Effect of grit blasting on fatigue life of aged 18Ni(300) maraging steel

Effect of grit blasting on fatigue life of aged 18Ni(300) maraging steel | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of grit blasting on fatigue life of aged 18Ni(300) maraging steel JOSE CARLOS FORTES PALAU, Dilermando Nagle Travessa This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1903680/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The fatigue life of aged 18Ni300 maraging steel was investigated in two different surface conditions: polished and grit-blasted. Axial fatigue tests results, plotted in the form of S-N curves were used to correlate the fatigue performance and the surface characteristics, carefully characterized using a combination of experimental techniques, including X-ray diffraction (XRD), contact profilometry, microhardness testing, scanning electron microscopy (SEM), and stereo optical microscopy (S-OM). The results show that turn-machining of round fatigue samples forms a thin recrystallized layer over the steel surface, which remains after polishing. Roughness increases significantly after grit-blasting, but strain hardening and compressive residual stresses developed after blasting outweigh the deleterious increase of roughness and the fatigue life increases in comparison to the polished condition. Both crack initiation and propagation are retarded under the effects of blasting. Maraging steel grit blasting fatigue life S-N curve Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction 18Ni(300) maraging are a special class of high-strength steels that differ from conventional steels in that their hardening mechanisms does not involve carbon [1]. They are based on the Fe-Ni system, with low-carbon content (< 0,03%) and highly alloyed, mainly with additions of cobalt, molybdenum, and titanium. The Fe-Ni system comprises a metastable transformation of austenite into a cubic tough low carbon martensite (α’) under normal cooling in the range of 300 to 200 o C, retaining in solid solution the other alloying elements. As the reversion of martensite into austenite under heating occurs typically above around 600 o C, this hysteresis enables to perform simple ageing treatments at around 500 o C to precipitate fine intermetallic phases like FeTi, Fe 2 Mo, Ni 3 Ti, and Ni 3 Mo, as investigated by [2], with high hardening effect. It results in a unique combination of high strength to weight ratio, moderate toughness, and good weldability [3, 4], making them attractive for a large range of applications [5]. The fatigue strength of maraging steels, on the other hand, is relatively low in comparison to the conventional high strength steels [6]. It has been reported that the fatigue strength of maraging steels with ultra-high strength is far below the expected value which is generally empirically estimated as 0.4–0.5 of the ultimate tensile strength (UTS) [7]. From the structural design point of view, fatigue failure is the most critical phenomenon in structural materials for many applications where cyclic loads are present. The occurrence of fatigue failure frequently results catastrophic accidents in service [8]. Fatigue life of structures is known to highly depend on the surface conditions because fatigue cracks generally start at the free surface of the material. As reported by McKelvey and Fatemi [9] fatigue failures typically initiate at the surface where micro-cracks form. Depending on the surface finish, micro-cracks may already be present, resulting in a significant reduction of fatigue life. Surface roughness is one of the most detrimental factors for fatigue life. Novovic et. al. [10] performed a literature review on the effect of machined surface topography and integrity on the fatigue life, evaluating different roughness parameters. They concluded that, although Ra is the most used parameter in describing fatigue behavior, Rt and Rz would be more suitable because they represent the worst defects present in the surface. On the other hand, Koster [11] states that when the surface roughness in the range of 2.5<Ra<5.0 µm, fatigue performance correlates better with the surface microstructure and residual stresses than with the roughness itself. Kumar et. al. [12] revealed that compressive residual stresses and plastic strain (cold work) are crucial to enhanced fatigue performance. Grit blasting is a cost-effective surface treatment method frequently used for surface cleaning and corrosion removal of metals, providing a suitable surface roughness for the adhesion of protective coatings [13]. Although it is similar to the shot peening process [14], grit-blasting employs irregular shapped steel blasting particles which can promote more sharply identations that make the metal’s surface susceptible to the fatigue pehnomena. During grit-blasting, however, the metal’s surface is repeatedly impacted by high-speed grit particles, and besides the cleaning effect, local plastic deformation and subsequent grain refinement are prone to occur on the surface, often accompanied by strain hardening and compressive residual stress in the subsurface layer [15, 16]. Both compressive residual stresses and strain hardening at the surface of metals can improve their fatigue resistance (if they surpass the roughness effect), as they tend to close short cracks and increase the yield strength at the crack tip, respectively [17]. It has been reported, however [18] that the effect of strain hardening is more significant, as cyclic loading can quickly reduce residual stresses at the surface. Fatigue performance of 18Ni(300) maraging steel in not frequently reported in the literature, mainly after grit-blasting that is commonly employed on parts after the aging heat treatment to remove oxide scales. In order to contribute to this subject, the present study reports the effect of grit-blasting on the fatigue life of aged 18Ni(300)maraging steel, in comparison to the machined condition. The results are discussed in terms of the magnitude of the surface roughness, residual stresses, and the strain-hardening produced by blasting. 1. Material And Experimental Methods 1.1 Materials Two batches of similar 18Ni(300) maraging steel were employed in this study, which the nominal chemical composition is shown in Table 1. The first batch, supplied by Böhler Edelstahl GmbH, was produced by vacuum induction melting-vacuum arc remelting (VIM-VAR) process, and hot rolled in the form of sheet 3.5 mm thick. This material was used specifically to investigate the residual stress profile obtained by grit-blasting. The second batch was produced by Eletrometal Aços Finos S.A (actually Villares Metals) by electric arc melting and double vacuum arc remelting (VAR), and hot rolled in form of plate 12.7 mm thick. This material was used specifically in the fatigue studies in blasted and machined conditions . Table 1 Chemical composition of the 18Ni(300) maraging steel (wt.%) Material C Ni Co Mo Ti Al Mn Fe Maraging (Böhler) < 0.007 18.58 9.35 4.67 0.75 0.087 0.01 Balance Maraging (Eletrometal) 0.010 18.0 8.90 4.62 < 0.60 0.092 < 0.02 Balance 1.2 Samples preparation Figure 1a shows a typical specimen, wire cut from the 3.5 mm thick sheet using an electrical discharge machining (EDM). The resulting samples had dimensions of 45.0 mm x 25.0 mm. The samples for fatigue tests were cut from the 12.5 mm thick plate, also by wire electrical discharge process. After cutting, samples were solution annealed at 860ºC for 1h followed by air cooling to room temperature, before turning in round specimens according to the dimension shown in Figure 1b. The main turning process parameters were: cutting speed (v=90 m/min), feed rate (f=0.05 mm/rev), depth cut (a=0.8 mm), with wet lubrication of the workpiece during the machining process. After cutting/machining, all samples were solution heat treated at 860ºC for 1h followed by air cooling to room temperature and then age hardened at 480°C for 3h. In the case of fatigue testing samples, the second solution heat treatment was performed to guarantee the elimination of any residual stress eventually present due to the machining process. Non blasted aged samples used in the fatigue testing were carefully ground longitudinally with 2000# sand paper, in order to remove any scale or deposit resulting from the heat-treatment, as well as to remove the machining lines perpendicular to the testing axis that could affect the fatigue test results. These samples were set as a baseline to evaluate the effect of grit-blasting on the fatigue life of aged 18Ni(300) maraging steel. Sub-sized tensile test samples were cut and machined parallel to the rolling direction of the sheet, according to ASTM-E8M and dimension show in Figure 1c. Their final dimensions at the gauge length were 3.5 mm x 7.0 mm x 26.0 mm. All samples were solution heat treated at 860 ◦ C for 1h followed by natural air cooling to room temperature, and subsequently aged at 480°C for 3h. 1.3 Grit-blasting process Grit-blasting was carried out at room temperature, using an industrial blast room system (Nortorf manufacturer) composed by an air compressor, a compressed air reservoir tank, a specimen holder, and a blasting nozzle. The grit-blasting media consisted of angular steel particles with 0.45 mm in average diameter (see Fig. 2a and 2b), hardness of 787 HV (it) , and density ρ=7.5 g/cm 3 . The blasting particles were used only once, as they can change their morphology after colliding with the samples. The blasting particles hit the samples surface at an angle of 75º, impelled by an air pressure of 0.80 MPa. This hit angle was observed to be the most severe in terms of the resulting roughness [19]. A venturi-type blasting gun with a 8.0 mm inner diameter was used, and the nozzle distance was kept constant at 150 mm. Fig. 3 shows the schematic representation of the main grit-blasting parameters. Plane sheet samples for residual stresses evaluation were manually blasted by four consecutive unidirectional passes, applied with an exposure time of 1.0 s per pass. The blasting hot spot of approximately 50.0 mm in diameter guarantee a minimum coverage of 100% along the sample width (25.0mm, see Figure 1a) after four passes. The coverage of the surface was checked using a stereo optical microscope (S-OM Zeiss, Discovery V8 model). For grit-blasting of the round fatigue test samples, each sample was fixed in a rotating device (15.8 RPS), while four consecutive manual passes of 1.0 s of exposition time (per pass) were performed along the main axis of the samples. This procedure resulted in a minimum coverage of 100%, as verified by S-OM analysis. All blasted samples were cleaned by dry air-blown for 30 s, followed by immersion in trichloroethylene solution for 60 s, to eliminate the residues of grit media, rinsed in water and dried at room temperature. 1.4 Tensile testing An Instron Ziwick 1474 universal tensile testing machine was used to straining the tensile testing samples at room temperature up to rupture. The testing machine was equipped with an Instron video-extensometer optic system with a high-resolution digital camera. A tensile test was performed on three samples at a constant crosshead speed of 3.0 mm min -1 , corresponding to an initial strain rate of 1.92 × 10 -3 s -1 . 