Welding Residual Stress Analysis and Fatigue Strength Assessment of Multi-Pass Dissimilar Material Welded Joint Between Alloy 617 and 12Cr Steel

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Article Welding Residual Stress Analysis and Fatigue Strength Assessment of Multi-Pass Dissimilar Material Welded Joint between Alloy 617 and 12Cr Steel

Haﬁz Waqar Ahmad * ID , Jeong Ho Hwang, Ju Hwa Lee and Dong Ho Bae * Graduate School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Korea; reﬂ[email protected] (J.H.H.); [email protected] (J.H.L.) * Correspondence: [email protected] (H.W.A.); [email protected] (D.H.B.); Tel.: +82-31-290-7443 (D.H.B.)

Received: 30 November 2017; Accepted: 27 December 2017; Published: 31 December 2017

Abstract: The reliability of welded structure can be evaluated through welding residual stress analysis and fatigue strength assessment. In this study, welding residual stresses of multi-pass dissimilar material welded joint between alloy 617 and 12Cr steel were analyzed numerically and experimentally. Fatigue strength was then assessed in the air. Based on results of welding residual stress analysis and fatigue strength assessment, a fatigue design method considering welding residual stress was investigated. Welding residual stresses at the weld of dissimilar welded joints distributed complicatedly on longitudinal and transverse directions, showing differences but a very similar distribution tendency between numerical and experimental results. Numerical and experimental peak values of welding residual stresses at HAZ of the weld on the 12Cr steel side were predicted to be 333 MPa and 282 MPa HAZ, respectively. The fatigue limit of dissimilar material welded joint between alloy 617 and 12Cr steel was assessed to be 306.8 MPa, which was 40% of tensile strength (767 MPa) of dissimilar material welded joint. However, the stress range including welding residual stress was assessed to be 206.9 MPa, which was 14% lower than that calculated by including the effect of residual stresses.

Keywords: dissimilar material welding; welding residual stress; ﬁnite element method; fatigue strength; modiﬁed Goodman equation; fatigue design; alloy 617; 12Cr steel

1. Introduction It is well-known that the most effective methodology for green power plant systems is to increase the generative efficiency of steam power plants [1]. The core technology for increasing the generative efficiency is to improve the performance of steam turbines. A basic method for improving the performance of thermal power plants is to elevate steam temperature [2]. Therefore, it is necessary to develop suitable materials for extreme environment of power plants. So far, Ni alloys have been candidates as available materials for high temperature steam power plants due to their incredible mechanical properties [3,4]. However, in order to apply Ni alloys (alloy 617 in this paper) to steam turbine materials such as turbine rotor and blades, it is necessary to do dissimilar material welding with the 12Cr steel alloy that is currently used as material for turbine rotor and blades. However, since Inconel alloy 617 and 12Cr steel alloy have different chemical compositions and mechanical properties, reliable welding technology needs to be developed to manufacture hybrid structures that can withstand extreme environmental conditions [4,5]. There are many suitable processes for the joining of dissimilar material as comprehensively summarized by Martinsen et al. [6]. These dissimilar material welded structures often experience variable loadings, ranging from cyclic to completely random fluctuation during their practical working.

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Therefore, engineering designers should sufficiently understand the welding metallurgical phenomena, as well as the mechanical properties of the weld, including the mechanism of welding residual stresses generated during the welding process. In particular, as the generation of welding residual stresses is formidable, as well as being the crack driving force of the welded structures, it is very important to understand and assess welding residual stresses in the design process of welded structures. Basically, as welding residual stresses are generated due to repetitive nonlinear thermal loading cycles by the welding heat input, assessing the distribution of these stresses may be too complex to be predicted accurately [7,8]. Residual stresses have a substantial effect on engineering properties of material and welded structures, particularly fatigue strength, corrosion resistance, dimensional stability, and distortion [9]. Many researchers have practiced various methods to measure residual stresses using experimental and numerical methods [10–12]. In recent years, as high computational power is available, many researchers have acknowledged the reliability of using the finite element method for this purpose [13,14]. In the present study, the hole-drilling method and the finite element method were used to measure welding residual stresses. Fatigue is considered the most common cause of failure in welded structures [15]. Dissimilar material welded joints are very sensitive to fatigue stress [16]. Therefore, fatigue testing with varying limits of high stresses can provide very useful information to assess the quality of welded joints [15]. Bae et al. have proposed a σa-r stress range including welding residual stresses by using a modified Goodman equation [17]. Their results showed that fatigue strength, including welding residual stresses, was lower than fatigue strength without considering residual stresses. The objective of the present study was to analyze the welding residual stresses of the multi-pass dissimilar material welded joint between alloy 617 and 12Cr steel, numerically and experimentally. Fatigue strength was then assessed in the air.

