Materials and Design 38 (2012) 7–18

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Materials and Design

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Strength enhancement of the welded structures by ultrasonic peening

Amir Abdullah, Massoud Malaki ⇑, Ahmad EskandariDepartment of Mechanical Engineering, Amirkabir University of Technology, No. 424, Hafez Ave., P.O. Box: 15875-4413, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 October 2011Accepted 21 January 2012Available online 3 February 2012

Keywords:WeldingFatigueStress relieving

0261-3069/$ - see front matter Crown Copyright � 2doi:10.1016/j.matdes.2012.01.040

⇑ Corresponding author. Tel.: +98 912 442 4739.E-mail address: Massoudmalaki@aut.ac.ir (M. Mala

Residual stresses exist in all manufacturing processes which use heat and/or force such as casting, form-ing, machining and welding. Sometimes they cause decrease in strength and life of components especiallyunder dynamic loads and vibration conditions. To improve fatigue strength, a number of post treatmentoperations are being used such as grinding, shot peening, re-melting and heat treatment. Ultrasonicpeening is a newly developed method for the improvement of fatigue strength of, mainly, welded jointsand structures. By employing this process, geometry of weld toe can be modified for reducing the stressconcentration. In addition, elimination of tensile residual stresses, exertion of compressive residual stres-ses and closing of cracks, voids and cavities are expected, too. The extra advantage of this technology is itsapplication on massive and large structures which cannot be treated by other procedures. For investigat-ing the effect of ultrasonic peening on stainless steel-304 welded parts, a series of experiments weredesigned and implemented. Ultrasonic peening is mostly used as a mechanical surface treatment methodin the automotive and aerospace industries. However, this paper comprises the results of experimentalfatigue strength tests along with metallography, micro hardness and corrosion resistance tests of weldedpieces with and without processing by ultrasonic peening. Experiments proved that under post treatmentby ultrasonic peening, a better mechanical and corrosion resistance is achieved.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Residual stresses are self-balanced internal stresses existing in afree body with no external forces or constraints on its boundaries.These stresses are elastic reaction of materials to a non-elastic ornon-homogenous strain such as plastic strains, acceleration, phasechanges (phase transformation), plastic thermal strains and so on[1].

Residual stresses are created in most manufacturing processeslike casting, machining, welding and all processes in which forceand/or heat are operative. Local residual stresses are a combinationof the following three types of stresses [2–4]:

Macro residual stresses (rI): These types of stresses, referred toas the primary-type stresses, are homogenous and are distrib-uted in an area bigger than grain size. These stresses are usuallytaken into account in calculations.Micro residual stresses (rII): Micro residual stresses or second-ary-type stresses are created by micro structure differences ofmaterials. The amount of this kind of stress is small, and thisis resulted from elastic and/or plastic interactions of grains

012 Published by Elsevier Ltd. All r

ki).

and phases. Maximum loading area of these stresses is in theorder of grain size.Submicron residual stresses (rIII): Third type of residual stres-ses is similar to the second one albeit smaller. These stressesare created by lattice defects with the influencing area in theorder of atomic dimensions.

Almost all mechanical, chemical and thermal processes lead tocreating the residual stresses, but their type is different in variousprocesses, for example, the first type of stresses is generated inmachining, welding, casting, forming, heat treatments and coatingprocesses, while the second one is generated in the ceramics andcomposite materials. Third type is generally created along withthe first and second ones of stresses [3].

Existence of tensile residual stresses in welded area decreasestensile load capacity and fatigue life as well. The welded part is dis-torted at the time of welding and gradual release of residual stres-ses results in dimensional and geometrical instabilities. Therefore,much effort is being made to reduce or at least eliminate theseharmful stresses [5]. Among these, ultrasonic peening usesperiodic impacts of peens with frequencies above 20 kHz on thesurface of weld and/or weld toe and this process creates a deforma-tion in tensile zone. By this action, tensile stresses can be relievedand cracks and voids are closed, furthermore, weld toe geometry ismodified, and also compressive residual stress is induced by theregional plastic deformation [5]. The extent of improvement

ights reserved.

Fig. 1. Mechanism of ultrasonic peening.

Fig. 2. Cross section of part improved by ultrasonic peening.

8 A. Abdullah et al. / Materials and Design 38 (2012) 7–18

depends on different factors like material properties, type of weldand loading type, too. For butt weld joints, 50–200% increase ofstrength has been reported [5].

