Article Strength Enhancement of Butt Welds by Means of Shot and Clean Blasting

Jonas Hensel * , Hamdollah Eslami, Thomas Nitschke-Pagel and Klaus Dilger

Institute of Joining and , Technische Universität Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany * Correspondence: [email protected]; Tel.: +49-531-391-95517

 Received: 7 June 2019; Accepted: 26 June 2019; Published: 1 July 2019 

Abstract: is a mechanical surface treatment to improve the fatigue strength of metallic components. Similarities exist between regular shot peening and conventional industrial clean blasting. However, the main difference between these two processes is the peening media used and the lack of control and documentation of peening parameters. The clean blasting process is not yet qualified to optimize fatigue enhancement, although it holds a similar potential to regular shot peening. Clean blasting is frequently applied to welded components, with the purpose of surface preparation for application of corrosion protection. This article presents the results of regular shot peened double V-groove (DV) butt welds made from construction steels S355N and S960QL, as well as the high strength aluminum alloy Al-6082. The peening parameters are varied widely. Furthermore, the effect of coverage and intensity is investigated to test the robustness of the peening processes. The data is completed with industrially clean blasted welds, representing typical workshop conditions. The overall objective of this work is to derive minimum peening parameters that still allow significant fatigue strength benefits. The presented data show a high robustness of the fatigue results to peening parameters.

Keywords: welded joints; fatigue strength; steel; aluminum; post-weld treatment; shot peening; clean blasting; residual stresses

1. Introduction

1.1. Post-Weld Treatment of Welds Post-weld treatment of welds for fatigue strength enhancement has become common in many industry applications. Grinding and thermal annealing, in particular, are widely used and accepted by technical standards, such as [1,2]. In the past few years, mechanical surface treatment methods have become more and more popular in the welding industry. Recently, the most discussed has been high frequency mechanical impact hammer peening (HFMI), which is locally applied to fatigue critical weld toes. Mechanical surface treatment methods, such as HFMI, utilize different fatigue beneficial effects to some degree [3,4]. These effects are the generation of compressive , cold , and geometric changes of the weld profile. The International Institute of Welding (IIW) has already adopted HFMI treatment in terms of a widely accepted guideline [5]. Further examples of accepted post-weld treatments are Tungsten Inert Gas (TIG)-dressing, as well as conventional hammer and needle peening [6]. However, all these mentioned methods have in common is that their application usually results in additional production costs. This article focusses on shot peening and clean blasting as alternative mechanical surface treatments. The main difference between these two processes is that shot peening is applied aiming at fatigue strength enhancements, mainly applied in industrial engineering, while clean blasting aims

Metals 2019, 9, 744; doi:10.3390/met9070744 www.mdpi.com/journal/metals Metals 2019, 9, 744 2 of 14 generally at surface cleaning. Furthermore, the surface roughness can be adjusted by clean blasting for the application of corrosion protection. Thus, shot peening is used under control of the peening parameters (e.g., shot media, peening intensity, coverage), while clean blasting is mainly controlled by the appearance of the surface (e.g., cleanliness, roughness). The peening media used for clean blasting is normally of lower quality (multiple re-use) and edged. Nevertheless, clean blasting has similar effects as shot peening in terms of the generation of compressive residual stress and cold work hardening. Shot peening is widely used in the automotive and aviation industries and has proven its potential for fatigue strength enhancement. A good overview on the principles and applications of shot peening can for instance be found in [7,8]. Unfortunately, the benefits of shot peening on fatigue strength cannot yet be used in the design of welded structures, because shot peening is not currently covered by design codes. This article presents fatigue test results from shot peened and clean blasted DV-butt welds. The objective of this work is to investigate the effects of different peening media and peening parameters on fatigue results of different construction metals. Finally, it should be possible to qualify regularly applied clean blasting as a post-weld treatment method. Hopefully, this would result in a way to increase the fatigue resistance of welded components without significantly increasing manufacturing costs.

1.2. Shot Peening as a Post-Weld Treatment Method of Welded Joints Shot peening is a flexible peening method for surfaces of various metallic and non-metallic components. Typical applications are the enhancement of surface roughness, cleanliness, or the enhancement of mechanical parameters and corrosion resistance [3]. The most important peening parameters of shot peening, in terms of fatigue improvement, are shot size, shot geometry, peening intensity, and coverage [9]. There are several shot peening media available, such as steel, glass, ceramic, or organic components. The user can influence the kinematic energy of the peening impacts by the choice of the peening media, shot size, impact angle, and shot velocity. Steel components are normally shot peened using steel balls. Glass beads are used in the case of demands for low peening intensity, for instance, low sheet thickness. Furthermore, they are used to smooth surfaces (finishing) or to avoid (chemical) reactions between peening media and peened components. Clean blasting is normally performed using cheaper peening media, like sand, corundum, or broken cast steel. The possibility for re-use of the peening media depends on the requirements regarding the surface quality of the peened component. The shot peening process can be further adjusted with the help of the peening parameters, which are controlled by the Almen test [10]. The peening intensity is a measure for the kinematic energy that is transferred from the shots to the surface. In practice, this is controlled by the empirical Almen strip test. The Almen strip is a steel specimen with defined thickness, , and mechanical parameters. Almen strips are peened using a defined parameter set, resulting in a specific plastic deformation of the strip. The intensity of the peening process is measured by the plastic deformation of the Almen strips and denoted in mm (or inch) and the Almen strip used. Coverage of the surface is defined as the percentage of the peened surface related to the un-peened surface. A coverage of 98% is the highest coverage value that can be experimentally determined, as the indentation spots of the shots are still distinguishable here. A coverage of 98% is normally denoted as “full” coverage. Higher or lower values of coverage are normally adjusted by a control of the peening time per surface area, e.g., 0.25 98% means a reduction × of the peening time to 25%. Shots should ideally hit the surface at 90◦ impact angle. Two effects are relevant for the fatigue improvement of shot peening: (1) the generation of compressive residual stress, and (2) cold work hardening. The reduction of surface roughness by plastic deformation may be of advantage in cases of high strength metals (hardened steels). The induced compressive residual stress field may retard fatigue micro cracks or slow down crack growth considerably. The magnitude of the induced compressive residual stress and the penetration depth of the residual stress field are important. The effects of residual stress on the fatigue strength of peened Metals 2019, 9, 744 3 of 14 surfaces depend on the ultimate strength and can be expressed by means of the sensitivity to residual stress m [11]. Cold work hardening can be utilized in un-notched or mildly notched specimens to enhance their fatigue strength. The fatigue strength of such specimens depends on the ultimate strength of the material. The significance of this effect is more pronounced in metals with low hardness. Hard metals show less sensitivity to . Furthermore, an increase in roughness lowers the effectiveness of beneficial fatigue effects through cold work hardening in the case of high hardness [3]. Investigations on fatigue strength improvements of welded steels by shot peening can be found in the literature, for instance [12–16]. The general success of shot peening is proven well. However, the peening parameters were not varied widely. The fatigue strength improvement of welded aluminum alloys by shot peening has also been proven already [17]. Peening with steel shots and standard peening parameters (0.2 mmA, 200% coverage) was applied to inert gas (MIG)- and TIG-welded Al-5083 alloys. The fatigue strength of welds was improved almost up to the base metal fatigue strength. However, the possibility of softening in the heat affected zone must always be considered in cases of high strength precipitation hardening alloys.

