8/12/2019 Dylatometria Ferryt Delta

1/9

The Thermal Expansion Characteristicso f Stainless Steel W e ld M eta lThermal expansion dataare establishedtohelpin theproperselectionof austeniticstainlesssteel filler metalsto beusedfor welding dissimilarmetal joints

BY J.VV.ELMER,D. L.OL S ONAND D. K. MATLOCK

ABSTRACT. Thermal expansion coeff icients for 28 stainless steel welds ofvary ing composi t ion have been measured. A graphic method of predict ingthe coeff icient of thermal expansion(CTE) for stainless steel welds centeredabout the DeLong d iagram has beenprepared f rom these data. An overalldescr ipt ionof thebehaviorof theCTEasa funct ion of composi t ion was accomplished by supplement ing the data withpublished thermal expansion data of allFe-Ni-Cr type alloys. Lines of constantexpansion were then mapped on theFe-Ni-Cr ternary diagram and were subsequent ly t ransposed to the Schaefflerdiagram. Using these diagrams, the CTEfo r awide rangeof ferrit icandau steniticalloyscan bepredic ted.

Residual delta ferrite in astainless steelwe ld wass h o wn to reduce the CTE ofthe duplex austenite-ferr ite microstructure from that of a ferr ite-free m icro-structure. Results predict the CTE ofresidual delta ferriteto be15.0 ^m/m/Covera 20 to400C(68to752F)temper ature range;inaddit ion, wi ththeThomascomposi te theory of thermal expansion,th e CTE of any stainless steel weld containing delta ferrite can be calculated.Adecrease in specif ic volume was shownto accompany the transformat ion ofmetastable delta ferrite to austeniteandsigma phase. This contractive dilationresults in a strainof 4.5 X 10 ~5 per percent ferr ite that t ransforms. Itshouldbeemphasized that this strainwi l l ,ingeneral,produce stresses thataretensileinnatureacross a weld jo int . Fur thermore, thetotal strain due to thetransformat ion isdetermined by the initial ferrite contentand the time that the joint isheld at anelevated temperature.

IntroductionThermal expansion is a fundamenta lmater ial property which relates dimen

sional changesof amaterial with changesin tempe rature.A convenient measureofthermal expansion is the mean linearcoeff icient of thermal expansion CTE)and isdef inedas:A

L o ( T 2 -T , )., =/ A L X 1\ Al (I)

wh e r eLiandL2are thespecimen lengthsat temperaturesT,andT2respect ively,L0istheinitial specimen lengthandcemis themean CTE. Fundamentally, theCTE m easurestheamount of strain, AL/L0 , whichaccompanies a material with achangeintemperature,AT.W he n dissimilar materials are joinedand restrained suchas in a weld ing p ro cess, changes in temperature wi l l a l lowstresses as high as the yield stress todeve lop in the joint . For example, Dalcheretal . Ref. 1)predic ted h oo p stressesup to 34 ksi(234 MPa)at therootof atypical dissimilar metal pipe weld due toth e CTE mismatch alone. The stressesdecay as a funct ion of posit ion awayf rom the interface so that far from theinterface the CTE mismatch stressesare

ze ro .The stress distr ibut ion is dete rmin edby thewe ld joint design and weldmentgeometry , whi le the stress level dependson bo th the dif ference in CTE betweenthe weld and base metalsand the temperature change from the stress freetemperature.The stress free temperature is thereference temperature at which no CTEstresses exist. Typically,in theas-weldedcond i t ion , this temperature wil l be closeto thesolidusof theweld meta l . H o w e v -

Basedon paper presentedat the62ndA WSAnnual Convention heldin Cleveland, Ohio,during April5-10, 1981.J. W.ELMER, D. L.OLSON and D. K. MAT-LOCK arewith the ColoradoSchoolofMines,Golden, Colorado.

