A Study on Microstructure, Residual Stresses and Stress Corrosion Cracking of Repair Welding on 304 Stainless Steel: Part I-Eﬀects of Heat Input

materials

Article A Study on Microstructure, Residual Stresses and Stress Corrosion Cracking of Repair Welding on 304 Stainless Steel: Part I-Eﬀects of Heat Input

Yun Luo 1,2,*, Wenbin Gu 1, Wei Peng 1, Qiang Jin 1, Qingliang Qin 3 and Chunmei Yi 4 1 College of New Energy, China University of Petroleum (East China), Qingdao 266580, China; [email protected] (W.G.); [email protected] (W.P.); [email protected] (Q.J.) 2 School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China 3 School of Automation and Electronics Engineering, Qingdao University of Science and Technology, Qingdao 266061, China; [email protected] 4 Shandong Meiling Chemical Equipment Co., Ltd., Zibo 255430, China; [email protected] * Correspondence: [email protected]

Received: 10 April 2020; Accepted: 19 May 2020; Published: 25 May 2020

Abstract: In this paper, the effect of repair welding heat input on microstructure, residual stresses, and stress corrosion cracking (SCC) sensitivity were investigated by simulation and experiment. The results show that heat input influences the microstructure, residual stresses, and SCC behavior. With the increase of heat input, both the δ-ferrite in weld and the average grain width decrease slightly, while the austenite grain size in the heat affected zone (HAZ) is slightly increased. The predicted repair welding residual stresses by simulation have good agreement with that by X-ray diffraction (XRD). The transverse residual stresses in the weld and HAZ are gradually decreased as the increases of heat input. The higher heat input can enhance the tensile strength and elongation of repaired joint. When the heat input was increased by 33%, the SCC sensitivity index was decreased by more than 60%. The macroscopic cracks are easily generated in HAZ for the smaller heat input, leading to the smaller tensile strength and elongation. The larger heat input is recommended in the repair welding in 304 stainless steel.

Keywords: 304 stainless steel; repair welding; heat input; residual stresses; stress corrosion cracking

1. Introduction Repair welding is the most important means to repair the defects of aging pressure vessel and piping components, and is thus used for life extension [1]. Meanwhile, because of non-uniform heating and cooling during repair welding, the inhomogeneous plastic deformation and residual stresses are generated in the repair zone, which is the weakest area of the repaired structure [2]. The presence of residual stress has a harmful effect on strength [3], creep [4], fatigue [5], corrosion [6], and fracture [7], etc. The effects of repair residual stresses on structure integrity are greater than that of initial weld residual stresses [8]. Especially, during corrosive environment, stress corrosion cracking (SCC) phenomena easily occur in the repaired zone. Over 40% of weld repairs resulted in subsequent cracking, according to the industry survey conducted by EPRI [9]. Therefore, an improved understanding of the effect of repair welding on residual stresses and SCC behavior becomes critically important for performing structural integrity assessment of repaired equipment. There have been numerous investigations employing experiment measurement techniques and numerical simulation methods for studying repaired residual stresses with various structural configurations. Dong et al. found that the tensile strength of repair was reduced by 30%–40% as compared to the initial welds due to the rather high level restraint in repair welds than initial welds [10]. The repaired

Materials 2020, 13, 2416; doi:10.3390/ma13102416 www.mdpi.com/journal/materials Materials 2020, 13, 2416 2 of 18 residual stresses are varied for different material, component geometry, and welding processes, including the repair length, depth, the repair welding heat input, pass sequence, etc. Dong et al. [11] found that the transverse residual stresses transit from tensile stresses to compressive stresses near repair ends. Song and Shao [12] carried out an in-depth investigation of repair weld geometry effects on residual stress distributions and concluded that a weld repair should be designed as long as possible, as narrow as possible, and as shallow as possible. Jiang also found that the residual stresses are decreased with an increase of repair width [13]. The residual stresses are decreased with the clad metal thickness increases and base metal thickness decreases [14]. Using multiple-layer and high heat input weld could be useful for decreasing the residual stress [15]. With the increase of repair length, the transverse stress is decreased, and the longitudinal stress has little changes [16]. The repaired residual stresses can be either enhanced or weakened by change in repair length, depth, and width. Except from residual stresses, the microstructure and microhardness of the repaired joint also played an important role in SCC incidents. Jiang et al. [17] investigated the effect of repair number on microstructure, hardness, and residual stress for a stainless steel clad plate. They found that the content of short ferrite and microhardness in weld increases as the repair number increases. The clad plate is recommended by repairing more than twice. Kessal et al. [18] concluded the hardness and residual stresses change with microstructure and ferrite content. Winiczenko et al. [19] studied the mechanical properties and microstructure of welded joint of ductile iron and stainless steel, the research results have good guidance to the friction welding of stainless steel. The corrosion resistance depends on the compressive residual stresses and the ferrite content, which are affected by the welding current and passes number. Reducing welding, machining, and surface strengthening technology can improve the corrosion resistance [20]. AghaAli [21] found that the HAZ of repair specimen is more sensitive to pitting corrosion and the corrosion sensitivity of HAZ was increased with repair number increases. As far as the corrosion resistance is concerned, repair welding can be safely employed as a means of extending the service life of the junction [22]. Raja et al. [23] compared the SCC behavior of as-welded and repair-welded joint while using the slow strain rate technique (SSRT). The SCC index for the repair welded joint is lower than that of the as-welded joint. Antunes et al. [24] also found that the corrosion resistance was depressed as the increases of repair number. Although many careful investigations were performed on as-weld joint [25–28], there are few studies on the residual stresses and SCC behavior of repair welding joint by experiment. The repair welding usually increases the residual stress of the local repair welding area, and it has a certain influence on the mechanical property, SCC behavior, and service life of the repaired area. The current large number of literature is mainly concentrated on the study on the effects of repair welding length, width, and times on structure integrity and pitting corrosion behavior, and little attention was paid to the effects of repair welding heat input and reinforcement height. It is still unclear how the repair welding heat input and reinforcement height affects the microstructure, residual stresses, and SCC behavior of repaired joint. Therefore, the aim of this study is to study the effects of repair welding technology (heat input, reinforcement height) on microstructure, residual stresses, and SCC behavior of repair welded joint, which will be of great significance for prolonging the service life of the repair welded equipment. This work is reported in two parts. The effects of heat input and reinforcement height were discussed in Part I and II, respectively. The heat input is an important welding parameter for ensuring the structure integrity of welded joints. An increase of welding heat input can reduce the occurrence of inadmissible welding imperfections in welded joints [29]. In general, with the increases of heat input, the molten zone and HAZ are broadly spread, and the maximum temperature welding pool is also increased. Too larger heat input might generate a harmful effect on the structure integrity of welded joint. For example, the residual stresses for dissimilar pipe weld joint are increased as the welding heat input increases [30,31]. The impact toughness was found to be decreasing [32] and the corrosive insensitivity increased [33] with an increase in heat input. Ravisankar [34] pointed out that the immediate and appropriate heat input was found to give better weld penetration with controlled Materials 2020, 13, x FOR PEER REVIEW 3 of 20

also increased. Too larger heat input might generate a harmful effect on the structure integrity of welded joint. For example, the residual stresses for dissimilar pipe weld joint are increased as the welding heat input increases [30,31]. The impact toughness was found to be decreasing [32] and the Materialscorrosive2020, 13 insensitivity, 2416 increased [33] with an increase in heat input. Ravisankar [34] pointed 3out of 18 that the immediate and appropriate heat input was found to give better weld penetration with controlled molten zone and good mechanical properties. The effect of heat input on repair weld joint is different molten zone and good mechanical properties. The effect of heat input on repair weld joint is different from that on first weld joint due to geometry constraint [10]. Thus, the effects of heat input should be from that on first weld joint due to geometry constraint [10]. Thus, the effects of heat input should be investigated in detail for every types of welded joint. The finite element modeling (FEM) is a good investigated in detail for every types of welded joint. The finite element modeling (FEM) is a good method for clarifying the effects of heat input on the weld pool and maximum welding temperature method for clarifying the effects of heat input on the weld pool and maximum welding temperature before welding trail [35–38]. Jiang et al. [39–48] made a larger number of investigations of welding before welding trail [35–38]. Jiang et al. [39–48] made a larger number of investigations of welding residual stress by FEM, and some key method of reducing residual stresses are obtained. residual stress by FEM, and some key method of reducing residual stresses are obtained. In thisIn this paper, paper, Part Part I, the I, the effects effects of repairof repair welding welding heat heat input input on on microstructure, microstructure, residual residual stresses, stresses, andand SCC SCC of 304of 304 stainless stainless steel steel were were investigated investigated by FEMby FEM and and experiment. experiment. Part Part I starts I starts with with a brief a brief descriptiondescription of theof the finite finite element element residual residual stress stress modeling modeling procedure procedure that that was was adopted adopted in in this this study. study. Subsequently,Subsequently, the the experimental experimental procedure procedure was was shown shown and and finite finite element element results results were were verified. verified. Afterwards,Afterwards, a finite a finite element element analysis analysis and and experiment experiment results results are presented are presented for identifying for identifying the e fftheect effect of heatof input. heat input. Finally, Finally, an optimized an optimized repair repair welding welding heat input heat input technology technology was proposed. was proposed.

2. Finite2. Finite Element Element Modeling Modeling 2.1. Geometry Model 2.1. Geometry Model The 304 stainless steel plate was repair welded by A102 electrode. The dimension of parent The 304 stainless steel plate was repair welded by A102 electrode. The dimension of parent specimen is 250 mm 100 mm 5 mm. The groove defect is extracted by machining in the center specimen is 250 mm× × 100 mm× × 5 mm. The groove defect is extracted by machining in the center of of the sample in order to make the surface defect. Figure1 shows the schematic diagram of repaired the sample in order to make the surface defect. Figure 1 shows the schematic diagram of repaired specimen. The defect depth is 2 mm and the groove angle is 75 . The width defect bottom is 5 mm. specimen. The defect depth is 2 mm and the groove angle is 75°.◦ The width defect bottom is 5 mm. L L = 40 mm is the repair welding length. = 40 mm is the repair welding length.