2. Experimental Work The effect of shot peening and clean blasting on the fatigue strength of general metal arc (GMA)-welded butt welds was investigated using three different materials, steel grades S355N (1.0545) and S960 (1.8933), as well as aluminum Al-6082 T6 (EN AW-AlSi1MgMn/3.2315). The sheet thickness was 10 mm (steel) and 5 mm (aluminum). The peening media were varied, reflecting regular shot blasting (steel shots S280) as well as clean blasting (glass beads and corundum). Furthermore, the coverage (peening time) was varied between 0.25 98% (low coverage) and 2 98% (double coverage), × × simulating different scenarios. A low coverage of 25% reflects the lower boundary of clean blasting of a time-optimized cleaning process, while 2 98% coverage is used in conventional shot peening × processes. Another parameter investigated was the influence of the peening intensity. This parameter was described by means of the Almen intensity, measured as the deflection of specific Almen strips of varying thickness, Table 3. The three peening intensities used here were 0.3 to 0.4 mmA, mmN, and mmC (0.3 to 0.4 mm deflection of Almen strips A, N and C respectively). An overview is given in Tables1–3. Specimens prepared by means of combinations of the aforementioned parameters were used for fatigue testing. The surface roughness, hardness, surface, and near surface residual stresses were documented.

Table 1. Base metals used for specimen preparation: mechanical and chemical parameters determined by tensile test respectively spectroscopy.

Tensile Elongation at Material Limit CEV Thickness Strength Fracture S355N 376 MPa 518 MPa 27% 0.41 10 mm S960QL 995 MPa 1033 MPa 16% 0.56 10 mm Al-6082 T6 317 MPa 340 MPa 17% - 5 mm

Table 2. Peening media used.

Parameter Steel Shots S280 Glass Beads Corundum NK 24 Particle size 600–1180 µm 300–400 µm 595–841 µm Shape spherical spherical edged Density 7.3–7.8 g/cm3 2.6 g/cm3 3.9 g/cm3 Hardness 400–520 HV 500–530 HV 1 8–9 1 (Mohs hardness) 1 Hardness taken from the literature [3]. Metals 2019, 9, 744 4 of 14

Table 3. Almen strips used.

Almen Strip N A C Thickness 0.79 0.02 mm 1.29 0.02 mm 2.39 0.02 mm ± ± ±

2.1. Specimen Preparation and Characterization

The specimens used for fatigue testing were prepared as DV-butt welds (60◦ opening angle). The power source was elmatech DV36 L(W) by elmatech, Germany. Plates of approximately 400 mm length were attached to each other by means of a robot-guided standard impulse MAG (metal active gas welding) or MIG (metal inert gas welding) process. The nominal welding energy per unit length of the MAG process was 10.2 kJ/cm (travel speed 35 cm/min, voltage 27.8 V, current 215 A). The energy per unit length of the MIG process was 4.6 kJ/cm (travel speed 52 cm/min, voltage 24 V, current 175 A). The shielding gases used were 82% Ar/18% CO2 for steel and 100% Ar for aluminum. The filler metals were EN ISO 14341-A-G4Si1 (S355N), G 89 4 M21 Mn4Ni2CrMo (S960QL), and EN ISO 18273:S Al 5356 (Al-6082 T6). Welding was conducted in a flat position, while the specimens were held in position by toggle clamps. Sheets of S960QL were welded at a pre-heating temperature of 100 C. Representing Metals 2019, 9, x FOR PEER REVIEW ◦ 5 of 14 macrographs and the hardness distribution (HV1) are shown in Figure1.

(a) (b) (c)

Figure 1. MacrographsMacrographs of DV-butt welds. ( a)) S355N, S355N, ( (b) S960QL, S960QL, ( c) Al-6082 T6.

The hardness was measured with the ultrasonic compact impedance method (UCI), according to ASTM A1038. The equipment used was made by a BAQ UT200 hardness scanner (BAQ, Braunschweig, Germany). Typically, steels show heat affected zones with fine- and coarse-grained zones. The aluminum shows some pores and some small hot cracks in the vicinity of the fusion zone. However, the inner irregularities did not affect the fatigue failure, which occurred at the geometric notches at the weld toes. The weld quality was “B” according to ISO 5817 and ISO 10024. The welded steels showed an increase in hardness in the heat affected zone to approximately 300 HV1 (S355N) and 400 HV1 (S960QL). S960QL further showed an area of slight softening from approximately 320 HV1 (base metal) to 280 HV1 in the recrystallized zone. Al-6082 showed a softening of the base metal due to the dissolution of the precipitations. The hardness in these zones decreased to 60 HV1 compared to 85 HV in the base metal. The 400 mm long welded plates were used to produce fatigue specimens by cutting and milling. The samples are shown in Figure2. As the picture shows, the samples had di fferent lengths, between 280 mm and 370 mm. However, this did not affect the fatigue test results, as the welding distortion was removed by means of a three-point bending device, resulting in nearly parallel surfaces in the area used for clamping.Figure The2. Fatigue plastic test zones samples of the (DV-weld) straightening shown processin as-welded were condition. located outside the fatigue

2.2. Residual Stress Measurements Residual stress measurements were carried out at the residual stress laboratories at the Institute of Joining and Welding at TU Braunschweig. The methods used were X-ray diffraction (XRD), according to the sin²-Ψ method [19], and incremental hole drilling (HD) [20]. Residual stress was determined from {211}-patterns of ferrite/martensite with the help of Cr-Kα-radiation at eleven Ψ- angles. Near-surface residual stresses were determined through incremental surface removal by means of electrolytic polishing and the hole drilling method. The collimator size was 2 mm for both steel and aluminum.

2.3. Fatigue Testing Fatigue testing was conducted on servo-hydraulic test rigs (Schenck S400, Schenck, Germany; w + b250, Walter + Bai AG, Löhningen, Switzerland) at the Institute of Joining and Welding at TU Braunschweig. The test load was applied force-controlled uniaxial perpendicular to the weld. The stress ratio applied was R = σmin/σmax = 0.1. All tests were stopped at specimen fracture. An overview of all test series is given in Table 4. The fatigue strength of untreated specimens was used as reference to quantify the effect of peening. Reference fatigue strength is available for all three base metals used, S355N, S960QL, and Al-6082. Within the 20 test series, the peening media, the intensity, and the

Metals 2019, 9, x FOR PEER REVIEW 5 of 14

Metals 2019, 9, 744 5 of 14 critical zone. Near-weld residual stresses were controlled before and after straightening to ensure that this had no effect on fatigue test results. For a more detailed explanation of the removal of distortion, see [18]. Furthermore, all samples were shot peened or clean blasted as a last preparation step before testing. All peening processes (test series 1 to 20) were performed air pressure-controlled, according to industrial standards at the facilities of OSK Kiefer GmbH in Oppurg, Germany. The peening of the samples led to (a) a local increase in hardness. (b) The near surface hardness (c) of Al-6082 was increased to >125 HV0.03 by shot peening at coverage rates above 50%. In the case of S355N and S960QL, Figure 1. Macrographs of DV-butt welds. (a) S355N, (b) S960QL, (c) Al-6082 T6. no significant increase in hardness was observed.

