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Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel
Accepted Manuscript Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel
Erfan Maleki, Okan Unal, Kazem Reza Kashyzadeh PII: DOI: Reference:
S0257-8972(18)30207-X doi:10.1016/j.surfcoat.2018.02.081 SCT 23152
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
5 September 2017 17 February 2018 21 February 2018
Please cite this article as: Erfan Maleki, Okan Unal, Kazem Reza Kashyzadeh , Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.02.081
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ACCEPTED MANUSCRIPT Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel Erfan Malekia,*, Okan Unalb, Kazem Reza Kashyzadeha a
Mechanical Engineering Department, Sharif University of Technology-International Campus, Kish Island, Iran b
Mechanical Engineering Department, Karabuk University, Karabuk, Turkey
*
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Corresponding author, email address: [email protected], Phone: +989125427218
Abstract
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The present study investigates experimentally the effects of different shot peening treatments, including conventional, severe, over, and re-shot peening on microstructure, mechanical
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properties, and fatigue behavior of AISI 1050 mild carbon steel. Different shot peening treatments were performed using various effective parameters by considering the influences of
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increasing Almen intensity and coverage. Optical microscopy and field emission scanning electron microscopy observations and X-Ray diffraction measurements were carried out to
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analyze grains refinement in each shot peening treatment. Microhardness and residual stress measurements were taken from shot peened surfaces to the core material to investigate the
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mechanical properties. The fatigue behaviors of the specimens were examined by using the axial fatigue test. The results indicated that post-grinding, re-shot peening, and severe shot peening
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processes have significant effects on fatigue life improvement.
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Key words: fatigue behavior, severe plastic deformation, shot peening, microstructure,
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mechanical properties,
1. Introduction
Most failures in engineering components are due to mechanical and metallurgical properties of the materials’ surface, and in most cases such failures originate from the exterior layers of components [1]. Therefore, improving surface layer properties has become a topic which has attracted considerable interest. In enhancing surface properties, especially in metallic materials, grains refinement and the creation of compressive residual stress (CRS) in the surface play important roles; these techniques have positive effects on materials’ behavior such as fracture, fatigue, corrosion, wear, and crack arrestment [2-4, 52].
ACCEPTED MANUSCRIPT Most metals are produced through thermo-mechanical processing, and grain sizes typically range between ~ 5- 10 µm [5]. However, it is now well-known that the strength of metals and alloys is strongly influenced by their grain size [6, 49]. The strength of metals is related to their microstructure (grain size) which is dependent on two relationships of low and high operating temperatures.
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At low temperatures (such as room temperature), the strength of the material is generally
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described by Hall–Petch (H-P) relationship as follows [7]:
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σy = σ0 + ky /d ½
(1)
Where σy is the yield stress, σo is a material constant for the starting stress required for
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dislocation movement, ky is the strengthening coefficient, and d is the average grain diameter.
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At high temperatures and under conditions of constant stress or load, the steady-state strain rate is given by the following relationship [8, 9]:
(2)
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ADGb b p n ( ) ( ) kT d G
Where D is the appropriate diffusion coefficient, T is the absolute temperature, G is the shear
modulus, b is the Burgers vector, k is Boltzmann’s constant, σ is the applied stress, p and n are
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respectively the exponents of the inverse grain size and the stress, and A is a dimensionless constant. For coarse-grained materials, the flow processes at elevated temperatures are
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intragranular in nature and p= 0, but when the grain size is reduced other flow processes which are dependent on the grain size become important [5]. Therefore, the strength of the material at
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both low and high temperatures increases when the grain size decreases; and grains refinement provide an opportunity for obtaining much better mechanical and metallurgical properties. By applying grain refinement, the size of coarse grains reaches a submicron (<1000 nm) scale. The grains with a size of 1- 100 nm and 100- 500 nm are called nanostructured (NS) and ultrafinegrained (UFG), respectively [10]. One of the common techniques of grain refinement and CRS creation as one of the severe plastic deformation (SPD) methods is shot peening (SP) [54]. SP is a cold working process in which the surface of a component is bombarded with small shots under controlled velocity [11]. The
ACCEPTED MANUSCRIPT capability of a metallic material to deform plastically depends on the ability of dislocations to move and enhance the rate of plastic strain. Restricting dislocation makes the material stronger, and strengthening in turn reduces ductility of the material. The cause of work hardening is the increase of dislocation density through plastic deformation [12, 13, 51]. In the SP process, plastic deformation is directly proportional to the amount of total kinetic energy of the shot stream transferred to the component. This energy is dependent to the values of Almen intensity and
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surface coverage. Based on the value of created kinetic energy, SP process is classified into three
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cases, including conventional shot peening (CSP), severe shot peening (SSP), and over shot
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peening (OSP). CSP employing conventional intensities and coverages is usually recognized to introduce of compression residual stresses in the subsurface layers of the material. SSP process,
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using unconventional high parameters, generates more compressive residual stresses and grain refinement of the treated surface layer and makes NS [14, 15]. OSP occurs when the created
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kinetic energy is not well selected and made suitable by using a high Almen intensity and coverage; such a disproportion may create defects such as producing points of stress
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concentration, superficial micro-cracks, and overlaps [16]. The effect of different types of SP on metallurgical and mechanical properties of various
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materials have been studied by many researchers [17-20]. The fatigue behaviors of conventionally and severely shot peened materials has been investigated by many authors as well
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[21-26]. However, few papers have explored the effects of OSP on the fatigue behavior of materials. Benedetti et al. [27, 28] have employed different intensities for SP of the same target.
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Their X-Ray diffraction measurement showed that higher intensities would be more beneficial in terms of residual stress distribution, but fatigue tests did not indicate any improvement in the life
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of the component. Fathallah et al. [29] have revealed a noticeable reduction of fatigue life in the case of coverage of 1000% against the common coverage of 100%. In addition, considering SP surface treatments with different intensities, Trško et al. [30] have studied fatigue life of an aluminum alloy in high and ultra-high cycle regions. Severe shot peening with a 9.6N Almen intensity and a coverage of 650 % increases fatigue life up to 9 %. However, using OSP parameters with a higher Almen intensity of 14.9A and a coverage of 650 % significantly shortens the fatigue life up to 21 %.
ACCEPTED MANUSCRIPT Furthermore, some SP post-treatment such as elimination of surface roughness [35], re-shot peening [36], and vibration polishing and peening [37] have been suggested to improve mechanical properties and fatigue behavior of materials. In this paper, microstructure, mechanical properties, and fatigue behavior of AISI 1050 mild carbon steel treated with CSP, SSP, OSP, re-shot peening (RSP), and post-grinding of the pre-
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shot peened specimens is investigated and compared experimentally. Different SP treatments
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having different Almen intensities and coverages were performed.
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Microstructure of the treated specimens has been characterized using optical microscopy (OM) and field emission scanning electron microscopy (FESEM) observations as well as X-Ray
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diffraction (XRD) analysis. Mechanical properties were evaluated by measuring hardness, residual stress, roughness, and fatigue lives. And the fatigue lives of specimens are obtained by
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using the axial fatigue test. Conventional and severe shot peened specimens were compared within the context of above measurements. The properties of treated specimens were investigated
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through various experimental methods as mentioned and the results revealed that, by increasing the severity of the SP process, plastic deformation of the surface layer is enhanced and NS grains
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are generated. In addition, both microhardness and residual stress are raised by increasing Almen intensity. Moreover, the deformed surface layers introduced by post-grinding, re-shot peening,
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2. Experimental setup
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and severe SP processes show significant effects on the improvement of fatigue life.
2.1. Material and specimens
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AISI 1050 mild carbon steel is selected as specimen material. AISI 1050 is used in general engineering applications because of its specific properties. Its unique mechanical properties enables this grade of steel to have a wide range of applications in the manufacture of forged shafts and gears. The chemical composition of AISI 1050 is shown in Table 1. Table 1. Chemical composition of AISI 1050 carbon steel (weight %). C
Mn
Si
Cr
Cu
Mo
Ni
S
Al
Fe
0.51
0.64
0.23
0.16
0.14
0.02
0.07
0.005
0.01
Bal.