2. Materials and Methods Materials used for dissimilar material welding included alloy 617 and 12Cr steel alloy. Alloy 617 has good resistance against high temperature and corrosion as it consists of Cr, Mo, and Co as major compositions, while 12Cr steel is being currently used as a material of turbine rotor and blade of steam power plant. Filler material used in this study was Thyssen 617. Its chemical composition is similar to alloy 617. Mechanical properties and chemical compositions of alloy 617, 12Cr steel, and Thyssen 617 are illustrated in Tables1 and2. Multi-pass direct current straight polarity (DCSP) tungsten inert gas (TIG) welding was carried out for multi-pass dissimilar material welding.

Table 1. Mechanical properties of alloy 617, 12Cr steel, and dissimilar material weld.

Yield Strenght Tensile Strenght Reduction in Melting Point Base Material Elongation (MPa) (MPA) Area (%) (◦C) Alloy 617 322 732 62 56 1330 12Cr 551 758 18 50 1375 Dissimilar material 490 767 48 - - welded joint

Table 2. Chemical composition of alloy 617, Thyssen 617, and 12Cr steel.

Chemical Composition (Weight %) Base/Filler Metal Ni Cr Co Mo Al C Fe Si Ti Cu Mn S Alloy 617 44.3 22 12.5 9.0 1.2 0.07 1.5 0.5 0.3 0.2 0.5 0.008 12Cr 0.43 11.6 - 0.04 - 0.13 Bal. 0.4 - 0.1 0.58 - Thyssen 617 45.7 21.5 11.0 9.0 1.0 0.05 1.0 0.1 1 - - - Metals 2018, 8, 21 3 of 11

Metals 2018, 8, 21 3 of 12 Real time monitoring system was used to control welding conditions (electrode shape, electrode to Real time monitoring system was used to control welding conditions (electrode shape, electrode workpiece distance, welding wave form, and welding heat input). Preliminary welding was performed to workpiece distance, welding wave form, and welding heat input). Preliminary welding was under various welding parameters (i.e., current, voltage, welding speed, shield gas compositions, and performed under various welding parameters (i.e., current, voltage, welding speed, shield gas ﬂow rates) to determine optimum dissimilar material welding conditions. Obtained optimum welding compositions, and flow rates) to determine optimum dissimilar material welding conditions. conditions for dissimilar material are listed in Table3[18]. Obtained optimum welding conditions for dissimilar material are listed in Table 3 [18]. Table 3. Optimum welding conditions. Table 3. Optimum welding conditions.

PassPass ShieldShield Gas Gas VoltageVoltage (V) (V) Current (A)(A) Welding Welding Speed Speed (cm (cm/min)/min) Heat Heat input Input (kJ/ (kJ/mm)mm) 11 Argon-2.5% Argon-2.5% H2 H2 1010 150 150 10 10 0.9 0.9 22 Argon-2.5% Argon-2.5% H2 H2 1313 150 150 10 10 1.17 1.17 33 Argon-2.5% Argon-2.5% H2 H2 1616 150 150 10 10 1.44 1.44 4 Argon-2.5% H2 16 150 10 1.44 4 Argon-2.5% H2 16 150 10 1.44 5 Argon-2.5% H2 16 150 10 1.44 5 Argon-2.5% H2 16 150 10 1.44 6 Argon-2.5% H2 16 150 10 1.44 6 Argon-2.5% H2 16 150 10 1.44 7 Argon-2.5% H2 16 150 10 1.44 7 Argon-2.5% H2 16 150 10 1.44

