Effect of Micro-Segregation on Impact Toughness of 2.25Cr-1Mo Steel After Post Weld Heat Treatment

metals

Article Effect of Micro-Segregation on Impact Toughness of 2.25Cr-1Mo Steel after Post Weld Heat Treatment

Hyesung Na 1 ID , Sanghoon Lee 2 and Chungyun Kang 1,*

1 Department of Materials Science and Engineering, Pusan National University, San 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea; [email protected] 2 Industrial Technology Support Division, Korea Institute of Materials Science, 797 Changwondaero, Changwon 641-831, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-51-510-2852

Received: 11 May 2018; Accepted: 21 May 2018; Published: 23 May 2018

Abstract: 2.25Cr-1Mo steel with high strength at high temperatures and superior hydrogen resistance is widely used as power generation boiler material in high-temperature and high-pressure environments. Following the test evaluation of the ASME Boiler and Pressure Vessel Code, specimens from the base metal of a boiler pipe were found to have impact toughness values of 386 and 28J, which are drastically different values. The analysis of the fracture surface of the 28J test specimen revealed MnS inclusions and it was found that cracks were initiated at the inclusions. Observation of the cross-section of the crack propagation front revealed that cracks propagated along the ferrite regions and precipitate voids. Inclusions were also found in the 386J impact specimen. However, the volume fraction of the inclusions was signiﬁcantly less than that of the 28J specimen. It was also found that the ferrite and carbide content of the 386J specimen was less than the 28J specimen. The reason that the inclusions, ferrite, and carbide content differed in the two adjacent impact test specimens was analyzed. The effects of micro-segregation such as MnS inclusions on ferrite and carbide were compared and analyzed.

Keywords: ferrite; carbide; bainite; continuous cooling transformation (CCT) curve; charpy impact test

1. Introduction The 2.25Cr-1Mo Steel is widely used as a power plant boiler pipe material [1–4] due to its high temperature creep strength, good hydrogen resistance, high toughness at high temperatures, and the ability to withstand high pressure hydrogen atmospheres (12–21 MPa) and high temperatures (400 ◦C to 450 ◦C) and pressure use environments. Due to its excellent mechanical properties, 2.25Cr-1Mo steel is widely used not only in thermal power plants but also in nuclear power plants where high stability is required [5]. However, 2.25Cr-1Mo steel is prone to reheating cracks after weld heat treatment (PWHT) is performed to remove residual stress after cracking due to the thermal stress from the heating and cooling cycles when the power plant is operating [6–8]. The integrity of the material is crucial since such cracks can cause plant shutdowns and secondary hazards. Inspection of the components of a power plant is carried out according to the US ASME Boiler and Pressure Vessel Code and Nuclear Power Plant Components Certiﬁcation [9]. An important quality test used to evaluate the material integrity is the Charpy V-notch impact test of fracture toughness [10]. For the base metal of boiler tubes, the impact test should also be tested under PWHT heat treatment conditions performed after the welding process. However, in the special case, the impact toughness values measured from the three impact tests can have a large variability even under the same conditions. This phenomenon occasionally occurs in impact specimens taken from the same pipe and even at similar locations. The impact toughness of the

Metals 2018, 8, 373; doi:10.3390/met8060373 www.mdpi.com/journal/metals Metals 2018, 8, 373 2 of 12 material is affected by many parameters such as microstructure, phases fractions and distributions, test temperature, grain size, impact velocity, specimen shape, and notch position (sampling position of test specimen)Metals 2018, [811, x –FOR18]. PEER In theREVIEW impact test of the base metal, not many variables affect the tests2 andof 12 they are completely linked to metallurgical aspects. If the material fails to meet the impact test requirements even at similar locations. The impact toughness of the material is affected by many parameters such duringas a microstructure, quality assurance phases inspection, fractions itand is necessary distribution tos, analyze test temperature, the cause grain and take size, measures impact velocity, to prevent recurrence.specimen In addition,shape, and it notch is possible position to (sampling prevent the positi recurrenceon of test by specimen) analyzing [11–18]. the cause In the of impact the variation test of theof impact the base test metal, and itnot can many be utilizedvariables as affect a basic the datatests and of the they quality are completely defect. In linked this study,to metallurgical we analyzed the variabilityaspects. If of the the material impact fails test resultsto meet of the 2.25Cr-1.0Mo impact test requirements steel in a power during plant a quality boiler pipeassurance to better understandinspection, the causeit is necessary of the variability. to analyze the Through cause and these take studies, measures we to have prevent found recurrence. that microsegregation In addition, (MnSit inclusion) is possible affects to prevent phase the transformation recurrence by analyzing in mztrix the structure cause of the and variation this result of the is consideredimpact test and to be a referenceit can material be utilized for as PWHT a basic steel. data of the quality defect. In this study, we analyzed the variability of the impact test results of 2.25Cr-1.0Mo steel in a power plant boiler pipe to better understand the cause 2. Experimentalof the variability. Procedure Through these studies, we have found that microsegregation (MnS inclusion) affects phase transformation in mztrix structure and this result is considered to be a reference material ◦ Thefor PWHT steel specimens steel. used in this study were SA335 GR.P22. Specimens were analyzed at the 0 , 90◦, and 180◦ positions. Composition analysis was done by optical emission spectrometer (LAB LAVM 10, SPECTRO2. Experimental Analytical Procedure Instruments GmbH, Kleve, Germany) and alloy components are shown in Table1. The steel specimens used in this study were SA335 GR.P22. Specimens were analyzed at the 0°, 90°, and 180° positions. Composition analysis was done by optical emission spectrometer (LAB Table 1. LAVM 10, SPECTRO AnalyticalChemical Instruments composition GmbH, of 2.25Cr-1Mo Kleve, Germany) steel (wt and %). alloy components are shown in Table 1. Chemical Composition (at. %) Material C SiTable Mn 1. Chemical Cu Ni composition Cr of Mo 2.25Cr-1Mo V steel Nb (wt %). Al P S Fe