1.5 Samples’s characterization Scanning electron microscopy (SEM) (Zeiss® LEO 435VPi field emission), and a Vickers microhardness tester (EMCO TEST, DuraScan, load of 0,01Kgf for 5 s) were used to characterize the microstructure and hardness of cross-sectioned samples. Prior the analysis, samples were metallographic prepared by usual techniques of grinding and polishing, according to the practices recommended by the ASTM E3-01 standard. Chemical etching with marble (10g CuSO4 + 50 ml HCl +50 ml H2O) according to the guidelines of the ASTME407-07 standard, was employed to reveal the microstructural details. The Vickers microhardness testing was performed on the cross-section of the samples in order to evaluate the hardness profile from the surface into the center of the samples. Indentations were performed starting close to the surface up to a depth of 0.966 mm, with intervals varying from 0.021 to 0.189. The roughness of the surface of samples (blasted and non-blasted) was measured using a Taylor Hobson Form Talysurf contact profilometer. Three measurements along the longitudinal direction were performed for each sample over a total evaluation length of 5.0mm and cut off lengths of 0.8mm. The obtained roughness parameters were: Ra (arithmetical mean deviation of the profile); Ry (the largest peak to valley height); Rz (ten point height of irregularities); Rt (maximum height of the profile); and Rsm (mean width of roughness profile features). X-ray diffraction technique (Rigaku Ultimate IV diffractometer) was used to evaluate the residual stresses at the surface, to a depth of 161 µm, using the sin 2 ψ method [20] on the biaxial surface stress field, idefined by the principal stresses, σ 1 and σ 2 , with no stress normal to the surface. The diffraction pattern was obtained with Cr kα radiation source (λ=2.291 Å) and vanadium filter on {211} plane of the martensite phase. The angle ψ was set between −45º and +45º and the residual stress profile was calculated along two orthogonal directions: σ 11 =0º;180º (axis x parallel to the blasting passes), and σ 22 =90º; 270º (axis y perpendicular to the blasting passes), see Figure 1a . For these calculations, the Young's modulus was considered as E=210 GPa and the Poisson's ratio considered as ν=0.28. X-ray patterns were recorded at room temperature in a Bragg-Brentano configuration, operating at 40 kV and 30 mA, in step scan mode with a step size of 0.02º and 3s per step, in the interval of 150º< 2θ < 160º. Under such conditions, the X-ray penetration was estimated to be around 5.0 µm [21]. Fur sub-surface measurements, the electro-polishing technique was used to remove successive layers of material without generating additional residual stresses. A Buehler EletroMet ® 4 system operating at 32 V at room temperature with an electrolyte of perchloric acid (75%) and ethanol (99.5%), in the proportion of 1:4 was used for electropolishing. The method proposed by Moore and Evans [22] was used to correct the residual stress measurements due to the removed volume of material. Room temperature axial fatigue testing was performed in fully reversed cycles (R= -1), with constant stress amplitude and frequency of 25 Hz, according to ASTM E-466. Testing was performed in a servo-hydraulic MTS 810.23M machine equipped with a 250 kN load cell. The samples were tested until failure, with a limit of 10 6 cycles as a run-out. S/N curves were obtained for samples in blasted and un-blasted conditions at various stress levels and stress-life (S-N) curves with a probability of failure of 50% where fitted using the least square method [23]. A statistical analysis considering 95% of survival life with 90% of confidence interval enables to rigorously compare the fatigue results for samples at different surface conditions [24]. The fracture surface of representative samples was observed by SEM and S-OM, aiming to identify the crack initiation. For fracture analysis, samples were ultrasonically cleaned in trichloroethylene solution for ten minutes. 2. Results And Discussion 2.1 Tensile properties The tensile properties of materials are shown in Table 2 . The tensile properties of material supplied by Eletrometal were obtained from the dissertation of Darcy dos Santos [25]. Table 2 Tensile properties material of samples after aged heat treatment at 480ºC/3h Heat treatment condition Material Tensile Strength Yield Strength Young’s Modulus Strain at Failure Hardness (MPa) (MPa) (GPa) (%) (HV0.01) Aged (480ºC for 3h) BÖEHLER 1911.71±1.44 1571.26±292.74 291.05±21.64 8.08±0.13 573.75±31.00 ELETROMETAL 2027.03±19.63 1994.56±27.36 183.18±3.13 8.27±0.17 560.50±25.77 2.2 Surface and sub-surface characterization Figure 4 shows the representative samples for fatigue testing for the two conditions: non-blasted (baseline) and grit-blasted surface. In the same figure, the roughness profile is also shown. Ra value for baseline (Figure 4a) is in the range of polished condition (0.1<Ra<0.3 μm) [10], ensuring that imperfections generated by turn-milling process were eliminated. After grit-blasting (Figure 4c), Ra increases in the order of 10, as well as Rt. The overall roughness profile after grit-blasting becomes irregular, resulting from the erosive nature of the process [26, 27] and from the heterogeneus morphology of the blasting media [19]. Furthermore, the deepest valleys resulting from turn-milling may not be removed during blasting, contributing to the heterogeneity of the profile. Although the Ra is the most widely parameter used to characterize the roughness of surfaces, it is not sufficiently precise for processes like grit-blasting and shot-peening [28][29]. In these cases, there is an intrinsic randomness of the impacting events, resulting in a heterogeneous profile highly dependent on the local aspects. As a consequence, additional parameters shall be evaluated. The roughness height parameters like Rt, Ry and Rz can be useful, as they reflect the worst defect present of the length measured, as well as the Rsm parameter that is related to the number of valleys in the assessment length. Table 3 resumes all these roughness parameters measured in both samples . Table 3 Summary of average values of roughness parameters measurements on the fatigue samples Parameter Polished Grit-Blasted Ra (µm) 0.29 ± 0.01 3.33 ± 0.26 Ry (µm) 2.82 ± 0.19 27.15 ± 4.34 Rz (µm) 2.50 ± 0.17 20.10 ± 0.49 Rt (µm) 3.03 ± 0.23 27.67 ± 4.09 Rsm (mm) 0,12 ± 0.01 0.22 ± 0.02 The effects surface topography and roughness on the fatigue life of metals are usually approached in terms of an equivalent stress concentration factor (Kt) [30]. In this context, the roughness parameters Rt (maximum height of the profile) and Rsm (mean spacing of adjacent local peaks) seem reasonable to describe the dimple’s depth and width to be used in the calculation of Kt. Li et. al. [31] proposed the following Eq. (1) to convert Rt and Rsm parameters into an equivalent Kt: Using the Eq. (1) and the measured parameters present in Table 3, an equivalent Kt in the order of 1.03 and 1.28 were obtained for polished and grit-blasted samples, respectively, an increase of around 24%. Figure 5 shows SEM cross-section images of 18Ni(300) aged steel turn-milled samples, close to the surface. It can be observed (Figure 5a) that the machining parameters used resulted in a recrystallized layer of 83.68µm. This can result from the significant plastic deformation imposed by the tool/material interaction during machining [32], and can affect significantly functional characteristics of metals, like corrosion and fatigue resistance [10]. The higher recrystallized grains close to the surface are supposed to soften the material. After grit-blasting, there is a clear modification of the microstructure, from the surface up to a depth of around 4 μm, as observed in Figure 5c. Whitin this layer, grit-blasting caused in remarkable deformation, and a consequent strain-hardening effect. Both sub-surface softening and hardening effects caused by machining and grit-blasting, respectively, are clearly seen through the microhardness profiles depicted in the Figure 6. Despite the significant scattering of data, it is observed that after grit-blasting the hardness was highest in the deformed sub-surface layer (approximately 618.0 HV 0.01 ), decreasing continuously and stabilizing at the typical values of the aged 18Ni(300) maraging steel (556 to 580 HV0.01), far below the extension of the deformed layer observed in Figure 5c (around 200 µm). Similar results were obtained by Wu et. al. [33] in GH4169 superalloy steel after shot-peening. In the machined/polished samples, hardness seems to be close to the typical values expected for this steel even close to the surface. On the other hand, there is a region between 300 and 500 μm below the surface where the hardness is slightly higher, reaching up to around 590 HV 0.01 . At this region, the deformation caused by is supposed to be high enough to increase locally the hardness, but insufficient to recrystallization to occur. The hardening effect observed in samples after grit-blasting can be related to two mechanisms: work hardening, and grain refinement [27, 34]. Both mechanisms are supposed to be highly favorable in retarding crack initiation and reducing the crack propagation rate during cyclic loading [16]. 2.3 Residual stresses profile Figure 7 shows the effect of grit-blasting on the residual stress’s depth profile, obtained by X-ray diffraction analysis along two directions in the sheet plane (X and Y, see Figure 1a) . It can be observed that the parameters used for the grit-blasting operation results in high compressive residual stresses at the surface of the 18Ni(300) maraging steel in the aged condition. Furthermore, this compressive state extends up to a depth of around 180 μm, as a result of unevenness plastic deformation [35]. The extension of these compressive residual stresses are in line with the hardness profile observed in Figure 6, being maximum (around 1.2GPa) at a depth of around 25 μm, decreasing to around 0.8GPa at the top surface. According to Kobayashi et. al. [36], in general two deformation types can be distinguished in metals subjected to shot-peening process: (i) plastic deformation due to the surface hammering, which is maximum at the top surface, and (ii) plastic deformation induced by Hertzian pressure, which is maximum at the sub-surface. The results obtained in the present work reflect this behavior, and the blasting operation probably reached a saturation point at the surface, for the actual conditions of intensity and time. Furthermore, when comparing the principal residual stresses profiles in X (σ 11 ) and Y(σ 22 ) directions, their isotropic nature is evident, resulted from a highly consistent process. Although the presence of compressive residual stresses corresponds to an obvious improvement in the fatigue behavior of metals by increasing the surface resistance to crack initiation as well as reducing the crack propagation rate, the extension of these benefits are unknown. It has been reported [37, 38] that the strain hardening, and the consequent residual stresses profile, can quickly change for some metals after few loading cycles or thermal exposure. Under these conditions, only the contribution from the grain refinement remains as a retarding factor for initial crack propagation [39]. 