Figure 2. Fatigue test samples (DV-weld) shown in as-welded condition. Figure 2. Fatigue test samples (DV-weld) shown in as-welded condition. 2.2. Residual Stress Measurements 2.2. Residual Stress Measurements Residual stress measurements were carried out at the residual stress laboratories at the Institute of JoiningResidual and Welding stress measurements at TU Braunschweig. were carried The methods out at the used residual were X-ray stress di laboraffractiontories (XRD), at the according Institute ofto theJoining sin2- Ψandmethod Welding [19], andat TU incremental Braunschweig. hole drilling The methods (HD) [20 ].used Residual were stress X-ray was diffraction determined (XRD), from Ψ {211}-patternsaccording to the of ferritesin²- /martensite method [19], with and the incremental help of Cr-K holeα-radiation drilling at (HD) eleven [20].Ψ-angles. Residual Near-surface stress was α Ψ determinedresidual stresses from were{211}-patterns determined of ferri throughte/martensite incremental withsurface the help removal of Cr-K by-radiation means of at electrolytic eleven - polishingangles. Near-surface and the hole residual drilling method.stresses were The collimator determined size through was 2 mm incremental for both steel surface and aluminum.removal by means of electrolytic polishing and the hole drilling method. The collimator size was 2 mm for both steel2.3. Fatigue and aluminum. Testing Fatigue testing was conducted on servo-hydraulic test rigs (Schenck S400, Schenck, Germany; 2.3. Fatigue Testing w + b250, Walter + Bai AG, Löhningen, Switzerland) at the Institute of Joining and Welding at TU Braunschweig.Fatigue testing The was test conducted load was applied on servo-hydraulic force-controlled test rigs uniaxial (Schenck perpendicular S400, Schenck, to the Germany; weld. The w +stress b250, ratio Walter applied + Bai was AG,R = Löhningenσmin/σmax ,= Switzerland)0.1. All tests wereat the stopped Institute at specimenof Joining fracture. and Welding An overview at TU Braunschweig.of all test series The is given test inload Table was4. Theapplied fatigue force-co strengthntrolled of untreated uniaxial specimens perpendicular was usedto the as weld. reference The stressto quantify ratio applied the effect was of peening.R = σmin/σ Referencemax = 0.1. All fatigue tests strengthwere stopped is available at specimen for all threefracture. base An metals overview used, ofS355N, all test S960QL, series is andgiven Al-6082. in Table Within4. The fatigue the 20 strength test series, of untreated the peening specimens media, thewas intensity, used as reference and the tocoverage quantify were the effect varied. of peening. The peening Reference conditions fatigue were strength controlled is available according for all three to state-of-the-art base metals used, shot peeningS355N, S960QL, processes. and It shouldAl-6082. be Within noted that the a20 coverage test series, below the 98% peening is normally media, not the used intensity, in shot peeningand the (test series 14–16 and 18 and 19). These test series were included in the test matrix to investigate unfavorable peening conditions, e.g., during clean blasting. Table4 further shows the surface roughness of peened samples (MarSurf M 400 surface measuring instrument, Mahr GmbH, Göttingen, Germany). Glass beads generally create a lower roughness than steel shots. The highest roughness is generated by corundum due to its edged form. The high-strength steel S960QL shows lower roughness than S355N and Al-6082 due to its higher hardness. Metals 2019, 9, 744 6 of 14

Table 4. Overview about fatigue test series of peened DV-butt welds.

Surface Test Series Base Metal Peening Media Intensity Coverage Roughness Rz Reference S355N - (as welded) - - Reference S960QL - (as welded) - - Reference Al-6082 - (as welded) - - 1 S355N Steel shots 0.3–0.4 mmA 1 98% 56.7 µm × 2 S355N Glass beads 0.3–0.4 mmA 1 98% 38.4 µm × 3 S355N Corundum 0.3–0.4 mmA 1 98% 71.9 µm × 4 S960QL Steel shots 0.3–0.4 mmA 1 98% 25.8 µm × 5 S960QL Glass beads 0.3–0.4 mmA 1 98% 30.1 µm × 6 S960QL Corundum 0.3–0.4 mmA 1 98% 80.2 µm × 7 Al-6082 Steel shots 0.3–0.4 mmA 1 98% 99.9 µm × 8 Al-6082 Glass beads 0.3–0.4 mmA 1 98% 63.9 µm × 9 Al-6082 Corundum 0.3–0.4 mmA 1 98% 107.5 µm × 10 S355N Steel shots 0.3–0.4 mmN 1 98% 57.3 µm × 11 S355N Steel shots 0.3–0.4 mmC 1 98% 61.3 µm × 12 Al-6082 Steel shots 0.3–0.4 mmN 1 98% 83.3 µm × 13 Al-6082 Steel shots 0.3–0.4 mmC 1 98% 96.5 µm × 14 S355N Steel shots 0.3–0.4 mmA 0.25 98% 35.8 µm × 15 S355N Steel shots 0.3–0.4 mmA 0.50 98% 37.1 µm × 16 S355N Steel shots 0.3–0.4 mmA 0.75 98% 30.7 µm × 17 S355N Steel shots 0.3–0.4 mmA 2 98% 63.8 µm × 18 Al-6082 Steel shots 0.3–0.4 mmA 0.50 98% 91.9 µm × 19 Al-6082 Steel shots 0.3–0.4 mmA 0.75 98% 88.2 µm × 20 Al-6082 Steel shots 0.3–0.4 mmA 2 98% 96.5 µm ×

Additional test series were prepared under less controlled conditions at industry workshops in a round-robin principle, Table5. These samples, made from S355N, were provided to these companies for conduction of a “regular” cleaning process. The test series consisted of twelve specimens. The fatigue data was evaluated by linear regression without consideration of run-outs.

Table 5. Additional test series of industrially clean blasted specimen (round robin principle).