First of all, this method was invented by Mukhanov and Mord-vintseva in 1950 [6]. By generating continuous ultrasonic vibra-tions at a carburized steel tool tip, the surface of treated partreceived direct impact. In 1960, Krylov proposed a spherical shotbetween a surface and ultrasonic transducer which imposed highplastic deformation, but free motion of spherical shot resulted neg-ative effects [7]. In 1970, Estatnikov used ultrasonic impact peensin the separate holes on the vibrating head for working on weldedand machined parts. He could exert high plastic deformation onthe surface. Later in 2006, he claimed ultrasonic peening as thebest method for large plastic deformations and fatigue strengthimprovement [8,9]. Under variable-amplitude load, in 2005, Huoet al. [10] studied the welded joints treated by ultrasonic peeningand TIG-dressing processes as well. In 2007, investigations ofMordyuk et al. showed that a Nano-crystalline layer can be gener-ated on an AISI-321 stainless steel by a severe plastic deformationvia ultrasonic peening treatment [11]. In 2009, a comparisonbetween shot-peening and ultrasonic peening was done by Leeand co-workers [12]. According to their results, ultrasonicallypeened parts showed an equal or even better resistance againstcorrosive environments. Ting Wang et al. [13] discussed on fatiguedesign differences between as-welded and ultrasonically peenedjoints. In this paper, stress ratio and its effects on fatigue life havealso been proposed and discussed. In 2010, Yin et al. [14] studiedthe fracture mechanism of welded joints treated by ultrasonicpeening process. Later, Zhao et al. [15] studied fatigue behaviorof welded joints through S–N curves. Moreover, in 2011, anotherresearch on the fatigue properties of ultrasonically peened frictionstir welded parts has been performed by Yang et al. [16]. In thisresearch, under stress ratio R = 0.5, fatigue life of welded joints ofAl7075-T651 was prolonged by 8–14 times.

1.1. Industrial applications

High power ultrasonic transducers have interestingly foundvarious practical applications in many industrial processes, ofwhich cleaning, welding and machining are well known instances.Other applications include ultrasonic assisted forming, moltenmetals’ treatments, chemical processing, and even therapeuticand surgical applications in medicine. In most industrial uses, anultrasonic system involves power levels of hundreds to thousandsof watts, and operating in the frequency ranges from 15 kHz to100 kHz. Moreover, typical amplitudes range from about 10 to40 lm.

One of the most promising directions in using of this technologyfor industrial purposes is the peening of welded joints. An ultra-sonic peening system includes a transducer, power supply, and acomputer kit for providing maximum possible increase in fatiguelife of parts and welded elements with minimum costs, laborsand energy consumption as well.

Ultrasonic peening produces a number of useful effects in microstructure of metallic materials. Foremost among these is increasingthe resistance of materials to surface-related failures, such as fati-gue and stress corrosion cracking. The ultrasonic peening technol-ogy can be effectively applied to elimination of distortions causedby welding and other technological processes, residual stress relief,and increasing of the hardness of materials.

In fatigue enhancement, the beneficial effects are achieved,mainly, by relieving of unfavorable tensile residual stresses andexerting compressive residual stresses into the surface layers,decreasing stress concentration of weld toe area, and also enhanc-ing mechanical properties of the surface layers of the weldedjoints. The fatigue results of welded pieces shows that ultrasonic

peening is the most efficient and economical compared with tradi-tional techniques such as grinding, TIG-dressing, shot peening andlike these. Ultrasonic peening is successfully applied in variousstructures like shipbuilding industries, different kinds of reservoirsof corrosive chemicals, turbines and aerospace applications. Thistechnology has become popular for many important reasons likeincreasing fatigue life of welded elements, eliminating distortionscaused by welding and other technological processes, relievingresidual stresses, increasing hardness of material surfaces and cre-ating nano-crystalline layers in the surface.

2. Mechanism of action of ultrasonic peening

Ultrasonic peening equipment generally includes high-fre-quency high-voltage electrical power supply, ultrasonic transducerhead, concentrator and impact tool.

Fig. 1 schematically shows the mechanism of action of ultra-sonic peening. Ultrasonic transducer ‘‘I’’ receives power from thegenerator and standing wave 1 is produced in the head. The ampli-tude of this wave is later amplified in concentrator ‘‘II’’ and thentransferred to the tool ‘‘III’’. Vibration of the tool tip impacts ontothe surface of the part ‘‘IV’’ (wave 2) and imparts plastic deforma-tion on the surface (zone 4). As a result of this plastic deformation,high compression residual stress is exerted on the surface of thepart, and the amount of which decreases in sub-layers. Penetration

Table 1The zones created in the part after ultrasonic peening process [17].

Zone Description Depth of zone from surface Improved properties

A Area of imposed plastic deformation and compressionresidual stresses

1–1.5 mm Corrosion, fatigue, wear, distortion improved

B Relieved zone of tensile residual stresses of the weld 1.5 mm and higher Crack growth, distortion improvedC Crystallized zone 0.01–0.1 mm Fatigue strength in high temperatures and corrosion resistance improved

Fig. 3. Comparison of methods for improving fatigue strength of weld, (1) weldspecimen without improving process, (2) ultrasonic peening by a tool of 5 mmdiameter, (3) hammering, (4) shot peening, (5) TIG dressing, (6) TIG dressing andultrasonic peening, (7) ultrasonic peening by a tool of 3 mm diameter.

Fig. 4. Distribution of residual stresses in Stainless Steel 304 plate, before and afterrolling.

Fig. 5. Dimensions of specimens which were cut by abrasive water jet.

A. Abdullah et al. / Materials and Design 38 (2012) 7–18 9

of vibration energy (wave 5) facilitates sub-layers move towardsresidual tensile stresses of former processes and helps release ofsuch stresses [8,9]. Fig. 2 and Table 1 show the approximatedimensions of the treated area in the welded specimen processedby ultrasonic peening [17].

Weld toe has been deformed by applying ultrasonic impacts gi-ven rise to less stress concentration [17]. Fig. 3 shows a comparisonamong improving methods of fatigue strength in welded structures[18].