ACCEPTED MANUSCRIPT Tensile and fatigue test specimens were fabricated from a sheet of hot rolled AISI 1050 with a thickness of 6mm according to ASTM E8M [40] and ASTM E466 [31] standards, respectively. The shape and size of specimens are given in Figure 1. The specimens were austenitized at 850°C for 1 h, immediately oil quenched, and then aged at 520°C for 1 h followed by air cooling. The average hardness of the resulting microstructure is 28
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HRC (285 HV). Figure 2a illustrates the microstructure of AISI 1050 before heat treatment (as
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received-annealed condition), and pearlite (dark phase) and ferrite (light phase) phases can be
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observed clearly. However, in Figure 2b after heat treatment (quenching and tempering) has been performed, the pearlite and ferrite boundaries vanish, and the ferrite proportion falls while
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pearlite structure fraction rise.
Figure 1. Shape and size of the specimens for (a) tensile and (b) fatigue test
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Figure 2. Microstructure of the AISI 1050 steel (a) before and (b) after accomplished heat treatment
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2.2. Shot Peening Treatments
SP treatments applied on the specimens were performed using an air blast shot peening (ABSP)
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device. Air pressure, shot type, and peening time are the main process parameters that affect the results. All treatments were carried out using standard steel shots with an average hardness of 50
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HRC, impingement angle of 90º, nozzle diameter of 6.35 mm (1/4 in), and a distance between nozzle and sample equal to 10 cm. Different SP treatments with varying Almen intensities and
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coverages were conducted on the specimens. Almen intensity was obtained according to SAE J443 standard [32]. Table 2 depicts the applied parameters of shot peening treatments on each
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considered specimen. In SCSP and SSSP1 specimens, the effect of increasing coverage was investigated, and in SSSP1, SSSP2, and SOSP specimens, the influence of enhancing Almen intensity was considered. SCSP specimen was treated via common and conventional parameters of SP process, and SSSP1, SSSP2, and SOSP specimens were shot peened with a higher kinetic energy. The effective parameters on the shot peening process are numerous and too hard to control exactly. They can easily be influenced by the humidity of the chamber and the quality of air compressor. Some of the parameters affecting the system are shot size, shot material, air pressure, and surface coverage. Both individual and total pre-conditions can alter the type of shot peening (conventional, severe, and over). There are no certain and distinctive borders between
ACCEPTED MANUSCRIPT severe and over shot peening methods. However, the macro and micro analysis (surface roughness, surface topography, surface yielding, and crack occurrence) of the shot peened surface are presented for the sake of distinguishing conventional and severe shot peenings. Besides, microhardness and residual stress analysis supports the results. Nevertheless, the purpose of employing “over shot peening” is to determine the border between the beneficial and hazardous effects rather than decreasing the grain size or increasing the fatigue limit through
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excessive plastic deformations. Over shot peening parameters were specified by investigating
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superficial micro-cracks and overlaps on and just beneath the surface. Some researchers [18]
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have pointed to the practical applications of over shot peening for aircraft and automotive engineering; they have also proposed A12-16 Almen intensity conditions as the conventional
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shot peening intensity. However, this can be broadened to the upper and lower values as A10-18 Almen intensity at lower 100%-200% coverages [19-20, 38]. Also, the intensity level used for
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conventional shot peening was converted to severe shot peening by increasing surface coverage up to 1500% [23-26, 30].
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Figure 3 represents shot peened fatigue test specimens.
Shot
Projection
Almen
Diameter
Pressure
Intensity
(mm)
(bar)
A strip (mm)
0.58
2.8
0.18
100
SSSP1
0.58
2.8
0.18
1500
SSSP2
0.71
3.5
0.22
1500
SOSP
0.71
5.5
0.26
1500
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SCSP
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Specimen ID
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Table 2. Different applied shot peening treatments parameters
Coverage (%)
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Figure 3. Prepared specimens for (a) tensile test and (b) fatigue test after SP
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In addition, in order to investigate the effect of re-shot peening one of the pre-shot peened specimens (SSSP2) was selected and dual step shot peening process was carried out on it. SP
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process can be accomplished in several ways such as single, dual or triple steps, all of which have their specific influences on relevant materials. Dual SP may even better improve fatigue
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resistance than a single peening treatment [36, 39]. In this process, a fully pre-shot peened specimen at the special intensity, re- shot peened (SSSP2+RSP) in a second operation with a
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lower intensity and a shorter peening duration were used. The parameters of the performed RSP process are shown in Table 3. Moreover, grinding process is employed to detoxify the probable created surface defects and reduce surface roughness on the over shot peened specimen (SOSP). The related specimen is called SOPS+G. Table 3. Parameters of the performed RSP process Shot
Projection
Almen
Diameter
Pressure
Intensity
(mm)
(bar)
(0.001 in A)
Coverage (%)
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2.4
10
100
2.3. Microscopic observations Microstructure observations are carried out via OM and FESEM. OM and FESEM observations were applied using Olympus and Mira 3-XMU respectively. Mira 3-XMU can reach high
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resolution up to 1.0 nm at 30 keV. Also, 200 V to 30 kV accelerating voltage exists with a BSE
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detector, EDX and EBSD facilities. Specimens for OM observations have been etched by 2%
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Nital.
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2.4. XRD Grains size measurements
To obtain the grain size after applying SP treatments with higher energy on the related specimens
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(SSSP1, SSSP2, and SOSP), X-ray diffraction (XRD) measurements were carried out. In XRD measurements, the full width at half maximum (FWHM) of the diffraction peaks were obtained and the crystallite sizes were calculated. For XRD analysis, X’Pert PRO MPD (PANalytical )X-
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ray diffractometer system and X’Pert High Score Plus (V. 3) analyzer are employed with Cu Kα
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radiation operated at 40 kV and 40 mA, scanning angle of 30º–150º, and irradiated area of 10 mm.
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2.5. Microhardness measurements
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In order to investigate the effects of accomplished SP treatments on the hardness of the specimens, microhardness tests were performed up to 800 µm on the cross sectional surface with intervals of 20 µm. Qness GmbH Q30 A microhardness tester at a load of 10 gf with a duration
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of 7 s using Vickers indenter.
2.6. Residual stress measurements To study the state of residual stresses on the specimens, different XRD analysis were applied via Xstress 3000 G2/G2R X-ray Stress Analyzer (radiation Cr Kα, irradiated area of 4 mm diameter, sin2ψ method, and diffraction angle (2θ) ~156 scanned between 45 and -45). Measurements were carried out in depth step by step by removing a very thin layer of material (~ 20 µm) through electro-polishing with a solution of acetic acid (94%) and perchloric acid (6%). The Voigt
ACCEPTED MANUSCRIPT method has been employed to assess microstructural and residual stress evaluations. Measured profile h is convoluted by physical profile g and broadened profile f. Also, structurally broadened profile is constructed by combining Cauchy and Gaussian components. This process leads to a slight stress relaxation and has to be corrected using the correlation methods [18, 35]. The removal layer and depth measurements after etching process are generally achieved by using a calibrated micrometer, dial gauge, and a travelling microscope. The removal layer of the
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specimens are determined with a calibrated travelling microscope which focuses on the exposed
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surface. Measurements are performed between the widths of the surface and mean value of the
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total is obtained.
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2.7. Tensile test
The STM-250 SANTAM universal testing machine is used to perform tensile test. The tensile
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rate is set and maintained at the range of 0.015 +/- 0.006 mm/mm/min based on Standard Test Methods for Tension Testing of Metallic Materials (ASTM E8). Also, and strains are measured
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through a clip gage extensometer with a length of 25 mm gage.
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2.8. Fatigue test
This test is a common method for determining the behavior of materials under fluctuating loads.