er, if apos twe ld heat t reatme nt (PWHT)operat ion has been used, stress reliefmay be sufficient to reduce the stressessuch that thePWHT tem perature w i l lbestress free. If welds of dissimilar metalsare usedat high operat ing temperatures,the thermally induced stressescandecayby creep mechanisms to low levels witht ime. The weldment at the operat ingtemperature w i l l thenbeessentially stressf ree . However, stresses wil l developw h e n the we ld iscoo led to r o o m t e m perature, apart icular conc ern if thermalcycling isfrequen t .Selection of theprop er f i ller metal toweld dissimilar metals that minimizesCTEmismatch stresses requires an understandingof the thermal expansion prop ertiesof thewe ld meta l .Acompi lat ionofpublished CTE databyBennett (Ref.2) forstainless steel weldsandbase metalswasan early at tempt to character ize thesepropert ies; however, l i t t le other informat ion isavailable on the CTEbehavior ofweld metal.Stainless steel filler metalsarefrequent ly used to weld austenit ic and ferrit icsteels. Typically, the austenitic stainless

steel hasaCTE th atis30 to40 greaterthan that of a ferrit ic steel. This CTEmismatch is freque nt ly responsible forproblems that occur in these type ofjoints.Onesuch prob lem exists withtheferr it ic Cr-Mo steel to austenitic stainlesssteel weld transit ion joint commonly usedin power plants Ref. 3, 4). Numerousservice failures resulting in cos t ly downt ime have occurred (Ref.5) in these weldjoints, because of fusion line crackingatthe ferr it ic weld interface when Type 310Cband other typesofstainless steel fillermetals were used.Thermal expansion mismatchhasbeensited Ref. 3-6) as one of the majorproblems with this part icular w e l d . Somedegree of success hasbeen obta ined inprevent ing or at least postponing thecracking problembyusing ERNiCr-3 nick-

WELDINC RESEARCH SUPPLEMENT|293-s

8/12/2019 Dylatometria Ferryt Delta

2/9

el-base filler metal which has a lower CTEthan Type 310 Cb stainless steel f iller, andhas a CTE be twe en that of the tw o basemetals (Ref. 5). Another problem thatexists is crackin g of w el d cladd ed m aterial(Ref. 7). Austenitic stainless steels are cladon ferr it ic steels to provide corrosionresistance. Typically these welds coverlarge surface areas relative to other w e l d ing processes, and an understanding ofhow the CTE var ies wi th d i lut ion wouldbe helpful in selecting proper filler metals.Before discussing the specific aspectsof this study, it is necessary to point outthat the CTE of a material is dependentupon the pr ior thermomechanical processing history and the temperaturerange over which it is measured. Forexample, a ferr ite-free wrought, stainlesssteel will have a different CTE than itswelded counterpart containing residualdelta ferr ite, or a 36 nickel- iron alloy(Invar) will have different CTE's de

pending on the mechanical processinghistory and the temperature range ofinterest. It is, therefore, necessary tocompare the CTE data for materials thathave had the same metallurgical processing historywelds , fo r exampleandwhose CTE were measured over ident icalcondit ions. Otherwise, the correlat ionscan only be considered approx imate. Ingeneral, however, these correlat ionsallow engineering trends to be established by means such as the plotting ofisoexpansion lines on diagrams as a funct ion of composit ion, using data acquiredfrom many sources.

Exper imental ProceduresWelding and Specimen Preparation

A three step weld alloying procedure,similar to that of Lippold and Savage (Ref.8) , was used to produce welds of desiredcomposit ions. This procedure is i l lustrated in Fig. 1. First, a GMAW processwas used to lay a bead on Vi in. thick(12.5 mm) Type 316 stainless steel plateusingY\bin. (1.6 mm) diam eter T ype 308,309, 310, 312, 330 or 410 stainless steelf i l ler metal wire. Welding grade argon-2oxyg en w as used as the shielding gas, andthe travel speed was var ied from 11 ipm(4.65 mm/s) to 28 ipm (11.9 mm/s) toproduce several dif ferent welds fromeach f i l ler metal. Next, the weld reinforcement was removed by machin ing,leaving the base metal intact.

This process lef t a str ip of the GMAWdeposited f i l ler metal down the center ofthe base plate. In the final step, thealloying filler metal and the base metalwere melted together us ing a CTAWpass. Welding grade argon was used asthe shielding gas w ith a 2 thor iate dtungsten e lect rode ground to a 90 degincluded angle. The travel speed of thispass was var ied from 2.5 ipm (1.07 m m /s) to 3.1 ipm (1.32 mm /s ) to prov ide