SinceSince thethe out of plane plane thermal thermal distortion distortion is is formidably formidably caused caused by by welding welding heat heat input input during during the themulti-pass multi-pass welding welding process, process, welding welding jigs were jigs wereused usedto fix toboth fix ends both of ends base of metals base metalsto prevent to prevent out-of- out-of-planeplane thermal thermal distortion. distortion. Multi-pass Multi-pass welding welding directions directions were wereparallel parallel to the to therolling rolling direction. direction. U- U-groovegroove shape shape was was machined machined for for narrow-gap narrow-gap welding welding [19]. [19 U-groove]. U-groove is economical, is economical, as it as requires it requires less lessfiller filler metal, metal, which which results results in less in distortion less distortion and re andsidual residual stress-related stress-related problems problems [20]. Figure [20]. Figure1 shows1 showsa welding a welding specimen specimen with U-groove with U-groove and narrow and narrowgap. In the gap. multi-pass In the multi-pass welding process, welding the process, specimen the ◦ specimenwas cooled was down cooled to be down under to 70 be °C under in the 70 intervalC in the of interval every welding of every process. welding After process. finishing After each finishing pass eachwelding, pass welding,the ultrasonic the ultrasonic testing was testing carried was carriedout following out following ASTM ASTME164-13 E164-13 standard standard to inspect to inspect weld welddefects defects of multi-pass of multi-pass welds. welds.

FigureFigure 1.1. SpecimenSpecimen withwith U-grooveU-groove (Unit:(Unit: mm).mm).

3.3. ResidualResidual StressStress AnalysisAnalysis ofof Multi-PassMulti-Pass DissimilarDissimilar MaterialMaterial WeldedWelded JointJoint between between Alloy Alloy 617 617 andand 12Cr12Cr SteelSteel

3.1.3.1. NumericalNumerical AnalysisAnalysis ofof WeldingWelding ResidualResidual StressesStresses AA seven-passseven-pass dissimilar dissimilar material material welded welded plate plate model model shown shown in Figure in Figure2 was used 2 was in thisused analysis. in this Itsanalysis. thickness, Its thickness, width, and width, weld lengthand weld were length 12.7, 300,were and 12.7, 300 300, mm, and respectively. 300 mm, respectively. Three-dimensional Three- ﬁnitedimensional element finite models element were usedmodels for were both thermalused for andboth mechanical thermal and analyses mechanical with aanalyses heat source with model. a heat Insource this study,model. ramp In this heat study, source ramp model heat was source used. model For welding, was used. 8-node For brickwelding, solid 8-node elements brick of typesolid DC3D8elements were of type used. DC3D8 Weld were beads used. of the Weld model beads represented of the model the weldrepresented bead determined the weld bead from determined the actual weldfrom condition.the actual Finerweld meshescondition. were Finer generated meshes in were weld metalgenerated regions in weld to handle metal the regions greater to nonlinearity handle the andgreater to obtain nonlinearity accurate and results. to obtain The accurate total number result ofs. elementsThe total andnumber nodes of wereelements 221,760 and and nodes 474,012, were respectively.221,760 and 474,012, Hyper-mesh respectively.® (10.0, Altair, Hyper-mesh Troy, MI,® (10.0, USA, Altair, 2008) was Troy, used MI, for USA, mesh 2008) generation. was used ABAQUS for mesh (6.10,generation. Dassault ABAQUS Systemes, (6.10, Johnston, Dassault RI,Systemes, USA, 2010) Johns waston, RI, employed USA, 2010) for transientwas employed temperature for transient and subsequenttemperature welding and subsequent residual stresswelding analysis. residual stress analysis.

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Figure 2. Finite element mesh of 3-D model. Figure 2. Finite element mesh of 3-D model. 3.1.1. Numerical Analysis Procedure 3.1.1. Numerical Analysis Procedure Uncoupled thermo-mechanical analysis was performed in this study as shown in Figure 3 [21]. Uncoupled thermo-mechanical analysis was performed in this study as shown in Figure3[ 21]. Temperature distribution at the weld was analyzed first by thermal analysis. It was then used as Temperature distribution at the weld was analyzed first by thermal analysis. It was then used input data for stress analysis. Some assumptions were taken into account in the process of thermal- as input data for stress analysis. Some assumptions were taken into account in the process of mechanical analysis. When performing the thermal analysis, the initial temperature of base and weld thermal-mechanical analysis. When performing the thermal analysis, the initial temperature of metal was assumed to be a room temperature (25 °C). The conductivity and specific heat in the molten base and weld metal was assumed to be a room temperature (25 ◦C). The conductivity and specific pool were assumed to be constant.. RadiationRadiation andand forcedforced convectionconvection duedue toto shieldingshielding gasgas flowflow werewere heat in the molten pool were assumed to be constant. Radiation and forced convection due neglected. Since the temperature is very high in thethe moltenmolten pool,pool, radiationradiation andand convectionconvection cancan to shielding gas flow were neglected. Since the temperature is very high in the molten pool, influence microstructures and cooling rates in weld metal. Heat losses or gains from phase radiation and convection can influence microstructures and cooling rates in weld metal. Heat transformation were neglected. The volume change effect due to phase transformation was neglected losses or gains from phase transformation were neglected. The volume change effect due to phase in the stress analysis. Apart from these assumptions mentioned above, in thermal-mechanical transformation was neglected in the stress analysis. Apart from these assumptions mentioned above, in analyses for dissimilar material welded model, influences of transient temperature field and thermal-mechanical analyses for dissimilar material welded model, influences of transient temperature temperature-dependent physical properties were considered as input data for accurate and reliable field and temperature-dependent physical properties were considered as input data for accurate assessment of thermal analysis. When performing the stress analysis, mechanical and physical and reliable assessment of thermal analysis. When performing the stress analysis, mechanical and properties of yield stress, elastic and plastic modules, and thermal expansion coefficient were physical properties of yield stress, elastic and plastic modules, and thermal expansion coefficient were considered to be temperature dependent. However, mechanical properties were assumed to be considered to be temperature dependent. However, mechanical properties were assumed to be constant constant above the melting point. The temperature dependency of Poisson’s ratio was neglected. above the melting point. The temperature dependency of Poisson’s ratio was neglected. Physical and Physical and mechanical properties of Alloy 617 and 12Cr steel are given in Figures 4 and 5, mechanical properties of Alloy 617 and 12Cr steel are given in Figures4 and5, respectively [22,23]. respectively [22,23].