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The SANTAM SAF-250 universal test machine is used to carry out fatigue test under tensioncompression loading conditions. The average stress is considered equal to zero and the stress
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ratio of R= -1. The loading frequency of 20 Hertz is used for all fatigue tests. The recommended number of fatigue test samples used to generate Wohler curve (S-N) and the number of
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replicated tests vary between 6-12 and 12-24 for preliminary and design allowable phases, respectively [41]. The minimum number of test specimens for constructing a S-N curve is 14 based on the fatigue testing standard which proposed by the Association of Mechanical Engineers of Japan in 1981 and by ASTM in 1998. The fatigue limit can be obtained by a stepwise procedure having 6 test specimens [42]. In the Wohler curve, the portion of curve with a negative slope constitutes the finite life region and represents fatigue strength of the material for a given number of stress cycles, while the horizontal portion represents infinite life region. The stress level corresponding to the horizontal portion is known as fatigue limit of the material. The endurance fatigue limit is obtained by using
ACCEPTED MANUSCRIPT JSME S 002 standard [53] and applying staircase method. The staircase technique known as upand-down method can estimate the endurance limit by taking into account its statistical nature. In this method, a specimen is tested at stress amplitude 𝑆𝑎 about 5% higher than the expected endurance limit. If the specimen fails before the stipulated number of cycles (around 1 million cycles), the next specimen is tested at a lower stress amplitude. Nevertheless, if a specimen survives, the test is suspended after the fulfillment of the specified number of cycles and the next
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specimen is tested at a greater amplitude of maximum alternating stress. Accordingly, the stress
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amplitude of each successive test is dependent on the outcome of its previous test. 2.9. Surface roughness measurements
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Surface roughness of specimens, a well-recognized side effect of SP process affecting fatigue behavior, is measured using SURFCORDER SE500. The roughness parameters were analyzed
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based on the definition of ISO 4287 [33].
3.1. Structural characterizations
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3. Results and discussion
After SP treatments were accomplished, the specimens were plastically deformed. OM images of
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the shot peened specimens are given in Figure 4. It can be seen that by increasing the kinetic energy of the SP process via enhancing the values of Almen intensity and coverage, the depth of
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the deformed layer from the surface rises as well. Figure 5 represents the FESEM images of the shot peened specimens that illustrate the created UFG and NS layer. Besides, the reduction of the
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grains size are clearly realized near the top surface layer. Also, it can be observed that in Figure 5.d, the specimen (SOSP) shot peened with very high kinetic energy which is not admissible,
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some nano-cracks are created in the top surface and the OSP phenomena has occurred.
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Figure 4. OM images of the different treated specimens: (a) SCSP, (b) SSSP1, (c) SSSP2 and (d) SOSP
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Figure 5. FESEM observations of the severely shot peened specimens: (a) SCSP, (b) SSSP1, (c) SSSP2 and (d) SOSP
As mentioned earlier, in order to achieve the grain size of the treated specimens with SP treatments at higher and unconventional rates of kinetic energy, the authors performed XRD
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analysis. The XRD patterns of treated specimens are presented in Figure 6. The crystallite sizes
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of the specimens in the surface layer were obtained using the most common method for determining the mean crystallite size. The full width half maximum (FWHM) of a diffraction θ
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peak at Scherer’s equation is as follows [34]:
(3)
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dXRD = K λ / β Cosθ
Where d is the apparent size of crystal, λ is the wavelength of the x-radiation (i.e. λCu-Kα1.54 Å),
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B is the corrected FWHM (i.e. area under the curve divided by maximum height, in radian), θ is the diffraction angle, and K is a constant close to unity (i.e. 0.94). β can be obtained from the
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observed FWHM by convoluting the Gaussian profile modeling the specimen broadening β r, as follows:
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βr 2 = β0 2 – βi 2
(4)
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Where β0 is the observed broadening and βi is the instrumental broadening. XRD is used to determine the peak broadening with the crystallite size due to high
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density of dislocations in nanocrystalline materials using Scherer’s formula [50].
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It is worth noting that the usage of Scherer’s equation is limited to nano-scale particles, and it is not applicable to grains larger than 100 nm, in which case the HRTEM (or TEM) gives more precise grain size distributions [35]. Based on XRD analysis, the values of FWHM and the crystallite size of SSP treated specimens obtained from the certain peak are presented in Table 4.
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Figure 6. Intensity distribution of all treated specimens
Table 4. FWHM and crystallite sizes of specimens peened with SSP treatments
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Specimen
Peak
2-Theta (º)
FWHM (º)
Crystallite size (nm)
α (1 1 0)
44.545
0.0968
89.2
SSSP2
α (1 1 0)
44.603
0.1570
55.5
SOSP
α (1 1 0)
44.523
0.1754
49.1
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SSSP1
3.2. Hardness Measurement Microhardness measurements were applied in order to investigate the effects of exposed plastic deformation on the specimens. Figure 7 represents the variation of microhardness through the interior for the as-received and shot peened specimens. The resultant data scattering was no more than 5%. It can be observed that increasing SP severity entails a rise in hardness and its
ACCEPTED MANUSCRIPT distribution in depth. In addition, RSP after SSP has very low effects on hardness improvement
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because of its low values of intensity and coverage.
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Figure 7. Variation of the microhardness for different SP treatments from top surface to interior
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3.3. Residual stress measurements
XRD stress measurements are used to obtain the residual stress distributions of as-received and
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treated specimens. Figure 8 compares the residual stress distribution of each specimen. As shown, the as-received specimen (not peened) has -20 MPa compressive residual stresses on the
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surface because of the performed heat treatment. However, more compressive residual stresses are revealed to the surface with the application of SP treatments. The results indicate that by increasing the values of Almen intensity (SSSP1, SSSP2, and SOSP), the depth of material affected by compressive residual stresses (CRS), values of surface, and maximum CRS rise, whereas increasing only the coverage (SCSP and SSSP1) has a lower effect on CRS distribution. Moreover, RSP has a slight influence on CRS creation after SSP. The results also displayed that by increasing SP severity and consequently the whole kinetic energy of SP process, the depth of material affected by CRS, values of surface, and maximum CRS rise.
ACCEPTED MANUSCRIPT Which treatment influences the deeper layer based on compressive stresses on the cross sections. For instance, the effective compressive stresses at -200 MPa remain up to 800 µm for severe and over shot peening. However, effective hardening vanishes away from the surface after approximately 500 µm. As a result, the treatments considered in this study, compared to hardening, are more beneficial for exposing compressive residual stress. Frankel et al. [43] have proposed a model for steel tubes and explored the application of much larger loads required to
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produce the adequate plastic deformation as a function of compressive or tensile surface residual
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stress. They observed that although alteration is maximum, the change brought on hardness
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could remain at certain levels. Gonzalez et al. [44] have reported similar results for the relationship between compressive residual stress and microhardness on Al 6063 alloy. In
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contrast, Bagherifard et al. [45] have revealed that both FWHM and compressive residual stresses vanish at 0.1 mm, and microhardness changes when plastic deformation exceeds at least
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Figure 8. Distribution of residual stresses obtained by XRD measurements
3.4. Tensile and axial fatigue test Generally, the values of yield and ultimate stresses change after performing heat treatment. However, the fatigue behavior of materials is dependent on the values of these stresses. Therefore, the values of yield stress and ultimate stress after heat treatment were obtained using
ACCEPTED MANUSCRIPT tensile test, which consisted of three samples. The stress-strain curves are shown in Figure 9. The details of the tensile test results are compared and presented in Appendix A. Based on these results, the values of yield stress and ultimate stress are obtained as 575 and 880 MPa,
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Figure 9. Stress-Strain curves of AISI 1050 steel
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Furthermore, as mentioned and revealed via FSESEM images, in the over shot peened specimens (SOSP), some nano-cracks were observed in the surface layer which may have detrimental
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influences on fatigue behavior. At this point, a solution is suggested and examined in the present paper to detoxify this problem through removing a thin layer of the treated specimen using grinding process. By performing grinding via an abrasive wheel, the created nano-cracks in the top surface are removed and the surface roughness generated by SP is reduced. Therefore, potential areas of crack initiation and nucleation decrease. EN 12413 grinder, one of the smoothest in its kind with a low speed of 3000 RPM and the concomitant soluble oil 3000 cooling is used to prevent residual stress creation in the ground pre-shot peened specimen (SOSP+G). After measuring the thickness of shot peened specimens before and after grinding, the thickness of the removed layer from two faces of specimens was determined approximately
ACCEPTED MANUSCRIPT as less than 300 µm. Thickness of 300 µm is totally removed from both sides of fatigue specimens which are under OSP conditions. In other words, after grinding, approximately 150 µm is removed from each side. This certain layer is meant to eliminate both rougher surfaces and sub-surface microcracks created under OSP conditions. Even the disturbed surface calculated and detected by Rt is removed, it is too difficult to enhance fatigue limit without eradicating subsurface microcracks. These cracks are very susceptible to causing a fatigue initiation cores and
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promoting propagation to the interior quite early. This is why the double of Rt distance is grinded
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for OSP conditions. In the grinding process, in order to omit the side effects on the residual stress
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field, thin layers of 5 µm were removed at each time, so that totally an amount of 150 µm was removed for each side through soluble oil cooling and a very low feed rate of the abrasive wheel.