0.102 0.218 0.433 0.120 0.1246Chemical 2.0948 Composition 0.930 0.011 (at. %) 0.0023 0.0251 0.0140 0.0048 Bal Material P22 0.097C 0.227Si 0.443Mn 0.124Cu 0.1239Ni 2.0305 Cr 0.931Mo 0.012V 0.0009 Nb 0.0247Al 0.0116 P 0.0039 S FeBal 0.1000.102 0.2290.218 0.4340.433 0.1160.120 0.1246 0.1230 2.00512.0948 0.9310.930 0.0110.0141 0.00230 0.02510.0246 0.0140 0.0098 0.0048 0.0047 BalBal AverageP22 0.1000.097 0.2250.227 0.4360.443 0.1200.124 0.1239 0.1238 2.04352.0305 0.931 0.0120.0124 0.0009 0.0011 0.0247 0.0248 0.0116 0.0118 0.0039 0.0047 BalBal 0.100 0.229 0.434 0.116 0.1230 2.0051 0.931 0.0141 0 0.0246 0.0098 0.0047 Bal Average 0.100 0.225 0.436 0.120 0.1238 2.0435 0.931 0.0124 0.0011 0.0248 0.0118 0.0047 Bal The base metal was PWHT annealed at 710 ◦C for 11 h and detailed heat treatment conditions are The base metal was PWHT annealed at 710 °C for 11 h and detailed heat treatment conditions shown in Table2. During the impact test of the PWHT specimens, the test specimens were randomly are shown in Table 2. During the impact test of the PWHT specimens, the test specimens were sampled at 1/4” thickness of the pipe (3 each × 6 set). The Charpy impact test method was tested by randomly sampled at 1/4” thickness of the pipe (3 each × 6 set). The Charpy impact test method was the standardtested by test the methodstandard of test ASTM method E 23of ASTM and the E 23 specimen and the specimen axis was axis produced was produced in the verticalin the vertical direction of thedirection pipe. The of sizesthe pipe. of the The pipe sizes and of the impact pipe specimensand impact arespecimens shown are in Figureshown 1in. TheFigure impact 1. The test impact (Zwick ◦ Roell-PSW750,test (Zwick Ulm, Roell-PSW750, Germany) Ulm, was Germany) carried out wa ats carried a temperature out at a temperature of 1 C. of 1 °C.

FigureFigure 1. Schematic 1. Schematic of pipe of pipe size size and and charpy charpy V-notch V-notchimpact impact specimen (a ()a pipe) pipe size size (b) ( bsingle-notch.) single-notch.

Two test specimens (referred to as 386 and 28J) with appreciable differences in impact values Twowere test selected specimens after (referredimpact testing to as 386and and the 28J) caus withe of appreciable the difference differences of the impact in impact values values was were selectedinvestigated. after impact The testingfracture andsurfaces the of cause the specimen of the differences was observed of the with impact a stereo values microscope was investigated. (optical The fracturemicroscope, surfaces OM, of Olympus the specimens BX-51M, was Tokyo, observed Japan) with and a scanning stereo microscope electron microscope (optical microscope, with energy OM, Olympusdispersive BX-51M, X-ray Tokyo, spectroscopy Japan) (SEM-EDX, and a scanning SUPRA40V electronP, Carl microscope Zeiss, Oberkochen, with energy Germany) dispersive and their X-ray spectroscopymicrostructures (SEM-EDX, were analyzed SUPRA40VP, with OM, Carl SEM-EDX, Zeiss, Oberkochen, and electron Germany) probe microanalysis and their (EPMA, microstructures JXA- were analyzed8530F, JEOL, with Tokyo, OM, SEM-EDX,Japan). Test andspecimens electron were probe etch microanalysised with Nital (3% (EPMA, nitric in JXA-8530F, alcohol) and JEOL, Ferrite Tokyo,

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Japan). Test specimens were etched with Nital (3% nitric in alcohol) and Ferrite and carbide counting was performed using ASTM E-562 (standard test for determining volume fraction) and image analysis software (Image Pro Plus5.0, Media Cybernetics, Washington, DC, USA). The programs Thermo-Calc

TCW5Metals (database: 2018, 8, x FOR TCFE6, PEER REVIEW Stockholm, Sweden, where Mats Hillert was a professor) and JMatPro3 of 12 5.1 software (Sente Software, Surrey Research Park, Guildford, UK) were used for state diagram and continuousand carbide cooling counting transformation was performed (CCT) using curve ASTM analysis. E-562 (standard test for determining volume fraction) and image analysis software (Image Pro Plus5.0, Media Cybernetics, Washington, DC, USA). The programs Thermo-CalcTable 2. PWHT TCW5 (Post (database: Weld Heat TCFE6, Treatment) Stockholm, conditions Sweden, of Cr-Mo where steel. Mats Hillert was a professor) and JMatPro 5.1 software (Sente Software, Surrey Research Park, Guildford, UK) were used for state diagram and continuousCondition cooling transformation (CCT) Actual curve analysis. Loading Temperature 20 ◦C Table 2. PWHT (Post Weld Heat Treatment) conditions of Cr-Mo steel. Heating Rate 20 ◦C/h Condition Actual Holding Temperature 700–710 ◦C Loading Temperature 20 °C HoldingHeating Time Rate 20 °C/h 11 h CoolingHolding Rate (above Temperature 425 ◦C) 700–710 20–30 °C◦C/h Holding Time 11 h Cooling Rate (above 425 °C) 20–30 °C/h 3. Results and Discussion

3.1. Cause3. Results Analysis and Discussion of Crack Initiation Point The3.1. Cause purpose Analysis of this of Crack study Initiation was to analyzePoint the variability in the impact values during impact testing. As mentionedThe purpose in the Experimentalof this study was Procedure to analyze section, the variability the test specimens in the impact were values randomly during pickedimpact from six setstesting. of three As mentioned sets of one in setthe and Experimental subjected Proced to theure impact section, test. the In test the specimens total six setwere test randomly piece, a test piecepicked in the from ﬁrst six set sets with of athree markedly sets of one different set and impact subjected value to the was impact found. test. Figure In the2 total shows six theset test impact valuespiece, of the a test ﬁrst piece set andin the the first macro-fracture set with a markedly surface different photographs impact value of each was test found. specimen. Figure 2 Theshows impact test valuesthe impact of were values 386, of the 379, first and set 28J, and respectively. the macro-fracture Two specimenssurface photographs had high of impact each test values specimen. (386 and 379J),The but impact one specimen test values hadof were a low 386, impact 379, and value 28J, respectively. of 28J. Only Two one specimens of the six had total high sets impact (18ea) values had an (386 and 379J), but one specimen had a low impact value of 28J. Only one of the six total sets (18ea) impact test value less than 135J (5.55%). The specimens with low impact test values is featured by a had an impact test value less than 135J (5.55%). The specimens with low impact test values is featured brittle fracture surface. The test specimens with high impact values were not completely destroyed nor by a brittle fracture surface. The test specimens with high impact values were not completely did brittledestroyed fractures nor did appear. brittle fractures appear.