Figure 3. NumericalNumerical analysis analysis procedure for the unco uncoupledupled thermo-mechanical analysis [[21].21].

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Figure 4. Physical and mechanical properties of Alloy 617. FigureFigure 4. 4.Physical Physical and and mechanicalmechanical properties of of Alloy Alloy 617. 617.

Figure 4. Physical and mechanical properties of Alloy 617.

Figure 5. Physical and mechanical properties of 12Cr steel. Figure 5. Physical and mechanical properties of 12Cr steel. Figure 5. Physical and mechanical properties of 12Cr steel. The element rebirth technique [24] was applied to simulate the multi-pass weld metal deposition effect.The With element this rebirth technique,Figure technique elements5. Physical [24] was andsimulati applied mechanicalng toeach simulateproperties weld the passof 12Crmulti-pass were steel. grouped weld metal at the deposition model effect.Thegeneration element With stage.this rebirth technique,During technique analys elements [is,24 ]these was simulati appliedelementng togroupseach simulate weld that thepasspresented multi-pass were weldgrouped weld passes metalat werethe depositionmodel first effect.generationremoved WithThe this elementand stage. technique, then rebirthDuring reactivated elements technique analys at ais, simulatingspecified [24] these was element appliedmoment each to weldgroupsto simulate simulate pass that the werea prgiven multi-passesented grouped deposition weld weld at the sequencepassesmetal model deposition were of generation weld first stage.removedeffect.passes. During WithWhen and analysis, thenthis a group reactivatedtechnique, these of weld element atelementselements a specified groups weresimulati moment that activateng presented eachtod, simulatespecific weld weld initial passa given passes temperatureswere deposition were grouped first weresequence removedat imposedthe ofmodel andweld at then reactivatedpasses.generationall nodes When at associated a stage. specified a group During with of moment weld the analys weld elementsis, to elements. simulatethese were element activate a given groupsd, specific deposition that initial presented sequence temperatures weld of passes weld were passes.imposedwere first Whenat removed and then reactivated at a specified moment to simulate a given deposition sequence of weld a groupall nodes of weld associated elements with werethe weld activated, elements. specific initial temperatures were imposed at all nodes passes.3.1.2. Heat When Source a group Model of weld for Numerical elements wereAnalysis activate d, specific initial temperatures were imposed at associatedall nodes with associated the weld with elements. the weld elements. 3.1.2. HeatIn this Source study, Model the heat for Numericalinput model Analysis using ramp function as shown in Figure 6 based on the 3.1.2.3.1.2.concept HeatIn Heat Sourcethis of Pavelic’sSourcestudy, Model Modelthe disc forheat model for Numerical input Numerical was model employed Analysis Analysis using [25,26]. ramp function as shown in Figure 6 based on the concept of Pavelic’s disc model was employed [25,26]. In thisIn study,this study, the heat the inputheat input model model using using ramp ramp function function as shown as shown in Figure in Figure6 based 6 based on the on conceptthe of Pavelic’sconcept disc of Pavelic’s model wasdisc model employed was employed [25,26]. [25,26].

Figure 6. Heat input model with ramp function.