Test Series Base Metal Peening Media Shape Participant 21 S355N Steel shots Edged 1 22 S355N Cast steel shots Edged 2 23 S355N Cast steel shots Edged 3

3. Results

3.1. Residual Stresses in Conventional and Peened Specimens The initial residual stresses were determined for all test series. The surface residual stresses in the as-welded condition (reference) are shown in Figure3. The stresses were determined by means of XRD. Shown here are the mean values, as well as the absolute minimum and maximum values. The weld seam width is indicated by the hachured area. Note that diffraction patterns could not be obtained within the weld of S960QL and Al-6082 due to coarse grains. Residual stresses varied between 50 MPa and 150 MPa in specimens made from S355N and between 10 MPa and 200 MPa in S960QL. − − Aluminum welds show significantly lower residual stress with a mean close to zero, ranging from 70 MPa to 50 MPa. Low residual stresses can be explained by the low restraint condition transverse − to the weld. This results in (relatively) free shrinkage and thus low residual stress. Compressive residual stress in the heat affected zones of the steels results from hindered volume increase due to phase transformation (austenite to ferrit/bainite/martensite) [21,22]. Metals 2019, 9, x FOR PEER REVIEW 7 of 14

The weld seam width is indicated by the hachured area. Note that diffraction patterns could not be obtained within the weld of S960QL and Al-6082 due to coarse grains. Residual stresses varied between 50 MPa and −150 MPa in specimens made from S355N and between 10 MPa and −200 MPa in S960QL. Aluminum welds show significantly lower residual stress with a mean close to zero, ranging from 70 MPa to −50 MPa. Low residual stresses can be explained by the low restraint condition transverse to the weld. This results in (relatively) free shrinkage and thus low residual stress. Compressive residual stress in the heat affected zones of the steels results from hindered volume increase due to phase transformation (austenite to ferrit/bainite/martensite) [21,22]. The residual stresses of the as-welded condition, shown in Figure 3, can normally be used as an indicator for residual stress effects on fatigue. However, this is not true for shot peened specimens. The reason for this is that the as-welded specimens show only a small residual stress gradient from surface into depth, while shot peening results in a relatively thin layer with distinctive residual stress gradients. The absolute value of the surface residual stress of shot peened samples at the surface is not a good measure for the beneficial compressive residual stress effect. It is recommended to determine the residual stress field within the first 1000 µm. Figure 4 shows the residual stress conditions in all three investigated materials after clean blasting using different media: steel shots, glass beads, and corundum. The coverage and intensity were kept constant at 98% and 0.3–0.4 mmA. The results were obtained from the hole drilling method in the vicinity of the weld toe. It must be noted that a direct measurement at the weld toe by means of the hole drilling method is generally not possible due to geometric restraints (attachment of strain gauges). Glass beads and corundum generated comparable residual stress fields in S355N, with a minimum of approximately −440 MPa. The use of steel shots led to slightly higher compressive residual stresses and deeper penetration of the residual stress field. Not shown here is the influence of coverage. Its influence was investigated using S280 steel shots at a constant intensity of 0.3–0.4 mmA. The effect of coverage on the residual stress field in S355N was found to be low. The residual stress fields were comparable to the one shown in Figure 4a. Generally, the compressive residual stress magnitudes in S960QL were found to be higher than in S355N due to its higher yield strength. The peening media had a significant effect on the residual stress fields. The use of corundum led to the smallest penetration depth of the compressive residual stress field, with −550 MPa at a 120 µm depth. Glass beads and steel shots led to higher compressive residual stresses, −600 MPa at deeper layers (180 µm and 300 µm, respectively). The effects of coverage and intensity on the residual stress fields were also investigated here, with similar results to S355N (low effect). Al-6082 showed generally lower compressive residual stresses due its lower yield strength. The influence of the shot media used was found to be relatively low. The highest compressive residual stresses were determined 200 µm below the surface. The magnitude was −200 MPa for all three media used. Not shown here is the influence of shot peening intensity. A higher intensity (0.3 mmA and 0.3 mmC) resulted in a deeper penetration of the compressive residual stress field than a low intensity (0.3 mmN). Furthermore, it was found that a low coverage of 75% and below resulted in lower Metalscompressive2019, 9, 744 residual stresses than 98%, at less penetration of the residual stress field. However,7 98% of 14 coverage and more (here up to 2 × 98%) did not further enhance the results.

(a) (b) (c)

FigureFigure 3.3. Residual stressstress atat surfacesurface of of DV-butt DV-butt welds welds made made from from S355N S355N (a), (a S960QL), S960QL (b), (b and), and Al-6082 Al-6082 (c). Hachures(c). Hachures indicate indicate the weldthe weld seam seam width. width.

The residual stresses of the as-welded condition, shown in Figure3, can normally be used as an indicator for residual stress effects on fatigue. However, this is not true for shot peened specimens. The reason for this is that the as-welded specimens show only a small residual stress gradient from surface into depth, while shot peening results in a relatively thin layer with distinctive residual stress gradients. The absolute value of the surface residual stress of shot peened samples at the surface is not a good measure for the beneficial compressive residual stress effect. It is recommended to determine the residual stress field within the first 1000 µm. Figure4 shows the residual stress conditions in all three investigated materials after clean blasting using different media: steel shots, glass beads, and corundum. The coverage and intensity were kept constant at 98% and 0.3–0.4 mmA. The results were obtained from the hole drilling method in the vicinity of the weld toe. It must be noted that a direct measurement at the weld toe by means of the hole drilling method is generally not possible due to geometric restraints (attachment of strain gauges). Glass beads and corundum generated comparable residual stress fields in S355N, with a minimum of approximately 440 MPa. The use of steel shots led to slightly higher compressive residual stresses and − deeper penetration of the residual stress field. Not shown here is the influence of coverage. Its influence was investigated using S280 steel shots at a constant intensity of 0.3–0.4 mmA. The effect of coverage on the residual stress field in S355N was found to be low. The residual stress fields were comparable to the one shown in Figure4a. Generally, the compressive residual stress magnitudes in S960QL were found to be higher than in S355N due to its higher yield strength. The peening media had a significant effect on the residual stress fields. The use of corundum led to the smallest penetration depth of the compressive residual stress field, with 550 MPa at a 120 µm depth. Glass beads and steel shots led to − higher compressive residual stresses, 600 MPa at deeper layers (180 µm and 300 µm, respectively). − The effects of coverage and intensity on the residual stress fields were also investigated here, with similar results to S355N (low effect). Al-6082 showed generally lower compressive residual stresses due its lower yield strength. The influence of the shot media used was found to be relatively low. The highest compressive residual stresses were determined 200 µm below the surface. The magnitude was 200 MPa for all three − media used. Not shown here is the influence of shot peening intensity. A higher intensity (0.3 mmA and 0.3 mmC) resulted in a deeper penetration of the compressive residual stress field than a low intensity (0.3 mmN). Furthermore, it was found that a low coverage of 75% and below resulted in lower compressive residual stresses than 98%, at less penetration of the residual stress field. However, 98% coverage and more (here up to 2 98%) did not further enhance the results. × Metals 2019, 9, x FOR PEER REVIEW 8 of 14

Metals Metals2019, 92019, 744, 9, x FOR PEER REVIEW 8 of 148 of 14

(a) (b) (c)

