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Boiler Grade DMV 310 N
DMV 310 NDMV 310 N
1 Introduction ___________________________________________________________________4 1.1 Salzgitter Mannesmann Stainless Tubes ________________________________________4 1.2 DMV 310 N _________________________________________________________________5 1.3 Trend ______________________________________________________________________5 1.4 Specifications(Standards) ____________________________________________________6 1.5 Available Sizes ______________________________________________________________6 1.6 Special Features _____________________________________________________________6
2 Material Properties _____________________________________________________________8 2.1 Microstructure ______________________________________________________________8 2.2 MaterialpropertiesaccordingtoASME _________________________________________9 2.2.1 Chemicalcomposition ___________________________________________________9 2.2.2 Mechanicalproperties _________________________________________________ 10 2.3 MaterialpropertiesaccordingtoVdTÜV ______________________________________ 12 2.3.1 Chemicalcomposition _________________________________________________ 12 2.3.2 Tensileproperties ______________________________________________________ 12 2.3.3 Creep strength ________________________________________________________ 14 2.3.4 Impact resistance _____________________________________________________ 15 2.3.5 Physicalproperties ____________________________________________________ 15 2.4 Hightemperatureproperties–Mechanism ____________________________________ 17 2.4.1 High temperature and creep rupture strength _____________________________ 17 2.4.2 Microstructurestability _________________________________________________ 18 2.5 Corrosionresistance _______________________________________________________ 19
3 Fabrication __________________________________________________________________ 20 3.1 Tube bending _____________________________________________________________ 20 3.2 Welding __________________________________________________________________ 20
4 References __________________________________________________________________ 22
Content
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1.1 Salzgitter Mannesmann Stainless TubesSalzgitter Mannesmann Stainless Tubes is oneoftheleadingmanufacturersofseam-lessstainlesssteelandnickelalloytubesandpipes.Ourcompanyhasoneofthelarg-estproductportfoliosinthissector.Ourtopqualityproductsandefficientservicecontributetothelong-termsuccessofourcustomers.Ourtoppriorityistoestablishandmaintainpositivelong-termrelationshipswithourbusinesspartners.Inordertosupportyouinthestockistandprojectbusiness,withgoodquality,innova-tivematerialsandmodernproductiontech-nologies,ourexpertsareconstantlyworking
tokeepupwiththelatesttrends.Asacompanyoperatingsuccessfullyonaninternationallevel,weunitemanynation-alitiesandculturesunderonebanner.Ournetworkcollaboratescloselyinallaspectsofprocurement,sales,productionandlogistics.Sinceourfoundationasajointventure,weholdourpositioninthetopleagueofcompa-niesinthissector.WeareamemberofthepowerfulSalzgit-terGroupandourstainlesssteelandnickelbaseproductsareanimportantadditiontotheGroup’soverallproductrange.Thankstoourimpressiveproductportfolio,wecanopenupattractivegrowthprospects.
1 Introduction
Headquarters Mülheim an der Ruhr, Germany
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1.2 DMV 310 NOurtubesandpipesareprimarilyusedforboilersinthermalpowerplantsandtheenergysector.Thetrendistowardshighersteamtemperaturestoincreasetheefficien-cyofpowerplantsontheonehandandtheuseofhighchlorineandsulphurcontainingcoalontheotherhand.Conventional18-8stainless steels such as grades 304 and 347 donotshowsufficientcorrosionresistanceinthesecases.ThehigherCrcontainingstainlesssteelsoftype310(25Cr-20Ni)ex-hibitahighercorrosionresistance.Thistypehasbeenconsideredasacandidatematerialincaseswherehighercorrosionresistanceisrequiredandthehighermaterialcostsareacceptable.However,thecreeprupturestrengthofconventional310stainlesssteelisratherlow.Thus,thesesteelscannotbeusedtoproducesuperheatertubesutilisedabove600°C(1110°F).Tofindaremedy,compositetubeshavebeenproducedcladwithtype310stainlesssteelasthefire-sidecomponentonacoresteeltubewhichpro-videssatisfactorycreepresistance.WithDMV310N,thedisadvantageoflowstrengthandcreepresistanceisovercome,whileretainingthehighcorrosionresistance.Theadditionofniobiumandnitrogenresultsin increased elevated temperature strength andcreepresistance.Theeffectsofsolidsolutionstrengtheningbynitrogentogetherwiththeprecipitationhardeningbyfineandstable NbCrN are used. 310 stainless steels canbepronetotheformationofcoarse
secondaryphasessuchassigmaphaseandCr2Nresultinginanembrittlementofthema-terial(reducedtoughness).Anoptimisationofthecomposition(mainlynickel,niobiumandnitrogencontent)wasnecessarytoin-creasethemicrostructuralstability.Thereforethealloycompositionisdesignedtofindtheoptimumbalancebetweenthevariousproperties.Inthisway,tubinginDMV310Nisnowsuitableforuseinsuperheaterboilersabove600°C(1110°F).