3. Experimental test steps

This section discusses the preparatory works on specimens,welding method, peening process, fatigue tests [19], metallographyworks (before and after ultrasonic peening process), micro-hard-ness tests and evaluation of chemical resistance and corrosion rate[20].

Table 2Chemical composition and mechanical properties of Stainless Steel 304 at room temperat

Chemical compositionMaterial Number (DIN) 1.4031Symbol X5CrNi18-10Trade name 304Chemical composition (wt.%) C Si Mn

Max 0.08 Max 0 Max 2.0

Mechanical propertiesDensity (Kg/m3) Hardness (HB 30) Tensile properties

Yield strength (Min MPa)800 215 205

3.1. Preparatory works for specimens

5 mm Thickness Stainless Steel 304 sheets were chosen for theexperiments. Chemical composition and mechanical properties ofthis kind of steel are as given in Table 2 [21].

The welding parameters were taken from Welding Handbook[22] then the sheets were butt welded on both sides by TIG weld-ing process. According to the instructions, tungsten electrode EWPwith polarization DCEN (DC current with negative electrode) wasused. At the next step, welding of both sides was performed afterpreheating of 250 �C and then maintaining at 100 �C and it should

ure [21].

P S Cr Ni FeMax 0.045 Max 0.03 Max 18 Max 10 68–74

Elongation in 2 in or 50 mm (Min %)

Tensile strength (Min MPa)515 40

Table 3Tensile test results.

Machinemodel

Machinecapacity

Specimennumber

Loadingrate(mm/s)

Yieldforce(kN)

Crosssection(mm2)

Yieldstress(MPa)

DARTEC 9600 5 ton 1 0.1 19.12 55 347.62 0.1 19.15 55 348.13 0.1 19.33 55 351.45

10 A. Abdullah et al. / Materials and Design 38 (2012) 7–18

be mentioned that the filler material was from St308. Furthermore,voltage of welding was set at 25 V and that the first side waswelded with 90 A and the second side was welded with 110 A.

Because of distortions created in welding of the plates, it wasnecessary to flatten the plates by a three-roller machine. Obvi-ously, the residual stresses change after rolling. Fig. 4 shows thetypical change of residual stresses after rolling process [20].

The fatigue test machine was of tension-tension type. Threespecimens were prepared for tensile test and twelve specimenswere prepared for fatigue tests (six of them without peening andsix of them with peening). Moreover, six specimens were preparedonly for metallography tests (three of them without peening andthree of them with peening).

Dimensions of the specimens were designed according toinstruction given in standard AWS B4.0-92 [23] (with regardingholder width of 24 mm of the test machine) as given in fig. 5.The specimens were cut by using abrasive water jet cutting.

3.2. Peening operation

After cutting of the specimens to the required size by abrasivewater jet, peening operation was done on nine specimens. 200 WLABSONIC power supply was used for this operation. Power usedwas 100 W with 20 kHz frequency. The impact tool of Titaniumwas a round rod with a flat tip of 3 mm diameter. For a better wavetransmission, a layer of acoustic wax was used, and then the peen-ing process was applied for 1 min on each weld toe of thespecimens.

4. Results of mechanical tests and discussions

4.1. Tensile test of specimens

Considering the necessity of determination of yield stress ofpieces for fatigue test, tensile test was performed on three speci-mens. Table 3 shows the result of the tests.

Table 4Fatigue test conditions and the values obtained for specimens without

No. 2rAmp (MPa) FMax (KN) FMin (KN)

Without peening1 300 17.325 0.825234 316 18.249 0.869567 330 19.057 0.90789

With peening1 300 17.325 0.82523 316 18.249 0.86945 330 19.057 0.9076

According to Table 3, mean yield stress was:

ðryÞave ¼ðryÞ1 þ ðryÞ2 þ ðryÞ3

3¼ 347:6þ 348:1þ 351:45

3¼ 349:08 MPa ð1Þ

4.1.1. Fatigue test of specimensBased on standard ASTM, E467 [24], the proportion of minimum

tensile force to maximum tensile force was taken 0.05. Having theyield stress of 349.08 MPa and loading frequency of 28 Hz, the ap-plied peak to peak stresses were taken 300 MPa, 316 MPa and330 MPa.

Fatigue tests were done by using DARTEC 9600 Machine (ForceCapacity 50 kN, Maximum frequency 100 Hz). For fastening ofspecimens to the machine, 500 N force was used. Regarding thecross sections of specimens, the maximum and minimum loadswere determined and adjusted on the machine. Table 4 showsthe conditions and results of the tests.

4.1.2. Discussion on fatigue test resultsPoint defects, atoms movements, lattice defects due to deforma-

tion, and defects in grain boundaries are the important factorsleading to creating microscopic residual stresses in a part. In fact,this kind of stress leads to an inhomogeneity in mechanical behav-iors of structure, so in structure maintenance, the main focus is onmacroscopic stresses.

Parameters such as specimen’s geometry, the degree of struc-ture’s constraints, the energy of weld, phase transformations etc.are too important in the level of residual stresses. When the tem-perature drops from the austenitic area to the room temperature,two types of dimensional changes are created. The first one is ther-mal shrinkage due to cooling, and the second one is transforma-tional expansion due to converting austenitic phase to the otherphases. Then, it can be declared that welding residual stressesnot only depend on welding energy and joint type but also onchemical properties of weld metal, base metal, and microstructureof them.