FigureFigure 1. The 1. The schematic schematic diagram diagram of repairedof repaired specimen. specimen.

A threeA three dimensional dimensional (3D) (3D) model model was was built built according according to theto the dimension dimension that that is shownis shown in Figurein Figure1. 1. FigureFigure2 shows 2 shows the t typicalhe typical ﬁnite finite element element meshing meshing of wholeof whole repair repair specimen. specimen The. The meshing meshing is is dense dense in in thethe weld,weld, and and then then it it becomes becomes coarse coarse gradually gradually far far away, away in, in order order to to reduce reduce the the computation computation time. time. The element types for the thermal analysis and mechanical analysis are three-dimensional eight node linear heat transfer element (DC3D8) and three-dimensional eight node linear brick reduced integration element (C3D8R), respectively. Materials 2020, 13, x FOR PEER REVIEW 4 of 20

The element types for the thermal analysis and mechanical analysis are three-dimensional eight node linear heat transfer element (DC3D8) and three-dimensional eight node linear brick reduced Materialsintegration2020, ele13,ment 2416 (C3D8R), respectively. 4 of 18

Figure 2. Finite element meshing of repaired specimen. Figure 2. Finite element meshing of repaired specimen. 2.2. Simulation of Residual Stress 2.2. Simulation of Residual Stress Here, the initial weld residual stresses are assumed to be negligible in repair weld modeling. RepairHere, welding the initial simulation weld isresidual composed stresses of welding are assumed temperature to be negligible analysis and in thenrepair followed weld modeling. by stress analysis,Repair welding which simulation employs the is composed temperature of welding that was temperature obtained from analysis thermal and analysis.then followed A sequential by stress couplinganalysis, analysiswhich employs was used the for temperature the ﬁnite element that was analysis. obtained The from element thermal birth analysis. and death A sequentialtechnique werecoupling adopted analysis to simulate was used the for weld the bead finite deposition element analysis. by ABAQUS The codeelement (6.10). birth and death technique were adopted to simulate the weld bead deposition by ABAQUS code (6.10). 2.2.1. Thermal Analysis 2.2.1.In Thermal thermal Anal analysis,ysis the welding process is primarily simulated by applying a heat source for the doubleIn ellipsoidalthermal analysis, distribution, the welding which process is expressed is primarily by the simulated following Goldakby applying equations a heat [ source49]: for the doubleFor ellipsoidal the front heat distribution, source: which is expressed by the following Goldak equations [49]:

6 √3 f Q For the front heat source: f 3x2/a2 3y2/b2 3(z vt z )2/c 2 q(x, y, z, t) = e− e− e− − − 0 1 (1) abc1π √π

63fQ 2 2 2 2 22 For the rear heat source: f 3x / a 3 y / b 3(z  vt  z01 ) / c q(,,,) x y z t e e e (1) abc  √1 6 3 frQ 3x2/a2 3y2/b2 3(z vt z )2/c 2 q(x, y, z, t) = e− e− e− − − 0 2 (2) For the rear heat source: abc2π √π where ff and fr give the fractions of the heat deposited in the front and the rear parts, respectively. 22 63fQr 3x2 / a 2 3 y 2 / b 2 3(z  vt  z ) / c It assumes that the ff is 1.5q(,,,) x and yf zr is t 0.5 when considering e e that the e temperature02 gradient in the front(2) leading part is steeper than that of the tailingabc2 edge. Q is the power of the welding heat source and the z0 is the position of the heat source in z-direction when t is zero. x, y, and z are the local coordinates where ff and fr give the fractions of the heat deposited in the front and the rear parts, respectively. It of the double ellipsoid model aligned with the welded plate. The heat source of double ellipsoidal assumes that the ff is 1.5 and fr is 0.5 when considering that the temperature gradient in the front distribution for the moving welding arc is modeled by a user subroutine DFLUX in ABAQUS compiled leading part is steeper than that of the tailing edge. Q is the power of the welding heat source and the by FORTRAN program. z0 is the position of the heat source in z-direction when t is zero. x, y, and z are the local coordinates Convection (q ) and radiation (q ) in the welding and subsequent cooling are both taken into of the double ellipsoida model alignedr with the welded plate. The heat source of double ellipsoidal account, which is simulated by: distribution for the moving welding arc is modeled by a user subroutine DFLUX in ABAQUS qa = ha(Ts Ta) (3) compiled by FORTRAN program. − − 4 4 qr = εk[(Ts + 273) (Ta + 273) ] (4) Convection (qa) and radiation (qr−) in the welding− and subsequent cooling are both taken into 2 whereaccount,ha whichis convection is simulated coeﬃ bycient: (15 W/m K) and ε is emissivity (0.8); k is the Stefan–Boltzmann 8 2 4 constant (5.67 10 J/(m K s). Ts and Ta are the current and room temperature (20 C), respectively. × − ◦ Table1 lists the chemical compositions ofqa the h 304 a() T stainlesss  T a steel. Additionally, Table2 lists the(3) temperature-dependent thermal physical properties of 304 stainless steel.

Materials 2020, 13, 2416 5 of 18

Table 1. Chemical compositions of 304 stainless steel (wt%).

C Si Mn P S Cr Mo Ni Cu Fe 0.048 0.419 1.228 0.031 0.002 18.08 0.011 8.113 0.009 Bal.

Table 2. The temperature-dependent thermal physical properties of 304 stainless steel.

Temperature (◦C) 20 200 400 600 800 Poisson’s ratio 0.28 0.28 0.28 0.28 0.28 Density (kg/m3) 8010 7931 7840 7755 7667 Speciﬁc heat (J/kg ◦C) 500 544.3 582 634 686 Young’s Modulus (GPa) 199 180 166 150 125 Yield strength (MPa) 206 153 108 82 69 Conductivity (W/m ◦C) 15.26 17.6 20.2 22.8 25.4 Thermal expansion coeﬃcient (1/ C 10 6) 16.0 17.2 18.2 18.6 19.5 ◦ × −

2.2.2. Residual Stress Analysis The residual stress is calculated by using the temperature distribution that was obtained from thermal analysis as input data. For 304 stainless steel, there is little change on microstructure, thus solid-state phase transformation does not occur. Therefore, the total strain can be decomposed into three components, as follows: ε = εe + εp + εts (5) where εe, εp, and εts stands for elastic strain, plastic strain, and thermal strain, respectively. Elastic strain is modeled while using the isotropic Hooke’s law with temperature-dependent Young’s modulus and Poisson’s ratio. The thermal strain is calculated using the temperature-dependent coeﬃcient of thermal expansion. For the plastic strain, a rate-independent plastic model is employed with Von Mises yield surface, temperature-dependent mechanical properties, and isotropic hardening model, which is commonly used for weld simulations due to the easily-conducted set of tests required for model calibration. The annealing eﬀect is also considered here, and the annealing temperature is assumed to be 800 ◦C for all of the materials in the present model. For stress analysis, temperature-dependent mechanical properties of the materials are incorporated, as shown in Table3. During the stress analysis, four end nodes on bottom surface are constrained in order to avoid the rigid body motion.

3. Experimental Details

3.1. Sample Preparation Manual arc welding technology is used to repair the defects of the sample. Before repair welding, the defects are cleaned to prevent impurities from entering the repair layer, and then preheating the areas around defects with 200 ◦C in order to keep the base metal dry during welding. In the process of repair welding, ﬁxtures should be used to ﬁx both ends of the sample in order to prevent excessive deformation of the specimen, as shown in Figure3. After repair welding, the sample was naturally cooledMaterials in2020 air., 13, x FOR PEER REVIEW 7 of 20

Figure 3. The constraint in the process of repair welding.

Table 3 lists the welding parameters of repaired specimens with different heat inputs. The repair length is 40 mm. The heat input of specimen A1, A2, A3, and A4 are 4.20, 4.67, 5.13, and 5.60 kJ/cm, respectively. The heat input is calculated by equation, as follows:

U  I q   (6) v where q is heat input; U is welding voltage; I is welding current; v is welding speed; and, η is arc efficiency.

Table 3. Welding parameters of repaired specimens with different heat inputs.

Repair Length Voltage Current Welding Speed Heat Input Specimen No. L/mm U/V I/A v/(mm/s) q/(kJ/cm)

A1 40 20 90 3 4.20

A2 40 20 100 3 4.67

A3 40 20 110 3 5.13

A4 40 20 120 3 5.60

3.2. Residual Stresses and SCC Procedure After repair welding, the metallographic test block is prepared from the repaired specimen. The microstructure of the weld, fusion line, and heat affected zone of metallographic specimen was observed by optical microscope. After observing the metallographic structure, the microhardness of the weld, fusion line, and HAZ of each sample was measured by Vickers hardness tester.

The residual stresses were measured by the X-ray diffraction technique. The X-ray diffraction measurement is based on Bragg’s law [50]:

2d sin   n (7) where λ is the wavelength, d is the interatomic lattice spacing, n is an integer, and θ is the diffraction angle.

The test method is 2θ – sin2θ, and the residual stress is calculated by [50]:

  K M (8)

E  K   cot (9) 2(1 v) 0 180

Materials 2020, 13, 2416 6 of 18

Table3 lists the welding parameters of repaired specimens with di ﬀerent heat inputs. The repair length is 40 mm. The heat input of specimen A1, A2, A3, and A4 are 4.20, 4.67, 5.13, and 5.60 kJ/cm, respectively. The heat input is calculated by equation, as follows:

U I q = · η (6) v · where q is heat input; U is welding voltage; I is welding current; v is welding speed; and, η is arc eﬃciency.