1.3 Trend Efficiencyisamajorperformancecriterionforpowerplantsandboilers.Increasingef-ficiencyessentiallyresultsinareductionofthefuelconsumptionandthusCO2emis-sions,whichisabigchallengeatthepresenttime.Higherefficiencyofsuchequipmentis reached by higher steam temperatures andpressures.Targetserviceconditionsarecurrentlysteamtemperaturesabove600°Cor1110°Fandpressuresofmorethan300baror4,350psi.Thematerialsmustwith-standtheseconditionsthroughoutthewholeservicelifeofthecomponent.Thisresultsinhigherdemandsoncreepresistance,elevated temperature strength and high temperaturecorrosionresistance.Theuseofcoalwithahighchlorineandsulphurcontentfurtherincreasestheneedforfire-sidecor-rosionresistance.ThesechallengesaremetbyDMV310N,inwhichthehighcorrosionresistanceoftype310materialsiscombinedwith increased high temperature strength
6
and creep resistance. Infuture,steeldevelopmentmustcontinuetofollowthenewdemandsandchallengesofboilerapplications.Itiswidelyexpectedthatnewpowerplantswillrunatevenhighertemperatures and pressures. Nickel based alloysinparticularwillbeabletomeetfuturerequirements,suchasDMV617(fordesigntemperatures>700°C).However,theseal-loyshavehighnickelandchromiumcontentandarethusmoreexpensive.
1.4 Specifications (Standards)DMV310Nfulfilstherequirementsaccord-ingtothefollowingspecifications:· 1.4952(X6CrNiNbN2520),accordingtoEN10216-5,EuropeanStandard
· TP310HCbN(25Cr-20Ni-Nb-N)accordingtoASMESA-213,USStandard
· ASMECodeCase2115-1(02.2000),UnitedStates
· VdTÜVmaterialdatasheet546(03.2007),FederalRepublicofGermany
1.5 Available SizesDMV 310 N austenitic stainless steel is used tomanufactureseamlessausteniticreheaterandsuperheaterboilertubes.Thisgradeissuitableforallcommonlyusedausteniticreheaterandsuperheaterboilertubesizesinthemostadvancedcoalfiredpowerstationswithsteamtemperaturesupto620°C (1150°F)andsupercriticalorultrasupercriti-
calboilerdesign.VdTÜVmaterialdatasheet546(03.2007)allowsamaximumouterdiameterof65mmandamaximumwallthicknessof12.5mm.ThestandardsizerangeaccordingtoEN-ISO1127aswellasothersizesarealsoavailableuponrequest.
1.6 Special Features· DMV310Nisanoptimisedausteniticstainlesssteeloftype310,suitableforuseas tubes in severe conditionsofmodernboilers(e.g.ultrasupercriticalboilers)withanappropriatecombinationofhighcorro-sionresistanceandhighelevatedtempera-ture strength.
· Production route of DMV 310 N: The materialisfirstsubjectedtoahotformingprocess(hotextrusion).Subsequently,coldfinishingiscarriedoutonthematerial,fol-lowedbysolutionannealingatatempera-turebetween1180°Cand1270°C (2155°Fand2320°F)accordingtoVdTÜVdatasheet546(03.2007).
· Ahighfire-sidehotcorrosion resistance andsteam-sideoxidationresistanceisen-suredbyahighchromiumcontentof25%.
· Satisfactoryelevated temperature strength and creep resistance is achievedbythecontrolledadditionsofnitrogenandniobium:solidsolutionstrengtheningandprecipitationhardeningareemployed.
1 Introduction
7
· TheoptimisedcompositionofDMV310N results in a microstructural stability reducingthetendencytowardsformationofcoarsesigmaandCr2Nphase,andthusreducingembrittlementofthematerial.