2.4 Fatigue test results Table 4 resumes the fatigue testing parameters and the number of cycles to failure (or runout) for all samples tested. The Basquin formula [40] was used to draw the fatigue life curves with a 50% of survival probability, as well as for 95% probability survival (reliability) and 90% of confidence level (R95C90) for all stress levels. S-N curves of stress amplitude (σ a ) as a function of the fatigue lifetime (2Nf) were plotted using the Eq. (2): Table 4 Fatigue experimental conditions and results obtained in fatigue testing of samples in both polished and grit-blasted conditions Condition σa (MPa) 2Nf (reversal) Status Polished surface 350 2,000,000 runout 350 2,000,000 runout 400 362,802 failed 400 354,772 failed 450 314,684 failed 450 291,930 failed 500 148,290 failed 500 142,352 failed 620 64,098 failed 620 49,890 failed Grit- Blasted surface 620 2,000,000 runout 700 713,282 failed 700 399,240 failed 750 366,186 failed 750 146,830 failed 800 119,404 failed 800 134,814 failed In Eq. (2), σ’ f is the fatigue strength coefficient defined as the stress intercept at 2N f =1, N f is the number of cycles to failure, 2Nf is the number of reversals to failure, and b is the fatigue strength exponent. Table 5 resumes the values obtained for σ’ f and b, using Eq. (2) and the Minitab ® 19.1 statistical software to calculate parameters for the least square method and Weibull’s distribution. Table 5 shows the values estimated for fatigue strength coefficient ( ) and fatigue strength exponent (b), for both curves using Eq. (2) Condition Reliability (R) and confidence level (C) R 2 Fatigue strenght coefficient, σ ' f (MPa) Fatigue strengtht expoent, b (-) Polished surface Median S-N Curve 0.96 7307.85 -0.225 R95C90 1 6515.57 Grit- Blasted surface Median S-N Curve 0.71 2377.08 -0.093 R95C90 1 2094.68 According to Dieter and Bacon [41], smaller values of |b| correspond to longer fatigue lifes. From Table 5 it can be observed that this parameter is much lower for grit-blasted samples, reflecting the overall better fatigue performance observed in Table 4, despite its higher equivalent stress concentration factor (Kt). The S-N curves plotted in Figure 8 highlight this difference, for both 50% of survive probability, as well as for R95C90 level. Furthermore, the slope of the curves changes considerably [42]. On the other hand, grit-blasted samples data show higher dispersion, as already expected due to the random nature of dimple’s dimensions and consequent roughness profile. Swan et. al. [6] performed fatigue tests in polished samples of solution heat treated/aged maraging steel, at similar conditions and with tested in dry argon. Their found a slightly better fatigue performance than the obtained in the present work for the polished condition tested in dry argon, however, for the condition tested in laboratory air the difference was not significant (see Figure 9). Sligh variations are common due to different starting material forms, gauges and production routes. Furthermore, differences in the amount of austenite (retained or reverted) can change the fatigue behavior, as the presence of a tougher phase can retard the fatigue crack propagation [43]. However, their results (in laboratory air) are below the performance of the grit-blasted samples, reinforcing our find that grit-blasting performed according to the parameters of the present work are beneficial for the fatigue life extension of the precipitation heat treated (aged) 18Ni(300) maraging steel. Fractographic analysis performed on fatigue tested samples revealed that mechanisms involved in the fracture are not exactly the same for polished and grit-blasted samples. Figure 10 shows the overall fracture morphology of one of the polished samples (σa=620 MPa, 2Nf=24,945), observed by S-OM. Turn mill marks perpendicular to the testing loading direction are still well visible even after polishing, which are the probably the crack initiation sites (at least three in this figure). Looking into more detail in SEM (Figure 11a), these initiation sites (four in this figure) are clearly seen, along with inclusions. Besides, two other distinct regions are also seen. The Region 2 is characterized by a stable fatigue crack propagation, with its typical crack-arrest marks (fatigue striations), see Figures 11b and 11c. Region 3 corresponds to the collapsing fracture and occurs when the residual strength approaches to the tensile component of the cyclic load, and fracture occurs suddenly. Looking into detail (Figure 11d), this region presents a large number of small dimples that are typical for ductile fracture. Figure 12a shows the fracture surface of one of the grit-blasted samples (σa=700 MPa; N f =199.620), which can also be divided into three distinct regions. However, in this case, crack initiation could not be associated to surface marks. An inclusion localized at ~85 μm below the top surface is the probable crack initiation site, as observed in Figure 12b. As this inclusion is within the compressive residual stress field and the work hardening layer (see Figures 6 and 7), it is supposed to retard the fatigue crack initiation and propagation, and improving the fatigue life of the sampleThe characteristic dimples of the ductile collapsing fracture in Region 3 are also seem (Figure 12c). In Figure 12d, few blasting particles impregnated in the sample’s surface are seem, but they apparently have no influence on the fatigue crack initiation. 3. Conclusions In the present work, the fatigue behavior of 18Ni(300) precipitation hardened maraging steel was evaluated after surface grit-blasting. The results were compared to a polished baseline condition. From the results obtained, the following conclusions can be drawn: Grit-blasting results in significant roughness increase, resulting from the impact of the irregular shaped blasting media. To the resulting roughness profile, is associated an equivalent stress intensity factor Kt of around 1.28. That corresponds to a significant increase over the polished baseline condition (Kt of around 1.03); As a result of grit-blasting, strain hardening, and compressive residual stresses, were observed in a layer up to about 180–200 µm below the blasted surface. The fatigue life of samples after grit-blasting increased substantially due to strain hardening and the presence of compressive residual stress, despite the increase on roughness and the consequent increase of Kt. Statements and Declarations Acknowledgments The authors are thankful to DCTA/AIE, in special to Research Ph.D. Vanderlei Oliveira and Elizeu do Nascimento Filho for collaboration on the fatigue tests conducted in this work. To DCTA/IEAv, in special to Research Ph.D. Davi Neves for the help in the measurements of the residual stress. The authors also would like to acknowledge the to DCTA/AMR, in special to Research Ph.D. Christian Dollinger and its staff for their assistance with scanning electron microscopy (SEM), and stereo optical microscopy (S-OM). Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author contributions José Carlos Fortes Palau: manufacturing samples, conducting experiments, collecting and analyzing data, and writing the manuscript; Dilermando Nagle Travessa: design of experiments, analyzing and arranging data, and reviewing the manuscript. Code availability Not applicable Conclusion In the present work, the fatigue behavior of 18Ni(300) precipitation hardened maraging steel was evaluated after surface grit-blasting. The results were compared to a polished baseline condition. From the results obtained, the following conclusions can be drawn: a) Grit-blasting results in significant roughness increase, resulting from the impact of the irregular shaped blasting media. To the resulting roughness profile, is associated an equivalent stress intensity factor Kt of around 1.28. That corresponds to a significant increase over the polished baseline condition (Kt of around 1.03); b) As a result of grit-blasting, strain hardening, and compressive residual stresses, were observed in a layer up to about 180-200 μm below the blasted surface. c) The fatigue life of samples after grit-blasting increased substantially due to strain hardening and the presence of compressive residual stress, despite the increase on roughness and the consequent increase of Kt. References Rohrbach K, Schmidt M (1990) Maraging Steels. In: Properties and Selection: Irons, Steels, and High-Performance Alloys. 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Procedia CIRP 71:374–379. https://doi.org/10.1016/j.procir.2018.05.044 Czepułkowska W, Wołowiec-Korecka E, Klimek L (2020) The Condition of Ni-Cr Alloy Surface After Abrasive Blasting with Various Parameters. J Mater Eng Perform 29:1439–1444. https://doi.org/10.1007/s11665-019-04399-z Hensel J, Eslami H, Nitschke-Pagel T, Dilger K (2019) Fatigue strength enhancement of butt welds by means of shot peening and clean blasting. Met (Basel) 9. https://doi.org/10.3390/met9070744 Tosha K, Lu J, Guelorget B, Nagashima E (2005) Shot peening and grit blasting-effects on surface integrity. Icsp9 Shot Peen 16:400–405 de los Rios ER, Walley A, Milan MT, Hammersley G (1995) Fatigue crack initiation and propagation on shot-peened surfaces in A316 stainless steel. Int J Fatigue 17:493–499. https://doi.org/https://doi.org/10.1016/0142-1123(95)00044-T Klotz T, Delbergue D, Bocher P et al (2018) Surface characteristics and fatigue behavior of shot peened Inconel 718. Int J Fatigue 110:10–21. https://doi.org/10.1016/j.ijfatigue.2018.01.005 Guechichi H, Castex L (2006) Fatigue limits prediction of surface treated materials. J Mater Process Technol 172:381–387. https://doi.org/10.1016/j.jmatprotec.2005.10.010 Poorna Chander K, Vashista M, Sabiruddin K et al (2009) Effects of grit blasting on surface properties of steel substrates. Mater Des 30:2895–2902. https://doi.org/10.1016/j.matdes.2009.01.014 Hauk V, Behnken H (1997) Structural and Residual Stress Analysis by Nondestructive Methods: Evaluation, Application. Assessment 132–148 Fitzpatrick ME, Fry A, Holdway P et al (2005) Measurement Good Practice Guide No. 52. Determination of Residual Stresses by X-ray Diffraction. Meas Good Pract Guid 74. https://doi.org/10.1007/s00028-005-0194-y Moore MG, Evans WP (1958) Mathematical Correction for Stress in Removed Layers in X-Ray Diffraction Residual Stress Analysis. SAE International Strzelecki P, Mazurkiewicz A, Musiał J et al (2019) Fatigue Life for Different Stress Concentration Factors for Stainless Steel 1.4301. Mater (Basel) 12:3677. https://doi.org/10.3390/ma12223677 Lee Y-L, Pan J, Hathaway R, Barkey M (2005) Fatigue testing and analysis: theory and practice. Butterworth-Heinemann dos Santos DR (2001) Otimização dos parâmetros de tratamento térmico e soldagem do aço Maraging 18Ni300. Dissertação (Mestrado em Engenharia Mecânica) - Faculdade de Engenharia de Guaratinguetá. Universidade Estadual Paulista, Guaratinguetá Ghara T, Paul S, Bandyopadhyay PP (2021) Influence of Grit Blasting on Residual Stress Depth Profile and Dislocation Density in Different Metallic Substrates. Metall Mater Trans A 52:65–81. https://doi.org/10.1007/s11661-020-06055-x Multigner M, Frutos E, González-Carrasco JL et al (2009) Influence of the sandblasting on the subsurface microstructure of 316LVM stainless steel: Implications on the magnetic and mechanical properties. Mater Sci Eng C 29:1357–1360. https://doi.org/10.1016/j.msec.2008.11.002 Bačová V, Draganovská D (2004) Analyses of the Quality of Blasted Surfaces. Mater Sci 40:125–131. https://doi.org/10.1023/B:MASC.0000042795.54319.a5 Bagherifard S, Ghelichi R, Guagliano M (2012) Numerical and experimental analysis of surface roughness generated by shot peening. Appl Surf Sci 258:6831–6840. https://doi.org/10.1016/j.apsusc.2012.03.111 AROLA D, WILLIAMS C (2002) Estimating the fatigue stress concentration factor of machined surfaces. Int J Fatigue 24:923–930. https://doi.org/10.1016/S0142-1123(02)00012-9 Li JK, Mei Y, Duo W, Renzhi W, AN ANALYSIS OF STRESS CONCENTRATIONS CAUSED BY SHOT PEENING AND ITS APPLICATION IN PREDICTING FATIGUE STRENGTH (1992) Fatigue Fract Eng Mater Struct 15:1271–1279. https://doi.org/10.1111/j.1460-2695.1992.tb01262.x Zhang W, Wang X, Hu Y, Wang S (2018) Quantitative Studies of Machining-Induced Microstructure Alteration and Plastic Deformation in AISI 316 Stainless Steel Using EBSD. J Mater Eng Perform 27:434–446. https://doi.org/10.1007/s11665-018-3129-9 Wu D, Yao C, Zhang D (2018) Surface characterization and fatigue evaluation in GH4169 superalloy: Comparing results after finish turning; shot peening and surface polishing treatments. Int J Fatigue 113:222–235. https://doi.org/10.1016/j.ijfatigue.2018.04.009 Chen M, Jiang C, Xu Z, Ji V (2019) Surface layer characteristics of SAF2507 duplex stainless steel treated by stress shot peening. Appl Surf Sci 481:226–233. https://doi.org/10.1016/j.apsusc.2019.03.045 Dalaei K, Karlsson B, Svensson L-E (2011) Stability of shot peening induced residual stresses and their influence on fatigue lifetime. Mater Sci Eng A 528:1008–1015. https://doi.org/10.1016/j.msea.2010.09.050 Kobayashi M, Matsui T, Murakami Y (1998) Mechanism of creation of compressive residual stress by shot peening. Int J Fatigue 20:351–357. https://doi.org/https://doi.org/10.1016/S0142-1123(98)00002-4 Foss BJ, Gray S, Hardy MC et al (2013) Analysis of shot-peening and residual stress relaxation in the nickel-based superalloy RR1000. Acta Mater 61:2548–2559. https://doi.org/10.1016/j.actamat.2013.01.031 Prevey PS, Cammett JT (2006) The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel. Shot Peening. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG, pp 295–304 Altenberger I, Scholtes B, Martin U, Oettel H (1999) Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304. Mater Sci Eng A 264:1–16. https://doi.org/10.1016/S0921-5093(98)01121-6 Basquin OH (1910) The exponential law of endurance tests. In:Proc Am Soc Test Mater. pp625–630 Dieter GE, Bacon D (1976) Mechanical metallurgy. McGraw-hill New York Gao Y, Li X, Yang Q, Yao M (2007) Influence of surface integrity on fatigue strength of 40CrNi2Si2MoVA steel. Mater Lett 61:466–469. https://doi.org/10.1016/j.matlet.2006.04.089 Kawagoishi N, Nagano T, Moriyama M, Kondo E (2009) Improvement of Fatigue Strength of Maraging Steel by Shot Peening. Mater Manuf Process 24:1431–1435. https://doi.org/10.1080/10426910903386055 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1903680","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":125551913,"identity":"21ae49bf-9907-47f5-a460-cb3a79b654ff","order_by":0,"name":"JOSE CARLOS FORTES PALAU","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA70lEQVRIiWNgGAWjYBACxgYQacPAwAdkPwAyefiI05LGwMDGwMBsANLCRpxdEC1sEiA2QS3M7e0PH/xIuCPPxn7GrPJrjp0MGwPzw0c38Dms54yxYU/CM8M2nhyz27LbkoEOYzM2zsGnZUYOmzTjj8OMbQxALZLbmIFaeNik8WtJf/6bIeGwfRv/G7NiyW31xGhJMGMGaklsk8gxY/y47TARWoB+kQT6JblN4lmxNOO24zxszAT8YggMsQ/AELPt50/e+PHntmp7fvbmh4/xamkAUwfAJDMPmMSjHATkGZC0MP4goHoUjIJRMApGJgAAYRRFYt9K+BcAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-9902-7016","institution":"Universidade Federal de São Paulo: Universidade Federal de Sao Paulo","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"JOSE","middleName":"CARLOS FORTES","lastName":"PALAU","suffix":""},{"id":125551914,"identity":"b096c073-f7ff-4e8f-8674-7312f36b1529","order_by":1,"name":"Dilermando Nagle Travessa","email":"","orcid":"","institution":"","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Dilermando","middleName":"Nagle","lastName":"Travessa","suffix":""}],"badges":[],"createdAt":"2022-07-28 04:17:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1903680/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1903680/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":24741608,"identity":"18ccfc41-96fd-409d-b0a0-f07c0226bbbc","added_by":"auto","created_at":"2022-08-03 18:16:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":185737,"visible":true,"origin":"","legend":"\u003cp\u003eShape and dimensions of samples. (a) the sample used to investigate the residual stress profile on the grit-blasted surface; (b) Sample of axial stress fatigue; (c) Sub-size tensile test sample\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/109262f81c6d95bcbe3cda7c.png"},{"id":24742722,"identity":"af5c5fa1-50d7-4fcb-a411-1e113e385ad1","added_by":"auto","created_at":"2022-08-03 18:26:02","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":228385,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image showing the morphology of the abrasive particles in (a). Cumulative size distribution of the grit blasting particles in (b), with an average equivalent diameter d50= 0.45 mm\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/ea7c764c0ff448565d084f14.png"},{"id":24742066,"identity":"e30e498c-5e0a-4212-9dcf-04b530a84ba1","added_by":"auto","created_at":"2022-08-03 18:21:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":38370,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the grit-blasting process\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/ed412ef9834869cc5d9ea6a4.png"},{"id":24741606,"identity":"d573c719-8e9a-4c5c-9eed-8e9e9ce7e1c9","added_by":"auto","created_at":"2022-08-03 18:16:02","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":615321,"visible":true,"origin":"","legend":"\u003cp\u003eSurface profiles corresponding to the surface topographies conditions of the fatigue sample showing the peaks and valleys of the surface roughness. (a) Surface profile after turning and polishing process; (b) Macroscopic view two different surface conditions; (c) Surface profile after Grit-blasting\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/64a91195beb2b611e3f63624.png"},{"id":24742072,"identity":"b5388152-2924-4ff2-bdc1-aa4c7089c744","added_by":"auto","created_at":"2022-08-03 18:21:02","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":502797,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image showing the circular cross-section of fatigue sample surface, (a) turned surface (magnificence 2000x), (b) turned+blasted surface (magnificence 2000x), (c) microstructure of subsurface layer after grit blasting, the 3.90µm thick layer is deformed layer (magnificence 19000x)\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/ea8ba1449b8cb1988f669d64.png"},{"id":24742068,"identity":"88c3fa73-9550-42f7-836c-759b4a0c03be","added_by":"auto","created_at":"2022-08-03 18:21:02","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":181695,"visible":true,"origin":"","legend":"\u003cp\u003eMicrohardness distributions on circular cross-section of the fatigue samples surface along the depth\u0026nbsp;\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/dfa4bd40ea8c6ce9ba42cb88.png"},{"id":24741603,"identity":"63f91b55-1d7c-4ff4-ae48-fe52ef3004b0","added_by":"auto","created_at":"2022-08-03 18:16:02","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":167591,"visible":true,"origin":"","legend":"\u003cp\u003eResidual stress profile along the depth below surface, after grit-blasting\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/4203a2a8308214578b948352.png"},{"id":24741611,"identity":"07142228-e8bd-4f6f-9d78-2a35c87a7b16","added_by":"auto","created_at":"2022-08-03 18:16:02","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":298717,"visible":true,"origin":"","legend":"\u003cp\u003eS–N curves for 18Ni(300) Maraging steel with 50% and R95C90 probability of failure in two different surface conditions\u003c/p\u003e","description":"","filename":"Figure8.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/23f3d279ecff136dc3127073.png"},{"id":24742070,"identity":"803a23ff-9c80-4bdc-8d26-cff5bbc40847","added_by":"auto","created_at":"2022-08-03 18:21:02","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":436652,"visible":true,"origin":"","legend":"\u003cp\u003eGraphic comparing the results obtained in the present work with the results obtained by Swam [6]\u003c/p\u003e","description":"","filename":"Figure9.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/19fc24bcee097b003905753d.png"},{"id":24743380,"identity":"b6973e8d-7dee-4ca8-8c6d-d7c102ffb42b","added_by":"auto","created_at":"2022-08-03 18:31:02","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":604728,"visible":true,"origin":"","legend":"\u003cp\u003eMacroscopic view of the fatigue fracture surface in turned condition (σa=620 MPa, 2Nf=24,945)\u003c/p\u003e","description":"","filename":"Figure10.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/446f96c3be1bad91540ee57d.png"},{"id":24741613,"identity":"97e5fdbb-80e2-4f31-bcd8-dbc66a50c081","added_by":"auto","created_at":"2022-08-03 18:16:02","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1179759,"visible":true,"origin":"","legend":"\u003cp\u003eFracture morphology of one of the the polished samples (σa=620 MPa; N\u003csub\u003ef \u003c/sub\u003e=24.945), (a)overall view of the fracture surface, (b) higher magnification image of the fatigue crack initiation sites (Region 1), (c) detail of the crack propagation region (Region 2), and (d) detail of the collapsing fracture region (Region 3)\u003c/p\u003e","description":"","filename":"Figure11.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/4bc9fb23fb16a2dc3da37aae.png"},{"id":24742723,"identity":"974cdc1c-d502-45ed-bfe1-bc59a2d603f7","added_by":"auto","created_at":"2022-08-03 18:26:02","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1269609,"visible":true,"origin":"","legend":"\u003cp\u003eFracture morphology turned sample (σa=700 MPa; N\u003csub\u003ef \u003c/sub\u003e=199.620): (a)macroscopic view of the fatigue fracture surface, (b) higher magnification image of the fatigue crack initiation point, (c) dimples, and (d) blasted surface with embedded grits\u003c/p\u003e","description":"","filename":"Figure12.png","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/d68eaa8dbca19d2d1eef6db6.png"},{"id":28062281,"identity":"3927746c-2505-47de-8878-e8b6a0efa260","added_by":"auto","created_at":"2022-10-20 20:10:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5503790,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1903680/v1/0a7a333c-3adc-45e5-bddd-600916b1584c.pdf"}],"financialInterests":"","formattedTitle":"Effect of grit blasting on fatigue life of aged 18Ni(300) maraging steel","fulltext":[{"header":"Introduction","content":"\u003cp\u003e18Ni(300) maraging are a special class of high-strength steels that differ from conventional steels in that their hardening mechanisms does not involve carbon [1]. They are based on the Fe-Ni system, with low-carbon content (\u0026lt; 0,03%) and highly alloyed, mainly with additions of cobalt, molybdenum, and titanium. The Fe-Ni system comprises a metastable transformation of austenite into a cubic tough low carbon martensite (\u0026alpha;\u0026rsquo;) under normal cooling in the range of 300 to 200\u003csup\u003eo\u003c/sup\u003eC, retaining in solid solution the other alloying elements. As the reversion of martensite into austenite under heating occurs typically above around 600\u003csup\u003eo\u003c/sup\u003eC, this hysteresis enables to perform simple ageing treatments at around 500\u003csup\u003eo\u003c/sup\u003eC to precipitate fine intermetallic phases like FeTi, Fe\u003csub\u003e2\u003c/sub\u003eMo, Ni\u003csub\u003e3\u003c/sub\u003eTi, and Ni\u003csub\u003e3\u003c/sub\u003eMo, as investigated by [2], with high hardening effect. It results in a unique combination of high strength to weight ratio, moderate toughness, and good weldability [3, 4], making them attractive for a large range of applications [5]. The fatigue strength of maraging steels, on the other hand, is relatively low in comparison to the conventional high strength steels [6]. It has been reported that the fatigue strength of maraging steels with ultra-high strength is far below the expected value which is generally empirically estimated as 0.4\u0026ndash;0.5 of the ultimate tensile strength (UTS) [7]. From the structural design point of view, fatigue failure is the most critical phenomenon in structural materials for many applications where cyclic loads are present. The occurrence of fatigue failure frequently results catastrophic accidents in service [8].