Table 3. Welding parameters of repaired specimens with diﬀerent heat inputs.

Specimen Repair Length Voltage Current Welding Speed Heat Input No. L/mm U/V I/A v/(mm/s) q/(kJ/cm) A1 40 20 90 3 4.20 A2 40 20 100 3 4.67 A3 40 20 110 3 5.13 A4 40 20 120 3 5.60

3.2. Residual Stresses and SCC Procedure After repair welding, the metallographic test block is prepared from the repaired specimen. The microstructure of the weld, fusion line, and heat affected zone of metallographic specimen was observed by optical microscope. After observing the metallographic structure, the microhardness of the weld, fusion line, and HAZ of each sample was measured by Vickers hardness tester. The residual stresses were measured by the X-ray diffraction technique. The X-ray diffraction measurement is based on Bragg’s law [50]:

2d sin θ = nλ (7) where λ is the wavelength, d is the interatomic lattice spacing, n is an integer, and θ is the diﬀraction angle. The test method is 2θ – sin2θ, and the residual stress is calculated by [50]:

σ = K M (8) · E π K = cot θ (9) −2(1 + v) 0 180 ∂(2θ) M = (10) ∂(sin2 θ) where K is the stress constant, θ0 is the diffraction angle under a stress-free state, and θ is the angle between the normal of the crystal surface and the material surface. A linear relationship exists between 2θ and sin2θ, and M is the slope between diffraction angle 2θ and sin2θ. M is calculated if more than three points (2θ, sin2θ) are determined. Before the measurement of residual stresses, the weld should be ground and leveled with a grinding wheel to remove the reinforcement height and oxide layer, as shown in Figure4a,b. The electrolytic polishing machine is then used for electrolytic polishing along the path of measuring point to remove the grinding influence layer of grinding wheel, as shown in Figure4c, and the polishing depth is 200 µm. Finally, the position of the measuring point should be marked on the specimen in advance. The X-ray diffraction (XRD) measurement was done by the X-350A type X-Ray stress gauge (Stress Technologies company, Handan, China). The voltage and current of the device in operation are 20 kV and 5 mA, respectively. Eleven points along P1 with an interval of 5 mm in weld and 10 mm in other area were measured, as shown in Figure4d. Here, the path P1 is located in the center line of the upper Materials 2020, 13, x FOR PEER REVIEW 8 of 20

(2) M  (10) (sin 2 )

Materials 2020, 13, x FOR PEER REVIEW 8 of 20 where K is the stress constant, θ0 is the diffraction angle under a stress-free state, and θ is the angle between the normal of the crystal surface and the material surface. A linear relationship exists (2) between 2θ and sin2θ, and M is theM slope between diffraction angle 2θ and sin2θ. M is (1calculated0) if (sin 2 ) more than three points (2θ, sin2θ) are determined.

Beforewhere the K is measurement the stress constant, of residualθ0 is the diffraction stresses, angle the weldunder shoulda stress- free be state,ground and andθ is theleveled angle with a between the normal of the crystal surface and the material surface. A linear relationship exists grinding wheel to remove the reinforcement height and oxide layer, as shown in Figure 4a,b. The between 2θ and sin2θ, and M is the slope between diffraction angle 2θ and sin2θ. M is calculated if electrolyticmore polishingthan three pointsmachine (2θ, sinis 2thenθ) are useddetermined. for electrolytic polishing along the path of measuring point to remove the grinding influence layer of grinding wheel, as shown in Figure 4c, and the polishing depthBefore is the 200 measurement μm. Finally, of residualthe position stresses, of the the weld measuring should be point ground should and leveled be marked with a on the grinding wheel to remove the reinforcement height and oxide layer, as shown in Figure 4a,b. The specimenelectrolytic in advance. polishing The machineX-ray diffraction is then used (XRD) for electrolytic measurement polishing was along done the by path the ofX- 350Ameasuring type X-Ray stress gaugepoint to(Stress remove Technologies the grinding influencecompany layer, Han ofdan, grinding China). wheel The, as vol showntage inand Figure current 4c, and of the device in operationpolishing are depth 20 kV is and200 μm.5 mA, Finally, respectively. the position Eleven of the measuringpoints along point P1 should with bean marked interval on of the 5 mm in weldMaterials andspecimen2020 10, 13 mm, 2416 in in advance. other areaThe X were-ray diffraction measured, (XRD) as shownmeasurement in Figure was done 4d. Here,by the Xthe-350A path type P1 X is-Ray located 7 of in 18 the centerstress line gauge of the (Stress upper Technologies surface perpendicularcompany, Handan, to China).the weld. The P2vol istage located and current in the of centerthe device line of the in operation are 20 kV and 5 mA, respectively. Eleven points along P1 with an interval of 5 mm in weld upper surface parallel to the weld. P3 is located in the upper surface of HAZ parallel to the weld. surfaceweld perpendicular and 10 mm in to other the weld.area were P2 ismeasured, located as in shown the center in Figure line 4d. of Here, the weld the path upper P1 is surface located parallelin to the weld.the P3center is located line of the in upper the upper surface surface perpendicular of HAZ to parallelthe weld. to P2 the is located weld. in the center line of the weld upper surface parallel to the weld. P3 is located in the upper surface of HAZ parallel to the weld.

Figure 4.Figure 4. SpecimensSpecimens 4. Specimens pre-treatment pre - pretreatment-treatment before before before X-ray XX--ray dirayﬀ ractiondiffractiondiffraction (XRD): (XRD): (XRD): (a )( beforea) before(a) polished;before polished; polished; (b ()b after) after (polished;b ) after polished;(c) electrochemicalpolished; (c) electrochemical (c) electrochemical polishing; polishing; and polishing; (d) labeling and and ( d(d) measure) labelinglabeling measure point. measure point. point.

After the theAfter residual residual the residual stress stress measurement, stress measurement, measurement, the test the the sample test test sample sample of slow of ofstrainslow slow strain rate strain tests rate (SSRT) tests rate (SSRT) tests is processed (SSRT) is by is processedwire cutprocessed electrical by wire by discharge wire cut electricalcut machiningelectrical discharge discharge (WEDM). machining machining Figure 5 (WEDM). (WEDM).shows theFigure processingFigure 5 shows 5 shows geometry. the processing the The processing tensile geometry. The tensile direction of SSRT specimen is perpendicular to the length of the repair weld, geometry.direction of The SSRT tensile specimen direction is perpendicular of SSRT specimen to the lengthis perpendicular of the repair to weld, the length and the of weldthe repair is located weld, at and the weld is located at the center of the SSRT specimen. Two SSRT samples were machined from andthe centerthethe weld same of theis repair located SSRT welding specimen.at the sample center Two, one of for SSRTthe the SSRT samplestensile specimen. test were in air machinedTwo(A1-1, SSRT A4-1) fromandsamples the the other were same (A1 machined- repair2, A4-2) welding from thesample, samefor onerepair the fortensile welding the test tensile in samplecorrosive test, in onemedium. air for (A1-1, the tensile A4-1) test and in the air other (A1-1, (A1-2, A4-1) A4-2) and the for other the tensile (A1-2, testA4-2) in forcorrosive the tensile medium. test in corrosive medium.

Figure 5. The geometry of slow strain rate tests (SSRT) specimen.

Figure 6 shows the SSRT machine. The corrosion solution is the 3.5% NaCl solution (wt. %). The FigureFigure 5. 5. TheThe geometry geometry of of s slowlow strain strain rate rate tests tests ( (SSRT)SSRT) specimen. strain rate is 10−6 s−1. Before the SSRT test, the SSRT sample was ground with sandpaper to remove Figurethe residual6 shows knife the mark SSRT in machine.the process, The and corrosionthen rinsed solutionwith distilled is the water. 3.5% A NaCl300 N solutionpre-load is (wt. %). Figure 6 shows the SSRT machine. The corrosion solution is the 3.5% NaCl solution (wt. %). The The strainapplied rate in isadvance 10 6 s to1 eliminate. Before the the clearance. SSRT test, the SSRT sample was ground with sandpaper to strain rate is 10−6 s−1. −Before− the SSRT test, the SSRT sample was ground with sandpaper to remove remove the residual knife mark in the process, and then rinsed with distilled water. A 300 N pre-load the residual knife mark in the process, and then rinsed with distilled water. A 300 N pre-load is is applied in advance to eliminate the clearance. appliedMaterials in advance 2020, 13 ,to x FOR eliminate PEER REVIEW the clearance. 9 of 20

FigureFigure 6. 6.Slow Slow strainstrain tensile tensile test test machine. machine.

The SCC sensitivity index Iδ is calculated by:

 cor (11) I  (1 )100%  air

where δcor and δair is the specimen elongation in inert medium and air, respectively. The higher the sensitivity index of SCC, the greater the SCC sensitivity of the specimen. After fracture test, the fracture surface of the specimen was observed by scanning electron microscope (SEM).

4. Validation Figure 7 compares the temperature field of the weld section by simulation and experiment. It is found that the simulated weld pool profile is roughly consistent with the actual situation. The temperature in weld center reaches 2014 °C and the temperature of the whole fusion zone has exceeded the melting point of stainless steel. The results show that the simulation method of temperature field in repair welding is feasible.

Figure 7. Comparison between the simulated and experimented weld pool shape.