8
2.1 MicrostructureThe material DMV 310 N is a type 310 austeniticstainlesssteelwithhighcorrosionresistance.Improvementsintheelevatedtemperature strength and creep resistance comparedtotheconventional310typesare mainly achieved by a balanced addi-tionofniobiumandnitrogen.Inparticularthenitrogencontentisincreasedcomparedtothe310typeausteniticsteelsaswellascomparedtootherboilertubegradessuchasDMV347HFGandDMV304HCu.Toincreasethestrengthofthematerial,primar-ilytheeffectofsolidsolution-strengtheningbynitrogenisused.Thecreepresistanceisfurtherincreasedduringservicebythefineprecipitationofcarbonitrides,Zphase(com-plexnitride)andM23C6 carbides. Neverthe-less,ahighamountofnitrogenremainsinsolutionintheausteniticmatrix[1]. Theproductionofboilertubesisdividedintothreemainprocessingsteps.Afirstthermaltreatmentisperformedduringthehotextru-sionofthematerial.Thisisfollowedbycolddeformationtoproducethefinaldimensionsofthetube(coldpilgeringorcolddrawing).Afinalheattreatmentcompletestheproduc-tionroute.Duringthissolutiontreatment,theprecipitatesaremainlydissolvedintheausteniticmatrix.Ahighsolutionrateisnecessarytoachievegoodcreepproperties.However,acertainamountofprecipitatesremains,restrictingthegraincoarseningbypinningthegrainboundaries.Thesetwoaims are achieved by using a thermal treat-
mentwhichisoptimisedintemperatureandtime.ThemicrostructureofDMV310Ninas-deliveredconditionisshowninFigure1.
Inthesolution-annealedconditionthedis-solvednitrogenincreasesthetensilestrengthatroomtemperatureandelevatedtempera-turesbythesolidsolution-strengtheningeffect.Undercreepconditionsinservice,precipitationofdifferentphasesbegins.Aswellasthewell-knownphasessuchasniobiumcarbonitrides,othermorecomplexchromiumcontainingnitridessuchastheZphase(NbCrN)areformed[2].Sinceniobiumismainlytiedtonitrogen,enoughcarbonisleftinsolutiontoformM23C6 carbides. Althoughtheamountofnitridesandcarboni-tridesincreaseswithtimeandtemperature,mostofthenitrogenremainsinsolution.The intragranular NbCrN precipitates are veryfineandstableevenafterlongaging
2 Material Properties
Figure 1: Microstructure of DMV 310 N in solution-annealed
condition.
9
timescontributingtothecreepresistancebyprecipitationhardening[3, 4].Ontheotherhand,M23C6alsomainlyformedintergranu-larlycouldleadtograinboundarychromiumdepletionandhenceslightlyenhancesintergranularcorrosion.Thisisacceptedduetotheadvantageofhigherstrength,makingitpossibletouseDMV310Nasamonobloctubingmaterialwithouttheneedforcladdingorcoextrusionwithamorecreepresistantsteelalloy.Grade310stainlesssteelsarepronetotheunwantedformationofcoarsesigma,Cr2N orPi(mixednitride)phases.Atcomparablylowlevelsofnitrogenandnickel(0.19%Nand17%Ni)precipitationofCr-richsigmaphaseoccurs.Conversely,ifthenitrogenandnickelcontentsarehigh(0.29%Nand23%Ni)theprecipitationofCr2N and Pi phases is detected,deterioratingimpactstrength[3]. Otheralloyingelementsalsoinfluencetheformationofthesedeleteriousphases.Forthisreason,optimisationofthealloycompo-sitionisperformedtoreducetheamountofthesecoarsesecondaryphasesandhencetominimiseembrittlementofthematerial.ThemicrostructureofDMV310Naftercreeprupturetestat750°Cand105MPaisshowninFigure2.Thecreepdamageinformofmicrocracksorientatedperpendiculartothestressdirectionistypical.Precipitationofnitrides has taken place inside the grains and atgrainboundaries.
2.2 Material properties according to ASMEThematerialmeetstherequirementsofTP310HCbNasspecifiedinASMESA-213.TherequiredmaterialpropertiesarefurtherdescribedintheASMECodeCase2115-1:25Cr-20Ni-Nb-N.
2.2.1 Chemical compositionTherequirementswithregardtothechemi-calcompositiongiveninASMESA-213andtheASMECodeCase2115-1aresum-marisedinTable1.Asolutiontreatmentatatemperatureofatleast2,000°F(1,100°C)isperformedbeforedelivery.
Figure 2: Microstructure of DMV 310 N after failure in creep
rupture test (750 °C, 105 MPa, 649 h).
10
2 Material Properties
2.2.2 Mechanical properties Tensilepropertiesatroomtemperature(RT)inthesolution-annealedconditionaccordingtoASMESA-213andtheASMECodeCase2115-1aregiveninTable2.Additionally,thehardnessshallnotexceed256HB(100HRB).
IntheASMECodeCase2115-1,themaximumallowablestressvaluesatdif-ferenttemperaturesaregiven(seeTable3,Figure3).Theyarebasedonfactorof3.5ontensilestrength,whereapplicable.Inthesecondcolumnhighervaluesaregivenforcasesinwhichslightlygreaterdeformation(1%)isacceptable.Thesevaluesexceed66.67%,butdonotexceed90%oftheyieldstrengthatelevatedtemperature.Theuseofthesestressvaluesmayresultindimensionalchangesduetopermanentstrain.Forthisreason,thesestressvaluesarenotrecom-mendedfortheflangesofgasketedjointsorotherapplicationswhereslightamountsofdistortioncancauseleakageormalfunction.