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFatigue life of structures is known to highly depend on the surface conditions because fatigue cracks generally start at the free surface of the material. As reported by\u0026nbsp;McKelvey and Fatemi [9]\u0026nbsp;fatigue failures typically initiate at the surface where micro-cracks form. Depending on the surface finish, micro-cracks may already be present, resulting in a significant reduction of fatigue life. Surface roughness is one of the most detrimental factors for fatigue life.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNovovic et. al. [10] performed a literature review on the effect of machined surface topography and integrity on the fatigue life, evaluating different roughness parameters. They concluded that, although Ra is the most used parameter in describing fatigue behavior, Rt and Rz would be more suitable because they represent the worst defects present in the surface. On the other hand, Koster [11]\u0026nbsp;states that when the surface roughness in the range of 2.5\u0026lt;Ra\u0026lt;5.0 \u0026micro;m, fatigue performance correlates better with the surface microstructure and residual stresses than with the roughness itself. Kumar et. al. [12]\u0026nbsp;revealed that compressive residual stresses and plastic strain (cold work) are crucial to enhanced fatigue performance.\u003c/p\u003e\n\u003cp\u003eGrit blasting is a cost-effective surface treatment method frequently used for surface cleaning and corrosion removal of metals, providing a suitable surface roughness for the adhesion of\u003c/p\u003e\n\u003cp\u003eprotective coatings [13]. Although it is similar to the shot peening process [14], grit-blasting employs irregular shapped steel blasting particles which can promote more sharply identations that make the metal\u0026rsquo;s surface susceptible to the fatigue pehnomena. During grit-blasting, however, the metal\u0026rsquo;s surface is repeatedly impacted by high-speed grit particles, and besides the cleaning effect, local plastic deformation and subsequent grain refinement are prone to occur on the surface, often accompanied by strain hardening and compressive residual stress in the subsurface layer [15, 16]. Both compressive residual stresses and strain hardening at the surface of metals can improve their fatigue resistance (if they surpass the roughness effect), as they tend to close short cracks and increase the yield strength at the crack tip, respectively [17]. It has been reported, however [18]\u0026nbsp;that the effect of strain hardening is more significant, as cyclic loading can quickly reduce residual stresses at the surface.\u003c/p\u003e\n\u003cp\u003eFatigue performance of 18Ni(300) maraging steel in not frequently reported in the literature, mainly after grit-blasting that is commonly employed on parts after the aging heat treatment to remove oxide scales. In order to contribute to this subject, the present study reports the effect of grit-blasting on the fatigue life of aged 18Ni(300)maraging steel, in comparison to the machined condition. The results are discussed in terms of the magnitude of the surface roughness, residual stresses, and the strain-hardening produced by blasting.\u003c/p\u003e"},{"header":"1.\tMaterial And Experimental Methods","content":"\u003cp\u003e1.1 Materials\u003c/p\u003e\n\u003cp\u003eTwo batches of similar 18Ni(300) maraging steel were employed in this study, which the nominal chemical composition is \u0026nbsp;shown in Table 1.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThe first batch, supplied by B\u0026ouml;hler Edelstahl GmbH, was produced\u0026nbsp;by vacuum induction melting-vacuum arc remelting (VIM-VAR) process, and hot rolled in the form of sheet 3.5 mm thick. This material was used specifically\u0026nbsp;to investigate the residual stress profile obtained by grit-blasting.\u0026nbsp;The second batch was produced by Eletrometal A\u0026ccedil;os Finos S.A (actually Villares Metals) by electric arc melting and double vacuum arc remelting (VAR), and hot rolled in form of plate 12.7 mm thick. This material\u0026nbsp;was used specifically in the fatigue studies in blasted and machined conditions\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u0026nbsp;\u003c/strong\u003eChemical composition of the 18Ni(300) maraging steel (wt.%)\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"21.48148148148148%\"\u003e\n \u003cp\u003e\u003cstrong\u003eMaterial\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eNi\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eCo\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eMo\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eTi\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eAl\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eMn\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"21.48148148148148%\"\u003e\n \u003cp\u003eMaraging (B\u0026ouml;hler)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u0026lt; 0.007\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e18.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e9.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e4.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e0.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e0.087\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003eBalance\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"21.48148148148148%\"\u003e\n \u003cp\u003eMaraging (Eletrometal)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e0.010\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e18.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e8.90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e4.62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u0026lt; 0.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e0.092\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003e\u0026lt; 0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.814814814814815%\"\u003e\n \u003cp\u003eBalance\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e1.2 Samples preparation\u003c/p\u003e\n\u003cp\u003eFigure 1a\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eshows a typical specimen, wire cut from the 3.5 mm thick sheet using an electrical discharge machining (EDM). The resulting samples had dimensions of 45.0 mm x 25.0 mm. The samples for fatigue tests were cut from the 12.5 mm thick plate, also by wire electrical discharge process. After cutting, samples were solution annealed\u0026nbsp;at 860\u0026ordm;C for 1h followed by air cooling to room temperature, before\u0026nbsp;turning in round specimens according to the dimension shown in Figure 1b.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThe main turning process parameters were: \u0026nbsp;cutting speed (v=90 m/min), feed rate (f=0.05 mm/rev), depth cut (a=0.8 mm), with wet lubrication of the workpiece during the machining process.\u003c/p\u003e\n\u003cp\u003eAfter cutting/machining, all samples were solution heat treated at 860\u0026ordm;C for 1h followed by air cooling to room temperature and then age hardened at 480\u0026deg;C for 3h. In the case of fatigue testing samples, the second solution heat treatment was performed to guarantee the elimination of any residual stress eventually present due to the machining process. Non blasted aged samples used in the fatigue testing were carefully ground longitudinally with 2000# sand paper, in order to remove\u0026nbsp;any scale or deposit resulting from the heat-treatment, as well as to remove the machining lines perpendicular to the testing axis that could affect the fatigue test results. These samples were set as a baseline to evaluate the effect of grit-blasting on the fatigue life of aged 18Ni(300) maraging steel.\u003c/p\u003e\n\u003cp\u003eSub-sized tensile test samples were cut and machined parallel to the rolling direction of the sheet, according to ASTM-E8M and dimension show in Figure 1c. Their final dimensions at the gauge length were 3.5 mm x 7.0 mm x 26.0 mm. All samples were solution heat treated at 860\u003csup\u003e◦\u003c/sup\u003eC for 1h followed by natural air cooling to room temperature, and subsequently aged at 480\u0026deg;C for 3h.\u003c/p\u003e\n\u003cp\u003e1.3 Grit-blasting process\u003c/p\u003e\n\u003cp\u003eGrit-blasting was carried out at room temperature, using an industrial blast room system\u0026nbsp;(Nortorf manufacturer)\u0026nbsp;composed by an air compressor, a compressed air reservoir tank, a specimen holder, and a blasting nozzle.\u0026nbsp;The grit-blasting media consisted of angular steel particles with 0.45 mm in average diameter (see Fig. 2a and 2b), hardness of 787 HV\u003csub\u003e(it)\u003c/sub\u003e, and density \u0026rho;=7.5 g/cm\u003csup\u003e3\u003c/sup\u003e. The blasting particles were used only once, as they can change their morphology after colliding with the samples.\u003c/p\u003e\n\u003cp\u003eThe blasting particles hit the samples surface at an angle of 75\u0026ordm;, impelled by an air pressure of 0.80 MPa. This hit angle was observed to be the most severe in terms of the resulting roughness [19]. A venturi-type blasting gun with a 8.0 mm inner diameter was used, and the nozzle distance was kept constant at 150 mm. Fig. 3 shows the schematic representation of the main grit-blasting parameters.\u003c/p\u003e\n\u003cp\u003ePlane sheet samples for residual stresses evaluation were manually blasted by four \u0026nbsp;consecutive unidirectional passes, applied with an exposure time of 1.0 s per pass. The blasting hot spot of approximately 50.0 mm in diameter guarantee a minimum coverage of 100% along the sample width (25.0mm, see Figure 1a) after four passes. The coverage of the surface was checked using a stereo optical microscope (S-OM Zeiss, Discovery V8 model).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor grit-blasting of the round fatigue test samples, each sample was fixed in a rotating device (15.8 RPS), while four consecutive manual passes of 1.0 s of exposition time (per pass) were performed along the main axis of the samples. This procedure resulted in a minimum coverage of 100%, as verified by S-OM analysis.\u003c/p\u003e\n\u003cp\u003eAll blasted samples\u0026nbsp;were cleaned by dry air-blown for 30 s, followed by immersion in trichloroethylene solution for 60 s, to eliminate the residues of grit media, rinsed in water and dried at room temperature.\u003c/p\u003e\n\u003cp\u003e1.4 Tensile testing\u003c/p\u003e\n\u003cp\u003eAn Instron Ziwick 1474 universal tensile testing machine was used to straining the tensile testing samples at room temperature up to rupture. The testing machine was equipped with an Instron video-extensometer optic system with a high-resolution digital camera. A tensile test was performed on three samples at a constant crosshead speed of 3.0 mm min\u003csup\u003e-1\u003c/sup\u003e, corresponding to an initial strain rate of 1.92 \u0026times; 10\u003csup\u003e-3\u003c/sup\u003e s \u003csup\u003e-1\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e1.5 Samples\u0026rsquo;s characterization\u003c/p\u003e\n\u003cp\u003eScanning electron microscopy (SEM) (Zeiss\u0026reg; LEO 435VPi field emission), and a Vickers microhardness tester (EMCO TEST, DuraScan, load of 0,01Kgf for 5 s) were used to characterize the microstructure and hardness of cross-sectioned samples. Prior the analysis, samples were metallographic prepared by usual techniques of grinding and polishing, according to the practices recommended by the ASTM E3-01 standard. Chemical etching with marble (10g CuSO4 + 50 ml HCl +50 ml H2O) according to the guidelines of the ASTME407-07 standard, was employed to reveal the microstructural details.