The ramp source heat inputFigure model 6. Heat for input simulation model with was ramp used function. for analysis. Heat flux is expressed in the following equation.Figure Figure 6. 6.Heat Heat input input modelmodel with ramp ramp function. function. The ramp source heat input model for simulation was used for analysis. Heat flux is expressed in the Thefollowing ramp sourceequation. heat input model for simulation was used for analysis. Heat flux is expressed The ramp source heat input model for simulation was used for analysis. Heat ﬂux is expressed in in the following equation. the following equation. Metals 2018, 8, 21 6 of 11

2 q(x) = q(0)e−Cx (1) q(0): maximum heat flux at the center of heating spot x: distance from the center of heating spot C: heat flux concentration coefficient Metals 2018, 8, 21 6 of 12 In this case, the total amount of heat input is indicated as the combination of surface heat input and body heat input as shown in Equation (2):() =(0) (1) q(0): maximum heat flux at the center of heating spot x: distance from the center of heating spot Q = Qs + Qb (2) C: heat flux concentration coefficient where In this case, the total amount of heat input is indicated as the combination of surface heat input Q: totaland heat body input heat input as shown in Equation (2):

QS: surface heat input = + (2) Qb: bodywhere heat input

TheQ: energy total heat input input per unit length (H, J/mm) can be shown as Equation (3): QS: surface heat input Qb: body heat input 1 H = Q × (3) The energy input per unit length (H, J/mm) can be vshown as Equation (3): 1 v: welding speed (mm/s) =× (3) 3 2 Body heat ﬂux (qb, W/mm ) and surface heat ﬂux (qs, W/mm ) are indicated in Equations (4) v: welding speed (mm/s) and (5): 3 2 Body heat flux (, W/mm ) and surface heat flux (, W/mm ) are indicated in Equations (4) and Qb (5): qb = (4) Ve r= (4) Qs 3 −3 x 2 q = e b (5) s bL π 3 = (5) where b and L are geometrical parameters of weld bead [25].

3.2. Experimentalwhere b and Analysis L are geometrical of Welding parameters Residual of Analysis weld bead [25].

Experimental3.2. Experimental analysis Analysis was of Welding performed Residual to Analysis calculate welding residual stresses and evaluate results of numericalExperimental analysis. Inanalysis this study,was performed a hole drillingto calculate method welding was residual used stresses according and evaluate to ASTM results E837 [27]. RS-200of milling numerical guide analysis. equipment In this study, shown a hole in drilling Figure method7a and was strain used rosette according (Micro to ASTM measurement E837 [27]. Co., EA-06-062RE-120RS-200 milling type, guide Kyunggi, equipment Korea) shown werein Figure used 7a and to measure strain rosette residual (Micro stresses. measurement We analyzedCo., EA- three points (5,06-062RE-120 20 and 35 mm)type, Kyunggi, apart from Korea) the weldwere center,used to respectively,measure residual on bothstresses. sides We as analyzed shown inthree Figure 7b. Surfacespoints of these (5, 20 specimens and 35 mm) fabricatedapart from the by weld the actualcenter, respectively, dissimilar materialon both sides welding as shown between in Figure alloy 617 7b. Surfaces of these specimens fabricated by the actual dissimilar material welding between alloy and 12Cr steel were carefully polished to reduce surface roughness error. 617 and 12Cr steel were carefully polished to reduce surface roughness error.

Figure 7. (a) RS-200 milling guide, (b) position of hole drilling. Figure 7. (a) RS-200 milling guide, (b) position of hole drilling.