[wt-%] C Si Mn P S Cr Ni Nb Nmin. 0.04 - - - - 24.0 17.0 0.20 0.15max. 0.10 0.75 2.00 0.03 0.03 26.0 23.0 0.60 0.35
Table 1: Chemical composition of DMV 310 N.
ksi MPa 1) %Minimum yield strength 43 295 -Minimum tensile strength 95 655 -Minimum elongation in 2 inch - - 30
1) calculated values
Table 2: Mechanical properties at RT in solution-annealed condition according to ASME SA-213, Code Case 2115-1.
maximum allowable stresses
with acceptable deformation
30
0
20
15
01000 1400
Maximum allowable stresses [ksi]
10
5
Temperature [°F]200 400 600 800 1200
25
Figure 3: Maximum allowable stresses according to ASME
Code Case 2115-1.
11
Temperature Maximum allowable stresses Maximum allowable stresses 2)
°F °C 1) ksi MPa 1) ksi MPa 1)
-20 to 100 -29 to 38 27.1 186.8 27.1 186.8200 93 24.0 165.5 26.9 185.5300 149 21.7 149.6 25.4 175.1400 204 20.2 139.3 24.6 169.6500 260 19.2 132.4 24.2 166.9600 316 18.5 127.6 24.0 165.5650 343 18.3 126.2 23.9 164.8700 371 18.1 124.8 23.8 164.1750 399 17.8 122.7 23.7 163.4800 427 17.6 121.3 23.6 162.7850 454 17.4 120.0 23.4 161.3900 482 17.1 117.9 23.1 159.3950 510 16.9 116.5 22.8 157.21000 538 16.6 114.5 22.4 154.41050 566 16.3 112.4 22.0 151.71100 593 16.1 111.0 16.1 111.01150 621 13.6 93.8 13.6 93.81200 649 10.1 69.6 10.0 69.61250 677 7.6 52.4 7.6 52.41300 704 5.7 39.3 5.7 39.31350 732 4.3 29.6 4.3 29.6
1) calculated values
2) slightly greater deformation acceptable
Table 3: Maximum allowable stresses at elevated temperatures according to ASME Code Case 2115-1, US customary units
and SI metric units.
12
2.3 Material properties according to VdTÜVInthefollowing,materialpropertiesasgivenintheVdTÜVmaterialdatasheet546(03.2007)aresummarised.
2.3.1 Chemical compositionTherequirementswithregardtochemicalcompositionaregiveninTable4.Thecom-positiongivenintheVdTÜVmaterialdatasheetfortheheatanalysisisnearlythesameasthatgiveninASMESA-213andtheASMECodeCase2115-1.However,nominimumcarboncontent,ahighersiliconcontentandalargerrangeforthechromiumcontentis
givenintheVdTÜVmaterialdatasheet.Ac-cordingtostandardpracticeinEurope,therequirementsfortheproductanalysisareslightlyextended.Solutionannealingofthefinishedtubesiscarriedoutatatemperaturebetween1180and1270°C(2155to 2320°F).
2.3.2 Tensile propertiesTensilepropertiesatroomtemperaturearegiveninTable5.Thevaluesforproofstrength are minimum values and are valid irrespectiveoflocationandpositionofthesample.
2 Material Properties
[wt-%] C Si Mn P S Cr Ni Nb NCast analysis
min. - - - - - 23.0 17.0 0.20 0.15max. 0.10 1.50 2.00 0.030 0.030 27.0 23.0 0.60 0.35
Product analysis
min. - - - - - 22.8 16.85 0.15 0.14max. 0.11 1.55 2.04 0.035 0.035 27.2 23.20 0.65 0.36
Table 4: Chemical composition of DMV 310 N.
MPa ksi 1) %0.2 % proof strength, min. 295 42.8 -1 % proof strength, min. 325 47.1 -Tensile strength 655 - 900 95.0 - 131 -Elongation at fracture - - 30
1) calculated values
Table 5: Tensile properties at room temperature according to the VdTÜV material data sheet 546 (03.2007).
13
Tensilepropertiesatelevatedtemperaturesare presented in Table 6 and Figure 4. The 0.2%and1%proofstrengthandthetensilestrength are summarised. The samples are takeninlongitudinaldirection.Theproofstrengthvaluesarerequirements,whereasthedatafortensilestrengthatelevatedtem-peratureareonlyguidelines.