FigureFigure 2. Absorbed 2. Absorbed energy energy of of Charpy Charpy impact impact testtest of steel specimens specimens (left (left): a): = a 385J, = 385J, b = 379J, b = 379J, c = 28J c = 28J and corresponding images of the fractured specimens (right). and corresponding images of the fractured specimens (right). To investigate the cause of the low impact test values, the fracture surface of the 28 and 386J test Tospecimens investigate were theexamined cause by of SEM. the lowThe SEM impact images test ar values,e shown the in Figure fracture 3. At surface position of X1 the in 28 Figure and 386J test specimens3a, the 28J test were specimen examined showed by SEM.mostly The brittle SEM fractu imagesre. It has are flat shown cleavage in Figure fractures3. At with position a size of X1 in Figure20~303a, theμm 28Jand test particles specimen with 2 showed μm diameters mostly show brittle that fracture. the river It patte hasrn ﬂat is cleavagespreading. fractures This part withis a judged as the crack initiation point. When the region around X1 is enlarged, it was found that the brittle fracture originates at a particle with about a 2 μm diameter. Particles with 2 μm diameters near

Metals 2018, 8, 373 4 of 12 size of 20~30 µm and particles with 2 µm diameters show that the river pattern is spreading. This part is judged as the crack initiation point. When the region around X1 is enlarged, it was found that the brittle fracture originates at a particle with about a 2 µm diameter. Particles with 2 µm diameters near the pointMetalsMetals X22018 2018 were, 8,, x8 ,FOR x foundFOR PEER PEER toREVIEW REVIEW be MnS (Mn: 51.33% and S: 36.06%) when analyzed by SEM-EDX.4 of4 12of 12 It has been reported in many studies that MnS inclusions have a negative effect on mechanical properties thethe point point X2 X2 were were found found to to be be MnS MnS (Mn:(Mn: 51.33%51.33% andand S:S: 36.06%)36.06%) when when analyzed analyzed by by SEM-EDX. SEM-EDX. It has It has (impact, strength, elongation) [19–21]. The low impact value of the 28J specimen is caused by the MnS beenbeen reported reported in in many many studies studies that that MnSMnS inclusions havehave a a negative negative effect effect on on mechanical mechanical properties properties inclusion.(impact,(impact, Figure strength, strength,3b shows elongation) elongation) SEM images[19–21]. [19–21]. ofTheThe the lowlow 386J impact test value value specimen, of of the the 28J 28J which specimen specimen has is a iscaused typical caused by ductile bythe the MnS MnS fracture zoneinclusion. whereinclusion. MnS Figure Figure inclusions 3b 3b shows shows were SEM SEM not images images observed. ofof thethe 386J Regions testtest specimen, specimen, wherethe which which inclusions has has a typicala typical were ductile ductile expected fracture fracture to be can be seenzonezone in where various where MnS MnS parts inclusions inclusions of the image.were were not not It observed. observed. can be seen Region thatss where where it is not the the aninclusions inclusions equiaxed were were dimple expected expected but to be anto canbe elongated can dimplebe be shape.seen seen in invarious It various can beparts parts seen of of the that the image. image. the elongated ItIt cancan be seen dimple thatthat it itis is is stretchednot not an an equiaxed equiaxed in the dimple dimple notch but direction.but an anelongated elongated The shear ledgesdimple coulddimple shape. also shape. be It Itcan observed. can be be seen seen that that the the elongatedelongated dimple isis stretched stretched in in the the notch notch direction. direction. The The shear shear ledgesledges could could also also be be observed. observed.

FigureFigure 3. SEM 3. SEM observation observation of of the thefracture fracture surface. surface. (a) ( a28J;) 28J; (b) 386J (b) 386Jspecimen. specimen. Figure 3. SEM observation of the fracture surface. (a) 28J; (b) 386J specimen. Cross-sections of the impact test specimens were examined to investigate the presence or absence of inclusions. Cross-sections of the impact test specimens were etched with Nital (3% nitric Cross-sectionsCross-sections of the of impactthe impact test specimenstest specimens were we examinedre examined to investigate to investigate the presencethe presence or absenceor in alcohol). Then inclusions in the 28J and 386J specimens were observed with SEM, which is shown of inclusions.absence of Cross-sections inclusions. Cross-sections of the impact of the test impact specimens test specimens were etched were with etched Nital with (3% Nital nitric (3% in nitric alcohol). in Figure 4a,b, respectively. Observation positions were chosen to be as close to the fracture surface Thenin inclusions alcohol). Then in the inclusions 28J and 386J in the specimens 28J and 386J were specimens observed were with observed SEM, whichwith SEM, is shown which in is Figureshown4 a,b, (red vertical lines) as possible. Inclusion shapes were examined in the region around the points X1, respectively.in X2,Figure Y1, Observation4a,b,and Y2respectively. in Figure positions 4.Observation The wereinclusion positions chosen sizes tovaried we bere aschosenfrom close 1 toμ to mbe theto as 10 close fracture μm to and the surfacethe fracture SEM-EDX (red surface vertical lines)(red ascomposition possible. vertical lines) Inclusionanalysis as possible. revealed shapes Inclusionthat were the examinedinclusions shapes were co innsisted the examined region of a complex in around the oxideregion the of pointsaround Al oxide, X1, the MnS, X2,points and Y1, X1, and Y2 in FigureX2,Al Y1,4 oxide.. The and The inclusionY2 inclusion in Figure sizes size 4. variedandThe volume inclusion from fraction 1 sizesµm of tovaried the 10 386Jµ mfrom impact and 1 the μspecimenm SEM-EDXto 10 were μm lessand composition than the thoseSEM-EDX of analysis revealedcompositionthe that 28J theimpact inclusionsanalysis specimen. revealed consisted that the of a inclusions complex co oxidensisted of Alof a oxide, complex MnS, oxide and of Al Al oxide.oxide, MnS, The inclusion and size andAl oxide. volume The fraction inclusion of size the and 386J volume impact fraction specimen of the were 386Jless impact than specimen those of were the 28Jless impactthan those specimen. of the 28J impact specimen.