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3.3. Results and Discussion The fundamental goal of this study was to developdevelop a reliable FEA model capable of predicting thermal history and residualresidual stressesstresses ofof dissimilardissimilar materialmaterial weldweld withwith higherhigher accuracy.accuracy. Figure 88 shows comparative results obtained from the FEA deve developedloped in this this study study and and experimental experimental analysis. analysis. Both resultsresults showed showed quantitatively quantitatively differences. differences. However, However, qualitatively, qualitatively, they showed they similarshowed tendency. similar Quantitativetendency. Quantitative difference difference was due was to the due fact to the that fa temperature-dependentct that temperature-dependent physical physical properties properties were notwere taken not intotaken account into account or neglected or neglected by some by assumptions. some assumptions. Welding Welding residual stressesresidual were stresses predicted were highlypredicted at HAZ highly on at both HAZ sides on both of the sides weld. of Residualthe weld. stresses Residual at stresses the HAZ at on the the HAZ side on of the 12Cr side steel of alloy12Cr weresteel alloy higher were than higher those than of the those alloy of 617 the side.alloy This617 side is due. This to theis due fact to that the Alloyfact that 617 Alloy has higher 617 has thermal higher expansionthermal expansion coefﬁcient coefficient than 12Cr than steel, 12Cr while steel, thermal while thermal conductivity conductivity of alloy of 617 alloy is lower 617 is than lower that than of 12Crthat of steel. 12Cr steel. In longitudinal direction (along the welding direction), numerical peak stress value on the 12Cr steel alloy side was predicted to bebe 333333 MPa.MPa. However,However, the experimental result showed a value of around 282282 MPa. MPa. In In transverse transverse direction direction (perpendicular (perpendicular to the to weldingthe welding direction), direction), numerical numerical peak stresspeak valuestress onvalue the 12Cron the steel 12Cr alloy steel side alloy was predictedside was topredicted be around to 306be MPa.around However, 306 MPa. the However, experimental the resultexperimental showed result a value showed of around a value 268 of MPa. around 268 MPa.

Figure 8. Comparison of FE analysis resultsresults with experimental measurements.

4. Fatigue Strength Assessment of Multi-Pass Dissimilar Material Welded Joint 4. Fatigue Strength Assessment of Multi-Pass Dissimilar Material Welded Joint

4.1. Specimen and Test ProcedureProcedure Tensile and fatigue fatigue test test specimen specimenss of of the the dissimilar dissimilar material material welded welded joint joint between between Alloy Alloy 617 and 617 and12Cr 12Crsteel steelalloy alloywere fabricated were fabricated from seven-pass from seven-pass welded welded plate in plateaccordance in accordance with ASTM-E8 with ASTM-E8 standard standard[28]. Figure [28 9]. shows Figure test9 shows specimen test specimenconfiguration. conﬁguration. Gauge length Gauge of the length specimen of the included specimen base included metals base(Alloy metals 617 and (Alloy 12Cr 617 steel), and their 12Cr HAZ, steel), and their weld HAZ, me andtal. Tensile weld metal. and fatigue Tensile strengths and fatigue of dissimilar strengths ofmaterial dissimilar welded material joint were welded evaluated joint were by using evaluated a material by using testing a materialsystem (INSTRON testing system 8801, (INSTRONLLC, Seoul 8801,Korea, LLC, 100 kN). Seoul Loading Korea, 100speed kN). for Loading tensile test speed was for controlled tensiletest by displacement was controlled of by1 mm/min. displacement Fatigue of tests were started from σmax = 690 MPa, corresponding to 90% of the static tensile strength (767 MPa) 1 mm/min. Fatigue tests were started from σmax = 690 MPa, corresponding to 90% of the static tensileof a multi-pass strength (767dissimilar MPa) ofmaterial a multi-pass weld dissimilarbetween Alloy material 617 weldand 12Cr between steel. Alloy Tests 617 were and performed 12Cr steel. Testsusing werea 10% performed load decreasing using a 10%method load until decreasing 106 cycles method as illustrated until 106 cycles in Table as illustrated 4. The fatigue in Table load4. frequency was 10 Hz in sine wave. Load ratio (R = Pmin/Pmax) was 0.1. The load range was constant.

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The fatigue load frequency was 10 Hz in sine wave. Load ratio (R = Pmin/Pmax) was 0.1. The load rangeMetals 2018 was, 8 constant., 21 8 of 12

Figure 9. ConfigurationConﬁguration of test specimen (ASTM E8M, unit:unit: mm).

Table 4.4. Fatigue test conditions.

Load Conditions Load Conditions σmax (MPa) 0.9σu = 690.3 0.8σu = 613.6 0.7σu = 536.9 0.6σu = 460.2 0.5σu = 383.5 0.4σu = 306.8 σmax (MPa)Stress Ratio0.9 σu = 690.3 0.8σu = 613.6 0.7σu = 536.9 R = 0.1 0.6σu = 460.2 0.5σu = 383.5 0.4σu = 306.8 FrEquation 10 Hz Stress Ratio R = 0.1

4.2.FrEquation Calculation of Stress Range Considering Welding Residual 10 Hz Stresses