Figure 8 shows the comparison of residual stresses along P1 by simulation and measurement. It can be seen that the distribution trend of residual stresses along P1 has good agreement with that by experiment. The residual stresses in weld and HAZ are greatly larger than those in other areas. For every heat input, the longitudinal residual stresses (TD) are larger than transverse stresses (TD). The

Materials 2020, 13, x FOR PEER REVIEW 9 of 20

Materials 2020, 13, 2416 8 of 18

Figure 6. Slow strain tensile test machine. The SCC sensitivity index Iδ is calculated by: The SCC sensitivity index Iδ is calculated by: δcor Iδ = (1  ) 100% (11) − δcorair × (11) I  (1 )100%  air where δcor and δair is the specimen elongation in inert medium and air, respectively. The higher the sensitivitywhere indexδcor and of δ SCC,air is the the specimen greater the elongation SCC sensitivity in inert medium of the specimen. and air, respectively. After fracture The test, higher the the fracture surfacesensitivity of the specimen index of SCC, was observed the greater by the scanning SCC sensitivity electron of microscope the specimen. (SEM). After fracture test, the fracture surface of the specimen was observed by scanning electron microscope (SEM). 4. Validation 4. Validation Figure7 compares the temperature field of the weld section by simulation and experiment. It is foundFigure that 7 thecompares simulated the temperature weld pool field profile of the isweld roughly section consistentby simulation with and theexperiment. actual situation.It is found that the simulated weld pool profile is roughly consistent with the actual situation. The The temperature in weld center reaches 2014 ◦C and the temperature of the whole fusion zone temperature in weld center reaches 2014 °C and the temperature of the whole fusion zone has has exceeded the melting point of stainless steel. The results show that the simulation method of exceeded the melting point of stainless steel. The results show that the simulation method of temperature field in repair welding is feasible. temperature field in repair welding is feasible.

FigureFigure 7. Comparison 7. Comparison between between thethe simulatedsimulated a andnd experimented experimented weld weld pool pool shape. shape.

FigureFigure8 shows 8 shows the the comparison comparison of of residualresidual stresses stresses along along P1 P1by bysimulation simulation and measurement. and measurement. It It cancan be be seen seen that that the the distributiondistribution trend trend of of residual residual stresses stresses along along P1 has P1 good has goodagreement agreement with that with by that by experiment.experiment. The residual residual st stressesresses in inweld weld andand HAZ HAZ are greatly are greatly larger largerthan those than in those other inareas. other For areas. Materials 2020, 13, x FOR PEER REVIEW 10 of 20 For everyevery heatheat input, input, the the longitudinal longitudinal residual residual stresses stresses (TD) (TD)are larger are largerthan transverse than transverse stresses (TD). stresses The (TD). The maximummaximum error between simulation simulation and and measurement measurement in weld in weld and and HAZ HAZ is below is below 9%, w 9%,hich which proves proves that thethat simulation the simulation method method on on residual residual stress stress isis available.available.

(a) 400 (b) 400 A1(4.20kJ/cm) A2(4.67kJ/cm) 300 300

200 200

100 100 0

-100 -100 TD-FEM Residual stress/MPa -200 TD-FEM TD-XRD Residual stress/MPa -200 TD-XRD LD-FEM LD-FEM -300 -300 LD-XRD LD-XRD -400 -400 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 Distance from weld center/mm Distance from weld center/mm

200 200

100 100

0 0

-100 -100 TD-FEM TD-FEM -200 -200

Residual stress/MPa Residual

TD-XRD stress/MPa Residual TD-XRD -300 LD-FEM -300 LD-FEM LD-XRD LD-XRD -400 -400 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 Distance from weld center/mm Distance from weld center/mm Figure 8. Comparison of residual stresses along P1 by simulation and measurement. (a) A1-4.2 kJ/cm, Figure 8. Comparison of residual stresses along P1 by simulation and measurement. (a) A1-4.2kJ/cm, (b) A2-4.67 kJ/cm, (c) A3-5.13 kJ/cm, (d) A4-5.60 kJ/cm. (b) A2-4.67kJ/cm, (c) A3-5.13kJ/cm, (d) A4-5.60kJ/cm.

5. Results and Discussions

5.1. OM Analysis Figure 9 shows the effect of heat input on the microstructure in weld. The microstructure in the weld is a composite of austenitic matrix and δ-ferrite. With the increase of heat input, the distribution of δ-ferrite is more uniform and the average grain width decreases. The ferrite content by quantitative metallography analysis for specimen A1, A2, A3, and A4 are 17.77%, 17.34%, 17.01%, and 16.27%, respectively. With the increase of heat input, the δ-ferrite in weld decreases slightly. The δ-ferrite in austenitic stainless steel weld is formed in a crystallization process and retained at room temperature. When the heat input increases, the retention time of weld metal at high temperature becomes longer, thus leading to the content of δ-ferrite in the weld decreasing. A certain content of δ-ferrite can prevent hot cracks and improve the mechanical properties of weld [51]. At the same time, the δ-ferrite can improve the resistance of intergranular corrosion and stress corrosion of weld. An excessive content of δ-ferrite will lead to embrittlement of austenite stainless steel.

Materials 2020, 13, 2416 9 of 18

5. Results and Discussion

5.1. OM Analysis Figure9 shows the e ﬀect of heat input on the microstructure in weld. The microstructure in the weld is a composite of austenitic matrix and δ-ferrite. With the increase of heat input, the distribution of δ-ferrite is more uniform and the average grain width decreases. The ferrite content by quantitative metallography analysis for specimen A1, A2, A3, and A4 are 17.77%, 17.34%, 17.01%, and 16.27%, respectively. With the increase of heat input, the δ-ferrite in weld decreases slightly. The δ-ferrite in austenitic stainless steel weld is formed in a crystallization process and retained at room temperature. When the heat input increases, the retention time of weld metal at high temperature becomes longer, thus leading to the content of δ-ferrite in the weld decreasing. A certain content of δ-ferrite can prevent hot cracks and improve the mechanical properties of weld [51]. At the same time, the δ-ferrite can improve the resistance of intergranular corrosion and stress corrosion of weld. An excessive content of δMaterials-ferrite 2020 will, 13 lead, x FOR to embrittlementPEER REVIEW of austenite stainless steel. 11 of 20

(a) (b)

Figure 9. Eﬀect of heat input on the microstructure in weld center. (a) A1—4.2 kJ/cm, (b) A2—4.67 kJ/cm, Figure 9. Effect of heat input on the microstructure in weld center. (a) A1-4.2kJ/cm, (b) A2-4.67kJ/cm, (c) A3—5.13 kJ/cm, (d) A4—5.60 kJ/cm. (c) A3-5.13kJ/cm, (d) A4-5.60kJ/cm. Figure 10 shows the eﬀect of heat input on the microstructure in the fusion line and the adjacent region.Figure With 10 the shows increases the effect of heat of heat input, input the on width the microstructure of fusion line merelyin the fusion has no line changes, and the while adjacent the austeniteregion. With grain the size increases in HAZ of is heat slightly input, increased, the width and of the fusion strip-shaped line merelyδ-ferrite has no content changes, in the while rolling the directionaustenite isgrain obviously size in reduced. HAZ is slightly This is becauseincreased, the and strip-shaped the strip-shapedδ-ferrite δ in-ferrite HAZ content is the residual in the rolling ferrite phasedirection at high is obviously temperature reduced. in the This forging is because process the of strip steel,-shaped which δ can-ferrite be properlyin HAZ is eliminated the residual by ferrite solid solutionphase at treatment.high temperature When the in heatthe forging input increases, process of the steel, retention which time can ofbe theproperly metal elimina near theted fusion by solid line becomessolution treatment. longer at a When high temperature, the heat input which increases, is equivalent the retention to the time prolongation of the metal of thenear holding the fusion time line of solidbecomes solution longer treatment, at a high thustemperature, leading to which the decrease is equivalent of strip-shaped to the prolongationδ-ferrite content of the holding in HAZ time [52]. of solid solution treatment, thus leading to the decrease of strip-shaped δ-ferrite content in HAZ [52].

(a) (b)

Materials 2020, 13, x FOR PEER REVIEW 11 of 20

(a) (b)

Figure 9. Effect of heat input on the microstructure in weld center. (a) A1-4.2kJ/cm, (b) A2-4.67kJ/cm, (c) A3-5.13kJ/cm, (d) A4-5.60kJ/cm.

Figure 10 shows the effect of heat input on the microstructure in the fusion line and the adjacent region. With the increases of heat input, the width of fusion line merely has no changes, while the austenite grain size in HAZ is slightly increased, and the strip-shaped δ-ferrite content in the rolling direction is obviously reduced. This is because the strip-shaped δ-ferrite in HAZ is the residual ferrite phase at high temperature in the forging process of steel, which can be properly eliminated by solid solution treatment. When the heat input increases, the retention time of the metal near the fusion line becomesMaterials 2020 longer, 13, 2416 at a high temperature, which is equivalent to the prolongation of the holding time10 of of 18 solid solution treatment, thus leading to the decrease of strip-shaped δ-ferrite content in HAZ [52].

Materials 2020, 13, x FOR PEER REVIEW 12 of 20 Materials 2020, 13, x FOR PEER REVIEW 12 of 20 (a) (b)

Figure 10. EEffectﬀect ofof heatheat inputinput on on the the microstructure microstructure in in the the fusion fusion line line and and adjacent adjacent region. region. (a) A1—4.2(a) A1- Figure 10. Effect of heat input on the microstructure in the fusion line and adjacent region. (a) A1- 4.2kJ/cm,kJ/cm, (b) ( A2—4.67b) A2-4.67kJ/cm, kJ/cm, ( c(c)) A3—5.13 A3-5.13kJ/cm, kJ/cm, (d (d) )A4 A4—5.60-5.60kJ/cm. kJ/cm. 4.2kJ/cm, (b) A2-4.67kJ/cm, (c) A3-5.13kJ/cm, (d) A4-5.60kJ/cm. Figure 11 shows the effect eﬀect of heat input on the micro micro-hardness-hardness of of weld, weld, fusion fusion line, line, and and HAZ. HAZ. The micro-hardnessmicroFigure-hardness 11 shows inin the allall threeeffect three regions ofregions heat isinput is about about on between thebetween micro 160–180 -160hardness–180 HV, HV, whichof weld,which is largerfusion is larger than line, than that and ofthat HAZ. base of basemetalThe micrometal (135–160 -(135hardness– HV).160 HV). in The all The microhardnessthree microhardness regions is of about theof the fusion between fusion zone zone 160 is– i smaller180s smaller HV, than whichthan that that is oflargerof weldweld than and that HAZ, HAZ, of becausebase metal ofof non-uniform(135non–-uniform160 HV). distribution distribution The microhardness of of materials materials of the and and fusion chemical chemical zone compositions. is compositions.smaller than With that With the of weld increasethe increase and of HAZ, heat of heatinput,because input, the of micro-hardness nonthe -microuniform-hardness distribution of weld of weld and of HAZ andmaterials HAZ decreases anddecreases chemical slightly slightly due compositions. to due the to slight the With slight decrease the decrease increase of δ ferrite of ofδ ferritecontentheat input, content and the the and micro slight the- increasehardnessslight increase of of austenite weld of austeniteand grain HAZ size. grain decreases size. slightly due to the slight decrease of δ ferrite content and the slight increase of austenite grain size.