According to researches and their metallography observations,initial fatigue cracks usually lead to an increase in the local stressconcentration. While a crack is initiating and growing, materialsin the toe expose harmfully adverse yield stresses. When this crackbecame bigger, cross section decreases and this leads stresses toincrease.

In general, since a crack initiates from a free surface and thenstarts to grow in depth; therefore, important factors effective onthe fatigue strength highly influence the first step of crack growth.

and with peening.

2FAmp (KN) N (cycle) Fracture location

16.5 159,552 Weld toe111,544211,686

17.38 110,985 Weld toe207,663

84,92618.15 60,231 Weld toe

103,36653,336

16.5 329,752 Base metal177,903 Base metal

17.38 228,433 Base metal238,235 Base metal

18.15 106,782 Weld toe118,691

Fig. 6. Left: location of fractured on specimens without peening. Right: location of fracture on specimens after peening.

Fig. 7. S–N curves of specimens with and without peening.

A. Abdullah et al. / Materials and Design 38 (2012) 7–18 11

So, the size of grains, the level of cold-working, residual stresses,dislocations’ energy, the distribution of alloy elements, and surfacetreatments have an important effect on crack growth. Furthermore,the second step of crack growth depends on geometrical shape,material properties, and applied stress conditions too.

In figures taken by electron microscope, since the parameter‘‘crack depth’’ is presented as an important criterion for crackgrowth, so crack depth, size, and how a crack is growing can beevaluated and then, by adjusting necessary parameters in ultra-sonic peening, either they are closed or at least further growth isbeing (or is) stopped.

In low cycles, for a lot of steals, fatigue strength is independentfrom grain size. On the other hand, for high cycles, it will decreasewhen grain size increase. The austenitic stainless steels cannot behardened by heat treatment, but significantly can be hardened bycold-working.

Since grain boundaries have a high level of energy and chemi-cally are more active, so when a surface of material is exposedby a corrosive chemical; first of all, boundaries are corroded. BothHAZ zone and weld material are not resilient against intergranularcorrosion, stress corrosion cracking, and pitting/crevice corrosion.By applying ultrasonic impacts on these regions, a deformed layerwith high density of compressed material is created. Under this cir-cumstance, the interaction between the grain boundaries and cor-rosive materials will be minimal; therefore, the process ofultrasonic peening leads to an increase in corrosion resistance aswell.

4.1.3. Without ultrasonic peeningFig. 6 (Left) shows the location of breakage due to fatigue for the

ordinary welded pieces. For all the specimens, the weld toe wasbroken, perhaps, due to surface micro cracks in this region, exces-sive tensile residual stresses created in welding and lower thick-ness of this zone compared with the thickness of weld point.

4.1.4. With ultrasonic peeningWhen a peening tool impacts a metal surface, kinetic energy in-

duces a severe plastic deformation so the compressive residualstresses are exerted on the weld region; therefore, in this zone,wear resistance and fatigue properties significantly enhance.

Fig. 6 (Right) shows the location of breakage due to fatigue forultrasonic peened welded pieces. For 300 MPa and 316 MPa loadedpieces, the location of fracture was on the base metal, at a distanceof about 12 mm from weld toe. For 330 MPa loaded pieces thebreakage point was on the weld toe. In all cases, the number of cy-cles to breakage was much higher for peened pieces. These couldbe, perhaps, due to shape modification of the weld toe, release ofresidual tensile stresses and their replacement by residual com-pressive stresses and closing of the micro cracks and voids by im-pacts of the tool.

Fig. 7 shows the S–N diagram of the results. The curve related tospecimens with peening has crossed the vertical axis at higherstresses. S–N curves show that in applied stress of 300 MPa, fatiguelife increases from 150,000 cycles for specimens without peeningto 330,000 cycles for specimens with peening which is equal to120% improvement in fatigue life. Similarly, for applied stress of330 MPa there is about 70% improvement in life. In 400 k-cycleloading, fatigue stress increases from 225 MPa to 290 MPa (about29% increase) when peening is applied.

On the fatigue life enhancement, there is different methods.Although a wide variety of welded joints with different materialsand technological conditions was investigated but, totally,20–80% increase of fatigue life has been reported in the recentpapers. What’s more, ultrasonic peening is nearly the best one.Results of this paper and also others in the literature show thesignificant effects of ultrasonic peening treatment. Indeed, byemploying this process on the weld toe, a layer of plastic defor-mation will be generated, moreover, geometry of weld toe canalso be intrestingly modified for reducing the stress concentra-tion. In addition, eliminating tensile residual stresses, exertingthe beneficial compressive residual stresses, closing of micro-and macro-cracks, voids and cavities are expected, too. Accordingto the results, fatigue life wes significantly extedned, for instance,in low and intermediate level of applied stresses, 120% improve-ment in fatigue life was achieved and that all of the breakagesoccurred in the base metal not weld toe. For higher level ofapplied stresses, 70% improvement was seen in the fatigue testexperiments.

Fig. 8. Cut specimens for metallographic test.

Table 5Numbering metallographic specimens cut from fatigue test pieces without and withpeening treatments.