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By removing this amount from the surface, a few parts of residual stress field were affected. In addition to the removing of created cracks, the amount of surface roughness decreased
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considerably, which has great effects on fatigue life improvements (see Appendix B).
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Surface morphology of shot peened, re-shot peened, and post-ground specimens are presented in Figure 10. The main roughness parameters, Ra (arithmetic mean), Rq (root mean square), and Rt
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(total height) were measured (lr = 1 mm and ln = 2 mm). The average values of three
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measurements applied on each specimen are given in Figure 11.
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Figure 10. Surface morphology of shot peened, re-shot peened and post-ground specimens (a) SCSP, (b) SSSP2, (c) SSSP2+RSP, (d) SOSP and (e) SOSP+G
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Figure 11. Surface roughness parameters of the treated specimens
In the present research, 30 fatigue test samples were used to create a S-N diagram for each
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treatment and SP conditions. The fatigue test was performed at 12 different stress levels. Therefore, the average fatigue life of the two specimens is treated as the fatigue life at the
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respective cyclic load level. The number of cycles to fracture increases with decreasing stress amplitude. The S-N curve becomes horizontal at a certain limiting stress. Below this limiting
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stress, known as the fatigue limit or endurance limit, the material can tolerate an infinite number of cycles without failure [46-48]. The staircase method, known as up-and-down method, is used
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to estimate fatigue endurance limit. Figure 12 represents the obtained S‐N curve of specimens. Figure 13 compares the fatigue limit of as-received and shot peened specimens. The fatigue test results revealed that the applied SP treatments on specimens SCSP, SSSP1, SSSP2, SSSP2+RSP, and SOSP+G have respectively improved the fatigue limit by 7.4, 14.8, 20.9, 44.4, and 48.1 % compared to the fatigue limit of the as-received specimens. Generally, SP treatments were able to increase the fatigue limit. In these cases, the positive effects of micro-structure refinement, hardness improvement and generation of compressive residual stresses are greater than the adverse influences of surface roughness alteration. It is interesting to note that re-shot peening and post-grinding on pre-shot
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areas as a result of the creation of nano-cracks in the treated surface.
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Figure 12. S–N curves obtained for as-received and different shot peened specimens
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Figure 13. Fatigue limit of as-received, shot peened, re-shot peened and post-ground specimens
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It can be observed from Figure 13 that shot peening process enhances high cycle fatigue life by generating a compressive residual stress. This effect can augment by increasing Almen intensity
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and coverage of shot peening process (Compare S-N curves of SCSP, SSSP1, and SSSP2 with SN curve of as-received specimens). But, this trend is not permanent and there are certain limits to
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increasing the value of shot peening parameters with the aim of prolonging high cycle fatigue life which should be obtained experimentally. The figure shows that the values of SP parameters
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related to SOSP specimens are more than the above limits which reduce fatigue life. The experiment results demonstrate that this problem can be overcome by applying post-grinding the
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overly shot peened specimens. The re-shot peening process can improve high cycle fatigue life of peened specimens by reducing surface roughness of the treated specimens through CSP and SSP techniques.
SEM observation was carried out to measure the fractured surface of treated specimens for assessing the effect of different SP processes on fatigue crack initiation and propagation. For instance Figures 14 and 15 depicted and compared the fractured surface of severely and conventionally shot peened specimens SSSP2 and SCSP. It can be observerd that in the both CSP and SSP, the fatigue crack initiated from the subsurface layer. In addition, different sections of the fracture surfaces are specified in the figures. In general, there are no strict and certain borders between crack occurrence-propagation and compressive residual stress state. Defect
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initiation and stress concentration densification could overcome the residual stress deposition, consequently, and and failure becomes unavoidable.
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ACCEPTED MANUSCRIPT Figure 14. SEM images of (a) fracture surface of severely shot peened specimen SSSP2, σa= 540 MPa, Nf = 2,180,000cycles, considering details of different stages of propagations (b) transcrystalline fatigue fracture and closer sites to final rupture, (c) fatigue fracture with fine striations, (d) increase of the spacing between striations closer to the final rupture, (e) final rupture, (f) transcrystalline ductile fracture with dimple
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morphology, (g) crack initiation under the surface layer and (h) crack initiation in higher magnification
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ACCEPTED MANUSCRIPT Figure 15. SEM images of (a) fracture surface of conventionally shot peened specimen SCSP, σ a= 440 MPa, Nf = 876,000cycles, considering details of different stages of propagations (b) transcrystalline fatigue fracture and closer sites to final rupture, (c) increase of the spacing between striations closer to the final rupture and transcrystalline ductile fracture and (d) fatigue cracks creation under the surface layer and (e) fatigue cracks under the surface layer with higher magnification
4. Conclusion
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In this paper, the AISI 1050 carbon steel specimens were treated by different SP processes to
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study the effects of CSP, SSP, OSP, RSP, and post-grinding on microstructure, mechanical
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properties, and fatigue behavior. Different shot peening treatments were performed under various conditions. The properties of treated specimens were investigated through several experimental
OM images showed that by increasing the severity of the SP process, plastic deformation of the surface layer rises as well. FESEM analysis demonstrated that NS layers are created in subsurface in both SSP and
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methods. On the basis of the obtained results, the following conclusions can be drawn:
OSP treatments.
By increasing severity excessively, OSP phenomena occurs, and it leads to the creation of
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nano/micro-cracks on the top surface.
According to XRD measurements, NS grain sizes decrease as a result of increasing the
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values of Almen intensity in the same coverage. The microhardness measurements revealed that hardness of the shot peened specimens
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has significantly increased due to the rise of Almen intensity. The exposed compressive residual stress on the shot peened specimens rises dramatically
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thanks to the growth of Almen intensity remarkably. In the case of overly shot peened specimens, fatigue life significantly drops because of the cracks created on the surface.
Post-grinding overly shot peened specimens can be used as an alternative method to detoxify the detrimental effects of OSP on fatigue behavior by removing the demolished surface layer.
Post- treatment of the shot peened specimens through techniques such as grinding or other methods used in the references [35-57] can be more effective in triggering fatigue life improvements.
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Appendix A
Table A. Comparison of tensile test results of AISI 1050 after performing heat treatment Peak point
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Sample
Break point
Yield point
Extension (mm)
Stress (MPa)
Strain (%)
Force (N)
Extension (mm)
Stress (MPa)
Strain (%)
Force (N)
Extension (mm)
Stress (MPa)
Strain (%)
1
66097.2
10.0226
884.37
0.176
27221.1
12.8547
363.25
0.2258
43741.2
0.52102
583.174
0.00913
2
66699.1
9.65842
890.045
0.1714
31633.1
12.5812
421.992
0.2201
42502.1
0.54578
565.917
0.00961
3
65273.7
10.55
869.45
0.18504
34723.4
12.9427
462.751
0.2269
43025.9
0.56208
573.544
0.00979
Delta
1425.4
0.89158
20.595
0.01364
7502.3
0.3615
99.501
0.0068
1239.1
0.04106
17.257
0.00066
Mean
675.76
10.0770
8.75667
0.00756
3530.87
0.14983
46.7533
0.00263
651.467
0.01912
8.9623
0.01045
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Force (N)
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Appendix B
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Figure B. Layer removed by grinding with respect to the value of the residual stress for each side of the over shot peened specimen.
ACCEPTED MANUSCRIPT Highlights •
Over shot peening generates micro-cracks in the surface layer.
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Fatigue behavior of the material is remarkably decreased after over shot
peening. Re-shot peening can be employ to improve the fatigue properties.