180 180 175 175 A1 170 A2A1 170 A3A2 165 A4A3 165 160 A4

160 155 155 150 Fusion zone 150 Weld

Microhardness (HV) 145 Weld Fusion zone HAZ

Microhardness (HV) 145 140 HAZ 140 135 135 0 2 4 6 8 10 12 0 2 4 6 8 10 12 Distance from weld center (mm) Distance from weld center (mm) Figure 11. Eﬀect of heat input on the micro-hardness for diﬀerent zones. Figure 11. Effect of heat input on the micro-hardness for different zones. Figure 11. Effect of heat input on the micro-hardness for different zones. 5.2. Residual Stress Analysis 5.2. Residual Stress Analysis Figure 12 shows the effect of heat input on the transverse and longitudinal residual stress distributionFigure 12along shows P1 by the finite effect element of heat simulation. input on theIt shows transverse that the and transverse longitudinal residual residua stressesl stress in thedistribution weld and along HAZ P1 is graduallyby finite element decreased simulation. with the Itincreases shows that of heat the transverseinput. When residual the heat stresses input inis increasedthe weld andfrom HAZ 4.20 tois 5.60gradually kJ/cm, decreased the maximum with t ransversethe increases residual of heat stress input. along When P1 is the decreased heat input from is 220increased to 196 fromMPa, 4.20 and to the 5.60 maximum kJ/cm, the value maximum is reduced transverse by about residual 11%. Thestress heat along input P1 hardlyis decreased affects from the longitudinal220 to 196 MPa, residual and the stress, maximum and the value longitudinal is reduced residual by about stress 11%. in The the heatweld input and hardlyHAZ is affects basically the maintalongitudinalined at residual approximately stress, and280 MPa.the longitudinal residual stress in the weld and HAZ is basically maintained at approximately 280 MPa.

Materials 2020, 13, 2416 11 of 18

5.2. Residual Stress Analysis Figure 12 shows the effect of heat input on the transverse and longitudinal residual stress distribution along P1 by finite element simulation. It shows that the transverse residual stresses in the weld and HAZ is gradually decreased with the increases of heat input. When the heat input is increased from 4.20 to 5.60 kJ/cm, the maximum transverse residual stress along P1 is decreased from 220 to 196 MPa, and the maximum value is reduced by about 11%. The heat input hardly affects the longitudinal residual stress, and the longitudinal residual stress in the weld and HAZ is basically Materialsmaintained 2020, 13 at, approximatelyx FOR PEER REVIEW 280 MPa. 13 of 20

(a) 250 (b) 400

300 200

MPa 200 150 100 HAZ Weld HAZ 100 HAZ Weld HAZ 0

-100 50 A1(4.20kJ/cm) -200 A1(4.20kJ/cm) 0 A2(4.67kJ/cm) A2(4.67kJ/cm)

Transverse residual stress/ A3(5.13kJ/cm) -300 A3(5.13kJ/cm) A4(5.60kJ/cm) stress/MPa residual Longitudinal A4(5.60kJ/cm) -50 -400 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 Distance from weld center/mm Distance from weld center/mm

Figure 12. EffectEﬀect of of heat heat input input on on the the residual residual stresses stresses along along P1. P1. (a) Transverse (a) Transverse stresses, stresses, (b) Longitudinal(b) Longitudinal stresses. stresses.

Figure 13 shows the effect of heat input on the residual stresses along P2. It can be seen that the Figure 13 shows the effect of heat input on the residual stresses along P2. It can be seen that the transverse residual stress at the starting point of the repair welding is less than that at ending point, transverse residual stress at the starting point of the repair welding is less than that at ending point, while the longitudinal residual stress is basically the same at the starting and ending point. This is while the longitudinal residual stress is basically the same at the starting and ending point. This is because the starting point of the repair welding is first heated, which is equivalent to the subsequent because the starting point of the repair welding is first heated, which is equivalent to the subsequent weld having a heat treatment effect on starting point. At the same time, the repair welding specimen is weld having a heat treatment effect on starting point. At the same time, the repair welding specimen isnot not horizontally horizontally constrained, constrained, so so that that the the transverse transverse residual residual stressstress atat thethe startingstarting point of the repair welding is partly released, and finally finally leads to the transverse transverse residual stress at the beginning beginning of of repair repair welding is less than that at end end point point of of repair repair welding. welding. The The longitudinal longitudinal residual residual str stressess at the the starting starting point of thethe repairrepair weldingwelding can can not not be be released released during during repair repair heating, heating, due due to theto the large large constraint constraint in the in thelongitudinal longitudinal direction, direction, thus thus leading leading to the to longitudinalthe longitudinal residual residual stress stress at start at start point point is the is samethe same as that as thatat ending at ending point. point. There There is a greatis a great transverse transverse stress stress gradient gradient at the at start the andstart endand point end point of repair of repair weld. weld.The transverse The transverse and longitudinal and longitudinal residual residual stresses stresses were increased were increased from compressive from compressive stress tostress tensile to tensilestress along stress thealong entire the length entire P2.length With P2. the With increases the increases of heat input, of heat the input, transverse the transverse residual residual stresses stressesin weld in were weld gradually were gradually decreased, decreased, while the while longitudinal the longitudinal stresses stresses merely havemerely no have changes. no changes. That is Thatbecause is because the peak the residual peak residual stresses stresses are determined are determined by the by yield the strengthyield strength instead instead of the of heat the input. heat input.The longitudinal The longitudinal stresses stresses in the weldin the andweld HAZ and haveHAZ reachedhave reached the yield the strength,yield strength, thus the thus heat the input heat inputaffects affects them little.them little.

(a) 300 (b)400 A1(4.20kJ/cm) A1(4.20kJ/cm) A2(4.67kJ/cm) A2(4.67kJ/cm) 200 Repair End A3(5.13kJ/cm) A3(5.13kJ/cm) MPa 300 start point point A4(5.60kJ/cm) A4(5.60kJ/cm) 100 200 0

100 -100

0 -200

Longitudinal residual stress/MPa

Transverse residual stress/

-300 -100 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Distance along P2/mm Distance along P2/mm

Figure 13. Effect of heat input on the residual stresses along P2. (a) Transverse stresses, (b) Longitudinal stresses.

Figure 14 shows the effect of heat input on the residual stresses in HAZ along P3. Similar to the residual stress in weld, the transverse residual stress was decreased as heat input increases and the

Materials 2020, 13, x FOR PEER REVIEW 13 of 20

(a) 250 (b) 400

300 200

MPa 200 150 100 HAZ Weld HAZ 100 HAZ Weld HAZ 0

-100 50 A1(4.20kJ/cm) -200 A1(4.20kJ/cm) 0 A2(4.67kJ/cm) A2(4.67kJ/cm)

Figure 12. Effect of heat input on the residual stresses along P1. (a) Transverse stresses, (b) Longitudinal stresses.

Figure 13 shows the effect of heat input on the residual stresses along P2. It can be seen that the transverse residual stress at the starting point of the repair welding is less than that at ending point, while the longitudinal residual stress is basically the same at the starting and ending point. This is because the starting point of the repair welding is first heated, which is equivalent to the subsequent weld having a heat treatment effect on starting point. At the same time, the repair welding specimen is not horizontally constrained, so that the transverse residual stress at the starting point of the repair welding is partly released, and finally leads to the transverse residual stress at the beginning of repair welding is less than that at end point of repair welding. The longitudinal residual stress at the starting point of the repair welding can not be released during repair heating, due to the large constraint in the longitudinal direction, thus leading to the longitudinal residual stress at start point is the same as that at ending point. There is a great transverse stress gradient at the start and end point of repair weld. The transverse and longitudinal residual stresses were increased from compressive stress to tensile stress along the entire length P2. With the increases of heat input, the transverse residual stresses in weld were gradually decreased, while the longitudinal stresses merely have no changes. That is because the peak residual stresses are determined by the yield strength instead of the heat Materialsinput. The2020 ,longitudinal13, 2416 stresses in the weld and HAZ have reached the yield strength, thus the12 heat of 18 input affects them little.

(a) 300 (b)400 A1(4.20kJ/cm) A1(4.20kJ/cm) A2(4.67kJ/cm) A2(4.67kJ/cm) 200 Repair End A3(5.13kJ/cm) A3(5.13kJ/cm) MPa 300 start point point A4(5.60kJ/cm) A4(5.60kJ/cm) 100 200 0

100 -100

0 -200

Longitudinal residual stress/MPa

Transverse residual stress/

-300 -100 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Distance along P2/mm Distance along P2/mm Figure 13. Eﬀect of heat input on the residual stresses along P2. (a) Transverse stresses, Figure 13. Effect of heat input on the residual stresses along P2. (a) Transverse stresses, (b) (b) Longitudinal stresses. Longitudinal stresses. Figure 14 shows the eﬀect of heat input on the residual stresses in HAZ along P3. Similar to the MaterialsFigure 2020, 1314, xshows FOR PEER the REVIEWeffect of heat input on the residual stresses in HAZ along P3. Similar 14to of the 20 residual stress in weld, the transverse residual stress was decreased as heat input increases and the residual stress in weld, the transverse residual stress was decreased as heat input increases and the longitudinal stress in HAZ hashas nono changes.changes. When the heat input was increased by 33%, the averageaverage transverse stress alongalong P3P3 inin weldweld andand HAZHAZ decreaseddecreased byby 30%30% andand 35%,35%, respectively.respectively.