Figure 4. OM and SEM images of inclusions in the cross-section of the (a) 28J and (b) 386J specimen.

Figure 4. OM and SEM images of inclusions in the cross-section of the (a) 28J and (b) 386J specimen. Figure 4. OM and SEM images of inclusions in the cross-section of the (a) 28J and (b) 386J specimen.

Metals 2018, 8, 373 5 of 12 Metals 2018, 8, x FOR PEER REVIEW 5 of 12 Metals 2018, 8, x FOR PEER REVIEW 5 of 12 ItIt has has been been reported reported in in the the literature literature that that the the inclusion inclusion volume volume fraction fraction affects affects the the impact impact value, value, whichIt hasmeans been the reported inclusion in the(MnS, literature AlO) volume that the fraction inclusion has volume been measured. fraction affects To To accurately the impact measure value, whichthethe inclusion inclusion means volumethe volume inclusion fraction fraction (MnS, of of the theAlO) two two volume test test spec specimens, fractionimens, has EPMA EPMA been back-scatter back-scatter measured. Toelectron electron accurately detector detector measure (BSE) (BSE) theimagesimages inclusion werewere collectedvolumecollected fraction and and are are of shown shownthe two in Figurein test Figure spec5. Inimens, 5. the In images theEPMA images from back-scatter thefrom EPMA-BSE, the electron EPMA-BSE, elements detector elements heavier (BSE) imagesheavieror lighter wereor thanlighter collected Fe canthan be andFe observed. canare beshown observed. The in elements Figure The 5.elements Al, In Mn, the S,imagesAl, and Mn, O from inS, theand the MnSO EPMA-BSE,in andthe AlOMnS inclusions elementsand AlO heavierinclusionsare lighter or arelighter than lighter Fe than and than Fe appear Fecan and be as appearobserved. black as regions bl Theack elementsregions in the EPMA-BSE in Al, the Mn, EPMA-BSE S, images. and Oimages. in For the 2.25Cr-1Mo For MnS 2.25Cr-1Mo and steel,AlO inclusionssteel,M23C6 M23C6 and are M6C and lighter precipitatesM6C than precipitates Fe and are observedappear are observed as [bl22ack,23 ].[22,23 regions One]. of One in the the of main EPMA-BSEthe elementsmain elements images. of the of precipitateFor the 2.25Cr-1Mo precipitate is Mo, steel,iswhich Mo, M23C6 which is heavier andis heavier M6C than FEprecipitates than and FE appears and are appears observed as white as white[22,23 regions regions]. One in the of in EPMA-BSEthe the main EPMA-BSE elements images images of [24 the]. [24]. Figureprecipitate Figure5a,b is5a,bshows Mo, shows which the 28 the is and heavier28 386J and specimens, than386J FEspecimens, and respectively. appears respective as Blackwhitely. inclusionsregions Black inclusionsin andthe EPMA-BSE white and precipitates white images precipitatesare [24]. observed Figure are 5a,bobservedin both shows specimens. in theboth 28 specimens. and However, 386J However,specimens, the 386J specimen the respective 386J sp showsecimenly. Black fewer shows inclusions black fewer inclusions blackand whiteinclusions than theprecipitates 28J than specimen. the are28J observedspecimen.To determine in To both determine the specimens. inclusion the inclusionHowever, (MnS, AlO) (MnS, the volume 386J AlO) sp fraction,ecimenvolume showsfraction, we measured fewer we measuredblack the inclusions total the size total than of size the the of black 28Jthe specimen.blackinclusion inclusion regionsTo determine regions in 12 BSE thein inclusion12 images BSE usingimages (MnS, image AlO)using analysisvolu imageme fraction, softwareanalysis wesoftware (image measured Pro (image Plus). the totalPro The Plus).size measured of Thethe blackmeasuredresults inclusion are results shown regions are in Figureshown in 612 .in BSEFigure images 6. using image analysis software (image Pro Plus). The measured results are shown in Figure 6.

FigureFigure 5. 5. CountingCounting inclusion inclusion using using EPMA-BSE EPMA-BSE image image ( (aa)) 28J 28J and and ( (bb)) 386J 386J Specimen. Specimen. Figure 5. Counting inclusion using EPMA-BSE image (a) 28J and (b) 386J Specimen. The inclusion volume fraction was about 0.8% for the 28J specimen and 0.1% for the 386J The inclusion volume fraction was about 0.8% for the 28J specimen and 0.1% for the 386J specimen. specimen.The inclusion Therefore, volume the inclusion fraction volume was about fraction 0.8% in forthe the28J specimen28J specimen is about and eight0.1% timesfor the greater 386J Therefore, the inclusion volume fraction in the 28J specimen is about eight times greater than the specimen.than the 386J Therefore, specimen. the Oneinclusion of the volume reasons fraction for the lowin the impact 28J specimen value of isthe about 28J specimen eight times is thatgreater the 386J specimen. One of the reasons for the low impact value of the 28J specimen is that the impact thanimpact the value 386J specimen.decreases asOne the of inclusion the reasons volume for the fraction low impact increases. value This of isthe consistent 28J specimen with ais previous that the value decreases as the inclusion volume fraction increases. This is consistent with a previous result impactresult reported value decreases in the literature as the inclusion [25]. It volumeis eviden fractiont that theincreases. inclusion This volume is consistent fraction with varies a previous greatly reported in the literature [25]. It is evident that the inclusion volume fraction varies greatly between resultbetween reported specimens in the even literature within [25].the same It is pipe.eviden Thist that ha sthe been inclusion attributed volume to a casting fraction defect varies caused greatly by specimens even within the same pipe. This has been attributed to a casting defect caused by the local betweenthe local specimensincorporation even of within inclusions the same during pipe. melting This haands been casting attributed [26]. to a casting defect caused by theincorporation local incorporation of inclusions of inclusions during melting during andmelting casting and [ 26casting]. [26].