Temperature 0.2 % proof strength 1 % proof strength Tensile strength°C °F 1) MPa ksi 1) MPa ksi 1) MPa ksi 1)
100 212 240 34.8 265 38.4 590 85.6200 392 205 29.7 230 33.4 540 78.3300 572 190 27.6 210 30.5 525 76.1350 662 190 27.6 210 30.5 525 76.1400 752 180 26.1 200 29.0 520 75.4450 842 175 25.4 195 28.3 520 75.4500 932 170 24.7 190 27.6 510 74.0550 1022 165 23.9 185 26.8 490 71.1600 1112 160 23.2 180 26.1 465 67.4650 1202 160 23.2 180 26.1 436 63.1700 1292 155 22.5 175 25.4 395 57.3750 1382 155 22.5 175 25.4 345 50.0
1) calculated values
Table 6: Minimum proof strength and tensile strength at elevated temperatures according to the VdTÜV material data sheet
546 (03.2007).
Tensile strength
0,2% proof strength
700
0
600
500
0
Stress [MPa]
400
100
Temperature [°C]200 400 600 800
200
300 1% proof strength
Figure 4: Minimum proof strength and tensile strength at
elevated temperatures according to the VdTÜV material data
sheet 546 (03.2007).
14
2.3.3 Creep rupture strengthThecreeprupturestrengthvaluesfor10,000hand100,000haresummarisedinTable7.Thesearelongtermvalues,basedonevalu-ationsofavailabletestresultstodate.Theyareaveragevaluesfromexistingscatterbands. These datasets are checked and may berevisedfromtimetotimeintheVdTÜVmaterial data sheet. It can be assumed that thelowerlimitofthescatterbandisabout20%lowerthanthegivenaverage.
InFigure5,theaveragevaluesofthecreepstrengthfor10,000and100,000haregiventogetherwiththeminimumvaluesofthe0.2%proofstrengthofDMV310Nshowing
thetypicalintersectionofthesevalues.
2 Material Properties
Temperature 10,000 h 100,000 h°C °F 1) MPa ksi 1) MPa ksi 1)
600 1112 284 41.2 184 26.7610 1130 260 37.7 170 27.7620 1148 238 34.5 154 22.3630 1166 212 30.7 140 20.3640 1184 190 27.6 126 18.3650 1202 171 24.8 114 16.5660 1220 154 22.3 102 14.8670 1238 142 20.6 90 13.1680 1256 130 18.9 82 11.9690 1274 118 17.1 73 10.6700 1292 108 17.7 66 9.6710 1310 98 14.2 59 8.6720 1328 89 12.9 53 7.7730 1346 79 11.5 48 7.0740 1364 71 10.3 43 6.2750 1382 64 9.3 39 5.7
1) calculated values
Table 7: Average creep strength values for 10,000 h and 100,000 h according to the VdTÜV material data sheet 546 (03.2007).
Creep strength, 10,000 h
0,2% proof strength
300
0
250
200
0
Stress [MPa]
150
Temperature [°C]200 400 600 800
50
100
Creep strength, 100,000 h
350
Figure 5: Creep strength for 10,000 and 100,000 h in
comparison to the 0.2 % proof strength at elevated
temperatures.
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2.3.4 Impact resistanceAccordingtotheVdTÜVmaterialdatasheet546(03.2007),theaverageimpactenergyinlongitudinaldirectionshouldbeatleast85Jatroomtemperature.Thisvalueistheaver-ageof3specimens.Onlyoneofthe3resultsispermittedtofallbelowtherequiredlevel,byamaximumof30%.Samplesagedinthetemperaturerangeof600to800°Cforupto10,000hdidnotbecomebrittle,astheformationofcoarsesecondaryphasessuchassigmaorCr2N phasewasavoided.Theprecipitationofthesephasesleadstoareductioninimpactstrength. IntheVdTÜVmaterialdatasheet546(03.2007)itisstatedthatdecreasedimpactvalueshouldbeconsideredduringdown-times,hydrostaticpressuretestsetc.
2.3.5 Physical propertiesInTable8andFigure6,thedynamicmodu-lusofelasticityisgiven.Thesummarisedvalues are guidelines.
Temperature Modulus of elasticity°C °F 1) 103 MPa 103 ksi 1)
20 68 193 28.0100 212 191 27.7200 392 184 26.7300 572 175 25.4400 752 167 24.2500 932 161 23.3600 1112 150 21.8700 1292 144 20.9750 1382 141 20.5
1) calculated values
Table 8: Modulus of elasticity according to the VdTÜV
material data sheet 546 (03.2007).
250
0
200
0
Modulus of elasticity [103 MPa]
150
Temperature [°C]200 400 600 800
50
100
Figure 6: Modulus of elasticity according to the VdTÜV
material data sheet 546 (03.2007).
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ThecoefficientofthermalexpansionisgiveninTable9andFigure7.Thereferencetemperatureis20°C.Thegivenvaluesareguidelines.