(a)250 (b) 300

250

MPa 200

200 150 150

100 100 A1(4.20kJ/cm) A1(4.20kJ/cm) 50 A2(4.67kJ/cm) A2(4.67kJ/cm) 50 A3(5.13kJ/cm)

A3(5.13kJ/cm) Longitudinal residual stress/MPa Transverse residual stress/ A4(5.60kJ/cm) A4(5.60kJ/cm) 0 0 0 10 20 30 40 50 0 10 20 30 40 50 Distance along P3/mm Distance along P3/mm Figure 14. Effect of heat input on the residual stresses along P3. (a) Transverse stresses, Figure 14. Effect of heat input on the residual stresses along P3. (a) Transverse stresses, (b) (b) Longitudinal stresses. Longitudinal stresses. 5.3. SCC Sensitivity Analysis 5.3. SCC Sensitivity Analysis Figure 15 shows the fractured SSRT specimens and fracture position. All of the specimens were fracturedFigure on 15 the shows side of the the fractured original defect,SSRT specimens as shown inand Figure fracture 15b, position. which indicated All of the indicating specimens that were the originalfractured defect on the edge side of of the the repair original welding defect, is as the shown dangerous in Figure position 15b, towhich cause indicated the equipment indicating to crack that again.the original The inhomogeneous defect edge of the materials repair andwelding residual is the stress dangerous might induce position this. to cause the equipment to crackFigure again. 16 The show inhomogeneous the stress-strain materials curve of and SSRT residual test for distressfferent might specimens. induce Undoubtedly,this. the tensile strength and elongation in corrosive solution is smaller than that in air. Whether in air or corrosive solution, the tensile strength and elongation for larger heat input is larger than those for smaller heat input. The higher heat input can enhance the strength and mechanical properties of repaired joint. Table3 lists the tensile strength, fracture time, and elongation for di fferent SSRT specimens. According to Equation (6), the SCC sensitivity index can be obtained, as listed in Table4. The SCC sensitivity index is smaller than 25%, indicating the repaired joint has no obvious SCC tendency. The SCC sensitivity index for heat input 5.60 kJ/cm (7.53) is greatly smaller than that for heat input 4.20 kJ/cm (19.37). When the heat input was increased by 33%, the SCC sensitivity index was decreased by more than 60%. Therefore, increasing the heat input can enhance the resistance ability to stress corrosion of repaired joint.

Figure 15. The fractured SSRT specimens (a) and fracture position (b).

Figure 16 show the stress-strain curve of SSRT test for different specimens. Undoubtedly, the tensile strength and elongation in corrosive solution is smaller than that in air. Whether in air or corrosive solution, the tensile strength and elongation for larger heat input is larger than those for smaller heat input. The higher heat input can enhance the strength and mechanical properties of repaired joint. Table 3 lists the tensile strength, fracture time, and elongation for different SSRT specimens. According to Equation (6), the SCC sensitivity index can be obtained, as listed in Table 4. The SCC sensitivity index is smaller than 25%, indicating the repaired joint has no obvious SCC tendency. The SCC sensitivity index for heat input 5.60 kJ/cm (7.53) is greatly smaller than that for heat input 4.20 kJ/cm (19.37). When the heat input was increased by 33%, the SCC sensitivity index was decreased by more than 60%. Therefore, increasing the heat input can enhance the resistance ability to stress corrosion of repaired joint.

Materials 2020, 13, x FOR PEER REVIEW 14 of 20 longitudinal stress in HAZ has no changes. When the heat input was increased by 33%, the average transverse stress along P3 in weld and HAZ decreased by 30% and 35%, respectively.

(a)250 (b) 300

250

MPa 200

200 150 150

100 100 A1(4.20kJ/cm) A1(4.20kJ/cm) 50 A2(4.67kJ/cm) A2(4.67kJ/cm) 50 A3(5.13kJ/cm)

A3(5.13kJ/cm) Longitudinal residual stress/MPa Transverse residual stress/ A4(5.60kJ/cm) A4(5.60kJ/cm) 0 0 0 10 20 30 40 50 0 10 20 30 40 50 Distance along P3/mm Distance along P3/mm

Figure 14. Effect of heat input on the residual stresses along P3. (a) Transverse stresses, (b) Longitudinal stresses.

5.3. SCC Sensitivity Analysis Figure 15 shows the fractured SSRT specimens and fracture position. All of the specimens were fractured on the side of the original defect, as shown in Figure 15b, which indicated indicating that Materialsthe original2020, 13defect, 2416 edge of the repair welding is the dangerous position to cause the equipment13 of 18to crack again. The inhomogeneous materials and residual stress might induce this.

Materials 2020, 13, x FOR PEER REVIEW 15 of 20 Figure 15. The fractured SSRT specimens (a) and fracture position (b). Figure 15. The fractured SSRT specimens (a) and fracture position (b). 800

Figure 16 show the 700stress-strain curve of SSRT test for different specimens. Undoubtedly, the tensile strength and elongation in corrosive solution is smaller than that in air. Whether in air or corrosive solution, the tensile600 strength and elongation for larger heat input is larger than those for smaller heat input. The 500higher heat input can enhance the strength and mechanical properties of repaired joint. Table 3 lists the tensile strength, fracture time, and elongation for different SSRT 400 specimens. According to Equation (6), the SCC sensitivity index can be obtained, as listed in Table 4.

The SCC sensitivity indexStress/MPa 300 is smaller than 25%, indicating the repaired joint has no obvious SCC tendency. The SCC sensitivity index for heat input 5.60 kJ/cm (7.53) A1-1 is greatly smaller than that for 200 A1-2 heat input 4.20 kJ/cm (19.37). When the heat input was increased A4-1 by 33%, the SCC sensitivity index was decreased by more 100than 60%. Therefore, increasing the heat A4-2 input can enhance the resistance ability to stress corrosion of repaired joint. 0 0 10 20 30 40 50 60 70 Strain/% Figure 16. The stress-strain curve of SSRT test. Figure 16. The stress-strain curve of SSRT test. Table 4. The mechanical parameters and SCC sensitivity index of SSRT test. Table 4. The mechanical parameters and SCC sensitivity index of SSRT test. Tensile Strength Fracture Time Elongation SCC Sensitivity Specimen No. Tensile SCC Specimen Rm/MPa Fracturet/h Elongationδ /% Index Iδ/% Strength Sensitivity A1-1No. 691.64Time 196.62 t/h δ/% 48.83 Rm/MPa Index Iδ/% 19.37 A1-2 642.72 158.32 39.37 A4-1 737.04 218.22 54.96 A1-1 691.64 196.62 48.83 7.53 A4-2 729.76 201.78 50.82 19.37 A1-2 642.72 158.32 39.37

Figure 17 showsA4-1 the macro737.04 fracture surface218.22 morphology54.96 of SSRT specimens. The fracture morphology is different for different specimens. The fracture surface of weld7.53 zone and base metal is smooth, while the fractureA4-2 surface729.76 of HAZ is irregular,201.78 which50.82 is composed of several grain inclined planes in different directions. There are macroscopic cracks in the HAZ for specimen A1-1 and A1-2, and noFigure macroscopic 17 shows cracks the macro are found fracture in the surface HAZ for morphology specimen A4-1of SSRT and specimens. A4-2. The macroscopicThe fracture cracksmorphology are easily is different generated for indifferent HAZ for specimens. the smaller The heat fracture input, surface leading of to weld the smaller zone and tensile base strengthmetal is andsmooth, elongation. while the fracture surface of HAZ is irregular, which is composed of several grain inclined planes in different directions. There are macroscopic cracks in the HAZ for specimen A1-1 and A1-2, and no macroscopic cracks are found in the HAZ for specimen A4-1 and A4-2. The macroscopic cracks are easily generated in HAZ for the smaller heat input, leading to the smaller tensile strength and elongation.

Materials 2020, 13, x FOR PEER REVIEW 15 of 20

800

700

600

500

400

Stress/MPa 300 A1-1 200 A1-2 A4-1 100 A4-2

0 0 10 20 30 40 50 60 70 Strain/%

Figure 16. The stress-strain curve of SSRT test.

Table 4. The mechanical parameters and SCC sensitivity index of SSRT test.

Tensile SCC Specimen Fracture Elongation Strength Sensitivity No. Time t/h δ/% Rm/MPa Index Iδ/%

A1-1 691.64 196.62 48.83 19.37 A1-2 642.72 158.32 39.37

A4-1 737.04 218.22 54.96 7.53 A4-2 729.76 201.78 50.82

Figure 17 shows the macro fracture surface morphology of SSRT specimens. The fracture morphology is different for different specimens. The fracture surface of weld zone and base metal is smooth, while the fracture surface of HAZ is irregular, which is composed of several grain inclined planes in different directions. There are macroscopic cracks in the HAZ for specimen A1-1 and A1-2, and no macroscopic cracks are found in the HAZ for specimen A4-1 and A4-2. The macroscopic cracksMaterials are2020 easily, 13, 2416 generated in HAZ for the smaller heat input, leading to the smaller tensile strength14 of 18 and elongation.

Materials 2020, 13, x FOR PEER REVIEW 16 of 20

Figure 17. TheThe macro macro fracture fracture surface surface morphology morphology for for the the specimen specimen (a) (A1a)- A1-1,1, (b) A1 (b)-2, A1-2, (c) A4 (c)-1, A4-1, and

and(d) A4 (d)-2. A4-2.