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Post-grinding detoxifies the detrimental effects of over shot peening.
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Erfan Maleki, Okan Unal, Kazem Reza Kashyzadeh PII: DOI: Reference:
S0257-8972(18)30207-X doi:10.1016/j.surfcoat.2018.02.081 SCT 23152
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
5 September 2017 17 February 2018 21 February 2018
Please cite this article as: Erfan Maleki, Okan Unal, Kazem Reza Kashyzadeh , Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.02.081
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ACCEPTED MANUSCRIPT Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel Erfan Malekia,*, Okan Unalb, Kazem Reza Kashyzadeha a
Mechanical Engineering Department, Sharif University of Technology-International Campus, Kish Island, Iran b
Mechanical Engineering Department, Karabuk University, Karabuk, Turkey
*
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Corresponding author, email address: [email protected], Phone: +989125427218
Abstract
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The present study investigates experimentally the effects of different shot peening treatments, including conventional, severe, over, and re-shot peening on microstructure, mechanical
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properties, and fatigue behavior of AISI 1050 mild carbon steel. Different shot peening treatments were performed using various effective parameters by considering the influences of
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increasing Almen intensity and coverage. Optical microscopy and field emission scanning electron microscopy observations and X-Ray diffraction measurements were carried out to
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analyze grains refinement in each shot peening treatment. Microhardness and residual stress measurements were taken from shot peened surfaces to the core material to investigate the
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mechanical properties. The fatigue behaviors of the specimens were examined by using the axial fatigue test. The results indicated that post-grinding, re-shot peening, and severe shot peening
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processes have significant effects on fatigue life improvement.
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Key words: fatigue behavior, severe plastic deformation, shot peening, microstructure,
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mechanical properties,
1. Introduction
Most failures in engineering components are due to mechanical and metallurgical properties of the materials’ surface, and in most cases such failures originate from the exterior layers of components [1]. Therefore, improving surface layer properties has become a topic which has attracted considerable interest. In enhancing surface properties, especially in metallic materials, grains refinement and the creation of compressive residual stress (CRS) in the surface play important roles; these techniques have positive effects on materials’ behavior such as fracture, fatigue, corrosion, wear, and crack arrestment [2-4, 52].
ACCEPTED MANUSCRIPT Most metals are produced through thermo-mechanical processing, and grain sizes typically range between ~ 5- 10 µm [5]. However, it is now well-known that the strength of metals and alloys is strongly influenced by their grain size [6, 49]. The strength of metals is related to their microstructure (grain size) which is dependent on two relationships of low and high operating temperatures.
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At low temperatures (such as room temperature), the strength of the material is generally
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described by Hall–Petch (H-P) relationship as follows [7]:
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σy = σ0 + ky /d ½
(1)
Where σy is the yield stress, σo is a material constant for the starting stress required for
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dislocation movement, ky is the strengthening coefficient, and d is the average grain diameter.
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At high temperatures and under conditions of constant stress or load, the steady-state strain rate is given by the following relationship [8, 9]:
(2)
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ADGb b p n ( ) ( ) kT d G
Where D is the appropriate diffusion coefficient, T is the absolute temperature, G is the shear
modulus, b is the Burgers vector, k is Boltzmann’s constant, σ is the applied stress, p and n are
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respectively the exponents of the inverse grain size and the stress, and A is a dimensionless constant. For coarse-grained materials, the flow processes at elevated temperatures are
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intragranular in nature and p= 0, but when the grain size is reduced other flow processes which are dependent on the grain size become important [5]. Therefore, the strength of the material at
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both low and high temperatures increases when the grain size decreases; and grains refinement provide an opportunity for obtaining much better mechanical and metallurgical properties. By applying grain refinement, the size of coarse grains reaches a submicron (<1000 nm) scale. The grains with a size of 1- 100 nm and 100- 500 nm are called nanostructured (NS) and ultrafinegrained (UFG), respectively [10]. One of the common techniques of grain refinement and CRS creation as one of the severe plastic deformation (SPD) methods is shot peening (SP) [54]. SP is a cold working process in which the surface of a component is bombarded with small shots under controlled velocity [11]. The
ACCEPTED MANUSCRIPT capability of a metallic material to deform plastically depends on the ability of dislocations to move and enhance the rate of plastic strain. Restricting dislocation makes the material stronger, and strengthening in turn reduces ductility of the material. The cause of work hardening is the increase of dislocation density through plastic deformation [12, 13, 51]. In the SP process, plastic deformation is directly proportional to the amount of total kinetic energy of the shot stream transferred to the component. This energy is dependent to the values of Almen intensity and
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surface coverage. Based on the value of created kinetic energy, SP process is classified into three
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cases, including conventional shot peening (CSP), severe shot peening (SSP), and over shot
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peening (OSP). CSP employing conventional intensities and coverages is usually recognized to introduce of compression residual stresses in the subsurface layers of the material. SSP process,
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using unconventional high parameters, generates more compressive residual stresses and grain refinement of the treated surface layer and makes NS [14, 15]. OSP occurs when the created
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kinetic energy is not well selected and made suitable by using a high Almen intensity and coverage; such a disproportion may create defects such as producing points of stress
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concentration, superficial micro-cracks, and overlaps [16]. The effect of different types of SP on metallurgical and mechanical properties of various
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materials have been studied by many researchers [17-20]. The fatigue behaviors of conventionally and severely shot peened materials has been investigated by many authors as well
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[21-26]. However, few papers have explored the effects of OSP on the fatigue behavior of materials. Benedetti et al. [27, 28] have employed different intensities for SP of the same target.
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Their X-Ray diffraction measurement showed that higher intensities would be more beneficial in terms of residual stress distribution, but fatigue tests did not indicate any improvement in the life
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of the component. Fathallah et al. [29] have revealed a noticeable reduction of fatigue life in the case of coverage of 1000% against the common coverage of 100%. In addition, considering SP surface treatments with different intensities, Trško et al. [30] have studied fatigue life of an aluminum alloy in high and ultra-high cycle regions. Severe shot peening with a 9.6N Almen intensity and a coverage of 650 % increases fatigue life up to 9 %. However, using OSP parameters with a higher Almen intensity of 14.9A and a coverage of 650 % significantly shortens the fatigue life up to 21 %.
ACCEPTED MANUSCRIPT Furthermore, some SP post-treatment such as elimination of surface roughness [35], re-shot peening [36], and vibration polishing and peening [37] have been suggested to improve mechanical properties and fatigue behavior of materials. In this paper, microstructure, mechanical properties, and fatigue behavior of AISI 1050 mild carbon steel treated with CSP, SSP, OSP, re-shot peening (RSP), and post-grinding of the pre-
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shot peened specimens is investigated and compared experimentally. Different SP treatments
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having different Almen intensities and coverages were performed.
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Microstructure of the treated specimens has been characterized using optical microscopy (OM) and field emission scanning electron microscopy (FESEM) observations as well as X-Ray
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diffraction (XRD) analysis. Mechanical properties were evaluated by measuring hardness, residual stress, roughness, and fatigue lives. And the fatigue lives of specimens are obtained by
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using the axial fatigue test. Conventional and severe shot peened specimens were compared within the context of above measurements. The properties of treated specimens were investigated
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through various experimental methods as mentioned and the results revealed that, by increasing the severity of the SP process, plastic deformation of the surface layer is enhanced and NS grains
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are generated. In addition, both microhardness and residual stress are raised by increasing Almen intensity. Moreover, the deformed surface layers introduced by post-grinding, re-shot peening,
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2. Experimental setup
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and severe SP processes show significant effects on the improvement of fatigue life.
2.1. Material and specimens
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AISI 1050 mild carbon steel is selected as specimen material. AISI 1050 is used in general engineering applications because of its specific properties. Its unique mechanical properties enables this grade of steel to have a wide range of applications in the manufacture of forged shafts and gears. The chemical composition of AISI 1050 is shown in Table 1. Table 1. Chemical composition of AISI 1050 carbon steel (weight %). C
Mn
Si
Cr
Cu
Mo
Ni
S
Al
Fe
0.51
0.64
0.23
0.16
0.14
0.02
0.07
0.005
0.01
Bal.