Temperature: between 20 °C and …
Coefficient of thermal expansion
°C °F 1) 10-6 / K 10-6 / °F 1)
100 212 13.4 7.4200 392 15.6 8.7300 572 16.0 8.9400 752 17.0 9.5500 932 17.2 9.5600 1112 17.5 9.7700 1292 17.9 9.9750 1382 18.0 10.0
1) calculated values
Table 9: Coefficient of thermal expansion (reference
temperature 20 °C (68 °F)) according to the VdTÜV material
data sheet 546 (03.2007).
InTable10andFigure8,thevaluesofthethermalconductivityaresummarised.Hereagain,thegivenvaluesareguidelines.
Temperature Thermal conductivity°C °F 1) W / (m·K) Btu / (ft ·h ·°F) 1)
20 68 12.1 7.0100 212 13.4 7.7200 392 15.1 8.7300 572 16.7 9.7400 752 18.2 10.5500 932 19.8 11.4600 1112 21.2 12.3700 1292 24.0 13.9750 1382 24.4 14.1
1) calculated values
Table 10: Thermal conductivity according to the VdTÜV
material data sheet 546 (03.2007).
2 Material Properties
20
0
18
10
Coefficient of themal expansion [10-6 /K]
16
Temperature [°C]200 400 600 800
13
1112
1415
17
1930
0
25
0
Themal conductivity [W/mK]
20
Temperature [°C]200 400 600 800
10
5
15
Figure 7: Coefficient of thermal expansion (reference
temperature 20 °C (68 °F)) according to the VdTÜV material
data sheet 546 (03.2007).
Figure 8: Thermal conductivity according to the VdTÜV
material data sheet 546 (03.2007).
17
2.4 High temperature properties – MechanismTheefficiencyofconventionalsteampowerplantsdependstoalargedegreeonthesteam temperature and the pressure. Steam temperaturehasincreasedbyabout60°C(110°F)inthelast30yearsandafurtherincreaseofupto100°C(180°F)isexpectedinthenext30years.Themajorlimitingfactorforfurtherincreasesinsteamtemperatureistheresistanceoftheutilisedmaterialtocor-rosionandcreep.Theneedforhighercorro-sionresistanceduetohighersteamtemper-aturesaswellashighchlorineandsulphurcontainingcoal(fire-sideatmosphere)ismetbythehigherchromiumcontentoftype310austenitic stainless steels. The high tempera-turestrengthandcreepresistanceofthesesteelsisimprovedinDMV310Nbyoptimis-ingthealloycomposition.Inparticulartheincreaseinnitrogencontenthasimprovedthecreepresistance,especiallyinthetemperaturerangebetween600and670°C(1110and1240°F).Thus,theuseofthistype310austeniticstainlesssteelispossibleforboilertubes,avoidingthehigherproductioneffortforcladdingorcoextrusionofdifferentmaterials.
2.4.1 High temperature and creep rupture strengthAnimprovementofmechanicalproper-ties,especiallyhightemperaturestrengthandcreepstrength,canbeachievedbysolidsolutionstrengtheningorprecipitation
hardeningeffect.Throughtheadditionofni-trogen,botheffectsareusedintheDMV310Nstainlesssteel.Inthesolution-annealedconditiononlyasmallamountofprecipi-tatesispresentinthematerial.Thenitrogenismainlydissolvedintheausteniticmatrixmaterial,androomtemperatureandelevatedtemperaturestrengthisenhancedbysolidsolutionstrengthening.Itisconsideredthatnitrogenismoresolublethancarbon,withamaximumsolubilityatchromiumcontentsofaround25%.Furthermore,theeffectofsolidsolutionstrengtheningisgreaterfornitrogenthanforcarbon,asnitrogenhasalargeratomicradius.Duringservice,theprecipitationofdifferentphasestakesplace.Inparticular,theprecipitationoffineandhomogenouslydistributedintragranularNb-CrNnitrides(Zphase)iseffectiveinincreas-ingthecreepresistancebyprecipitationhardening.Additionally,theprecipitationofniobiumcarbonitridesisseen [1, 3-4]. As the niobiumcontentinDMV310Nislowerthanintheniobiumcontainingmaterial310Nb,theamountofniobiumprecipitatesisalsoreduced.Inthemain,morecomplexandchromiumcontainingnitridesareformed.Additionally,M23C6 carbides are precipi-tatedastheMXcarbideformingelementsare used in the nitrides. These carbides aremainlyprecipitatedintergranularlyonthegrainboundaries,sothattheeffectonprecipitationhardeningislow.However,theymaystrengthenthegrainboundaries.De-spitetheformationofnitrideprecipitates,a
18
highamountofnitrogenremainsdissolvedintheausteniticmatrix.Thus,thestrengtheningeffectsofbothsolidsolutionandprecipita-tionhardeningareexploitedsimultaneouslyduringserviceoftheboilertubes.
