.1016/j.surfcoat. 2012.08.048. [46] C. Correa, D. Peral, J.A. Porro, M. Díaz, L. Ruiz de Lara, A. García-Beltrán, et al., Random-type scanning patterns in laser shock peening without absorbing coating

[47]

[48]

[49]

[50]

[51]

145

in 2024-T351 Al alloy: a solution to reduce residual stress anisotropy, Opt. Laser Technol. 73 (2015) 179–187, http://dx.doi.org/10.1016/j.optlastec.2015.04.027. F.Z. Dai, J.Z. Lu, Y.K. Zhang, K.Y. Luo, Q.W. Wang, L. Zhang, et al., Effect of initial surface topography on the surface status of LY2 aluminum alloy treated by laser shock processing, Vacuum 86 (2012) 1482–1487, http://dx.doi.org/10.1016/j.vacuum.2012.02.001. Y. Sano, M. Obata, T. Kubo, N. Mukai, M. Yoda, K. Masaki, et al., Retardation of crack initiation and growth in austenitic stainless steels by laser peening without protective coating, Mater. Sci. Eng. A 417 (2006) 334–340, http://dx.doi.org/10.1016/j.msea. 2005.11.017. S. Sathyajith, S. Kalainathan, S. Swaroop, Laser peening without coating on aluminum alloy Al-6061-T6 using low energy Nd:YAG laser, Opt. Laser Technol. 45 (2013) 389–394, http://dx.doi.org/10.1016/j.optlastec.2012.06.019. A.A. Bugayev, M.C. Gupta, R. Payne, Laser processing of inconel 600 and surface structure, Opt. Lasers Eng. 44 (2006) 102–111, http://dx.doi.org/10.1016/j.optlaseng.2005. 04.014. L. Wagner, M. Mhaede, M. Wollmann, I. Altenberger, Y. Sano, Surface layer properties and fatigue behavior in Al 7075‐T73 and Ti‐6Al‐4 V, Int. J. Struct. Integr. 2 (2011) 185–199, http://dx.doi.org/10.1108/17579861111135923.