4.2. CalculationFor the appraisal of Stress of Range stress Considering range, a Weldingtechnique Residual was published Stresses by Bae et al. that incorporated welding residual stresses [17]. They used modified Goodman equation to calculate stress range by For the appraisal of stress range, a technique was published by Bae et al. that incorporated taking welding residual stresses into account. Maximum stress (σmax) values were taken from Table welding residual stresses [17]. They used modiﬁed Goodman equation to calculate stress range by 4. Minimum stress (σmin) was calculated using Equation (6): taking welding residual stresses into account. Maximum stress (σmax) values were taken from Table4. σ Minimum stress (σmin) was calculated using EquationR= (6): (6) σ σ Equation (7) represents the Goodman Requation= min used to measure the stress range without(6) σ considering welding residual stresses max

Equation (7) represents the Goodmanσ equationσ used to measure the stress range without + =1 considering welding residual stresses (7) σ σ a + mean = 1 (7) where Se Su where σ(stress amplitude) = ; σmax − σmin σa(stress amplitude) = ; and 2 and σ +σ σmax + σmin σσ(mean(mean stress stress)) = = mean 2

Se:: fatigue fatigue strength strength Su:: ultimate ultimate strength strength The proposedproposed modiﬁedmodified Goodman Goodman equation equation taking taking into into account account the the welding welding residual residual stresses stresses can becan represented be represented as Equation as Equation (8): (8): σa−r (σmean + σr) σ+ (σ +σ) = 1 (8) Se + Su =1 (8) where σa-r is the stress taking into account the welding residual stresses. It can be calculated by using Equationwhere σa-r (9): is the stress taking into account the welding residual stresses. It can be calculated by using Equation (9): (σmean + σr) σ − = S 1 − (9) a r e S (σu +σ) σ = 1 − (9) 4.3. Results and Discussion σ 4.3. ResultsFigure and10 showsDiscussion the relationship between the stress ( ) and the fatigue life (Nf) of a dissimilar material multi-pass welded joint between alloy 617 and 12Cr steel. Fatigue tested specimens mostly Figure 10 shows the relationship between the stress (σ) and the fatigue life (Nf) of a dissimilar material multi-pass welded joint between alloy 617 and 12Cr steel. Fatigue tested specimens mostly failed at HAZ of 12Cr steel alloy as shown in Figure 11, even though welding residual stresses were reduced during machining for specimen fabrication. This might be due to higher welding residual

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Metalsfailed 2018 at,HAZ 8, 21 of 12Cr steel alloy as shown in Figure 11, even though welding residual stresses9 of 12 wereMetals 2018 reduced, 8, 21 during machining for specimen fabrication. This might be due to higher welding9 of 12 stressresidual distribution stress distribution as compared as compared to those to thoseof allo ofy alloy 617, 617, and and the the metallurgical metallurgical microstructure microstructure and stress distribution as compared to those of alloy 617, and the metallurgical microstructure and composition changes during multi-pass welding heat cycle [[18].18]. composition changes during multi-pass welding heat cycle [18].

Figure 10. S-N curve for fatigue test result. Figure 10. S-N curve for fatigue test result.

Figure 11. The fatigue tested specimen (failed at HAZ of 12Cr steel alloy). Figure 11. The fatigue tested specimen (failed at HAZ of 12Cr steel alloy).