The micro fracture surface morphology in weld by SEM is shown in Figure 18 inin order to reveal the eeffectﬀect ofof heatheat inputinput onon fracturefracture mechanismmechanism inin weld.weld. It is found that the microfracture surfaces in weldweld ofof thethe fourfour specimensspecimens showshow dimpledimple fracturefracture characteristics.characteristics. For the specimen A1 A1-1-1 and A1 A1-2,-2, the dimplesdimples are shallowshallow andand smallersmaller thanthan thatthat forfor thethe specimenspecimen A4-1A4-1 andand A4-2.A4-2. The higher the heat input,input, thethe deeperdeeper andand biggerbigger fracturefracture dimples,dimples, whichwhich resultsresults inin betterbetter mechanicalmechanical properties.properties.

Figure 18. The micro fracture surfacesurface morphologymorphology inin weld weld for for the the specimen specimen ( a()a) A1-1, A1-1, (b (b) A1-2,) A1-2, (c ()c A4-1,) A4- and1, and (d )(d A4-2.) A4-2.

Based on the above studies, we can conclude that a higher heat input is beneficialbeneficial for reducing residual stress and and enhan enhancingcing microstructure, microstructure, mechanical mechanical properties properties of of repair repair joint. joint. Heat Heat input input is an is anessential essential factor factor in quality in quality control control in repair in repair welding. welding. It influences It influences the cooling the cooling rate after rate welding, after welding, which whichaffects atheffects mechanical the mechanical properties properties and microstructure and microstructure of the weld of the and weld HAZ and and, HAZ consequently, and, consequently, affects atheffects residual the residual stress stressdistribution. distribution. With With the heat the heat input input increases, increases, the the maximum maximum temperature temperature at at weld pool is increased. On a micro scale, the δ-ferrite in weld and the average grain width decreases slightly with the increases of heat input, because the retention time of weld pool at a high temperature becomes longer. On the macro scale, the larger heat input leads to the increases of plastic zone, then resulting in the more residual stresses are released, which is similar to the results that were found by Jiang [15] and Unnikrishnana [33]. The tensile residual stress is main factor for resulting in SCC of welded joint at corrosive environment. The larger the residual stress, the easier to occur SCC. Thus, the SCC sensitivity index was decreased for the larger heat input, which was also verified by Mohammed [32]. To sum up, the larger heat input will be beneficial to improve the entire performance of welded joints, including strength, microstructure, corrosion resistance, etc. In reality, factories often meet production schedules by providing higher heat input to achieve higher welding speeds. Here, in this study, the heat input larger than 5 KJ/cm is recommended in the repair welding in 304 stainless steel in the case of no welding defects.

6. Conclusions In this paper, the effect of repair welding heat input on microstructure, residual stresses, and SCC sensitivity were investigated by simulation and experiment. An optimized repair welding heat input was proposed. The main conclusions are drawn, as follows:

Materials 2020, 13, 2416 15 of 18 pool is increased. On a micro scale, the δ-ferrite in weld and the average grain width decreases slightly with the increases of heat input, because the retention time of weld pool at a high temperature becomes longer. On the macro scale, the larger heat input leads to the increases of plastic zone, then resulting in the more residual stresses are released, which is similar to the results that were found by Jiang [15] and Unnikrishnana [33]. The tensile residual stress is main factor for resulting in SCC of welded joint at corrosive environment. The larger the residual stress, the easier to occur SCC. Thus, the SCC sensitivity index was decreased for the larger heat input, which was also veriﬁed by Mohammed [32]. To sum up, the larger heat input will be beneﬁcial to improve the entire performance of welded joints, including strength, microstructure, corrosion resistance, etc. In reality, factories often meet production schedules by providing higher heat input to achieve higher welding speeds. Here, in this study, the heat input larger than 5 KJ/cm is recommended in the repair welding in 304 stainless steel in the case of no welding defects.

6. Conclusions In this paper, the effect of repair welding heat input on microstructure, residual stresses, and SCC sensitivity were investigated by simulation and experiment. An optimized repair welding heat input was proposed. The main conclusions are drawn, as follows: (1) The heat input influences the microstructure. With the increase of heat input, the δ-ferrite in weld and the average grain width decrease slightly, while the austenite grain size in HAZ is slightly increased. The heat input has little effect on the micro-hardness. (2) With the increases of heat input, the transverse residual stresses in the weld and HAZ is gradually decreased, while the longitudinal stresses merely have no changes. When the heat input was increased by 33%, the average transverse stresses were decreased by more than 30%. (3) All of the specimens under SSRT were fractured on the side of the original defect. The higher heat input can enhance the tensile strength and elongation of repaired joint. When the heat input was increased by 33%, the SCC sensitivity index was decreased by more than 60%. (4) A different heat input has a different fracture mode in weld and HAZ. The macroscopic cracks are easily generated in HAZ for the smaller heat input, leading to the smaller tensile strength and elongation. The heat input larger than 5KJ/cm is recommended in the repair welding in 304 stainless steel.

Author Contributions: Conceptualization, Y.L.; Investigation, Y.L., W.G., W.P. and Q.J.; Software, Q.J. and Q.Q.; Data curation, W.G. and W.P.; Validation, Y.L.; Resources, C.Y.; Writing—original draft, Y.L.; Writing—review & editing, Q.Q. All authors have read and agreed to the published version of the manuscript. Funding: The authors gratefully acknowledge the support provided by National Natural Science Foundation of China (51905545), China Postdoctoral Science Foundation (2018M642723), National Key R&D Program of China (2018YFC0808801), Qingdao Postdoctoral Applied Research Project (qd20180070), the Young Scholar of the Yangtze River Scholars Program (Q2017137), Taishan Scholar Foundation (ts201511018), Fundamental Research Funds for the Central Universities (17CX05019; 18CX05002A) and the Natural Science Foundation of Shandong Province (ZR2019MEE108). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Bouchard, P.J. Special issue on residual stresses at repair welds. Int. J. Pres. Ves. Pip. 2005, 82, 243. [CrossRef] 2. Brown, T.B.; Dauda, T.A.; Truman, C.E.; Smith, D.J.; Pfeiﬀer, W. Predictions and measurements of residual stress in repair welds in plates. Int. J. Pres. Ves. Pip. 2006, 83, 809–818. [CrossRef] 3. Dong, P.; Hong, J.K.; Zhang, J.; Rogers, P.; Bynum, J.; Shah, S. Eﬀects of repair weld residual stresses on wide-panel specimens loaded in tension. Trans. ASME J. Press. Vessel. Technol. 1998, 120, 122–128. [CrossRef] 4. Nezhad, H.Y.; O’Dowd, N.P. Creep relaxation in the presence of residual stress. Eng. Fract. Mech. 2015, 138, 250–264. [CrossRef] Materials 2020, 13, 2416 16 of 18

5. Zhang, W.; Jiang, W.; Zhao, X.; Tu, S.-T. Fatigue life of a dissimilar welded joint considering the weld residual stress: Experimental and finite element simulation. Inter. J. Fatigue 2018, 109, 182–190. [CrossRef] 6. Xu, S.; Wei, Y.; Guo, D.; Zhang, L.; Wang, W. Numerical investigation of thermo-mechanical stress in U-tube including forming effect for the SCC failure analysis. Eng. Fail. Anal. 2017, 77, 126–137. [CrossRef] 7. Jun, H.-K.; Seo, J.W.; Jeon, I.S.; Lee, S.H.; Chang, Y.S. Fracture and fatigue crack growth analyses on a weld-repaired railway rail. Eng. Fail. Anal. 2016, 59, 478–492. [CrossRef] 8. Song, S.; Dong, P. Residual Stresses in Weld Repairs and Mitigation by Design. In Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering, San Francisco, CA, USA, 8–13 June 2014; Paper No: OMAE2014-24547, V005T03A038. 9. Shi, X.; Zhang, Y.L.; Zhao, J.P.; Gou, R.B. Evaluation of Welding Residual Stress Based on Temper Bead Welding Technique. Adv. Mater. Res. 2011, 418–420, 1208–1212. [CrossRef] 10. Dong, P. On repair weld residual stresses and significance to structural integrity. Weld. World 2018, 62, 351–362. [CrossRef] 11. Dong, P.; Zhang, J.; Bouchard, P.J. Effects of repair weld length on residual stress distribution. Trans. ASME J. Press. Vessel. Technol. 2002, 124, 74–80. [CrossRef] 12. Song, S.; Dong, P. Residual stresses at weld repairs and effects of repair geometry. Sci. Technol. Weld. Join. 2016, 22, 1–13. [CrossRef] 13. Jiang, W.; Gong, J.M.; Tu, S.T.; Li, G.C. Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate. Int. J. Pres. Ves. Pip. 2010, 87, 457–463. [CrossRef] 14. Jiang, W.; Yang, B.; Gong, J.M.; Tu, S.T. Effects of clad and base metal thickness on residual stress in the repair weld of a stainless steel clad plate. Trans. ASME J. Press. Vessel. Technol. 2011, 133, 061401. [CrossRef] 15. Jiang, W.C.; Wang, B.Y.; Gong, J.M.; Tu, S.T. Finite element analysis of the effect of welding heat input and layer number on residual stress in repair welds for a stainless steel clad plate. Mater. Des. 2011, 32, 2851–2857. [CrossRef] 16. Jiang, W.; Xu, X.P.; Gong, J.M.; Tu, S.T. Influence of repair length on residual stress in the repair weld of a clad plate. Nucl. Eng. Des. 2012, 246, 211–219. [CrossRef] 17. Jiang, W.; Luo, Y.; Zhang, G.; Woo, W.; Tu, S.T. Experimental to study the effect of multiple weld-repairs on microstructure, hardness and residual stress for a stainless steel clad plate. Mater. Des. 2013, 51, 1052–1059. [CrossRef] 18. Kessal, B.A.; Fares, C.; Meliani, M.H.; Alhussein, A.; François, M. Effect of gas tungsten arc welding parameters on the corrosion resistance and the residual stress of heat affected zone. Eng. Fail. Anal. 2020, 107, 104200. [CrossRef] 19. Winiczenko, R.; Kaczorowski, M. Friction welding of ductile iron with stainless steel. J. Mater. Proces. Technol. 2013, 213, 453–462. [CrossRef] 20. Li, X.; Xie, F.; Wang, D.; Xu, C.; Qi, J. Effect of residual and external stress on corrosion behaviour of X80 pipeline steel in sulphate-reducing bacteria environment. Eng. Fail. Anal. 2018, 91, 275–290. [CrossRef] 21. AghaAli, I.; Farzam, M.; Golozar, M.A.; Danaee, I. The effect of repeated repair welding on mechanical and corrosion properties of stainless steel 316L. Mater. Des. 2014, 54, 331–341. [CrossRef] 22. Katsas, S.; Nikolaou, J.; Papadimitriou, G. Corrosion resistance of repair welded naval aluminium alloys. Mater. Des. 2007, 28, 831–836. [CrossRef] 23. Venugopal, A.; Sreekumar, K.; Raja, V.S. Effect of repair welding on electrochemical corrosion and stress corrosion cracking behavior of TIG welded AA2219 aluminum alloy in 3.5 wt pct NaCl solution. Metal. Mater. Trans. A 2010, 41, 3151–3160. [CrossRef] 24. Salvador, C.F.; Antunes, R.A. FCAW repair welding cycles, HAZ microstructure and corrosion resistance of 2304 duplex stainless steel. Corro. Eng. Sci. Technol. 2016, 51, 573–580. [CrossRef] 25. Jiang, W.; Yu, Y.; Zhang, W.; Xiao, C.; Woo, W. Residual stress and stress fields change around fatigue crack tip: Neutron diffraction measurement and finite element modeling. Int. J. Pres. Ves. Pip. 2020, 179, 104024. [CrossRef] 26. Jiang, W.; Chen, W.; Woo, W.; Tu, S.T.; Zhang, X.C.; Em, V. Effects of low-temperature transformation and transformation-induced plasticity on weld residual stresses: Numerical study and neutron diffraction measurement. Mater. Des. 2018, 147, 65–79. [CrossRef] Materials 2020, 13, 2416 17 of 18