Figure 4. Residual stress in the surface layer obtained at the weld toes by means of hole drilling method. DV-butt welds made from S355N (a), S960QL (b), and Al-6082 (c). (a) (b) (c) 3.2. Cyclic Stability of Shot Peening Residual Stress FigureFigure 4. Residual 4. Residual stress stress in the in surfacethe surface layer layer obtained obtained at the at weldthe weld toes toes by means by means of hole of hole drilling drilling method. method.Residual DV stress-butt welds stability made under from cyclic S355N lo ading(a), S960QL was investigated (b), and Al-6082 using (c ).S355N samples. These were DV-buttpeened welds using made steel shots from at S355N a normal (a), S960QLintensity ( bof), 0.3 and–0.4 Al-6082 mmA, while (c). the coverage was varied at 98% and 25%, Figure 5. Residual stresses were determined at the surface by means of X-ray diffraction. 3.2. Cyclic3.2. Cyclic Stability Stability of Shot of Shot Peening Peening Residual Residual Stress Stress After assessing the initial residual stresses, the measurements were repeated after N = 1, 10, … 10,000 ResidualloadResidual cycles. stress Thestress stability fatigue stability loading under under was cyclic cyclic applied loading loading at a stress waswas investigated ratio of R = 0.1 using, using with S355N a S355Nmaximum samples. samples. stress These of These 360 were were peenedMPa. usingThis was steel chosen shots to at test a normal the stability intensity of the of residual0.3–0.4 mmAstresses, while under the worst coverage case conditions. was varied It atcan 98% peened using steel shots at a normal intensity of 0.3–0.4 mmA, while the coverage was varied at 98% andbe 25%, seen Figurefrom the 5 .diagram Residual that stresses a coverage were of determined 98% led to considerablyat the surface more by stablemeans residual of X-ray stress diffraction. than and 25%,Aftera coverage Figureassessing5 of. Residualthe25%. initial Although residual stresses the stresses, wereinitial determinedconditions the measurements were at comparable, the were surface repeated stresses by means after in theN of = less1, X-ray 10, covered … di 10,000ff raction. Afterload assessingsamp cycles.le were the The already initial fatigue residualrelaxed loading at stresses, wasthe firstapplied load the atcycle. measurements a stress ratio of were R = 0.1 repeated, with a maximum after N = stress1, 10, of... 36010,000 load cycles.MPa. ThisIn The the was following, fatigue chosen loading tothe test in- depththe was stability residual applied of stress the at residual a of stress a pre -stressesloaded ratio of S355N underR = specimen 0.1,worst with case were a conditions. maximum determined, It stresscan of 360 MPa.be asseen seen This from in was Figure the chosendiagram 6. The coverage to that test a coverage the was stability 98% of using 98% of steelled the to shots residual considerably at 0.3– stresses0.4 mmA. more under stableThis test worstresidual was conducted case stress conditions. than It cana be coverageunder seen the from ofsame 25%. the loading diagramAlthough conditions that the ainitial as coverage described conditions of above. 98% were ledThe tocomparable,stabilized considerably residual stresses more stress in stablewasthe lessmeasured residual covered stress here using the hole drilling method and X-ray diffraction. It can be seen from the diagram that the than asamp coveragele were of already 25%. Althoughrelaxed at the the first initial load conditions cycle. were comparable, stresses in the less covered entireIn the compressive following, residual the in-depth stress fieldresidual is partly stress relaxed. of a pre However,-loaded S355Nthe compressive specimen residual were determined, stress of sampleapproximately were already − relaxed300 MPa atremain the firsted effective. load cycle. as seen in Figure 6. The coverage was 98% using steel shots at 0.3–0.4 mmA. This test was conducted under the same loading conditions as described above. The stabilized residual stress was measured here using the hole drilling method and X-ray diffraction. It can be seen from the diagram that the entire compressive residual stress field is partly relaxed. However, the compressive residual stress of approximately −300 MPa remained effective.

Metals 2019, 9, x FOR PEER REVIEW 9 of 14

Figure 5.FigureResidual 5. Residual stress stress relaxation relaxation in in shot shot peened peened DV-buttDV-butt welds. welds. Use Use of steel of steel shots shots at constant at constant intensity.intensity. Variation Variation of the of coverage the coverage of 98%of 98% (top (top)) and and 25%25% ( bottom). ).Cyclic Cyclic loading loading at R = at 0.1R with= 0.1 with maximummaximum stress of stress 360 of MPa. 360 MPa.

Figure 6. Change of residual stress depth profiles in shot peened (steel shots, intensity 0.3–0.4mmA, coverage of 98%) DV-butt welds under cyclic loading. Cyclic loading at R = 0.1 with maximum stress of 360 MPa.

3.3. Fatigue Test Results During fatigue testing, it was noted that the different shot peening parameters had only very little effect on the resulting fatigue strength enhancement of S355N DV-butt welds. Because of this, all results from test series 1–3, 10, 11, and 14–17 (all test series using S355N) were treated as the same population and analyzed together, as seen in Figure 7. All these test results are summarized here under the simplified term “clean blasted”, as no further distinction between the test series was made. The nominal fatigue strength range at 2 million load cycles was determined to ∆σ = 254 MPa (50% POS—probability of survival). Additionally, shown here are the test series peened by industry partners, according to a round robin principle (participants 1 to 3). These results show similar fatigue strength to the “clean blasted” condition. Overall, the test data covers the finite life region between 105 and 106 well. The regression line was determined to k ≈ 10. Run-outs (specimen without failure) were stopped at 5 million load cycles. The fatigue strength results from as-welded samples were determined in the IBESS-project and are given as a reference here [23]. Without going into details, the IBESS specimens were produced using the same welding parameters at the same facility. However, it must be pointed out that these tests were conducted at R = 0 and are not corrected to R = 0.1 due to its uncertain sensitivity to mean stress. The diagram further contains IIW FAT 90 (weld toe failure of untreated DV-butt welds) as a

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Metals 2019, 9, 744 9 of 14

In the following, the in-depth residual stress of a pre-loaded S355N specimen were determined, as seen in Figure6. The coverage was 98% using steel shots at 0.3–0.4 mmA. This test was conducted under the same loading conditions as described above. The stabilized residual stress was measured Figure 5. Residual stress relaxation in shot peened DV-butt welds. Use of steel shots at constant here using the hole drilling method and X-ray diffraction. It can be seen from the diagram that the intensity. Variation of the coverage of 98% (top) and 25% (bottom). Cyclic loading at R = 0.1 with entire compressive residual stress field is partly relaxed. However, the compressive residual stress of maximum stress of 360 MPa. approximately 300 MPa remained effective. −