Without peening treatments With peening treatments

No. Appliedstress

Fatigue lifecycle

No. Appliedstress

Fatigue lifecycle

7 300 111,544 13 300 177,9038 300 159,552 14 300 329,7529 316 84,926 15 316 228,433

10 316 110,985 16 316 238,23511 330 53,336 17 330 106,78212 330 60,231 18 330 118,691

12 A. Abdullah et al. / Materials and Design 38 (2012) 7–18

5. Metallography of specimens

To study the influences of heat of welding and the mechanicalimpacts applied by peening the steps given bellow were followedaccording to standard ASTM E3-95 [25].

To reduce the effects of cutting process on the metallurgicalproperties of specimens required for metallography, fine electro-discharge wire cutting Charmills Robofil 6020 was used whichcould generate smooth surface and better condition for furtherpreparation and treatment works. To have a section from weld me-tal, peened area, and the parent material, the specimens were cutnormal to the welding direction in the middle of the width of thefatigue test-piece. The length of the cut of the separated specimenswas selected 30 mm in order to cover the cross section of all weldtoes of the fatigue test piece. Fig. 8 shows the cut section of thespecimens. The cut specimens were divided into four groups:

Fig. 9. Images of weld toe and base metal for specimens without peening and withou100 lm).

(1) Three specimens cut from welded and peened test pieces,numbered 1, 2 and 3.

(2) Three specimens cut from welded but not-peened testpieces, numbered 4, 5 and 6.

(3) Six specimens cut from welded and not peened test pieces(after implementing fatigue test on the pieces under differ-ent stresses), numbered according to Table 5.

(4) Six specimens cut from welded and peened test pieces (afterimplementing fatigue test on the pieces under differentstresses), numbered according to Table 5.

The cut specimens were marked with their number andmounted in plastic body such that the cut face was exposed. Thenthe cut face layer was removed and semi polished by silicon car-bide emery clothes meshed 120, 150, 220, 400, 600, 800, 1000,1200, 1500, 2000, 2500, and 3000 on an inclined surface with purewater flow. The thickness of the layer removed was about 400micrometers (to make sure no effect from wire-cut operation).

For final polishing, Alumina suspension 0.05 lm (from BohlerCo.-USA), soluble in pure water was used on a rotary disk of Poli-Cel1 polish numbers 1 & 2 (Struers A/S Co.-Denmark) rotating at200 rev/min.

According to instructions given in metallography handbookASM, vol. 9 [26], the specimens were submerged and etched for10 min in Marble solution and were immediately washed by purewater after etching. Then, they were washed by pure alcohol anddried by clean hot air. After specimen preparation, optical micro-scope Olympus, BH-2 was used for studying microstructure on dif-ferent zones. For Electron Microscope investigation, the surface ofthe pieces were gold plated by sputter coater model SCD 005 ofBAL-TEC (Swiss), for 10 nm thickness of gold under vacuum condi-tion 10�5 mbar and electrical current of 60 mA in 60 s. Also, thescanning electron microscope used was Philips XL30 of Nether-lands. It was employed for accurate determination of the deformedlayer thickness of peening.

5.1. Discussion on metallographic results

First of all, in some austenitic stainless steels such as AISI 304SS,applying ultrasonic impacts exerts a severe plastic strain; there-fore, martensite phase is going to be revealed at room temperature.

By comparing metallography (or graphic) results of the paperwith other ones, taken by optical and electron microscopes, itcan be concluded that after processing by ultrasonic peening thereis not only no trace of microscopic cracks but also because of highfrequency impacts on weld toe, geometrical modification can oc-cur, too. Therefore, the use of ultrasonic peening treatment is a

t fatigue test (specimen 5), taken by optical microscope (magnification of 20 and

Fig. 10. Crack in weld toe of specimen without peening, without fatigue test(specimen 5).

Fig. 12. Depth of penetration induced by peening operation, (specimen 2).

Fig. 13. Width of the affected layer by peening process in weld toe, 860 lm(specimens 2).

A. Abdullah et al. / Materials and Design 38 (2012) 7–18 13

good method in order to exert compressive residual stresses, clos-ing cracks, preventing the growth of cracks, preventing the failureof welded structures, and all of all, it significantly leads to fatiguestrength enhancement.

5.2. Comparison of weld toe and base metal without ultrasonicpeening, without fatigue test

As it is shown in Fig. 9, surface micro cracks are observed in thearea of weld toe and base metal where the stress concentration canbe expected. Fig. 10 of electron microscope shows a crack of 21 lmlength in the weld toe in a non-peened specimen before applyingfatigue test.

5.3. Comparison of weld toe and base metal after ultrasonic peening,without fatigue test

As it is observed in Fig. 11a, there is not any trace of crack in theweld toe. In the meantime, by applying peening impacts, the curva-ture of weld toe has been modified and as a result there is less con-centration of stress in this area. Far from the peened area (Fig. 11b),there are traces of microscopic cracks in the base metal (Fig. 11).

Fig. 12, a figure taken by electron microscope, shows the depthof deformed layer in the affected area which is about 60–100 lm.

Fig. 11. Comparison of images of weld toe and base metal for specimen with peening andmetal.