Figure 6. The experimentally measured specimens and the amount of inclusions. FigureFigure 6. 6. TheThe experimentally experimentally measured measured specimens specimens and and the the amount amount of of inclusions. inclusions. 3.2. Cause Analysis of Crack Propagation 3.2. Cause Analysis of Crack Propagation As stated above, crack initiation starts at an inclusion. Cross-sections of the impact specimens wereAs observed stated above, by SEM crack to identify initiation the starts path ofat thane inclusion.cracks generated Cross-sections by the inclusions. of the impact The specimensOM image wereshown observed in Figure by 7aSEM shows to identify the macrostructure the path of th ofe cracks the cross-section generated by of thethe inclusions. 28J impact The test OM specimen. image shownFigure 7bin showsFigure an7a SEMshows image the macrostructureof the region highlight of the edcross-section by the red square of the in28J Figure impact 7a. test The specimen. specimen Figure 7b shows an SEM image of the region highlighted by the red square in Figure 7a. The specimen

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3.2. Cause Analysis of Crack Propagation As stated above, crack initiation starts at an inclusion. Cross-sections of the impact specimens were observed by SEM to identify the path of the cracks generated by the inclusions. The OM image shown in Figure7a shows the macrostructure of the cross-section of the 28J impact test specimen. Figure7b shows an SEM image of the region highlighted by the red square in Figure7a. The specimen was tilted to show the fracture surface well and then was observed by SEM. By observing from the side ofMetals the fracture 2018, 8, x FOR surface, PEER REVIEW it is seen that the movement path of the crack can be one of6 of two 12 types. One type of crack propagation occurs across the ferrite, which is seen in Figure7 (cracks across bainite was tilted to show the fracture surface well and then was observed by SEM. By observing from the were not observed while crack propagation across ferrite was easily observed). side of the fracture surface, it is seen that the movement path of the crack can be one of two types. InOne the secondtype of crack type propagation of crack propagation, occurs across the a micro-void ferrite, which is is generated seen in Figure around 7 (cracks the across carbide bainite located at the grainwere boundary not observed and while cracks crack migrate propagatio alongn across the micro-void.ferrite was easily Impact observed). values are known to correlate with grainIn size the[ second17]. A type smaller of crack grain propagation, size yields a micro-void better is impact generated values around and the bainite carbide located with ﬁne at grain size performsthe grain well boundary in impact and cracks tests. migrate This along is the the reason micro-void. that Impact the crack values propagates are known to along correlate the ferrite. When precipitateswith grain size are [17]. present, A smaller it isgrain known size yields that better the coarse impact M23C6 values and carbide bainite and with M7C3 fine grain are size formed at performs well in impact tests. This is the reason that the crack propagates along the ferrite. When the grain boundary [22,23]. M C carbides and M C carbide are incoherent boundaries with the precipitates are present, it is23 known6 that the coarse M23C67 3 carbide and M7C3 are formed at the grain matrix structure.boundary [22,23]. Due to M the23C6 incoherentcarbides and boundaries, M7C3 carbide there are incoherent is a small boundaries gap between with the the matrix matrix and the precipitate.structure. (The Due size to of the the incoherent gap is an boundaries, atomic size.) there When is a small a small gapforce between is applied the matrix to theseand the gaps, the gap growsprecipitate. into micro (The voidssize of bythe stressgap is concentration.an atomic size.) When Therefore, a small our force results is applied show tothat these cracks gaps, the propagate along thegap micro-voids grows into micro around voids theby stress precipitate concentration. of the Therefore, grain boundaries our results orshow across that cracks the ferrite. propagate Fromalong the the results micro-voids shown around in Figure the precipitate7, the effect of the of grain the ferriteboundaries and or carbide across the on ferrite. the impact properties From the results shown in Figure 7, the effect of the ferrite and carbide on the impact properties was estimatedwas estimated by comparing by comparing and and analyzing analyzing the the texturestextures of of the the 28J 28J and and 386J 386J impact impact test specimens. test specimens.

Figure 7. Microstructure of the 28J specimen cross-section. (a) OM image; (b) SEM image; (c,d) SEM Figure 7.imageMicrostructure of regions enlarged of the by 28J thespecimen red squares cross-section. in (b). (a) OM image; (b) SEM image; (c,d) SEM image of regions enlarged by the red squares in (b). 3.2.1. Effect of Ferrite on Impact Value 3.2.1. EffectFigure of Ferrite 8 is a on microstructure Impact Value photograph of the (a) 28J impact test specimen and (b) 386J test specimen. Figure 8c shows the elongated ferrite + bainite structure of the 28J specimen and (d) shows Figure8 is a microstructure photograph of the (a) 28J impact test specimen and (b) 386J test the unstretched ferrite + bainite structure of the 386J specimen. The bainite + ferrite structure in Figure specimen.8C is Figure elongated8c shows in the thedirection elongated of the ferriteimpact +test bainite while structureFigure 8d shows of the that 28J specimenthe bainite and+ ferrite (d) shows the unstretchedstructure is ferrite not elongated. + bainite The structureferrite fraction of the of (c 386J) was specimen. higher than Thethat of bainite (d). The + fraction ferrite of structure the in Figure8ferritec is elongated was measured in the by direction the ASTM-E562 of the method impact by test taking while 16 OM Figure images8d shows from the that regions the bainiteleft of the + ferrite structurered is lines not in elongated. Figure 8a,b. The ferrite fraction of (c) was higher than that of (d). The fraction of the Figure 9 shows the fraction of ferrite, which is measured from OM images. For the 386J test ferrite was measured by the ASTM-E562 method by taking 16 OM images from the regions left of the specimens, the average ferrite fraction was 65.15% while the 28J specimen showed a high average red linesferrite in Figure fraction8a,b. of 83.51%. Therefore, we conclude that a higher ferrite fraction yields a lower impact Figurevalue.9 Theshows reason the for fraction this is that, of ferrite,as the fraction which of isferrite measured increases, from the fraction OM images. of bainite For decreases. the 386J test specimens,The impact the average value is lowered ferrite fractiondue to the was decrease 65.15% in the while fraction the of 28Jthe bainite, specimen which showed has a satisfactory a high average ferrite fractionimpact value. of 83.51%. The ferrite Therefore, fraction of we theconclude test specimens that having a higher an impact ferrite value fraction of 135J yields was measured a lower impact for 18 specimens by the same method. In Figure 9b, the reliability is low due to the small number of test specimens. However, it can be seen that the correlation between ferrite fraction and the impact value is linear. It is also possible to present a guideline for the fraction of ferrite required to yield the