Figure 6. Change of residual stress depth profiles in shot peened (steel shots, intensity 0.3–0.4mmA, Figure 6. Change of residual stress depth profiles in shot peened (steel shots, intensity 0.3–0.4mmA, coverage of 98%) DV-butt welds under cyclic loading. Cyclic loading at R = 0.1 with maximum stress coverage of 98%) DV-butt welds under cyclic loading. Cyclic loading at R = 0.1 with maximum stress of 360 MPa. of 360 MPa. 3.3. Fatigue Test Results 3.3. Fatigue Test Results During fatigue testing, it was noted that the different shot peening parameters had only very littleDuring effect on fatigue the resulting testing, fatigue it was strengthnoted that enhancement the different of shot S355N peening DV-butt parameters welds. Because had only of very this, alllittle results effect fromon the test resulting series 1–3,fatigue 10, strength 11, and 14–17enhancement (all test of series S355N using DV-butt S355N) welds. were Because treated of as this, the sameall results population from test and series analyzed 1–3, 10, together, 11, and 14–17 as seen (a inll test Figure series7. Allusing these S355N) test resultswere treated are summarized as the same herepopulation under theand simplified analyzed termtogether, “clean as blasted”,seen in Figu as nore further7. All these distinction test results between are thesummarized test series here was made.under the The simplified nominal fatigueterm “clean strength blasted”, range as at no 2 million further loaddistinction cycles wasbetween determined the test toseries∆σ =was254 made. MPa Δσ (50%The nominal POS—probability fatigue strength of survival). range Additionally,at 2 million load shown cycles here was are determined the test series to peened = 254 by MPa industry (50% partners,POS—probability according of to survival). a round robin Additionally, principle (participantsshown here 1are to 3).the These test resultsseries peened show similar by industry fatigue strengthpartners, to according the “clean to blasted” a round condition.robin principle Overall, (parti thecipants test data 1 to covers 3). These the finite results life show region similar between fatigue 105 andstrength 106well. to the The “clean regression blasted” line condition. was determined Overall, to thek test10. data Run-outs covers (specimen the finite without life region failure) between were ≈ stopped105 and 106 at 5 well. million The load regression cycles. line was determined to k ≈ 10. Run-outs (specimen without failure) wereThe stopped fatigue at strength5 million results load cycles. from as-welded samples were determined in the IBESS-project and are givenThe as afatigue reference strength here [results23]. Without from as-welded going into samp details,les thewere IBESS determined specimens in the were IBESS-project produced using and theare samegiven welding as a reference parameters here at[23]. the Without same facility. going However, into details, it must the be IBESS pointed specimens out that were these testsproduced were conductedusing the same at R =welding0 and are parameters not corrected at the to sameR = 0.1 faci duelity. to However, its uncertain it must sensitivity be pointed to mean out stress. that these The diagramtests were further conducted contains at R IIW = 0 FATand 90are (weld not corrected toe failure to ofR untreated= 0.1 due to DV-butt its uncertain welds) sensitivity as a design to fatigue mean strength.stress. The The diagram as-welded further fatigue contains strength IIW was FAT calculated 90 (weld totoe∆ σfailure= 162 of MPa untreated at 2 million DV-butt load welds) cycles withas a k 4. The benefit of clean blasting (respectively shot peening) is most prominent at higher numbers ≈ of load cycles. The fatigue strength increase was determined to ∆σ = 254 MPa/162 MPa = 156% at 2 million load cycles. Both peened and un-peened test series show comparable fatigue strengths below approximately 200,000 load cycles. Figure8 shows the test data from peened and un-peened specimens made from S960QL. Here, the peened test series are distinguished according to the peening media used (test series 4–6). Again, all results of peened specimens show similar fatigue strength between 260 MPa and 290 MPa at 2 million load cycles. Peening by means of steel shots and glass beads led to slightly better results, Metals 2019, 9, x FOR PEER REVIEW 10 of 14 design fatigue strength. The as-welded fatigue strength was calculated to Δσ = 162 MPa at 2 million load cycles with k ≈ 4. The benefit of clean blasting (respectively shot peening) is most prominent at higher numbers of load cycles. The fatigue strength increase was determined to Δσ = 254 MPa/162 MPa = 156% at 2 million load cycles. Both peened and un-peened test series show comparable fatigue strengths below approximately 200,000 load cycles. Figure 8 shows the test data from peened and un-peened specimens made from S960QL. Here, the peened test series are distinguished according to the peening media used (test series 4–6). Again, all results of peened specimens show similar fatigue strength between 260 MPa and 290 MPa at 2 million load cycles. Peening by means of steel shots and glass beads led to slightly better results, which may be a consequence of the beneficial surface roughness. The inclination in the finite life region was determined to values between approximately k = 6 to 8. This can be explained by the higher yield strength of the base metal in comparison to S355N. The reference fatigue test data was taken from the IBESS-project as well. The fatigue strength as-welded was determined to Δσ = 146 MPa at 2 million load cycles with k ≈ 4. In a direct comparison, shot peening led to an increase in the fatigue strength of approximately 180% to 200%. The fatigue strength increase is most prominent at high numbers of load cycles but also provable at higher stress amplitudes. This is explained by the high material strength and, thus, a higher residual stress stability. The differences in fatigue strength results between the three peened test series is most likely a result of the surface roughness. High strength materials usually show higher sensitivity to notch effects. Metals 2019, 9, 744 10 of 14 Figure 9 shows the results from tested Al-6082 specimens. Like results from S355N, the fatigue data of all aluminum test series was very similar and, hence, was analyzed as one group, without furtherwhich maydistinction be a consequence of the individual of the beneficial peening surfaceparame roughness.ters. The group The inclination is therefore in thereferred finite lifeas “clean region blasted”was determined (no peening to values parameters between gi approximatelyven). Here, run-outsk = 6 to were 8. This tested can be toexplained 10 million by load the highercycles. yieldThe determinedstrength of thefatigue base strength metal in at comparison 2 million lo toad S355N. cycles Theof clean reference blasted fatigue specimens test data was was Δσ taken= 94 MPa. from The the regressionIBESS-project line aswas well. determined The fatigue to k strength≈ 9. Hence, as-welded the fatigue was strength determined benefit to due∆σ to= 146post MPa weld at treatment 2 million isload mostly cycles effective with k at 4.high In anumbers direct comparison, of load cycles. shot This peening is also led explained to an increase by the in relatively the fatigue low strength yield ≈ strengthof approximately of the material. 180% toThe 200%. reference The fatiguefatigue strengthdata (as-welded) increase iswere most taken prominent from a previous at high numbers project [24].of load These cycles specimens but also were provable tested at at higher R = 0.1, stress too. amplitudes. The fatigue Thisstrength is explained at 2 million by the load high cycles material was determinedstrength and, to thus,Δσ = a58 higher MPa residual(k ≈ 3) and stress can stability. be described The di byfferences IIW’s FAT in fatigue 50 quite strength well. The results increase between in fatiguethe three strength peened through test series clean is mostblasting likely was a resultdetermined of the surfaceto 162% roughness. at 2 million High load strengthcycles. However, materials theusually increase show is higherlimited sensitivity again by the to notchinclination effects. of the finite life regression line.

Figure 7. Fatigue test data of clean blasted DV-butt welds made from S355N. “Clean blasted” are Figureresults 7. from Fatigue test seriestest data 1–3, of 10, clean 11, 14–17. blasted Samples DV-butt peened welds by made participants from S355N. 1 to 3 were“Clean clean blasted” blasted are in industrial facilities and fatigue tested in Braunschweig (round robin principle). Metalsresults 2019, 9 , fromx FOR test PEER series REVIEW 1–3, 10, 11, 14–17. Samples peened by participants 1 to 3 were clean blasted in11 of 14 industrial facilities and fatigue tested in Braunschweig (round robin principle).

Figure 8. Fatigue test data of clean blasted DV-butt welds made from S960QL. Shown here are test Figure 8. Fatigue test data of clean blasted DV-butt welds made from S960QL. Shown here are test series 4–9. series 4–9. Figure9 shows the results from tested Al-6082 specimens. Like results from S355N, the fatigue data of all aluminum test series was very similar and, hence, was analyzed as one group, without further distinction of the individual peening parameters. The group is therefore referred as “clean

Figure 9. Fatigue test data of clean blasted DV-butt welds made from Al-6082. Shown here are test series 7–8, 12 ,13 and 18–20 named as “clean blasted” without further distinction.