From Figure 10, the low fatigue limit that was the stress value did not fail until 106 cycles of a From Figure 1010,, the low fatiguefatigue limit that was thethe stressstress valuevalue diddid notnot failfail untiluntil 10106 cycles of a dissimilar material multi-pass welded joint between Alloy 617 and 12Cr steel alloy, σ L, were were assessed assessed dissimilar material multi-pass welded joint between Alloy 617 and 12Cr steel alloy, σLL, were assessed to be around 306.8 MPa. This was 40% of the static tensile strength (767 MPa) of the dissimilar to bebe aroundaround 306.8 306.8 MPa. MPa. This This was was 40% 40% of the of staticthe static tensile tensile strength strength (767 MPa) (767 of MPa) the dissimilar of the dissimilar material. material. Since welding residual stresses commonly become crack driving factors in the welded Sincematerial. welding Since residual welding stresses residual commonly stresses becomecommonly crack become driving crack factors driving in the factors welded in structure, the welded it is structure, it is very important to take into account welding residual stresses for safe design of welded verystructure, important it is very to takeimportant into account to take weldinginto account residual welding stresses residual for safestresses design for safe of welded design structures.of welded structures. However, as mentioned above, since welding residual stresses were relieved during However,structures. asHowever, mentioned as above,mentioned since above, welding since residual welding stresses residual were stresses relieved were during relieved fabrication during of fabrication of fatigue specimen, it is difficult to assess the effect of welding residual stress on fatigue fatiguefabrication specimen, of fatigue it isspecimen, difﬁcult it to is assess difficult the to effect assess of the welding effect of residual welding stress residual on fatiguestress on strength fatigue strength from Figure 12. In this study, stress range including welding residual stresses was calculated fromstrength Figure from 12 Figure. In this 12. In study, this study, stress stress range ra includingnge including welding welding residual residual stresses stresses was was calculated calculated by by modified Goodman equation represented by Equation (9). Figure 12 shows comparison of stress modiﬁedby modified Goodman Goodman equation equation represented represented by Equation by Equati (9).on Figure (9). Figure 12 shows 12 shows comparison comparison of stress of ranges stress ranges including and not including the welding residual stress. Stress rangesσ (σa) that do not include includingranges including and not and including not including the welding the welding residual residual stress. stress. Stress Stress ranges ranges ( a) that(σa) that do not do includenot include the the welding residual stresses are 14% higher than those that include the effect of welding residual weldingthe welding residual residual stresses stresses are 14% are higher14% higher than those than thatthose include that include the effect the of effect welding of welding residual residual stresses. stresses. The low stress range limit not including the residual stresses was 276.1 MPa. The low stress Thestresses. low stressThe low range stress limit range not includinglimit not including the residual th stressese residual was stresses 276.1 MPa.was 276.1 The low MPa. stress The range low stress limit range limit including the welding residual stress was predicted to be 206.9 MPa. This means that includingrange limit the including welding residualthe welding stress residual was predicted stress was to be predicted 206.9 MPa. to This be 206.9 means MPa. that weldingThis means residual that welding residual stresses should be taken into account for fatigue design of the welded structure. stresseswelding should residual be stresses taken into should account be taken for fatigue into acco designunt for of thefatigue welded design structure. of the welded structure.

Metals 2018, 8, 21 10 of 11 Metals 2018, 8, 21 10 of 12

FigureFigure 12. 12. ComparisonComparison of of Δσ∆σ-Nf-Nf relations. relations.

5.5. Conclusions Conclusions InIn this study,study, wewe investigated investigated residual residual stresses stresse ofs of multi-pass multi-pass dissimilar dissimilar material material welded welded joint joint both bothexperimentally experimentally and by and numerical by numeri simulation.cal simulation. Fatigue Fatigue life assessment life assessment of the dissimilarof the dissimilar material material welded weldedjoint was joint also was carried also out.carried Our out. conclusions Our conclusions are summarized are summarized in the following: in the following:

(1)(1) WeldingWelding residual residual stresses stresses at at the the weld weld of of dissimilar welded welded joint distributed complicatedly complicatedly on on longitudinallongitudinal and and transverse transverse directions. directions. Results ofof numericalnumerical andand experimentalexperimental analysis analysis showed showed a agood good agreement agreement qualitatively. qualitatively. (2)(2) NumericalNumerical and and experimental experimental peak peak values values of of welding welding residual residual stresses stresses at at HAZ HAZ of of the the weld weld on on the the 12Cr12Cr steel steel side side were were predicted predicted to to be be 333 333 and and 282 282 MPa MPa HAZ, HAZ, respectively. respectively. (3)(3) TheThe low low fatigue fatigue limit limit of of dissimilar dissimilar material material welded welded joint joint was was assessed assessed to to be be 306.8 306.8 MPa, MPa, which which waswas 40% 40% of of tensile tensile strength strength (767 (767 MPa). MPa). (4)(4) StressStress range range values values without without including including welding welding residual residual stresses stresses were were 14% 14% higher higher than than those those calculated by including the effect of residual stresses. calculated by including the effect of residual stresses.

Acknowledgments: This work was supported by the Reliability Evaluation Lab of the Mechanical Engineering Department,Acknowledgments: SungkyunkwanThis work University, was supported Suwon, by Korea. the Reliability Evaluation Lab of the Mechanical Engineering Department, Sungkyunkwan University, Suwon, Korea. Author Contributions: H.W.A. conceived and designed the experiments; H.W.A. and J.H.H. performed the Author Contributions: H.W.A. conceived and designed the experiments; H.W.A. and J.H.H. performed the experiments and analyzed the data under the supervision of D.H.B. All experiments were performed in experiments and analyzed the data under the supervision of D.H.B. All experiments were performed in SungkyunkwanSungkyunkwan University, University, Kore Korea;a; H.W.A. wrote thethe paper.paper. ConflictsConﬂicts of Interest: The authors have no conflic conﬂictt of interest to disclose.

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