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Vickers microhardness testing was performed on the cross-section of the samples in order to evaluate the hardness profile from the surface into the center of the samples. Indentations were performed starting close to the surface up to\u0026nbsp;a depth of 0.966 mm, with intervals varying from 0.021 to 0.189.\u0026nbsp;\u0026nbsp;The roughness of the surface of samples (blasted and non-blasted) was measured using a Taylor Hobson Form Talysurf contact profilometer. Three measurements along the longitudinal direction were performed for each sample over a total evaluation length\u0026nbsp;of 5.0mm and cut off lengths of 0.8mm. The obtained\u0026nbsp;roughness parameters were: Ra (arithmetical mean deviation of the profile); Ry (the largest peak to valley height); Rz (ten point height of irregularities); Rt (maximum height of the profile); and Rsm (mean width of roughness profile features).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;X-ray diffraction technique (Rigaku Ultimate IV diffractometer) was used to evaluate the residual stresses at the surface, to a depth of 161 \u0026micro;m, using the sin\u003csup\u003e2\u003c/sup\u003e\u0026psi; method [20] on \u0026nbsp;the biaxial surface stress field, idefined by the principal stresses, \u0026sigma;\u003csub\u003e1\u003c/sub\u003e and \u0026sigma;\u003csub\u003e2\u003c/sub\u003e, with no stress normal to the surface. The diffraction pattern was obtained with Cr k\u0026alpha; radiation source (\u0026lambda;=2.291 \u0026Aring;) and vanadium filter on {211} plane of the martensite phase. The angle \u0026psi; was set between \u0026minus;45\u0026ordm; and +45\u0026ordm; and the residual stress profile was calculated along two orthogonal directions:\u0026nbsp;\u0026sigma;\u003csub\u003e11\u003c/sub\u003e=0\u0026ordm;;180\u0026ordm; (axis x parallel to the blasting passes), and\u0026nbsp;\u0026sigma;\u003csub\u003e22\u003c/sub\u003e=90\u0026ordm;; 270\u0026ordm; (axis y perpendicular to the blasting passes), see Figure 1a\u003cstrong\u003e.\u003c/strong\u003e \u0026nbsp;For these calculations, the Young\u0026apos;s modulus was considered as E=210 GPa and the Poisson\u0026apos;s ratio considered as \u0026nu;=0.28. \u0026nbsp;X-ray patterns were recorded at room temperature in a Bragg-Brentano configuration, operating at 40 kV and 30 mA, in step scan mode with a step size of 0.02\u0026ordm; and 3s per step, in the interval of 150\u0026ordm;\u0026lt; 2\u0026theta; \u0026lt; 160\u0026ordm;. Under such conditions, the X-ray penetration was estimated to be around 5.0 \u0026micro;m [21].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFur sub-surface measurements, the electro-polishing technique was used to remove successive layers of material without generating additional residual stresses.\u0026nbsp;A Buehler EletroMet\u003csup\u003e\u0026reg;\u003c/sup\u003e 4 system operating at 32 V at room temperature with an electrolyte of perchloric acid (75%) and ethanol (99.5%), in the proportion of 1:4 was used for electropolishing. The method proposed by Moore and Evans [22] was used to correct the residual stress measurements due to the removed volume of material.\u003c/p\u003e\n\u003cp\u003eRoom temperature axial fatigue testing was performed in fully reversed cycles (R= -1), with constant stress amplitude and frequency of 25 Hz, according to ASTM E-466. Testing was performed in a servo-hydraulic MTS 810.23M machine equipped with a 250 kN load cell. The samples were tested until failure, with a limit of 10\u003csup\u003e6\u003c/sup\u003e cycles as a run-out. S/N curves were obtained for samples in blasted and un-blasted conditions at various stress levels and stress-life (S-N) curves with a probability of failure of 50% where fitted using the least square method [23]. A statistical analysis considering 95% of survival life with 90% of confidence interval enables to rigorously compare the fatigue results for samples at different surface conditions [24]. The fracture surface of representative samples was observed by SEM and S-OM, aiming to identify the crack initiation. For fracture analysis, samples were ultrasonically cleaned in trichloroethylene solution for ten minutes.\u003c/p\u003e"},{"header":"2. Results And Discussion","content":"\u003cp\u003e2.1 Tensile properties\u003c/p\u003e\n\u003cp\u003eThe tensile properties of materials are shown in Table 2\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eThe tensile properties of material supplied by Eletrometal were obtained \u0026nbsp;from the dissertation of \u0026nbsp;Darcy dos Santos [25].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2\u0026nbsp;\u003c/strong\u003eTensile properties material of samples after aged heat treatment at 480\u0026ordm;C/3h\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" width=\"12.371134020618557%\"\u003e\n \u003cp\u003e\u003cstrong\u003eHeat treatment condition\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" width=\"11.34020618556701%\"\u003e\n \u003cp\u003e\u003cstrong\u003eMaterial\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.49484536082474%\"\u003e\n \u003cp\u003e\u003cstrong\u003eTensile Strength\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.52577319587629%\"\u003e\n \u003cp\u003e\u003cstrong\u003eYield Strength\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.463917525773196%\"\u003e\n \u003cp\u003e\u003cstrong\u003eYoung\u0026rsquo;s Modulus \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.34020618556701%\"\u003e\n \u003cp\u003e\u003cstrong\u003eStrain at Failure\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.463917525773196%\"\u003e\n \u003cp\u003e\u003cstrong\u003eHardness\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"21.62162162162162%\"\u003e\n \u003cp\u003e\u003cstrong\u003e(MPa)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.972972972972972%\"\u003e\n \u003cp\u003e\u003cstrong\u003e(MPa)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.27027027027027%\"\u003e\n \u003cp\u003e\u003cstrong\u003e(GPa)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.864864864864865%\"\u003e\n \u003cp\u003e\u003cstrong\u003e(%)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.27027027027027%\"\u003e\n \u003cp\u003e\u003cstrong\u003e(HV0.01)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" width=\"12.371134020618557%\"\u003e\n \u003cp\u003e\u003cstrong\u003eAged (480\u0026ordm;C for 3h)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.34020618556701%\"\u003e\n \u003cp\u003eB\u0026Ouml;EHLER\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.49484536082474%\"\u003e\n \u003cp\u003e1911.71\u0026plusmn;1.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.52577319587629%\"\u003e\n \u003cp\u003e1571.26\u0026plusmn;292.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.463917525773196%\"\u003e\n \u003cp\u003e291.05\u0026plusmn;21.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.34020618556701%\"\u003e\n \u003cp\u003e8.08\u0026plusmn;0.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.463917525773196%\"\u003e\n \u003cp\u003e573.75\u0026plusmn;31.00\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.941176470588236%\"\u003e\n \u003cp\u003eELETROMETAL\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"18.823529411764707%\"\u003e\n \u003cp\u003e2027.03\u0026plusmn;19.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20%\"\u003e\n \u003cp\u003e1994.56\u0026plusmn;27.36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.647058823529413%\"\u003e\n \u003cp\u003e183.18\u0026plusmn;3.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"12.941176470588236%\"\u003e\n \u003cp\u003e8.27\u0026plusmn;0.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.647058823529413%\"\u003e\n \u003cp\u003e560.50\u0026plusmn;25.77\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e2.2 Surface and sub-surface characterization\u003c/p\u003e\n\u003cp\u003eFigure 4 shows the representative samples for fatigue testing for the two conditions: non-blasted (baseline) and grit-blasted surface. \u0026nbsp;In the same figure, the roughness profile is also shown. Ra value for baseline (Figure 4a) is in the range of polished condition (0.1\u0026lt;Ra\u0026lt;0.3 \u0026mu;m) [10], ensuring that imperfections generated by turn-milling process were eliminated. After grit-blasting (Figure 4c), Ra increases in the order of 10, as well as Rt. The overall roughness profile after grit-blasting becomes irregular, resulting from the erosive nature of the process [26, 27] and from the heterogeneus morphology of the blasting media [19]. Furthermore, the deepest valleys resulting from turn-milling may not be removed during blasting, contributing to the heterogeneity of the profile. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough the Ra is the most widely parameter used to characterize the roughness of surfaces, it is not sufficiently precise for processes like grit-blasting and shot-peening [28][29].\u0026nbsp; In these cases, there is an intrinsic randomness of the impacting events, resulting in a heterogeneous profile highly dependent on the local aspects. As a consequence, additional parameters shall be evaluated.\u0026nbsp;\u0026nbsp;The roughness height parameters like Rt, Ry and Rz can be useful, as they reflect the worst defect present of the length measured, as well as the Rsm parameter that is related to the number of valleys in the assessment length. Table 3 resumes all these roughness parameters measured in both samples\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3\u0026nbsp;\u003c/strong\u003eSummary of average values of roughness parameters measurements on the fatigue samples\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003e\u003cstrong\u003eParameter\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e\u003cstrong\u003ePolished\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e\u003cstrong\u003eGrit-Blasted\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003eRa (\u0026micro;m)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e0.29 \u0026plusmn; 0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e3.33 \u0026plusmn; 0.26\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003eRy (\u0026micro;m)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e2.82 \u0026plusmn; 0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e27.15 \u0026plusmn; 4.34\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003eRz (\u0026micro;m)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e2.50 \u0026plusmn; 0.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e20.10 \u0026plusmn; 0.49\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003eRt (\u0026micro;m)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e3.03 \u0026plusmn; 0.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e27.67 \u0026plusmn; 4.