27. Jiang, W.; Woo, W.; Wan, Y.; Luo, Y.; Xie, X.; Tu, S.T. Evaluation of Through-Thickness Residual Stresses by Neutron Diffraction and Finite-Element Method in Thick Weld Plates. Trans. ASME J. Press. Vessel. Technol. 2017, 139, 031401. [CrossRef] 28. Wan, Y.; Jiang, W.; Luo, Y. Using X-ray diffraction and FEM to analyze residual stress of tube to tubesheet welded joints in a shell and tube heat exchanger. ASME J. Press. Vessel. Technol. 2017, 139, 051405. [CrossRef] 29. Nowacki, J.; Rybicki, P. The influence of welding heat input on submerged arc welded duplex steel joints imperfections. J. Mater. Proces. Technol. 2005, 164–165, 1082–1088. [CrossRef] 30. Akbari, D.; Sattari-Far, I. Effect of the welding heat input on residual stresses in butt-welds of dissimilar pipe joints. Int. J. Pres. Ves. Pip. 2009, 86, 769–776. [CrossRef] 31. Hemmatzadeh, M.; Moshayedi, H.; Sattari-Far, I. Influence of heat input and radius to pipe thickness ratio on the residual stresses in circumferential arc welded pipes of API X46 steels. Int. J. Pres. Ves. Pip. 2017, 150, 62–71. [CrossRef] 32. Mohammed, A.M.; Shrikrishna, K.A.; Sathiya, P.; Goel, S. The impact of heat input on the strength, toughness, microhardness, microstructure and corrosion aspects of friction welded duplex stainless steel joints. J. Manuf. Process. 2015, 18, 92–106. 33. Unnikrishnana, R.; Idurya, K.S.N.S.; Ismaila, T.P.; Bhadauriaa, A.; Shekhawatb, S.K.; Khatirkara, R.K.; Sapatea, S.G. Effect of heat input on the microstructure, residual stresses and corrosion resistance of 304L austenitic stainless steel weldments. Mater. Charact. 2014, 93, 10–23. [CrossRef] 34. Ravisankar, A.; Velaga, S.K.; Rajput, G.; Venugopal, S. Influence of welding speed and power on residual stress during gas tungsten arc welding (GTAW) of thin sections with constant heat input: A study using numerical simulation and experimental validation. J. Manuf. Process. 2014, 16, 200–211. [CrossRef] 35. Jiang, W.; Luo, Y.; Li, J.H.; Woo, W. Residual Stress Distribution in a Dissimilar Weld Joint by Experimental and Simulation Study. Trans. ASME J. Press. Vessel. Technol. 2016, 139, 011402. [CrossRef] 36. Jiang, W.; Luo, Y.; Wang, B.; Woo, W.; Tu, S.T. Neutron diffraction measurement and numerical simulation to study the effect of repair depth on residual stress in 316L stainless steel repair weld. Trans. ASME J. Press. Vessel. Technol. 2015, 137, 041406. [CrossRef] 37. Luo, Y.; Jiang, W.; Chen, D.; Wimpory, R.C.; Li, M.; Liu, X. Determination of Repair Weld Residual Stress in a Tube to Tube-Sheet Joint by Neutron Diffraction and the Finite Element Method. Trans. ASME J. Press. Vessel. Technol. 2018, 140, 021404. [CrossRef] 38. Xie, X.-F.; Jiang, W.; Luo, Y.; Xu, S.; Gong, J.-M.; Tu, S.-T. A model to predict the relaxation of weld residual stress by cyclic load: Experimental and finite element modeling. Int. J. Fatigue 2017, 95, 293–301. [CrossRef] 39. Jiang, W.; Luo, Y.; Wang, B.Y.; Tu, S.T.; Gong, J.M. Residual stress reduction in the penetration nozzle weld joint by overlay welding. Mater. Des. 2014, 60, 443–450. [CrossRef] 40. Luo, Y.; Jiang, W.; Wan, Y.; Woo, W.; Tu, S.-T. Effect of helix angle on residual stress in the spiral welded oil pipelines: Experimental and finite element modeling. Int. J. Pres. Ves. Pip. 2018, 168, 233–245. [CrossRef] 41. Luo, Y.; Jiang, W.; Zhang, Q.; Wan, Y.; Zhang, W.Y.; Wang, Y.J. Experimental and numerical study on the reduction of residual stress in the fillet weld by overlay welding and cutting method. Trans. ASME J. Press. Vessel. Technol. 2016, 138, 061405. [CrossRef] 42. Jiang, W.; Luo, Y.; Wang, H.; Wang, B.Y. Effect of impact pressure on reducing the weld residual stress by high pressure water jetting for 304 stainless steel clad plate. Trans. ASME J. Press. Vessel. Technol. 2015, 137, 031401. [CrossRef] 43. Wan, Y.; Jiang, W.; Song, M.; Huang, Y.; Li, J.; Sun, G.; Shi, Y.; Zhai, X.; Zhao, X.; Ren, L. Distribution and formation mechanism of residual stress in duplex stainless steel weld joint by neutron diffraction and electron backscatter diffraction. Mater. Des. 2019, 1815, 108086. [CrossRef] 44. Wan, Y.; Jiang, W.; Li, J.; Sun, G.; Kim, D.-K.; Woo, W.; Tu, S.-T. Weld residual stresses in a thick plate considering back chipping: Neutron diffraction, contour method and finite element simulation study. Mater. Sci. Eng. A 2017, 69924, 62–70. [CrossRef] 45. Jiang, W.; Guan, X.-W. A study of the residual stress and deformation in the welding between half-pipe jacket and shell. Mater. Des. 2013, 51, 1052–1059. [CrossRef] 46. Jiang, W.; Woo, W. Neutron diffraction and finite element modeling to study the weld residual stress relaxation induced by cutting. Mater. Des. 2013, 51, 415–420. [CrossRef] 47. Jiang, W.; Zhang, Y.; Woo, W. Using heat sink technology to decrease residual stress in 316L stainless steel welding joint finite element simulation. Int. J. Pres. Ves. Pip. 2012, 92, 56–62. [CrossRef] Materials 2020, 13, 2416 18 of 18

48. Jiang, W.; Gong, J.M.; Wang, Y.F.; Woo, W.; Tu, S.T. Control of welding residual stress and deformation of the butt welded ultra-thick tube-sheet in a large scale EO reactor: Effect of applied load. Trans. ASME J. Press. Vessel. Technol. 2012, 134, 061406. [CrossRef] 49. Goldak, J.; Chakravarti, A.; Bibby, M. A new finite element model for welding heat sources. Metal. Mater. Trans. B 1984, 15, 299–305. [CrossRef] 50. GB7704-2008, Non-Destructive Testing Practice for Residual Stress Measurement by X-Ray; China Standard Press: Beijing, China, 2008. 51. Chandra, K.; Kain, V.; Bhutani, V.; Raja, V.S.; Tewari, R.; Dey, G.K.; Chakravartty, J.K. Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties. Mater. Sci. Eng. A 2012, 5341, 163–175. [CrossRef] 52. Zhang, Y.; Fan, M.; Ding, K.; Zhao, B.; Zhang, Y.; He, Y.; Wang, Y.; Wu, G.; Wei, T.; Gao, Y. Formation and control of the residual δ-ferrite in 9% Cr-HAZ of Alloy 617/9% Cr dissimilar welded joint. Sci. Technol. Weld. Join. 2020.[CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).