2.4.2 Microstructure stabilityDuringservice,theaimisforhomogeneousandfineprecipitationofnitridestoincreasecreepresistance.InthecaseofDMV310N,acomplexnitride(ZphaseorNbCrN)isformed.Aftertheinitialprecipitationahighstabilityofthemicrostructureisdesirabletokeepthepropertiesstable.Precipitationcanusuallybedividedintothreestages:nucleation,growthandcoarsening.SothefindingsforDMV310Nmaterial,thatthenitrogencontentinnitridesandcarbonitridestendtoincreasewithagingtemperatureandtime,areasexpected.Nevertheless,itcanbeseenthatthegrowthofthenitridesisrelativelyslowandstableevenafterlongterm and higher temperature aging [1]. These precipitatestogetherwithsolutenitrogenareconsideredtobeeffectiveinimprovingthecreeprupturestrengthofthissteel.Duringaging,furtherprecipitatesareformedasmentionedbefore.Theformationofcarbides such as M23C6occurspreferentiallyongrainboundarieswhich,asaresult,arestrengthened.However,depletionofcarboninthematrixpromotestheformationofinter-metallicphasessuchassigmaphase.BytheformationofthechromiumcarbideM23C6,a
remarkableamountofchromiumisremovedfromthematrixandisnolongeravailabletorestrictcorrosion.TheamountofM23C6 carbideisrestrictedbyacertainamountofniobiumwhichisincludedinthematerial.NbCformsinpreferencetoM23C6 carbide reducingthechromiumdepletionofthegrainboundaries.Duringservice,othercoarsesecondaryphasesmayformwhichcanleadtoadropinductilityandtoughnessandcancauseembrittlementofthematerial.Type310stainlesssteelsarepronetosigmaphaseformation[5]. Sigma phase is an intermetallic phaseconsistingmainlyofironandchro-mium.Duringserviceconditions,theforma-tionofthisphasetakesplaceaccordingtothethermodynamicstabilityandkineticsofprecipitation.Theseareinfluencedbythecompositionofthealloyandthegrainsize(diffusionalongthegrainboundariesismuchfasterthanthroughthegrains) [6]. In par-ticular,anincreaseofnickeland/ornitrogencontentiseffectiveinsuppressingsigmaphaseformation.ButanexcessofbothelementsgivesrisetotheformationofCr2N andPiphase(mixednitride)resultingalsoinareductionoftoughness.ItisevidentthatthechromiumcontentisalsoimportantwithrespecttotheCr2N as well as sigma phase formation.Furthermore,themodificationofotherelementssuchassiliconandmanga-neseiseffective.InDMV310Nmaterial,thecompositionisadjustedinthiswaytomini-
2 Material Properties
19
misetheformationofthesecoarsesecond-aryphases.Thesolutenitrogeninthematrixcontributestomicrostructuralstability.Theformationoftheundesirablecoarseprecipi-tatesislimitedevenafteragingfor10,000hat600to750°C.The creep rupture strength values as well as thecharpyimpactpropertiesoftheopti-misedDMV310Nmaterialaresuperiortothoseofother310stainlesssteels.
2.5 Corrosion resistanceChromiumisthekeyelementinfluencingthecorrosionbehaviourofsteels.Thesensitiv-ityofthematerialtosteam-sideoxidationaswellasfire-sidecorrosionisanimportantfactorforuseinthefieldofboilers.Theuseofhighsulphurcoalmakesthefire-sidecor-rosionresistanceofevenhigherimportance.Thecorrosionrateisincreasedbythedepo-sitionofsulphidesonthetubesurface.Intherangeof600to650°Cthesesulphidesareinliquidstate.Freesulphurtrioxidedissolvestheprotectiveoxidescaletoformironandchromiumbasedsulphateswhichrapidlyincreasesthecorrosionrate.Asufficientamountofchromiumisnecessaryinordertoreformthechromialayer.Thisisdeterminedbytheoverallamountofchromiumontheonehandandthekinetics(fastdiffusionalonggrainboundaries)ontheother.Thecorrosionresistanceisimprovedbyanincreasedchromiumcontent,resultinginrapidformationofadenseandadhesive
chromiascale.Ahighamountofchromiumalsoensuresafastreformationofthisscaleafterdeterioration.Asaresult,thecorrosionresistanceoftype310stainlesssteelswithachromiumcontentofaround25%issuperiortothatof18-8stainlesssteelswithabout18%ofchromiumsuchasDMV304HCuorDMV347HFG.Ingeneral,theprecipita-tionofchromiumcontainingnitridesandespeciallytheformationofM23C6 with a high amountofchromiumresultsinadepletionofchromium.Theformationofachromiascaleafterdetachmentisthereforeslowerthanfortheothergrades.Nevertheless,resultsofsteamoxidationtestsandhotcorrosiontestsinsynthesizedcoalashenvironmentbetween650and700 °Cforupto1000hexhibitsimilarbehaviourtothatofother310stainless steels [1]. EvenifDMV310Ncouldhaveslightlylowercorrosionresistancethanthehighernio-biumalloyedgrade310Nbafterlongtermtrialsduetotheprecipitationofchromiumenrichedphases,theadvantagesoftheclearlyincreasedmechanicalproperties(creepresistance,hightemperaturestrength)outweighthis.