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"29.528535980148884%\"\u003e\n \u003cp\u003eRsm (mm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.29528535980149%\"\u003e\n \u003cp\u003e0,12 \u0026plusmn; 0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"43.17617866004963%\"\u003e\n \u003cp\u003e0.22 \u0026plusmn; 0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe effects surface topography and roughness on the fatigue life of metals are usually approached in terms of an equivalent stress concentration factor (Kt) [30]. In this context, the roughness parameters Rt (maximum height of the profile) and Rsm (mean spacing of adjacent local peaks) seem reasonable to describe the dimple\u0026rsquo;s depth and width to be used in the calculation of Kt. \u0026nbsp;Li et. al. [31] proposed the following Eq. (1) to convert Rt and Rsm parameters into an equivalent Kt:\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\"\u003e\u003c/p\u003e\n\u003cp\u003eUsing the Eq. (1) and the measured parameters present in Table 3, an equivalent Kt in the order of 1.03 and 1.28 were obtained for polished and grit-blasted samples, respectively, an increase of around 24%.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 5 shows SEM cross-section images of 18Ni(300) aged steel turn-milled samples, close to the surface. It can be observed (Figure 5a) that the machining parameters used resulted in a recrystallized layer of 83.68\u0026micro;m. This can result from the significant plastic deformation imposed by the tool/material interaction during machining [32], and can affect significantly functional characteristics of metals, like corrosion and fatigue resistance [10]. The higher recrystallized grains close to the surface are supposed to soften the material. After grit-blasting, there is a clear modification of the microstructure, from the surface up to a depth of around 4 \u0026mu;m, as observed in Figure 5c. Whitin this layer, grit-blasting caused in remarkable deformation, and a consequent strain-hardening effect. \u0026nbsp;Both sub-surface softening and hardening effects caused by machining and grit-blasting, respectively, are clearly seen through the microhardness profiles depicted in the Figure 6.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDespite the significant scattering of data, it is observed that after grit-blasting\u0026nbsp;the hardness was highest in the deformed sub-surface layer (approximately 618.0 HV\u003csub\u003e0.01\u003c/sub\u003e), decreasing continuously and\u0026nbsp;stabilizing at the typical values of the aged 18Ni(300) maraging steel (556 to 580 HV0.01),\u0026nbsp;far below the extension of the deformed layer observed in Figure 5c (around 200\u0026nbsp;\u0026micro;m). Similar results were obtained by Wu et. al. [33] in GH4169 superalloy steel after shot-peening. \u0026nbsp;\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eIn the machined/polished samples, hardness seems to be close to the typical values expected for this steel even close to the surface. On the other hand, there is a region between 300 and 500\u0026nbsp;\u0026mu;m below the surface where the hardness is slightly higher, reaching up to around 590 HV\u003csub\u003e0.01\u003c/sub\u003e. At this region, the deformation caused by is supposed to be high enough to increase locally the hardness, but insufficient to recrystallization to occur. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe hardening effect observed in samples after grit-blasting can be related to two mechanisms: work hardening, and grain refinement [27, 34]. Both mechanisms are supposed to be highly favorable in retarding crack initiation and reducing the crack propagation rate during cyclic loading [16].\u003c/p\u003e\n\u003cp\u003e2.3 Residual stresses profile\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Figure 7 shows the effect of grit-blasting on the residual stress\u0026rsquo;s depth profile, obtained by X-ray diffraction analysis along two directions in the sheet plane (X and Y, see Figure 1a)\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eIt can be observed that the parameters used for the grit-blasting operation results in high compressive residual stresses at the surface of the 18Ni(300) maraging steel in the aged condition. Furthermore, this compressive state extends up to a depth of around 180 \u0026mu;m, as a result of unevenness plastic deformation [35]. The extension of these compressive residual stresses are in line with the hardness profile observed in Figure 6, being maximum (around 1.2GPa) at a depth of around 25 \u0026mu;m, decreasing to around 0.8GPa at the top surface. \u0026nbsp;According to Kobayashi et. al. [36], in general two deformation types can be distinguished in metals subjected to shot-peening process: (i) plastic deformation due to the surface hammering, which is maximum at the top surface, and (ii) plastic deformation induced by Hertzian pressure, which is maximum at the sub-surface. The results obtained in the present work reflect this behavior, and the blasting operation probably reached a saturation point at the surface, for the actual conditions of intensity and time. Furthermore, when comparing the principal residual stresses profiles in X (\u0026sigma;\u003csub\u003e11\u003c/sub\u003e) and Y(\u0026sigma;\u003csub\u003e22\u003c/sub\u003e) directions, their isotropic nature is evident, resulted from a highly consistent process. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough the presence of compressive residual stresses corresponds to an obvious improvement in the fatigue behavior of metals by increasing the surface resistance to crack initiation as well as reducing the crack propagation rate, the extension of these benefits are unknown. It has been reported [37, 38] \u0026nbsp; that the strain hardening, and the consequent residual stresses profile, can quickly change for some metals after few loading cycles or thermal exposure. Under these conditions, only the contribution from the grain refinement remains as a retarding factor for initial crack propagation [39].\u003c/p\u003e\n\u003cp\u003e2.4 Fatigue test results\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 4 resumes the fatigue testing parameters and the number of cycles to failure (or runout) for all samples tested. The Basquin formula [40]\u0026nbsp;was used to draw the fatigue life curves with a 50% of survival probability, as well as for 95% probability survival (reliability) and 90% of confidence level (R95C90) for all stress levels.\u0026nbsp;S-N curves of stress amplitude (\u0026sigma;\u003csub\u003ea\u003c/sub\u003e) as a function of the fatigue lifetime (2Nf) were plotted using the Eq. (2):\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 4\u003c/strong\u003e Fatigue experimental conditions and results obtained in fatigue testing of samples in both polished and grit-blasted conditions\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"26.98961937716263%\"\u003e\n \u003cp\u003eCondition\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"23.18339100346021%\"\u003e\n \u003cp\u003e\u0026sigma;a (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"29.757785467128027%\"\u003e\n \u003cp\u003e2Nf (reversal)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.069204152249135%\"\u003e\n \u003cp\u003eStatus\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"10\" width=\"26.98961937716263%\"\u003e\n \u003cp\u003ePolished surface\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"23.18339100346021%\"\u003e\n \u003cp\u003e350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"29.757785467128027%\"\u003e\n \u003cp\u003e2,000,000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.069204152249135%\"\u003e\n \u003cp\u003erunout\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e2,000,000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003erunout\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e400\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e362,802\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e400\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e354,772\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e450\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e314,684\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e450\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e291,930\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e148,290\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e142,352\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e620\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e64,098\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e620\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e49,890\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"7\" width=\"26.98961937716263%\"\u003e\n \u003cp\u003eGrit- Blasted surface\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"23.18339100346021%\"\u003e\n \u003cp\u003e620\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"29.757785467128027%\"\u003e\n \u003cp\u003e2,000,000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.069204152249135%\"\u003e\n \u003cp\u003erunout\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e700\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e713,282\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e700\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e399,240\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e750\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e366,186\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e750\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e146,830\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e119,404\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"31.753554502369667%\"\u003e\n \u003cp\u003e800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"40.758293838862556%\"\u003e\n \u003cp\u003e134,814\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"27.488151658767773%\"\u003e\n \u003cp\u003efailed\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eIn Eq. (2),\u0026nbsp;\u0026sigma;\u0026rsquo;\u003csub\u003ef\u003c/sub\u003e is the fatigue strength coefficient defined as the stress intercept at 2N\u003csub\u003ef\u003c/sub\u003e =1, N\u003csub\u003ef\u003c/sub\u003e is the number of cycles to failure, 2Nf is the number of reversals to failure, and b is the fatigue strength exponent. Table 5\u0026nbsp;resumes the values obtained for\u0026nbsp;\u0026sigma;\u0026rsquo;\u003csub\u003ef\u0026nbsp;\u003c/sub\u003eand b,\u0026nbsp;using Eq. (2) and the Minitab\u003csup\u003e\u0026reg;\u003c/sup\u003e 19.1 statistical software to calculate parameters for the least square method and Weibull\u0026rsquo;s distribution.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 5\u003c/strong\u003e shows the values estimated for fatigue strength coefficient (\u0026nbsp;\u0026nbsp;) and fatigue strength exponent (b), for both curves using Eq. (2)\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" cellspacing=\"0\" width=\"92%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"14.43298969072165%\"\u003e\n \u003cp\u003e\u003cstrong\u003eCondition\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003e\u003cstrong\u003eReliability (R) and confidence level (C)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.309278350515465%\"\u003e\n \u003cp\u003e\u003cstrong\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003e\u003cstrong\u003eFatigue strenght coefficient,\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e\u0026sigma;\u003c/strong\u003e\u003cstrong\u003e\u0026apos;\u003csub\u003ef\u0026nbsp;\u003c/sub\u003e(MPa)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"23.711340206185568%\"\u003e\n \u003cp\u003e\u003cstrong\u003eFatigue strengtht expoent, b (-)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" width=\"14.43298969072165%\"\u003e\n \u003cp\u003ePolished surface\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003eMedian S-N Curve\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.309278350515465%\"\u003e\n \u003cp\u003e0.96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003e7307.