Metals 2018, 8, 373 7 of 12 value. The reason for this is that, as the fraction of ferrite increases, the fraction of bainite decreases. The impact value is lowered due to the decrease in the fraction of the bainite, which has a satisfactory impact value. The ferrite fraction of the test specimens having an impact value of 135J was measured for 18 specimens by the same method. In Figure9b, the reliability is low due to the small number of test specimens. However, it can be seen that the correlation between ferrite fraction and the impact Metals 2018, 8, x FOR PEER REVIEW 7 of 12 value isMetals linear. 2018 It, 8,is x FOR also PEER possible REVIEW to present a guideline for the fraction of ferrite required7 toof 12 yield the minimumminimum required required impact impact value, value, but but this this wouldwould require the the analysis analysis of ofmore more specimens. specimens. The two The two minimum required impact value, but this would require the analysis of more specimens. The two specimensspecimens shown shown in Figurein Figure9a 9a are are from from the the same same pipe.pipe. The reason reason for for the the large large difference difference in their in their specimens shown in Figure 9a are from the same pipe. The reason for the large difference in their volume fractions is discussed below. volumevolume fractions fractions is discussed is discussed below. below.

Figure 8. Microstructure of the base metal: (a) 386J specimen and (c) is (a) part enclosed with a red FigureFigure 8. Microstructure 8. Microstructure of of the the base base metal: metal: ((aa)) 386J386J specimen specimen and and (c) ( cis) is(a)( apart) part enclosed enclosed with with a red a red square; (b) 21 specimen and (d) is (a) part enclosed with a red square. square;square; (b) 21 (b specimen) 21 specimen and and (d )(d is) is (a ()a part) part enclosed enclosed withwith a a red red square. square.

Figure 9. Relationship between volume fraction of ferrite and impact value (a) Volume fraction of ferrite for 386J and 28J specimens and (b) liner analysis of impact value and fraction of ferrite. Figure 9. Relationship between volume fraction of ferrite and impact value (a) Volume fraction of Figure 9. Relationship between volume fraction of ferrite and impact value (a) Volume fraction of ferrite for 386J and 28J specimens and (b) liner analysis of impact value and fraction of ferrite. ferrite forThe 386J 28J and and 28J 386J specimens test specimens and (b )contained liner analysis different of impact volume value fractions and fraction of inclusions, of ferrite. which contained MnS. The amount of Mn in MnS is about 60%. Therefore, it is reported that a depleted zone The 28J and 386J test specimens contained different volume fractions of inclusions, which of Mn is formed around MnS and this depleted part promotes ferrite formation [27,28]. Therefore, contained MnS. The amount of Mn in MnS is about 60%. Therefore, it is reported that a depleted zone Thethe 28J volume and 386J fraction test specimensof ferrite was contained higher in the different 28J impact volume test specimen fractions because of inclusions, it contained which more contained MnS.of The MnMnS amount is inclusions formed of around Mnthan inthe MnS 386J andspecimen. is about this depleted 60%.The matrix Therefore, pa rtcomposition promotes it is reported hasferrite been formation described that a depleted [27,28]. previously Therefore, zone as a of Mn is formedthechange aroundvolume in MnSfractionthe Mn and content of thisferrite of depleted the was matrix higher part due in promotesto the the 28J volume impact ferrite fraction test formation specimen in MnS inclusions. [ 27because,28]. Therefore, Theit contained composition the more volume fractionMnSof of inclusionsMn ferrite in the was matrix than higher the when 386J in the the specimen. volume 28J impact fraction The testmatrix of specimen inclusions composition becauseare 0% has and been it containedabout described 0.3% morewas previously calculated. MnS inclusions as a change in the Mn content of the matrix due to the volume fraction in MnS inclusions. The composition than theThe 386J calculated specimen. alloy The composition matrix composition is shown in Figu hasre been10 by describeda CCT curve previously created using as Jmat-Pro. a change The in the Mn of Mn contentin the matrix of the matrixwhen theat 0% volume and 0.3% fraction inclusions of inclusions decreases arefrom 0% 0.436 and to about 0.256%. 0.3% was calculated. content of the matrix due to the volume fraction in MnS inclusions. The composition of Mn in the The calculated alloy composition is shown in Figure 10 by a CCT curve created using Jmat-Pro. The matrix when the volume[Total fraction measured of Mn inclusions content (0.436%) are 0% = and0.05% about X Mn 0.3%content was in MnS calculated. (60%) + The calculated Mn content of the matrix at 0% and 0.3% inclusions decreases from 0.436 to 0.256%. alloy composition is shown in FigureMn 10content by a of CCT matrix curve at 0.3% created inclusions] using Jmat-Pro. The Mn content of the [Total measured Mn content (0.436%) = 0.05% X Mn content in MnS (60%) + matrix at 0%As and a result, 0.3% the inclusions ferrite CCT decreases curve moves from to the 0.436 left to as 0.256%.the amount of Mn decreases from 0.436 to Mn content of matrix at 0.3% inclusions] 0.256%. At the same cooling rate (0.1 °C/s), ferrite content increases as the ferrite moves toward [Total measured Mn content (0.436%) = 0.05% X Mn content in MnS (60%) + shorterAs a result,times along the ferrite the curve. CCT That curve is, movesas the amount to the left of inclusions as the amount increases, of Mn the decreases amount of from Mn in0.436 the to 0.256%. At the same coolingMn rate content (0.1 °C/s), of matrix ferrite at content 0.3% inclusions]increases as the ferrite moves toward shorter times along the curve. That is, as the amount of inclusions increases, the amount of Mn in the

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As a result, the ferrite CCT curve moves to the left as the amount of Mn decreases from 0.436 to 0.256%. At the same cooling rate (0.1 ◦C/s), ferrite content increases as the ferrite moves toward shorterMetals times 2018 along, 8, x FOR the PEER curve. REVIEW That is, as the amount of inclusions increases, the amount8of of Mn12 in the matrix decreases.Metals 2018, 8, x As FOR the PEER amount REVIEW of Mn in the matrix decreases, the ferrite CCT curve moves8 of to12 the left and thematrix content decreases. of ferrite As increasesthe amount even of Mn at in the the same matrix cooling decreases, rate. the ferrite CCT curve moves to the left and the content of ferrite increases even at the same cooling rate. matrix decreases. As the amount of Mn in the matrix decreases, the ferrite CCT curve moves to the left and the content of ferrite increases even at the same cooling rate.