dif ferent amounts of di lut ion of the CMAdeposit with the Type 316 stainless steelbase metal.The d i f ferent GMAW deposi t ion ratesand GTAW dilut ion rates were used toprod uce a total of 28 welds of d if ferentcompositions from the six f iller metals.For each filler me tal, a series of w elds wasproduced which var ied in composi t ionfrom the base metal to the f i l ler metal.The welding parameters and result ingchemical composit ions of the test weldsare presented in Table 1. The chemicalcomposit ions were measured using emission spectroscopy and interst it ial analysis.In Table 1, the three nu mbers of thespecimen designat ion refer to the stainless steel f iller metal type, and the lettersrefer to high, medium, low or zero deposit ion rate (GMAW) and dilut ion rate(GTAW). For example, 308HL refers to308 f i l ler metal, high deposit ion (GMAW),low dilut ion (GTAW). Note that a highheat input was necessary to produce aweld bead of sufficient size, such that aspecimen for di latometr ic analysis couldbe removed. The a l l -weld-meta l d i la tometr ic specimen machined from the center l ine of the w eld bead as show n in Fig.IC measures 1.5 in. (38 mm) long and0.15 in. (3.8 mm) in diameter.Testing Procedure

Thermal expansion measurementswere made using a hor izontally posit ioned vit reous sil ica push rod dilatometer. Dilat ion curves were recorded asspecimen length change versus temperature using an X-Y recorder. NBS standardreference mater ials SRM 736 (copper)and SRM 737 (tungsten) were used to

calibrate the dilatometer. The dilatometercalibrat ion curve was established usingthe tungsten standard. The copper standard was then tested and from its corrected dilat ion curve the accuracy of thedi la tometer was determined to be 1 . 8 error in the mean CTE as determined f rom 11 data points taken at 25C(45F) intervals in the ran ge fr om 275 to525C (527 to 977F).All test ing was conducted in an argonatmosphere to prevent oxidat ion of thespecimen. The heat ing rate was cont rol led at 5.5C/min (10F/min) andcool ing curves were recorded dur ing afurnace cool at cooling rates less than5C/min (9F/min). The specimen temperature w as reco rded using a single typeK thermocouple that was kept in contactwith the center of the specimen dur ingthe test . The length change, accurate to1 X 1 0- 4 in. (2.5 X 10 ~3 mm), wasrecorded us ing an LVDT mounted in amicrometer stage which was separated

f rom the furnace wi th a water cooledbarrier.The experimental thermal expansiondata were cor rected to obta in the t ruedilation curve using the established calibrat ion curve. A microprocessor assembly equipped with a digit izer was used toanalyze and f it the true dilat ion curve to afour th order polynomial o f the form:L = L0+ aT + bT2 4-cT3 4- dT4 (2)

where specimen length, L, at temperature , T, can be calculated from the original length,L0,and polynom ial c oeff icientsa, b, c, d. Both instantaneous and meanCTE data were calculated from this equat ion. The polynomial est imat ion typicallyf it the actual data with a coeff icient of

G M A B e a d 308,309,310,312,330,410 Filler316 Stainless Steel

0.5

Specimen0.15x(inch)G T A P a s s

ipm

Fig.1 Threestep weldalloyingprocedure used to produce stainlesssteel welds

294-s ISEPTEMBER 19 82

8/12/2019 Dylatometria Ferryt Delta

3/9

Table 1Summary of Welding Procedures and Resulting Compos

Specimen3 0 8 H *308HL308HH3 0 9 H *309HL3 0 9 HM309HH3 0 9 M M309LH31OH*310MM310MH310LH3 3 0 H *330HL3 3 0 HM330HH330MM4 1 0 H *410HL4 1 0 HM410HH410MH410LH3 1 2 H *312HH312MM312MH I current in

I300300300300300300300300290290290290290290290290290300280300300300300300320320320320

ampere*

GMAV

28-2928-2928-2929-3029-3029-3029-3029-3029-3029-3028-2928-2928-2929-3029-3029-3029-3029-3029-3029-3029-3029-3029-3029-3029-3029-3028-2929-30

; Vvoltage

s11111111111111213811212138111111112111111111213811112121

in volts

iN / A310310N / A310310310310310N / A300310310N / A305310310310N / A310310310310310N / A310310310S we l d

GTA(aV

N / A16-1716-17N / A16-1716-1716-1716-1716-17N / A17-1817-1817-18N / A17-1817-1817-1818-19N / A18-1918-1918-1918-1918-19N / A18-1918-1918-19ng speed in

sN / A3.11.7N / A3.12.72.12.72.1N / A2.72.12.1N / A3.12.71.72.7N / A3.12.71.71.71.7N / A1.72.71.7

ipm.