4. Discussion Shot peening generates compressive residual stresses in all three materials. The surface residual stresses vary from test series to test series. Due to distinctive stress gradients, these values are not a good indicator for fatigue benefits through peening. The in-depth compressive residual stress field varies slightly in some of the test series. Steel shots with a coverage of at least 98% led to good penetration depth. However, the fatigue data did not reflect these differences of penetration depth. The peening process appears to be robust in terms of fatigue strength benefits. Nevertheless, the determination of cyclic residual stress relaxation indicates that a full coverage is beneficial for residual stress stability. This in good agreement with the literature [3]. Transferring these results to a possible utilization of clean blasting as a fatigue-enhancing post weld treatment method, full coverage should be always achieved. An explanation for the higher residual stress stability with increasing coverage is the more pronounced cold work hardening. The fatigue data evaluated here (Figure 7 to Figure 9) cannot directly be compared to IIW FAT- values because the data was not corrected for the stress ratio of R = 0.5 (IIW). Furthermore, the data was analyzed based on a 50% probability of survival. This study clearly shows the beneficial effect of shot peening on the fatigue strength of DV-butt welds made from a wide range of construction metals

Metals 2019, 9, x FOR PEER REVIEW 11 of 14

Metals 2019, 9, 744 11 of 14 blasted” (no peening parameters given). Here, run-outs were tested to 10 million load cycles. The determined fatigue strength at 2 million load cycles of clean blasted specimens was ∆σ = 94 MPa. The regression line was determined to k 9. Hence, the fatigue strength benefit due to post weld ≈ treatment is mostly effective at high numbers of load cycles. This is also explained by the relatively low yield strength of the material. The reference fatigue data (as-welded) were taken from a previous project [24]. These specimens were tested at R = 0.1, too. The fatigue strength at 2 million load cycles was determined to ∆σ = 58 MPa (k 3) and can be described by IIW’s FAT 50 quite well. The increase ≈ in fatigueFigure strength 8. Fatigue through test data clean of clean blasting blasted was DV-butt determined welds made to 162% from at S960QL. 2 million Shown load cycles.here are However, test the increaseseries 4–9. is limited again by the inclination of the finite life regression line.

Figure 9. Fatigue test data of clean blasted DV-butt welds made from Al-6082. Shown here are test Figure 9. Fatigue test data of clean blasted DV-butt welds made from Al-6082. Shown here are test series 7–8, 12, 13 and 18–20 named as “clean blasted” without further distinction. series 7–8, 12 ,13 and 18–20 named as “clean blasted” without further distinction. 4. Discussion 4. Discussion Shot peening generates compressive residual stresses in all three materials. The surface residual stressesShot vary peening from testgenerates series tocompressive test series. Dueresidual to distinctive stresses in stress all three gradients, materials. these The values surface are not residual a good stressesindicator vary for from fatigue test benefits series to through test series. peening. Due to The distinctive in-depth compressivestress gradient residuals, these stress values field are variesnot a goodslightly indicator in some for of fatigue the test benefits series. Steelthrough shots peening. with a coverage The in-depth of at leastcompressive 98% led residual to good penetrationstress field variesdepth. slightly However, in thesome fatigue of the data test did series. not reflectSteel shots these with differences a coverage of penetration of at least depth. 98% led The to peening good penetrationprocess appears depth. to However, be robust inthe terms fatigue of fatiguedata did strength not reflect benefits. these Nevertheless, differences of the penetration determination depth. of Thecyclic peening residual process stress relaxationappears to indicates be robust that in aterm fulls coverage of fatigue is beneficialstrength benefits. for residual Nevertheless, stress stability. the determinationThis in good agreement of cyclic residual with the literaturestress relaxation [3]. Transferring indicates thesethat a results full coverage to a possible is beneficial utilization for of residualclean blasting stress stability. as a fatigue-enhancing This in good agreement post weld with treatment the literature method, [3]. full Transf coverageerring shouldthese results be always to a possibleachieved. utilization An explanation of clean for theblasting higher as residual a fatigue-enhancing stress stability withpost increasingweld treatment coverage method, is the morefull coveragepronounced should cold be work always hardening. achieved. An explanation for the higher residual stress stability with increasingThe fatigue coverage data is the evaluated more pronounced here (Figures cold7–9 work) cannot hardening. directly be compared to IIW FAT-values becauseThe fatigue the data data was evaluated not corrected here for(Figure the stress7 to Figu ratiore of9) cannotR = 0.5 directly (IIW). Furthermore, be compared theto IIW data FAT- was valuesanalyzed because based the on data a 50% was probability not corrected of survival. for the Thisstress study ratio clearly of R = shows0.5 (IIW). the Furthermore, beneficial effect the of data shot waspeening analyzed on the based fatigue on a strength 50% probability of DV-butt of survival. welds made This study from aclearly wide rangeshows ofthe construction beneficial effect metals of shotin direct peening comparison on the fatigue to as-welded strength of butt DV-butt welds. weld Thes made fatigue from strength a wide enhancement range of construction was proven metals for industrially clean blasted specimens as well. Furthermore, the beneficial effect of peening on fatigue was proven, especially in the high cycle region of S-N curves, see Table6. Tests using high strength steel S960QL additionally proved a fatigue strength enhancement at lower load cycles. The reason for this is the high yield strength of this material. Furthermore, the high strength of the base metal allows greater fatigue strength improvement, which may be explained by a combination of higher Metals 2019, 9, 744 12 of 14 compressive residual stress and high hardness. The high yield strength leads to a higher sensitivity to residual stress, resulting in a higher fatigue strength.

Table 6. Fatigue strength enhancement of the S-N curve in dependence of the base metal after shot peening.

Fatigue Strength Enhancement at Base Metal at N = 2,000,000 N = 100,000 S355N - 156% ≈ S960QL 125–140% 180–200% Al-6082 110% 162% ≈ ≈

The peening process generated compressive residual stress in all test series. The fatigue data obtained does not indicate any advantage of peening media and/or parameters used for the soft metals S355N and Al-6082. The high strength steel showed some sensitivity to surface roughness induced by peening. The smallest fatigue strength improvement of S960QL was determined after the use of sharp edged corundum with a corresponding high roughness. This effect was not observed in the results from S355N or Al-6082, although the roughness was considerably higher after the use of corundum. All results shown here were determined using butt welds with a relatively low stress concentration. General recommendations for bonus factors cannot be made yet as the data base, especially using weld joint types with higher stress concentration, such as cruciform joints, is missing. However, in a previous study, shot peening effects on the fatigue strength of longitudinal stiffeners made from S355N was investigated [25]. In contrast to the butt welds investigated here, the fatigue strength of longitudinal stiffeners was not also enhanced. The reason for this was found in the relaxation of compressive residual stresses at certain load amplitudes. As a result, the fatigue strength of as-welded and shot peened samples was comparable at 1 million load cycles and below. A benefit was proven only at load stress amplitudes of 60 MPa and less (R = 1), due to a higher residual stress stability. These − results show the effect of stress concentration at the weld on the success of shot peening as a post-weld treatment method. This should be studied further in future.