Figure 10.FigureCCT 10. curves CCT curves for different for different volume volume fraction fraction of alloyingof alloying elements elements calculated calculated byby a Jmat-ProJmat-Pro simulation. simulation. Figure 10. CCT curves for different volume fraction of alloying elements calculated by a Jmat-Pro simulation. 3.2.2. Effect3.2.2. ofEffect Carbide of Carbide onImpact on Impact Value Value The3.2.2. microstructureThe Effect microstructure of Carbide in on regions in Impact regions aroundValue around thethe carbidecarbide in in the the 28J 28J and and 386J 386J specimens specimens was observed. was observed. Figure 11 shows a cross-sectional SEM image of both specimens. Inter-granular white spherical Figure 11 showsThe microstructure a cross-sectional in regions SEM around image the carbid of bothe in specimens.the 28J and 386J Inter-granular specimens was observed. white spherical precipitates are observed and precipitates are observed in the prior austenite grain boundary. The precipitatesFigure are11 shows observed a cross-sectional and precipitates SEM image are of observed both specimens. in the Inter-granular prior austenite whitegrain spherical boundary. precipitates of Figure 11d have greater spacing and are larger in size than the prior austenite grain precipitates are observed and precipitates are observed in the prior austenite grain boundary. The The precipitatesboundary ofprecipitates Figure 11 ofd (c). have greater spacing and are larger in size than the prior austenite grain boundaryprecipitates precipitates of Figure of (c). 11d have greater spacing and are larger in size than the prior austenite grain boundary precipitates of (c).

Figure 11. SEM image of the base metal (a) 386J and enlarged red square (c); (b) 28J and enlarged red square (d). Figure 11.FigureSEM 11. imageSEM image of the of the base base metal metal (a ()a) 386J 386J and enlarged enlarged red red square square (c); ( (bc)); 28J (b and) 28J enlarged and enlarged red red square (d). square (d).

Metals 2018, 8, 373 9 of 12 Metals 2018, 8, x FOR PEER REVIEW 9 of 12

ForFor reliablereliable measurements, measurements, Murakami Murakami etching etching and aand precipitate a precipitate etching etching solution solution of prior austeniteof prior grainaustenite boundary grain boundary was performed. was performed. The amount The amount and size and of thesize precipitates of the precipitates were measured were measured by using by Imageusing Image Pro plus. Pro The plus. size The and size size and distributions size distributions of the precipitates of the precipitates are shown are in shown Figure 12in .Figure The results 12. The in results in Figure 12 show that the size and amount of 28J specimen precipitates are larger than the Figure 12 show that the size and amount of 28J specimen precipitates are larger than the 386J specimen. 386J specimen. These results suggest that a large amount of large precipitates in the 28J specimen is These results suggest that a large amount of large precipitates in the 28J specimen is generated by generated by micro-voids due to the incoherent boundary with the matrix and then micro-voids are micro-voids due to the incoherent boundary with the matrix and then micro-voids are combined combined to propagate cracks. From the results shown in Figures 11 and 12, the difference of the to propagate cracks. From the results shown in Figures 11 and 12, the difference of the precipitates precipitates present in the two impact test specimens was investigated. present in the two impact test specimens was investigated.

FigureFigure 12.12. Histogram ofof aa carbidecarbide inin 386J386J andand 28J28J specimens.specimens.

FigureFigure 1313aa showsshows aa well-knownwell-known Fe-CFe-C phasephase diagram.diagram. InIn the Fe-C phase diagram, the C solubility inin thethe ferriteferrite isis 0.025%.0.025%. Figure 1313bb is a statestate diagramdiagram ofof carboncarbon contentcontent inin 2.25Cr-1Mo2.25Cr-1Mo steelsteel usingusing Thermo-calc.Thermo-calc. Additionally,Additionally, thethe CC solubilitysolubility inin thethe ferriteferrite isis 0.009%.0.009%. TheThe amountamount ofof CC solubilitysolubility variesvaries dependingdepending onon thethe alloyingalloying elements.elements. Solubility of C was low in the state calculated by Thermo-Calc. ThisThis isis probablyprobably duedue toto thethe phasephase ofof M23C6,M23C6, whichwhich takestakes carboncarbon fromfrom multiplemultiple systems.systems. InIn orderorder toto makemake aa moremore conservativeconservative judgment,judgment, modelingmodeling waswas performedperformed byby calculatingcalculating thethe amountamount ofof ferriteferrite caboncabon solubilitysolubility asas 0.025%.0.025%. ModelingModeling ofof thethe carboncarbon chemicalchemical concentrationconcentration showsshows thatthat thethe CC contentcontent ofof ferriteferrite isis 0.025%0.025% andand thethe CC contentcontent ofof thethe basebase metalmetal isis 0.1%0.1% (see(see FigureFigure 1313c).c). TheThe contentcontent ofof CC ((0.1%0.1% −− 0.025% = 0.075%0.075%)) mustmust bebe containedcontained inin thethe otherother constituentconstituent otherother thanthan ferrite.ferrite. The higher the ferrite content, the higher thethe content of of C C is is in in the the other other constituent constituent includ includinging calculating calculating the theamount amount of carbon of carbon in the in other the otherconstituent. constituent.