C r19.518.317.622.621.021.520.018.718.522.519.519.017.517.316.316.816.016.015.817.316.017.017.217.922.022.019.520.0

tion of 28 Stainless Steel Welds

Si.38.70.78.39.44.41.36.40.42.40.40.35.39.41.33.35.35.31.37.35.32.32.32.33.50.47.45.39

M o1.271.682.001.271.441.441.801.852.001.12.12.02.11.171.501.641.931.501.201.661.581.621.802.051.321.631.751.65

N bTiZr< . 0 1< . 0 1

8/12/2019 Dylatometria Ferryt Delta

4/9

Table 2CTE,Creq,

Specimen308H308HL308HH3 0 9 H *309HL3 0 9 HM309HH3 0 9 M M309LH31OH*310MM310MH310LH3 3 0 H *330HL330HM330HH330MM41OH*410HL4 1 0 HM410HH410MH410LH3 1 2 H *312HH312MM312MH

N ieq and Ferrite Content for 28 Stainless Steel WeldsMean Coef f ic ientThermal Expansion,/ i m / m / C , 2 0

18.018.518.417.318.218.318.518.918.617.718.318.118.718.918.418.418.6I8.413.918.919.019.318.818.717.017.518.418.7

400C Creq21.321.020.824.523.123.622.321.221.124.222.221.520.119.118.319.018.518.017.619.518.119.119.520.524.124.322.221.9

Nieq14.813.914.017.117.318.519.417.117.820.716.017.815.729.622.924.320.320.010.114.113.716.813.815.215.615.715.616.5

Ferrite, %10.47.25.612.18.15.40.90.50.20

8/12/2019 Dylatometria Ferryt Delta

5/9

Fig. 4 Optical micrographsh owingmartensitic stainless steel weld. Type410filler metal;Type 316 base metal; GMA weld. Fed} inHNOj etchant. X625

present to influence the CTE of fullyaustenit ic welds, there should be asmooth transit ion in the CTE from 18.5/*m/m/C to 15.5/um/m/C as the nickel content is increased to that of 100nickel.Th erefo re, a large increase in nickel content is necessary to reduce the CTEof a fully austenitic alloy by a significantamount.

In contrast, the 410 filler metal series isdifferent in the sense that a small co mp osit ion variation changes the CTE from 19/ttm/m/C to values as low as 11 gm/m/C because of the presence of martensite and ferri te in the microstructure.Thus the 410 GMA weld conta in ing fe rrite and martensite has a CTE of 13.9/tim/m/C while the remaining welds inthe 410 series, which were heavily di luted with the Type 316 base metal, haveCTE values around 19 f tm/m/CFe-Ni-Cr Ternary and Schaeffler Diagram

The narrow composi t iona l range o f theDeLong diagram limits its use to stainless

steels. Therefore, to obtain an overal lunderstanding of the variation in CTEwith composi t ion , CTE data was collected from the l i terature (Ref. 11-20)pertaining to all Fe-Ni-Cr containingalloys. Similar to the procedure usedabove wi th the DeLong d iagram, the datawere p lo t ted on the te rnary d iagram andisoexpansion l ines were drawn usingthese data as a guideline. CTE as a funct ion of composit ion for the Fe-Ni binarysystem (Ref. 17, 18) and for the Fe-Crbinary system (Ref. 19, 20) was particularly helpful in establ ishing the contours.Regions where the CTE data exists areshaded. Extrapolation of the contoursoutside these regions was accomplishedby understanding the phases which arepresent (Ref. 12) and the CTE end pointsestablished from work on the binarysystems (Ref. 17-20).The terna ry diagram is show n in Fig. 5.The region of h ighest thermal expansionis again seen in the conventional stainless

steel a l loys center ed abou t 14N ie q and 18Creq. This region represents a peak ofCTE a t approximate ly 19 nm /m / C .Reduction of the Nie q will result in deltaferri te being present and wil l decreasethe CTE, while a reduction of both theN ieq and theCreq will result in delta ferriteand martensite being present and wil la lso decrease the CTE. The low est thermal expansion coeff icients are observedin the Fe-36Ni, Invar-type, al loys; however, these al loys are not frequently usedfor we ld ing .Large variations in the CTE are appar