ACCEPTED MANUSCRIPT Tensile and fatigue test specimens were fabricated from a sheet of hot rolled AISI 1050 with a thickness of 6mm according to ASTM E8M [40] and ASTM E466 [31] standards, respectively. The shape and size of specimens are given in Figure 1. The specimens were austenitized at 850°C for 1 h, immediately oil quenched, and then aged at 520°C for 1 h followed by air cooling. The average hardness of the resulting microstructure is 28
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HRC (285 HV). Figure 2a illustrates the microstructure of AISI 1050 before heat treatment (as
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received-annealed condition), and pearlite (dark phase) and ferrite (light phase) phases can be
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observed clearly. However, in Figure 2b after heat treatment (quenching and tempering) has been performed, the pearlite and ferrite boundaries vanish, and the ferrite proportion falls while
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pearlite structure fraction rise.
Figure 1. Shape and size of the specimens for (a) tensile and (b) fatigue test
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Figure 2. Microstructure of the AISI 1050 steel (a) before and (b) after accomplished heat treatment
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2.2. Shot Peening Treatments
SP treatments applied on the specimens were performed using an air blast shot peening (ABSP)
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device. Air pressure, shot type, and peening time are the main process parameters that affect the results. All treatments were carried out using standard steel shots with an average hardness of 50
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HRC, impingement angle of 90º, nozzle diameter of 6.35 mm (1/4 in), and a distance between nozzle and sample equal to 10 cm. Different SP treatments with varying Almen intensities and
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coverages were conducted on the specimens. Almen intensity was obtained according to SAE J443 standard [32]. Table 2 depicts the applied parameters of shot peening treatments on each
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considered specimen. In SCSP and SSSP1 specimens, the effect of increasing coverage was investigated, and in SSSP1, SSSP2, and SOSP specimens, the influence of enhancing Almen intensity was considered. SCSP specimen was treated via common and conventional parameters of SP process, and SSSP1, SSSP2, and SOSP specimens were shot peened with a higher kinetic energy. The effective parameters on the shot peening process are numerous and too hard to control exactly. They can easily be influenced by the humidity of the chamber and the quality of air compressor. Some of the parameters affecting the system are shot size, shot material, air pressure, and surface coverage. Both individual and total pre-conditions can alter the type of shot peening (conventional, severe, and over). There are no certain and distinctive borders between
ACCEPTED MANUSCRIPT severe and over shot peening methods. However, the macro and micro analysis (surface roughness, surface topography, surface yielding, and crack occurrence) of the shot peened surface are presented for the sake of distinguishing conventional and severe shot peenings. Besides, microhardness and residual stress analysis supports the results. Nevertheless, the purpose of employing “over shot peening” is to determine the border between the beneficial and hazardous effects rather than decreasing the grain size or increasing the fatigue limit through
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excessive plastic deformations. Over shot peening parameters were specified by investigating
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superficial micro-cracks and overlaps on and just beneath the surface. Some researchers [18]
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have pointed to the practical applications of over shot peening for aircraft and automotive engineering; they have also proposed A12-16 Almen intensity conditions as the conventional
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shot peening intensity. However, this can be broadened to the upper and lower values as A10-18 Almen intensity at lower 100%-200% coverages [19-20, 38]. Also, the intensity level used for
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conventional shot peening was converted to severe shot peening by increasing surface coverage up to 1500% [23-26, 30].
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Figure 3 represents shot peened fatigue test specimens.
Shot
Projection
Almen
Diameter
Pressure
Intensity
(mm)
(bar)
A strip (mm)
0.58
2.8
0.18
100
SSSP1
0.58
2.8
0.18
1500
SSSP2
0.71
3.5
0.22
1500
SOSP
0.71
5.5
0.26
1500
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SCSP
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Specimen ID
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Table 2. Different applied shot peening treatments parameters
Coverage (%)
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Figure 3. Prepared specimens for (a) tensile test and (b) fatigue test after SP
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In addition, in order to investigate the effect of re-shot peening one of the pre-shot peened specimens (SSSP2) was selected and dual step shot peening process was carried out on it. SP
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process can be accomplished in several ways such as single, dual or triple steps, all of which have their specific influences on relevant materials. Dual SP may even better improve fatigue
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resistance than a single peening treatment [36, 39]. In this process, a fully pre-shot peened specimen at the special intensity, re- shot peened (SSSP2+RSP) in a second operation with a
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lower intensity and a shorter peening duration were used. The parameters of the performed RSP process are shown in Table 3. Moreover, grinding process is employed to detoxify the probable created surface defects and reduce surface roughness on the over shot peened specimen (SOSP). The related specimen is called SOPS+G. Table 3. Parameters of the performed RSP process Shot
Projection
Almen
Diameter
Pressure
Intensity
(mm)
(bar)
(0.001 in A)
Coverage (%)
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2.4
10
100
2.3. Microscopic observations Microstructure observations are carried out via OM and FESEM. OM and FESEM observations were applied using Olympus and Mira 3-XMU respectively. Mira 3-XMU can reach high
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resolution up to 1.0 nm at 30 keV. Also, 200 V to 30 kV accelerating voltage exists with a BSE
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detector, EDX and EBSD facilities. Specimens for OM observations have been etched by 2%
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Nital.
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2.4. XRD Grains size measurements
To obtain the grain size after applying SP treatments with higher energy on the related specimens
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(SSSP1, SSSP2, and SOSP), X-ray diffraction (XRD) measurements were carried out. In XRD measurements, the full width at half maximum (FWHM) of the diffraction peaks were obtained and the crystallite sizes were calculated. For XRD analysis, X’Pert PRO MPD (PANalytical )X-
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ray diffractometer system and X’Pert High Score Plus (V. 3) analyzer are employed with Cu Kα
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radiation operated at 40 kV and 40 mA, scanning angle of 30º–150º, and irradiated area of 10 mm.
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2.5. Microhardness measurements
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In order to investigate the effects of accomplished SP treatments on the hardness of the specimens, microhardness tests were performed up to 800 µm on the cross sectional surface with intervals of 20 µm. Qness GmbH Q30 A microhardness tester at a load of 10 gf with a duration
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of 7 s using Vickers indenter.
2.6. Residual stress measurements To study the state of residual stresses on the specimens, different XRD analysis were applied via Xstress 3000 G2/G2R X-ray Stress Analyzer (radiation Cr Kα, irradiated area of 4 mm diameter, sin2ψ method, and diffraction angle (2θ) ~156 scanned between 45 and -45). Measurements were carried out in depth step by step by removing a very thin layer of material (~ 20 µm) through electro-polishing with a solution of acetic acid (94%) and perchloric acid (6%). The Voigt
ACCEPTED MANUSCRIPT method has been employed to assess microstructural and residual stress evaluations. Measured profile h is convoluted by physical profile g and broadened profile f. Also, structurally broadened profile is constructed by combining Cauchy and Gaussian components. This process leads to a slight stress relaxation and has to be corrected using the correlation methods [18, 35]. The removal layer and depth measurements after etching process are generally achieved by using a calibrated micrometer, dial gauge, and a travelling microscope. The removal layer of the
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specimens are determined with a calibrated travelling microscope which focuses on the exposed
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surface. Measurements are performed between the widths of the surface and mean value of the
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total is obtained.
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2.7. Tensile test
The STM-250 SANTAM universal testing machine is used to perform tensile test. The tensile
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rate is set and maintained at the range of 0.015 +/- 0.006 mm/mm/min based on Standard Test Methods for Tension Testing of Metallic Materials (ASTM E8). Also, and strains are measured
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through a clip gage extensometer with a length of 25 mm gage.
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2.8. Fatigue test
This test is a common method for determining the behavior of materials under fluctuating loads.