-- 28J: 0.1% C Base Metal Composition = 83.51% Ferrite Volume FractionFraction XX 0.025%0.025% CC ++ CC content ofof 16.49% the other constituent ==> C C content content of of 16.49% 16.49% the the other other constituent constituent = = 0.4798% 0.4798% - 386J: 0.1% C Base Metal Composition = 64.76% Ferrite Volume Fraction X 0.025% C + C content - 386J: 0.1% C Base Metal Composition = 64.76% Ferrite Volume Fraction X 0.025% C + C content of 35.24% the other constituent ==> C content of 35.24% the other constituent = 0.2378% of 35.24% the other constituent ==> C content of 35.24% the other constituent = 0.2378% For this reason, the amount of precipitate generated is due to the fact that the carbon content of For this reason, the amount of precipitate generated is due to the fact that the carbon content of the other constituent in PWHT is twice as much as that of the other constituent of 386J. the other constituent in PWHT is twice as much as that of the other constituent of 386J. Figure 14 shows the results of EPMA analysis of the 28J impact specimen. It can be seen that Figure 14 shows the results of EPMA analysis of the 28J impact specimen. It can be seen that there is a large amount of precipitate in the prior austenite grain boundaries. The precipitates consist there is a large amount of precipitate in the prior austenite grain boundaries. The precipitates consist of Cr, Mo, Mn, and C and, in particular, precipitates with a higher content Cr and C were found. of Cr, Mo, Mn, and C and, in particular, precipitates with a higher content Cr and C were found. Usually, the precipitates with a higher content of Cr and C than M23C6 precipitates are supposed to Usually, the precipitates with a higher content of Cr and C than M23C6 precipitates are supposed to be be M7C3. In addition to forming incoherent boundaries between the matrix and precipitate by the M7C3. In addition to forming incoherent boundaries between the matrix and precipitate by the HCP HCP crystal structure, they form a surrounding Cr-depleted zone due to the high interrelation of Cr crystal structure, they form a surrounding Cr-depleted zone due to the high interrelation of Cr and C. and C. Excess Cr-rich precipitates in the prior austenite grain boundaries cause Cr-depleted zone formation around the precipitate, which softens the grain boundaries and becomes vulnerable to

Metals 2018, 8, 373 10 of 12

Excess Cr-rich precipitates in the prior austenite grain boundaries cause Cr-depleted zone formation Metals 2018, 8, x FOR PEER REVIEW 10 of 12 aroundMetals 2018 the, 8, precipitate,x FOR PEER REVIEW which softens the grain boundaries and becomes vulnerable to stress corrosion10 of 12 stresscracking corrosion when thecracking Cr content when decreases the Cr content [29,30 ].decreases It should [29,30]. also be It noted should that also these be precipitatesnoted that these may stress corrosion cracking when the Cr content decreases [29,30]. It should also be noted that these precipitatesaffect the physical may affect properties the physical of the HAZproperties during of weldingthe HAZ [ 11during]. welding [11]. precipitates may affect the physical properties of the HAZ during welding [11].

Figure 13. Schematic of calculation of carbon contents in the other constituent (a) Fe-C phase diagram Figure 13. 13. SchematicSchematic of calculation of calculation of carbon of carbon contents contents in thein other the constituent other constituent (a) Fe-C (phasea) Fe-C diagram phase (b) 2.25Cr-1Mo Steel + C phase diagram calculated using Thermo-calc (c) modeling of carbon diagram(b) 2.25Cr-1Mo (b) 2.25Cr-1Mo Steel + SteelC phase + C phasediagram diagram calculated calculated using using Thermo-calc Thermo-calc (c) ( cmodeling) modeling of of carbon concentration between ferrite and the other constituent. concentration between ferrite and the other constituent.

Figure 14. EPMA images of the 28J impact specimen. (a) composite image (b) chromium (c) carbon Figure 14. EPMA images of the 28J impact specimen. (a) composite image (b) chromium (c) carbon (d) molybdenumEPMA ( imagese) manganese of the 28J(f) sulfur. impact specimen. ( ) composite image ( ) chromium ( ) carbon (d) molybdenum ((e)) manganesemanganese ((ff)) sulfur.sulfur. 4. Conclusions 4. Conclusions The MnS and other inclusions in the 28J impact specimen were found to be larger than the 386J The MnS and other inclusions in the 28J impact specimen were found to be larger than the 386J specimen.The MnS At the and same other cooling inclusions rate, the in the ferrite 28J impactcontentspecimen of the base were metal found is increased to be larger as the than ferrite the CCT 386J specimen. At the same cooling rate, the ferrite content of the base metal is increased as the ferrite CCT curvespecimen. moves At to the the same left coolingdue to the rate, Mn the deficiency ferrite content caused of by the the base MnS metal formation. is increased As the as theferrite ferrite content CCT curve moves to the left due to the Mn deficiency caused by the MnS formation. As the ferrite content increases,curve moves the toimpact the left value due is to lowered the Mn deﬁciency due to the caused decrease by of the the MnS bainite, formation. whichAs has the high ferrite impact content test increases, the impact value is lowered due to the decrease of the bainite, which has high impact test values.increases, The the increase impact of value ferrite is loweredcontent increases due to the the decrease C content of theof the bainite, other whichconstituent has high due impact to the low test values. The increase of ferrite content increases the C content of the other constituent due to the low values.C content The (0.025%) increase of of ferrite ferrite (about content 0.4798%). increases This the results C content in a large of the amount other constituent of precipitate due (M23C6) to the low in C content (0.025%) of ferrite (about 0.4798%). This results in a large amount of precipitate (M23C6) in theC content prior austenite (0.025%) grain of ferrite boundaries (about 0.4798%). and coarse This Cr resultsprecipitates in a large are the amount migration of precipitate path of cracks (M23C6) in the prior austenite grain boundaries and coarse Cr precipitates are the migration path of cracks in micro-voidin the prior generation. austenite grain The boundariesoccurrence andof Cr-depleted coarse Cr precipitates zones leads are to softening the migration of the path prior of austenite cracks in micro-void generation. The occurrence of Cr-depleted zones leads to softening of the prior austenite grainmicro-void boundaries, generation. which The can occurrence be expected of to Cr-depleted weaken the zones stress leads corrosion to softening cracking of of the these prior areas. austenite grain boundaries, which can be expected to weaken the stressstress corrosioncorrosion crackingcracking ofof thesethese areas.areas. Author Contributions: S.L. and C.K. conceived and designed the experiments. H.N. performed the experiments Author Contributions: S.L. and C.K. conceived and designed the experiments. H.N. performed the experiments Authorand analyzed Contributions: the data. S.L. and C.K. conceived and designed the experiments. H.N. performed the experiments and analyzed the data. Acknowledgments: This work was supported by a National Research Foundation of Korea (NRF) grant funded Acknowledgments: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2012R1A5A1048294). by the Korean government (MSIP) (No. 2012R1A5A1048294). Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors 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 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. decision to publish the results.

Metals 2018, 8, 373 11 of 12

Acknowledgments: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2012R1A5A1048294). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding sponsors 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.

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