20
3.1 Tube bendingBoilertubesmanufacturedinsteelgradeDMV310Naregenerallysuitableforcoldandhotdeformation.Ifhotformingisnotperformedusingacontrolledtemperatureprocessbetween1175°Cand1250°C(2150and2280°F)anadditionalsolutionanneal-ingisrequired.Cold-formedtubesmustbenewlysolutionannealedifthecolddeforma-tionistoohigh.IfcolddeformationexceedsthevaluesgiveninTable11,additionalsolutionannealingafterdeformationismandatory.
Inordertomaintainthecorrosionresistance,anewsolutionannealingisrecommended,evenafterasmalldegreeofcolddeforma-tion.
3.2 WeldingThe material DMV 310 N is weldable using state-of-the-arttechnologies.Thefollow-ingfusionweldingtechniquesarepossible:metal gas-shielded welding with welding wires,weldingsticksorwithcoredwireelectrodesandmetalarcweldingwithlimealkalineenclosedelectrodes.Itisneces-sarytouseapprovedfillermaterialswhicharealsotestedattheforeseenapplication
temperature. Preheating and heat treatment aftertheweldingprocessinthefabricationofDMV310Nisnotmandatory.However,ifthematerialissensitisedafterwelding,thenapostweldingtreatment(solutiontreatment)canbedonetorestoretheproperties,mainlytoincreasethecorrosionresistance[VdTÜVmaterialdatasheet546(03.2007)].Generally,austeniticstainlesssteelshaveahighsusceptibilitytohotcrackingintheweld. Welding tests have indicated simi-laritiesbetweenniobiummodifiedtype310stainless steels as well as DMV 310 N and theniobiumcontainingDMV347HFGmate-
rial [1].Itispossibletoachievesoundweldjoints,butthemarginofsafetyisnarrowerthanforthemorecrackresistantsteels[3]. Weldmetalcrackingcanbecontrolledbytheuseofappropriatefillermaterials.MostcommonfillerwiresareNi-materialsbasedonalloy617.Torestrictheataffectedzonecracking,alowheatinputshouldbeusedandcareshouldbetakentoavoidatmo-sphericcontaminationofthemoltenmetal.
3 Fabrication
Norm Max. cold deformation Radius to wall thickness ratio
Additional solution annealing temperature
VdTÜV Datasheet 546 >20 % < 2,51180-1270°C (2156-2318 °F)
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[1]SAWARAGI,Y.;TERANISHI,H.;YOSHIKAWA,K.:Thedevelopmentofnewlysteelwithhighelevatedtemperaturestrengthandhighcorrosionresistanceforboiler.SumitomoMetalIndustries,Ltd.ProceedingsoftheInternationalConferenceonCREEP,Tokyo,(1986),pp.239-244.
[2]JACK,D.H.;JACK,K.H.:StructureofZ-phase,NbCrN.JournaloftheIronandSteelInsti-tute,(1972),pp.790-792.
[3]YOSHIKAWA,K.;SAWARAGI,Y.;YUZAWA,H.:Developmentofnewtubeswithhightem-peraturestrengthandhighcorrosionresistance.ProceedingsoftheInternationalConferenceonImprovedCoal-FiredPowerPlants,PaloAlto,California,(1986),EPRI,pp.166-182.
[4]NATORI,A.:HR3Ctubesincoal-firedboiler.StainlessSteelEurope,(1992),pp.20-23.
[5]TISINAI,G.F.;STANLEY,J.K.;SAMANS,C.H.:EffectofnitrogenonsigmaformationinCr-Nisteelsat1200°F(650°C).JournalofMetals,(1954),pp.1259-1267.
[6]SCHWIND,M.;KÄLLQVIST,J.;NILSSON,J.-O.;ÅGREN,J.;ANDRÉN,H.-O:σ-Phase precipitationinstabilizedausteniticstainlesssteels.ActaMaterialia,Vol.48,(2000),pp.2473-2481.
4 References
Salzgitter Mannesmann Stainless Tubes GmbHWiesenstraße 3645473 Mülheim an der RuhrGermanyTel.: +49(0)20845801Fax: +49(0)2084582640
SMST-Tubes,D
MV310N04/2008www.heselsvom
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