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The SANTAM SAF-250 universal test machine is used to carry out fatigue test under tensioncompression loading conditions. The average stress is considered equal to zero and the stress
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ratio of R= -1. The loading frequency of 20 Hertz is used for all fatigue tests. The recommended number of fatigue test samples used to generate Wohler curve (S-N) and the number of
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replicated tests vary between 6-12 and 12-24 for preliminary and design allowable phases, respectively [41]. The minimum number of test specimens for constructing a S-N curve is 14 based on the fatigue testing standard which proposed by the Association of Mechanical Engineers of Japan in 1981 and by ASTM in 1998. The fatigue limit can be obtained by a stepwise procedure having 6 test specimens [42]. In the Wohler curve, the portion of curve with a negative slope constitutes the finite life region and represents fatigue strength of the material for a given number of stress cycles, while the horizontal portion represents infinite life region. The stress level corresponding to the horizontal portion is known as fatigue limit of the material. The endurance fatigue limit is obtained by using
ACCEPTED MANUSCRIPT JSME S 002 standard [53] and applying staircase method. The staircase technique known as upand-down method can estimate the endurance limit by taking into account its statistical nature. In this method, a specimen is tested at stress amplitude 𝑆𝑎 about 5% higher than the expected endurance limit. If the specimen fails before the stipulated number of cycles (around 1 million cycles), the next specimen is tested at a lower stress amplitude. Nevertheless, if a specimen survives, the test is suspended after the fulfillment of the specified number of cycles and the next
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specimen is tested at a greater amplitude of maximum alternating stress. Accordingly, the stress
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amplitude of each successive test is dependent on the outcome of its previous test. 2.9. Surface roughness measurements
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Surface roughness of specimens, a well-recognized side effect of SP process affecting fatigue behavior, is measured using SURFCORDER SE500. The roughness parameters were analyzed
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based on the definition of ISO 4287 [33].
3.1. Structural characterizations
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3. Results and discussion
After SP treatments were accomplished, the specimens were plastically deformed. OM images of
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the shot peened specimens are given in Figure 4. It can be seen that by increasing the kinetic energy of the SP process via enhancing the values of Almen intensity and coverage, the depth of
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the deformed layer from the surface rises as well. Figure 5 represents the FESEM images of the shot peened specimens that illustrate the created UFG and NS layer. Besides, the reduction of the
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grains size are clearly realized near the top surface layer. Also, it can be observed that in Figure 5.d, the specimen (SOSP) shot peened with very high kinetic energy which is not admissible,
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some nano-cracks are created in the top surface and the OSP phenomena has occurred.
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Figure 4. OM images of the different treated specimens: (a) SCSP, (b) SSSP1, (c) SSSP2 and (d) SOSP
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Figure 5. FESEM observations of the severely shot peened specimens: (a) SCSP, (b) SSSP1, (c) SSSP2 and (d) SOSP
As mentioned earlier, in order to achieve the grain size of the treated specimens with SP treatments at higher and unconventional rates of kinetic energy, the authors performed XRD
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analysis. The XRD patterns of treated specimens are presented in Figure 6. The crystallite sizes
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of the specimens in the surface layer were obtained using the most common method for determining the mean crystallite size. The full width half maximum (FWHM) of a diffraction θ
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peak at Scherer’s equation is as follows [34]:
(3)
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dXRD = K λ / β Cosθ
Where d is the apparent size of crystal, λ is the wavelength of the x-radiation (i.e. λCu-Kα1.54 Å),
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B is the corrected FWHM (i.e. area under the curve divided by maximum height, in radian), θ is the diffraction angle, and K is a constant close to unity (i.e. 0.94). β can be obtained from the
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observed FWHM by convoluting the Gaussian profile modeling the specimen broadening β r, as follows:
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βr 2 = β0 2 – βi 2
(4)
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Where β0 is the observed broadening and βi is the instrumental broadening. XRD is used to determine the peak broadening with the crystallite size due to high
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density of dislocations in nanocrystalline materials using Scherer’s formula [50].
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It is worth noting that the usage of Scherer’s equation is limited to nano-scale particles, and it is not applicable to grains larger than 100 nm, in which case the HRTEM (or TEM) gives more precise grain size distributions [35]. Based on XRD analysis, the values of FWHM and the crystallite size of SSP treated specimens obtained from the certain peak are presented in Table 4.
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Figure 6. Intensity distribution of all treated specimens
Table 4. FWHM and crystallite sizes of specimens peened with SSP treatments
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Specimen
Peak
2-Theta (º)
FWHM (º)
Crystallite size (nm)
α (1 1 0)
44.545
0.0968
89.2
SSSP2
α (1 1 0)
44.603
0.1570
55.5
SOSP
α (1 1 0)
44.523
0.1754
49.1
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SSSP1
3.2. Hardness Measurement Microhardness measurements were applied in order to investigate the effects of exposed plastic deformation on the specimens. Figure 7 represents the variation of microhardness through the interior for the as-received and shot peened specimens. The resultant data scattering was no more than 5%. It can be observed that increasing SP severity entails a rise in hardness and its
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Figure 7. Variation of the microhardness for different SP treatments from top surface to interior
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3.3. Residual stress measurements
XRD stress measurements are used to obtain the residual stress distributions of as-received and
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treated specimens. Figure 8 compares the residual stress distribution of each specimen. As shown, the as-received specimen (not peened) has -20 MPa compressive residual stresses on the
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surface because of the performed heat treatment. However, more compressive residual stresses are revealed to the surface with the application of SP treatments. The results indicate that by increasing the values of Almen intensity (SSSP1, SSSP2, and SOSP), the depth of material affected by compressive residual stresses (CRS), values of surface, and maximum CRS rise, whereas increasing only the coverage (SCSP and SSSP1) has a lower effect on CRS distribution. Moreover, RSP has a slight influence on CRS creation after SSP. The results also displayed that by increasing SP severity and consequently the whole kinetic energy of SP process, the depth of material affected by CRS, values of surface, and maximum CRS rise.
ACCEPTED MANUSCRIPT Which treatment influences the deeper layer based on compressive stresses on the cross sections. For instance, the effective compressive stresses at -200 MPa remain up to 800 µm for severe and over shot peening. However, effective hardening vanishes away from the surface after approximately 500 µm. As a result, the treatments considered in this study, compared to hardening, are more beneficial for exposing compressive residual stress. Frankel et al. [43] have proposed a model for steel tubes and explored the application of much larger loads required to
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produce the adequate plastic deformation as a function of compressive or tensile surface residual
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stress. They observed that although alteration is maximum, the change brought on hardness
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could remain at certain levels. Gonzalez et al. [44] have reported similar results for the relationship between compressive residual stress and microhardness on Al 6063 alloy. In
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contrast, Bagherifard et al. [45] have revealed that both FWHM and compressive residual stresses vanish at 0.1 mm, and microhardness changes when plastic deformation exceeds at least
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0.2 mm.
Figure 8. Distribution of residual stresses obtained by XRD measurements
3.4. Tensile and axial fatigue test Generally, the values of yield and ultimate stresses change after performing heat treatment. However, the fatigue behavior of materials is dependent on the values of these stresses. Therefore, the values of yield stress and ultimate stress after heat treatment were obtained using
ACCEPTED MANUSCRIPT tensile test, which consisted of three samples. The stress-strain curves are shown in Figure 9. The details of the tensile test results are compared and presented in Appendix A. Based on these results, the values of yield stress and ultimate stress are obtained as 575 and 880 MPa,
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Figure 9. Stress-Strain curves of AISI 1050 steel
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Furthermore, as mentioned and revealed via FSESEM images, in the over shot peened specimens (SOSP), some nano-cracks were observed in the surface layer which may have detrimental
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influences on fatigue behavior. At this point, a solution is suggested and examined in the present paper to detoxify this problem through removing a thin layer of the treated specimen using grinding process. By performing grinding via an abrasive wheel, the created nano-cracks in the top surface are removed and the surface roughness generated by SP is reduced. Therefore, potential areas of crack initiation and nucleation decrease. EN 12413 grinder, one of the smoothest in its kind with a low speed of 3000 RPM and the concomitant soluble oil 3000 cooling is used to prevent residual stress creation in the ground pre-shot peened specimen (SOSP+G). After measuring the thickness of shot peened specimens before and after grinding, the thickness of the removed layer from two faces of specimens was determined approximately
ACCEPTED MANUSCRIPT as less than 300 µm. Thickness of 300 µm is totally removed from both sides of fatigue specimens which are under OSP conditions. In other words, after grinding, approximately 150 µm is removed from each side. This certain layer is meant to eliminate both rougher surfaces and sub-surface microcracks created under OSP conditions. Even the disturbed surface calculated and detected by Rt is removed, it is too difficult to enhance fatigue limit without eradicating subsurface microcracks. These cracks are very susceptible to causing a fatigue initiation cores and
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promoting propagation to the interior quite early. This is why the double of Rt distance is grinded
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for OSP conditions. In the grinding process, in order to omit the side effects on the residual stress
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field, thin layers of 5 µm were removed at each time, so that totally an amount of 150 µm was removed for each side through soluble oil cooling and a very low feed rate of the abrasive wheel.