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" width=\"23.711340206185568%\"\u003e\n \u003cp\u003e-0.225\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"41.666666666666664%\"\u003e\n \u003cp\u003eR95C90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.666666666666668%\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"41.666666666666664%\"\u003e\n \u003cp\u003e6515.57\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" width=\"14.43298969072165%\"\u003e\n \u003cp\u003eGrit- Blasted surface\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003eMedian S-N Curve\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.309278350515465%\"\u003e\n \u003cp\u003e0.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.77319587628866%\"\u003e\n \u003cp\u003e2377.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd rowspan=\"2\" width=\"23.711340206185568%\"\u003e\n \u003cp\u003e-0.093\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"41.666666666666664%\"\u003e\n \u003cp\u003eR95C90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.666666666666668%\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"41.666666666666664%\"\u003e\n \u003cp\u003e2094.68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eAccording to Dieter and Bacon [41], smaller values of |b| correspond to longer fatigue lifes. From Table 5 it can be observed that this parameter is much lower for grit-blasted samples, reflecting the overall better fatigue performance observed in Table 4, despite its higher equivalent stress concentration factor (Kt). The S-N curves plotted in Figure 8\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003ehighlight this difference, for both 50% of survive probability, as well as for R95C90 level. Furthermore, the slope of the curves changes considerably [42]. On the other hand, grit-blasted samples data show higher dispersion, as already expected due to the random nature of dimple\u0026rsquo;s dimensions and consequent roughness profile. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSwan et. al. [6] performed fatigue tests in polished samples of solution heat treated/aged maraging steel, at similar conditions and with tested in dry argon. Their found a slightly better fatigue performance than the obtained in the present work for the polished condition tested in dry argon, however, for the condition tested in laboratory air the difference was not significant (see Figure 9).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSligh variations are common due to different starting material forms, gauges and production routes. Furthermore, differences in the amount of austenite (retained or reverted) can change the fatigue behavior, as the presence of a tougher phase can retard the fatigue crack propagation [43]. However, their results (in laboratory air) are below the performance of the grit-blasted samples, reinforcing our find that grit-blasting performed according to the parameters of the present work are beneficial for the fatigue life extension of the precipitation heat treated (aged) 18Ni(300) maraging steel.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFractographic analysis performed on fatigue tested samples revealed that mechanisms involved in the fracture are not exactly the same for polished and grit-blasted samples. Figure 10 shows the overall fracture morphology of one of the polished samples (\u0026sigma;a=620 MPa, 2Nf=24,945), observed by S-OM. \u0026nbsp;Turn mill marks perpendicular to the testing loading direction are still well visible even after polishing, which are the probably the crack initiation sites (at least three in this figure). Looking into more detail in SEM (Figure 11a), these initiation sites (four in this figure) are clearly seen, along with inclusions. Besides, two other distinct regions are also seen. The Region 2 is characterized by a stable fatigue crack propagation, with its typical crack-arrest marks (fatigue striations), see Figures 11b and 11c. \u0026nbsp;Region 3 corresponds to the collapsing fracture and occurs when the residual strength approaches to the tensile component of the cyclic load, and fracture occurs suddenly. Looking into detail (Figure 11d), this region presents a large number of small dimples that are typical for ductile fracture.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 12a shows the fracture surface of one of the grit-blasted samples (\u0026sigma;a=700 MPa; N\u003csub\u003ef\u0026nbsp;\u003c/sub\u003e=199.620), which can also be divided into three distinct regions. However, in this case, crack initiation could not be associated to surface marks. An inclusion localized at ~85 \u0026mu;m below the top surface is the probable crack initiation site, as observed in Figure 12b. As this inclusion is within the compressive residual stress field and the work hardening layer (see Figures 6 and 7), it is supposed to retard the fatigue crack initiation and propagation, and improving the fatigue life of the sampleThe characteristic dimples of the ductile collapsing fracture in Region 3 are also seem (Figure 12c). In Figure 12d, few blasting particles impregnated in the sample\u0026rsquo;s surface are seem, but they apparently have no influence on the \u0026nbsp;fatigue crack initiation.\u003c/p\u003e"},{"header":"3. Conclusions","content":"\u003cp\u003eIn the present work, the fatigue behavior of 18Ni(300) precipitation hardened maraging steel was evaluated after surface grit-blasting. The results were compared to a polished baseline condition. From the results obtained, the following conclusions can be drawn:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eGrit-blasting results in significant roughness increase, resulting from the impact of the irregular shaped blasting media. To the resulting roughness profile, is associated an equivalent stress intensity factor Kt of around 1.28. That corresponds to a significant increase over the polished baseline condition (Kt of around 1.03);\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAs a result of grit-blasting, strain hardening, and compressive residual stresses, were observed in a layer up to about 180\u0026ndash;200 \u0026micro;m below the blasted surface.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe fatigue life of samples after grit-blasting increased substantially due to strain hardening and the presence of compressive residual stress, despite the increase on roughness and the consequent increase of Kt.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Statements and Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are thankful to DCTA/AIE, in special to Research Ph.D. Vanderlei Oliveira and Elizeu do Nascimento Filho for collaboration on the fatigue tests conducted in this work. To DCTA/IEAv, in special to Research Ph.D. Davi Neves for the help in the measurements of the residual stress. The authors also would like to acknowledge the to DCTA/AMR, in special to Research Ph.D. Christian Dollinger and its staff for their assistance with scanning electron microscopy (SEM), and stereo optical microscopy (S-OM).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJos\u0026eacute; Carlos Fortes Palau: manufacturing samples, conducting experiments, collecting and analyzing data, and writing the manuscript; Dilermando Nagle Travessa: design of experiments, analyzing and arranging data, and reviewing the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n"},{"header":"Conclusion","content":"\u003cp\u003eIn the present work, the fatigue behavior of 18Ni(300) precipitation hardened maraging steel was evaluated after surface grit-blasting. The results were compared to a polished baseline condition. From the results obtained, the following conclusions can be drawn:\u003c/p\u003e\n\u003cp\u003ea) Grit-blasting results in significant roughness increase, resulting from the impact of the irregular shaped blasting media. To the resulting roughness profile, is associated an equivalent stress intensity factor Kt of around 1.28. That corresponds to a significant increase over the polished baseline condition (Kt of around 1.03);\u003c/p\u003e\n\u003cp\u003eb) As a result of grit-blasting, strain hardening, and compressive residual stresses, were observed in a layer up to about 180-200 \u0026mu;m below the blasted surface. \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n\u003cp\u003ec) The fatigue life of samples after grit-blasting increased substantially due to strain hardening and the presence of compressive residual stress, despite the increase on roughness and the consequent increase of Kt.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRohrbach K, Schmidt M (1990) Maraging Steels. In: Properties and Selection: Irons, Steels, and High-Performance Alloys. ASM International, pp\u0026nbsp;793\u0026ndash;800\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSha W, Cerezo A, Smith GDW (1993) Phase chemistry and precipitation reactions in maraging steels: Part I. 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Mater Manuf Process 24:1431\u0026ndash;1435. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1080/10426910903386055\u003c/span\u003e\u003cspan address=\"10.1080/10426910903386055\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Maraging steel, grit blasting, fatigue life, S-N curve","lastPublishedDoi":"10.21203/rs.3.rs-1903680/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1903680/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe fatigue life of aged 18Ni300 maraging steel was investigated in two different surface conditions: polished and grit-blasted. Axial fatigue tests results, plotted in the form of S-N curves were used to correlate the fatigue performance and the surface characteristics, carefully characterized using a combination of experimental techniques, including X-ray diffraction (XRD), contact profilometry, microhardness testing, scanning electron microscopy (SEM), and stereo optical microscopy (S-OM). The results show that turn-machining of round fatigue samples forms a thin recrystallized layer over the steel surface, which remains after polishing. Roughness increases significantly after grit-blasting, but strain hardening and compressive residual stresses developed after blasting outweigh the deleterious increase of roughness and the fatigue life increases in comparison to the polished condition. Both crack initiation and propagation are retarded under the effects of blasting.\u0026nbsp;\u003c/p\u003e","manuscriptTitle":"Effect of grit blasting on fatigue life of aged 18Ni(300) maraging steel","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-08-03 18:15:59","doi":"10.21203/rs.3.rs-1903680/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5678f34d-bf2f-42ff-99b6-b18416e5d30f","owner":[],"postedDate":"August 3rd, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2022-10-20T20:10:07+00:00","versionOfRecord":[],"versionCreatedAt":"2022-08-03 18:15:59","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1903680","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1903680","identity":"rs-1903680","version":["v1"]},"buildId":"rHA-KDH7Qsr4HCuvH75dn","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}