Therefore, due to imposed deformation in the structure of this areaand creating finer grains near the surface, increase of fatiguestrength is predictable. Fig. 13 shows that the width of the affectedlayer is about 800–1000.

After ultrasonic peening treatment, severe plastic deformationwas found on the surface, multi-directional mechanical twins wereobserved in the deformed area of ultrasonically peened specimens.Figs. 12 and 13 show depth and width of the affected layer inducedby peening operation, respectively.

5.4. Comparison of weld toe with and without peening, before fatiguetest

Fig. 14 shows the weld toe of specimens with and without peen-ing and before fatigue tests. With study and comparison of these

without fatigue test (specimen 3) taken by optical microscope. (a) Weld toe, (b) base

Fig. 14. Comparison of optical microscope images of weld toe, before fatigue test. (a) Micro cracks in weld toe of non-peened specimen (specimen 6), (b) elimination of cracksand modification of curvature of weld toe in peened specimen (specimen 3).

Fig. 15. Existence of notch in the weld toe of specimen without peening beforefatigue test (specimen 4).

Fig. 16. Closed cracks in the weld toe of specimen with peening before fatigue test(specimen 1).

14 A. Abdullah et al. / Materials and Design 38 (2012) 7–18

optical microscope images, it is clear that in the specimen withoutpeening, micro cracks of different sizes exist in HAZ zone and weldtoe, whilst there is no crack present in the weld toe of the peened testpiece. In the meantime, the weld toe curvature was modified too.

Comparison of the images obtained by electron microscope(Figs. 15 and 16) show that in the weld toe of the part withoutpeening, there is a notch with 7 lm opening. However, in the pee-ned specimen, the cracks have been forged and closed under actionof ultrasonic impacts.

5.5. Comparison of weld toe with and without peening, after fatiguetest

Fig. 17 shows the weld toe of specimens with and withoutpeening and after fatigue test. With study and comparison of theseoptical microscope images, it is observed that in the specimenwithout peening, at the weld toe, there is a big crack with largeopening after fatigue test, whilst no trace of crack or void can beobserved in the peened test piece.

Comparison of the images obtained by electron microscope(Figs. 18 and 19) show that in the weld toe of the part without peen-ing, there is a long breakage with 4 lm opening, whilst in the caseof peened specimen, no crack or unfavorable effect can be observed.

6. Micro-hardness test on specimens

Standards ASTM E92-82 [27] and ASTM E384-99 [28] were ob-served for micro-hardness test on specimens using Vickers scale.Eq. (2) was used to calculate the value.

HV ¼ 1:000� 103 � P=AS ¼ 2:000� 103 � P Sinða=2Þ=d2

HV ¼ 1854:4� P=d2ð2Þ

where P is the load in gf, As is the mean area of indentation in lm2, dis the mean diameter of the indentation in lm and a is the angle ofindentation tip (136�).

Shimadzu-Type M, micro-hardness tester was used with a loadof 25 gf in 5 s by diamond tip of 13 lm diameter. Micro-hardnesstest was done in several different depths from the free surface oftest pieces in three regions (weld area, weld toe and 7 mm fromthe weld toe on base material) and the hardness variations wasplotted against distance from the free surface of each test piece.

6.1. Discussion on the results of micro-hardness tests

By applying ultrasonic vibration on the weld toes, a layer ofplastic deformation was generated which caused refined grainsin depth of less than 100 lm. Furthermore, micro hardness andmechanical strength improved in that area.

Because of ultrasonic treatments, the plastic deformation in thesurface layer of the pieces which led to work-hardening is created.In fact, the strength and micro-hardness in this region are signifi-cantly enhanced. Fig. 20 shows the micro-hardness distributionalong the thickness direction processing by ultrasonic peening pro-cess. It can be clearly seen that the largest value of the hardness isin top layer of the workpiece, and then it is gradually decreased

Fig. 17. Comparison of optical microscope images of weld toe, after fatigue test under applied stress 300 MPa. (a) Specimen with peening (specimen 13), (b) specimenwithout peening (specimen 7).

Fig. 18. Existence of notch in the weld toe of specimen without peening, afterfatigue test with applied stress 300 MPa (specimen 7).

Fig. 19. Absence of crack in weld toe of specimen with peening, after fatigue testunder applied stress 300 Mpa (specimen 14).

150

200

250

300

350

400

450

500

0 0.5 1 1.5 2 2.5

Distance from surface, mm (in weld toe until 2.5 mm depth)

Mic

ro-H

ardn

ess,

HV

Sample 1

Sample 2

Sample 4

Sample 5

Sample 7

Sample 10

Sample 11

Sample 13

Sample 16

Sample 17

Fig. 20. Comparison of the distribution of micro hardness at weld toe against depthfrom the surface of specimens: with peening (1 and 2), without peening (4, 5),without peening and with fatigue test (7, 10, and 11), with peening and fatigue test(13, 16 and 17).

Fig. 21. Obtained curve from Potentiostat output curve and tangent drawingmethod on the curve.

A. Abdullah et al. / Materials and Design 38 (2012) 7–18 15

with depth. This plastic deformation is achieved by dislocation mo-tion and that the density of dislocations increases in the process ofpeening. When the amount of plastic deformation increases, mate-rial density increases, too. Meanwhile, this process provides thegrain crushing, grain refining and the boundaries reforming. As aresult, the surface micro-hardness and strength of the welded partsincrease together.