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By removing this amount from the surface, a few parts of residual stress field were affected. In addition to the removing of created cracks, the amount of surface roughness decreased
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considerably, which has great effects on fatigue life improvements (see Appendix B).
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Surface morphology of shot peened, re-shot peened, and post-ground specimens are presented in Figure 10. The main roughness parameters, Ra (arithmetic mean), Rq (root mean square), and Rt
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(total height) were measured (lr = 1 mm and ln = 2 mm). The average values of three
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measurements applied on each specimen are given in Figure 11.
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Figure 10. Surface morphology of shot peened, re-shot peened and post-ground specimens (a) SCSP, (b) SSSP2, (c) SSSP2+RSP, (d) SOSP and (e) SOSP+G
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Figure 11. Surface roughness parameters of the treated specimens
In the present research, 30 fatigue test samples were used to create a S-N diagram for each
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treatment and SP conditions. The fatigue test was performed at 12 different stress levels. Therefore, the average fatigue life of the two specimens is treated as the fatigue life at the
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respective cyclic load level. The number of cycles to fracture increases with decreasing stress amplitude. The S-N curve becomes horizontal at a certain limiting stress. Below this limiting
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stress, known as the fatigue limit or endurance limit, the material can tolerate an infinite number of cycles without failure [46-48]. The staircase method, known as up-and-down method, is used
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to estimate fatigue endurance limit. Figure 12 represents the obtained S‐N curve of specimens. Figure 13 compares the fatigue limit of as-received and shot peened specimens. The fatigue test results revealed that the applied SP treatments on specimens SCSP, SSSP1, SSSP2, SSSP2+RSP, and SOSP+G have respectively improved the fatigue limit by 7.4, 14.8, 20.9, 44.4, and 48.1 % compared to the fatigue limit of the as-received specimens. Generally, SP treatments were able to increase the fatigue limit. In these cases, the positive effects of micro-structure refinement, hardness improvement and generation of compressive residual stresses are greater than the adverse influences of surface roughness alteration. It is interesting to note that re-shot peening and post-grinding on pre-shot
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areas as a result of the creation of nano-cracks in the treated surface.
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Figure 12. S–N curves obtained for as-received and different shot peened specimens
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Figure 13. Fatigue limit of as-received, shot peened, re-shot peened and post-ground specimens
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It can be observed from Figure 13 that shot peening process enhances high cycle fatigue life by generating a compressive residual stress. This effect can augment by increasing Almen intensity
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and coverage of shot peening process (Compare S-N curves of SCSP, SSSP1, and SSSP2 with SN curve of as-received specimens). But, this trend is not permanent and there are certain limits to
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increasing the value of shot peening parameters with the aim of prolonging high cycle fatigue life which should be obtained experimentally. The figure shows that the values of SP parameters
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related to SOSP specimens are more than the above limits which reduce fatigue life. The experiment results demonstrate that this problem can be overcome by applying post-grinding the
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overly shot peened specimens. The re-shot peening process can improve high cycle fatigue life of peened specimens by reducing surface roughness of the treated specimens through CSP and SSP techniques.
SEM observation was carried out to measure the fractured surface of treated specimens for assessing the effect of different SP processes on fatigue crack initiation and propagation. For instance Figures 14 and 15 depicted and compared the fractured surface of severely and conventionally shot peened specimens SSSP2 and SCSP. It can be observerd that in the both CSP and SSP, the fatigue crack initiated from the subsurface layer. In addition, different sections of the fracture surfaces are specified in the figures. In general, there are no strict and certain borders between crack occurrence-propagation and compressive residual stress state. Defect
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initiation and stress concentration densification could overcome the residual stress deposition, consequently, and and failure becomes unavoidable.
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ACCEPTED MANUSCRIPT Figure 14. SEM images of (a) fracture surface of severely shot peened specimen SSSP2, σa= 540 MPa, Nf = 2,180,000cycles, considering details of different stages of propagations (b) transcrystalline fatigue fracture and closer sites to final rupture, (c) fatigue fracture with fine striations, (d) increase of the spacing between striations closer to the final rupture, (e) final rupture, (f) transcrystalline ductile fracture with dimple
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morphology, (g) crack initiation under the surface layer and (h) crack initiation in higher magnification
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ACCEPTED MANUSCRIPT Figure 15. SEM images of (a) fracture surface of conventionally shot peened specimen SCSP, σ a= 440 MPa, Nf = 876,000cycles, considering details of different stages of propagations (b) transcrystalline fatigue fracture and closer sites to final rupture, (c) increase of the spacing between striations closer to the final rupture and transcrystalline ductile fracture and (d) fatigue cracks creation under the surface layer and (e) fatigue cracks under the surface layer with higher magnification
4. Conclusion
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In this paper, the AISI 1050 carbon steel specimens were treated by different SP processes to
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study the effects of CSP, SSP, OSP, RSP, and post-grinding on microstructure, mechanical
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properties, and fatigue behavior. Different shot peening treatments were performed under various conditions. The properties of treated specimens were investigated through several experimental
OM images showed that by increasing the severity of the SP process, plastic deformation of the surface layer rises as well. FESEM analysis demonstrated that NS layers are created in subsurface in both SSP and
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methods. On the basis of the obtained results, the following conclusions can be drawn:
OSP treatments.
By increasing severity excessively, OSP phenomena occurs, and it leads to the creation of
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nano/micro-cracks on the top surface.
According to XRD measurements, NS grain sizes decrease as a result of increasing the
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values of Almen intensity in the same coverage. The microhardness measurements revealed that hardness of the shot peened specimens
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has significantly increased due to the rise of Almen intensity. The exposed compressive residual stress on the shot peened specimens rises dramatically
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thanks to the growth of Almen intensity remarkably. In the case of overly shot peened specimens, fatigue life significantly drops because of the cracks created on the surface.
Post-grinding overly shot peened specimens can be used as an alternative method to detoxify the detrimental effects of OSP on fatigue behavior by removing the demolished surface layer.
Post- treatment of the shot peened specimens through techniques such as grinding or other methods used in the references [35-57] can be more effective in triggering fatigue life improvements.
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Appendix A
Table A. Comparison of tensile test results of AISI 1050 after performing heat treatment Peak point
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Sample
Break point
Yield point
Extension (mm)
Stress (MPa)
Strain (%)
Force (N)
Extension (mm)
Stress (MPa)
Strain (%)
Force (N)
Extension (mm)
Stress (MPa)
Strain (%)
1
66097.2
10.0226
884.37
0.176
27221.1
12.8547
363.25
0.2258
43741.2
0.52102
583.174
0.00913
2
66699.1
9.65842
890.045
0.1714
31633.1
12.5812
421.992
0.2201
42502.1
0.54578
565.917
0.00961
3
65273.7
10.55
869.45
0.18504
34723.4
12.9427
462.751
0.2269
43025.9
0.56208
573.544
0.00979
Delta
1425.4
0.89158
20.595
0.01364
7502.3
0.3615
99.501
0.0068
1239.1
0.04106
17.257
0.00066
Mean
675.76
10.0770
8.75667
0.00756
3530.87
0.14983
46.7533
0.00263
651.467
0.01912
8.9623
0.01045
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Force (N)
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Appendix B
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Figure B. Layer removed by grinding with respect to the value of the residual stress for each side of the over shot peened specimen.
ACCEPTED MANUSCRIPT Highlights •
Over shot peening generates micro-cracks in the surface layer.
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Fatigue behavior of the material is remarkably decreased after over shot
peening. Re-shot peening can be employ to improve the fatigue properties.
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Post-grinding detoxifies the detrimental effects of over shot peening.
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•