5. Conclusions Fatigue strength can be significantly improved by shot peening. Round robin tests with S355N specimens have further proven the high potential of regular industrial clean blasting processes to achieve similar results. The effect of shot peening on fatigue strength increases with increasing yield strength of the base metal. Coverage should be controlled and chosen to be at least 98% to improve residual stress stability under mechanical loading. The peening media affects the surface roughness. In the case of high strength steels, roughness becomes important, as an increase in roughness lowers the fatigue strength. Stress concentration may influence the residual stress stability and thus the fatigue strength. This should be further investigated. It is recommended to qualify industrial clean blasting processes with the help of the Almen test, roughness measurements and, as far as possible, residual stress measurements and component fatigue tests. By such a qualification of the clean blasting, designers may use bonus factors for the fatigue design of welded components.

Author Contributions: Conceptualization, T.N.-P. and J.H.; methodology, T.N.-P. and J.H.; validation, T.N.-P.; investigation, H.E. and J.H.; resources, K.D.; data curation, H.E. and J.H.; writing—original draft preparation, J.H.; writing—review and editing, T.N.-P..; visualization, J.H.; supervision, T.N.-P.; project administration, T.N.-P.; funding acquisition, T.N.-P. and K.D. Funding: This IGF project IGF-Nr.: 18.985 N of the “Forschungsvereinigung Schweißen und verwandte Verfahren e. V. des DVS” was supported via AiF within the program for promoting the Industrial Collective Research (IGF) of the German Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament. Acknowledgments: The authors thank OSK Kiefer, Oppurg, Germany, for the peening of samples. We acknowledge support by the Open Access Publication Funds of the Technische Universität Braunschweig. Metals 2019, 9, 744 13 of 14

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Hobbacher, A. Recommendations for Fatigue Design of Welded Joints and Components; Welding Research Council: New York, NY, USA, 2009. 2. Beuth Verlag. DIN EN 1993-1 Eurocode 3: Bemessung und Konstruktion von Stahlbauten; Beuth Verlag: Berlin, Germany, 2010. 3. Wohlfahrt, H.; Krull, P. Mechanische Oberflächenbehandlung; Wiley-VCH: Weinheim, Germany, 2000. (In German) 4. Schulze, V. Modern Mechanical Surface Treatment; Wiley-VCH: Weinheim, Germany, 2006. 5. Marquis, G.; Barsoum, Z. A Guideline for Fatigue Strength Improvement of High Strength Steel Welded Structures Using High Frequency Mechanical Impact Treatment. Procedia Eng. 2013, 66, 98–107. [CrossRef] 6. Haagensen, P.; Maddox, S. IIW Recommendations on Post Weld. Improvement of Steel and Aluminium Structures; The International Institute of Welding: Paris, France, 2011. 7. Marsh, K.J. Shot Peening: Techniques and Applications; Chameleon Press Ltd: London, France, 1993. 8. Wagner, L. Shot Peening; Wiley-VCH: Weinheim, Germany, 2003. 9. Zinn, W.; Scholtes, B. Mechanical Surface Treatments of Lightweight Materials—Effects on Fatigue Strength and Near-Surface Microstructures. J. Mater. Eng. Perform. 1999, 2, 145–151. [CrossRef] 10. SAE International. S. J. 442, Test. Strip, Holder, and Gage for Shot Peening; SAE International: Warrendale, PA, USA, 2013. 11. Macherauch, E.; Wohlfahrt, H. Eigenspannungen und Ermüdung, In Ermüdungsverhalten metallischer Werkstoffe; Deutsche Gesellschaft für Metallkunde: Oberursel, Germany, 1985; pp. 237–283. 12. Nitschke-Pagel, T.; Wohlfahrt, H.; Dilger, K. Application of the local fatigue strength concept for the evaluation of post weld treatments. Weld. World 2007, 51, 65–75. [CrossRef] 13. Nitschke-Pagel, T.; Dilger, K. Wirkungsweise mechanischer Oberflächenbehandlungen zur Schwingfestigkeitsverbesserung von Schweißverbindungen und Qualitätssicherungsmethoden. In DVS-Berichte Band 250; DVS-Verlag: Düsseldorf, Germany, 2008; pp. 174–182. 14. Cheng, X.; Fisher, J.W.; Prask, H.J.; Gnäupel-Herold, T.; Yen, B.T.; Roy, S. Residual stress modification by post-weld treatment and its beneficial effect on fatigue strength of welded structures. Int. J. Fatigue 2003, 25, 1259–1267. [CrossRef] 15. Kirkhope, K.; Bell, R.; Caron, L.; Basu, R.; Ma, K.-T. Weld detail fatigue life improvement techniques. Part 1: Review. Mar. Struct. 1999, 12, 447–474. [CrossRef] 16. Maddox, S. Improving the fatigue strength of welded joints by peening. Met. Constr. 1985, 17, 220–224. 17. Nitschke-Pagel, T.; Wohlfahrt, H. Fatigue Strength Improvement of Welded Aluminium Alloys by Different Post Weld Treatment Methods. In Shot Peening; Wiley-VCH: Weinheim, Germany, 2006; pp. 360–366. 18. Hensel, J.; Nitschke-Pagel, T.; Dilger, K. Effects of Residual Stresses and Compressive Mean Stresses on the Fatigue Strength of Straightened Longitudinal Stiffeners IIW Document XIII-2541-14; International Institute of Welding: Paris, France, 2014. 19. Scholtes, B. Röntgenographisches Verfahren, In Handbuch für experimentelle Spannungsanalyse, C. Rohrbach (Hrsg.); VDI-Verlag: Düsseldorf, Germany, 1989; pp. 435–464. 20. ASTM International. ASTM E837-13a, Standard Test. Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method; ASTM International: West Conshohocken, PA, USA, 2013. 21. Macherauch, E.; Wohlfahrt, H. Different sources of residual stress as a result of welding. In Proceedings of the Conference on Residual Stresses in Welded Constructions and Their Effects, London, UK, 15–17 November 1977. 22. Wohlfahrt, H. Die Bedeutung der Austenitumwandlung für die Eigenspannungsentstehung beim Schweißen (in German). Härtereitechnische Mitt. 1986, 41, 248–257. 23. Zerbst, U. Analytische Bruchmechanische Ermittlung der Schwingfestigkeit; BAM: Berlin, Germany, 2015. 24. Nitschke-Pagel, T.; Eslami, H.D.K. Influence of the Deformation Intensity on the Fatigue Strength of Aluminum Welds with Different Mechanical Surface Treatments. IIW-Doc. XIII-2483-13; International Institute of Welding: Paris, France, 2013. Metals 2019, 9, 744 14 of 14

25. Hensel, J.; Nitschke-Pagel, T.; Dilger, K. Effects of residual stresses and compressive mean stresses on the fatigue strength of longitudinal fillet welded gussets. Weld. World 2016, 60, 267–281. [CrossRef]

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