6.2. Comparison of micro-hardness curves of weld toe for differentspecimens

Fig. 20 shows the comparison of micro-hardness variations inthe weld toe, before and after ultrasonic peening, and before and

after fatigue tests. After study of micro-hardness distributioncurves obtained from weld toe, it was seen that the hardness ofthe peened area near the surface is high due to applied cold work-ing. It was almost as twice as hardness of non-peened pieces (the

Table 6Potentiostat test results, for base metal and weld toe of specimens with peening(specimens 1 and 2) and specimens without peening (specimens 4 and 5).

Sample Icorr(A) Icorr(A/cm2) Rp(X) Ecorr(V) Corrosionrate(mm/year)

Base metal0.4294 � 10�8 2.862 � 10�8 6.709 � 10+6 �0.148 3.847 � 10�3

1 0.3726 � 10�8 2.484 � 10�8 5.173 � 10+6 �0.149 3.338 � 10�3

2 0.7411 � 10�8 4.941 � 10�8 7.301 � 10+6 �0.151 6.139 � 10�3

4 1.232 � 10�8 7.251 � 10�8 1.727 � 10+6 �0.171 32.25 � 10�3

5 1.284 � 10�8 8.024 � 10�8 1.912 � 10+6 �0.135 11.78 � 10�3

16 A. Abdullah et al. / Materials and Design 38 (2012) 7–18

hardness on the surface was as high as 450 HV and was dropping to225 HV in depth of 100 lm).

It was seen that the hardness of weld metal was more than weldtoe and the hardness of weld toe was more than the hardness ofbase metal. In addition, by applying fatigue load, no change wasobserved in the micro-hardness of non-peened specimens.

Micro-hardness behavior of peened materials is in correlationwith: (a) the grain refinement accompanied by (b) strain-inducedmartensitic, and (c) the grain boundary activities. According to re-cent works, generally, about 30–80% increase of micro-hardnesshas been reported in depth of 150 lm, but in this paper, about100% increase of micro-hardness has been measured. The hardnessof peened joints of St304 is almost as twice as hardness of non-pee-ned joints, for example, the hardness on the surface was as high as450 HV and was dropping to 225 HV in depth of 100 lm. Finally, itis very important that amount of micro-harness for a specific test isaffected by so many factors and conditions, for example, vibrationamplitude and frequency, temperature, material properties andmicro-structures, peening time are crucial factors in this regard.

Fig. 22. Final comparison of polarization curves of different specimens obtainedfrom Potentiostat system, (for base metal of Stainless Steel 304; 1 and 2- weld toe ofspecimens with peening; and 4 and 5- weld toe of specimens without peening), inNaCl3.5% solution.

7. Evaluation of chemical resistance and corrosion rate ofspecimens

To investigate the ultrasonic peening effect on behavior,potential and corrosion resistance of welded joints and to deter-mine corrosion rate in peened zones, the peened and non-peenedspecimens were cut from the test pieces. After that they were pol-ished by emery cloth of different mesh sizes up to 3000 to thedepth of 400 lm and after final polishing and finishing, copperwire was soldered to each specimen. After determining an areaon the surface of each specimen which includes weld metal, weldtoe and base metal; the rest of the surface was insulated by Bizouxliquid (a combination of Bizoux powder and Klavny liquid with 3 to1 ratio). Standard ASTM G5-87 [29] was used for all preparatoryworks and drawing the polarization curves. For this purpose, atfirst, NaCl 3.5% was prepared using Merk salt and distilled water.Before starting the tests, the specimens were immersed in the pre-pared NaCl for 25 min for stability. Then, in a three-electrode cell[comprising work electrode (specimen), auxiliary electrode (Plati-num) and reference electrode (Silver–Silver Chloride1)], the con-nections were made by copper wires. Using the Potentiostatsystem, model PG Stat12 made by AutoLab of Netherland, first theopen loop potential of nine specimens was measured. Then, on thebasis of this potential, the measuring range for corrosion potentialsin cathode and anode were determined, too. After performing thetests, Potentiostat system calculated the rate of corrosion using TA-FEL extrapolation and linear polarization methods. Based on stan-dard ASTM G102-89 [30], Fig. 21 shows a sample of polarizationcurve obtained from experimental test. It also shows method of tan-gent drawing onto the curve for determining the corrosion rate bysystem software.

7.1. Evaluation of corrosion test results

Preliminary polarization curves in the form of logarithm of cor-rosion current density against electrical potential have been shownin Fig. 22. The corrosion current density (Icorr) and corrosion poten-tial (Ecorr) of each specimen was determined through the polariza-tion curves. Table 6 summarizes the results of the tests.

According to Faraday’s law and graphical analysis of polariza-tion curves, higher corrosion potential is equivalent to higher cor-rosion resistance and lower corrosion current density means lowercorrosion rate.

1 Ag/AgCl.

Corrosion current density (Icorr) and polarization resistance (Rp)are good parameters for comparisons. According to the results ofTable 6, polarization resistance of peened pieces (specimen 1 and2) is higher and corrosion rate of them is lower than specimenswithout processing by peening (specimen 4 and 5). Corrosion rateof peened area is near to the base metal.

In ultrasonically peened specimen, the corrosion current den-sity decreased, and the corrosion potential increased. The smalleramounts of the corrosion current density indicate a higher corro-sion resistance. In fact, the more positive values of the corrosionpotential are seen, the higher corrosion resistance is achieved.Therefore, an ultrasonically peened specimen shows the best cor-rosion resistance than another as-received specimen.

After evaluating and comparing the corrosion results with othertechnical reports, a better corrosion resistance was achieved forultrasonically peened welded joints. In fact, because the non-pee-ned welded joints relatively include more residual stresses (highlevel of energy), their corrosion resistance will be higher than pee-ned welded parts. After processing by ultrasonic peening, a layer ofplastic cold work is formed. This compressed layer has not any de-fect like micro cracks and voids, and also has a better surface qual-ity; therefore, peened surfaces are more resilient against corrosivematerials. By applying ultrasonic peening, about 100% improve-ment was seen in corrosion resistanc improvement of St304 thathas not been reported till now.

8. Conclusions

In this section, the main conclusions of the fatigue, metallogra-phy, micro-hardness and corrosion results have been resented.

A. Abdullah et al. / Materials and Design 38 (2012) 7–18 17

8.1. Fatigue tests

For fatigue test results, the main results are:

� Tensile residual stress is normally a serious cause of fatiguestrength reduction in weld toe. On the contrary compressiveresidual stress can lead to increased fatigue strength of weldedjoints.� Ultrasonic peening leads to geometrical modification of weld

toe and reduction of stress concentration in this zone. In addi-tion, it closes cracks and voids. Normally, ultrasonic vibrationsrelieve residual tensile stresses and peening action inducescompressive residual stresses. Consequently, all these contrib-ute to increased fatigue strength.� By using ultrasonic peening on welded stainless steel 304, fati-

gue life was increased by 120% in 300 MPa peak to peakdynamic loading and the rate of increase was 70% in 330 MPadynamic loading range. At 400 k-cycles, the dynamic loadingrange was increased from 225 MPa to 290 MPa (29% improve-ment in fatigue strength) when ultrasonic peening was used.� Fatigue failure of pieces without peening occurred in weld toe

whilst fatigue failure of parts with ultrasonic peening occurredin base metal about 12 mm far from weld toe (except for330 MPa dynamic loading range which was occurring in weldtoe).� Avoidance of a long term plan for extensive hot work, avoidance

of long and unscheduled operational disruptions, increasedstructural safety for the installation during the remaining ser-vice life are the economical benefits due to reduced mainte-nance as a result of the ultrasonic peening treatment.� Application of ultrasonic peening for fatigue life extension has

two main grounds. First, for some treated welded joints, ultra-sonic peening treatment are the only solution left to enhancethe fatigue life since no structural modification option is possi-ble. Second, for other welded joints, this process was donebefore any problem. Indeed, it is an easy and low-cost solutionto improve weld properties rather than waiting for a crack andthen peen it.

8.2. Metallography tests

For metallography test results, the main results are:

� There was no detectable difference in structure of the piecesbefore and after ultrasonic peening, with and without fatiguetests on the front and back surfaces of the specimens.� In images taken by optical and electron microscopes, micro-

superficial cracks were observed in base metal and weld toe,before ultrasonic peening. However, after ultrasonic peening,cracks of weld toe were eliminated and weld toe curvaturewas modified.� Detectable depth of penetration of ultrasonic peening was up to

100 lm and the width was up to 1000 lm.� In the specimens affected by fatigue test, extensive crack prop-

agation was observed when no peening was used. However, inspecimens with peening there was no trace of cracks even afterfatigue test.� After ultrasonic peening treatment, nano sized grains, multi –

directional twins and strain – induced martensite will beformed in surface layers. An ultrasonically peened specimenhas more strain – induced martensite than the other ones,because it receives more plastic strain energy.

8.3. Micro-hardness tests

For micro-hardness test results, the main results are:

� Hardness of weld metal is more than weld toe and hardness ofweld toe is more than base metal.� By applying ultrasonic impacts on weld toe, superficial hardness

was increased and the amount of hardness in surface was 450HV. This was falling to 225 HV in depth of 100 lm from the freesurface.� After fatigue test, there is no detectable change in hardness of

weld toe in non-peened specimens.� The generation of mechanical twins and strain- induced mar-

tensite give rise to increase in the hardness of the surface layers.

8.4. Corrosion tests

For corrosion resistance, the main results are:

� Corrosion rate of weld toe of specimens without peening wasmore than base metal and the corrosion of specimens withpeening was near to base metal.� Regarding the results obtained from electrochemical polariza-

tion tests of the weld toe (by using Potentiostat system andNaCl 3.5% solution), the mean electrical resistance and corro-sion rate for specimens without peening were 1.8 � 10+6 Xand 0.022 mm/yr respectively and for specimens with peeningthese were 6.4 � 10+6 X and 0.0044 mm/yr respectively. Theseindicate the increased corrosion resistance of peened pieces.� The ultrasonically peened specimens showed equal or even bet-

ter corrosion resistance. Specimens having more strain- inducedmartensite, showed general and localized corrosion resistancesthan as-received specimens, irrespective of galvanic effect.

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