ent over the ternary diagram. The mostsignif icant changes that occur for ferri t ic-austenitic stainless steel welds are thosethat occur due to the presence of deltaferri te and martensite in the microstructure. Therefore, the isoexpansion l ines

drawn on the te rnary d iagram weretransposed to the conventional Schaeffler Diagram (Ref. 10) so that both theconsti tut ion of the weld and its CTE canbe predicted from one diagram. Thisdiagram is sh ow n in Fig. 6. The DeLongdiagram is outl ined to i l lustrate whe re theinit ia l data on welds is located, while theshaded areas again represent thoseregions where CTE data was compiledfro m the l i terature. This is a clearer repr esentation of how ferri te and martensiteinfluence the CTE. The martensite causesa trough of low expansion in the diagramwhile ferri te results in a more gradualdecrease in CTE.The Influence of Delta Ferrite on the CTE

Stainless steel welds will contain residual delta ferri te in the as-welded micro-structure i f the composit ion of the weldhas a sufficiently high Cr e q /N i e q ratio. Theresidual delta ferrite has a BCC crystallinestructure, and can be present in variousmorphologies (Ref. 21) with in the FCCaustenit ic matrix. On sol id if ication, thereis a partitioning of ferrite stabilizing elements to the delta ferri te and austenitestabilizing elements to the austenitematrix (Ref. 21). It has already beenshown that sol id solution effects on theCTE are small and that the importantfactor in determ ining the CTE of thematerial is the CTE of the phases that arepresent.

To determine the CTE of residual deltaferri te and thus i ts contrib ution to the CTEof the du plex stru cture, a series of d i latometric tests were conducted on a singlespecimen, 312H, initially containing34.2 delta ferri te. The residual deltaferri te is metastable and wil l transform toaustenite and sigma phase at elevatedtemperatures. This phase transformation

C r

Mean Coefficient ofThermal Expansion20 -4 00 C/im/m)/C

4 6 8Fig. 5 Mean coefficient thermal expansion(fim/m/C, 20 -400 C, i.e.,68-752 F)plotted as isoexpansion contours on the Fe-Nieq-Creq ternarydiagram.Shaded areasrepresentregionswhere data were acquired

HZUJ_ loUJ- JUtto

M e a n C o e f f i c i e n t o f T h e r m a l E x p a n s i o n| j i m /m )/ C , 20-400C

810S^2l4;i6^17

Ferritej I8 16 24 32C H R O M I U M E Q U I V A L E N TFig.6Mean coefficient thermal expansion(nm/m/C, 20 -400 C, i.e., 68-752F)plottedas isoexpansioncontours on theSchaefflerconstitution diagram. Shaded areasrepresentregionswhere data were acquired

WELDING RESEARCH SUPPLEMENT|297-s

8/12/2019 Dylatometria Ferryt Delta

6/9

Table3FerriteCon tent, CTE, and Length Measurements After Each of Six Dilatom etric Tests Co nducted on a High Ferrite W eld

Test no.vswe l d e d

123456

M a x i m u mtempera tu reofdi la tometr ic

test, C-

550650700750800800

Ferritecon ten tafter testvo l34.232.726.418.07.03.12.0

Mean coeff ic ient o fthermal expansion,(Aim/m)/C 2 0 ^17.217.417.517.818.118.2-

400C

SpecimenAs de te rm ined

f romdi la tometr icmeasurements,

in.

1.50681.50681.50581.50501.50481.5047

(mm)-

(38.273)(38.273)(38.247)(38.227)(38.222)(38.219)

length

Mic rome te rmeasurements,in.

1.50701.50681.50621.50581.50511.50491.5048

(cm)(38.278)(38.273)(38.257)(38.247)(38.230)(38.224)(38.222)

has been studied (Ref. 22-24), and thekinetics of this interface controlled dif fusion transformation (Ref. 22) allow it tooccur under the thermal cycl ingcondit ions of a di latometric test. Therefore, the specimen was run through several d i latometric cycles that served twopurposes:

1. To heat the specimen and transform only part of the ferri te.2. To mea sure the CTE of the mate rial,as a func tion of ferri te conte nt.A separate metal lographic specimenwas placed in the di latometer and subjected to the same thermal cycl ing as thedilatometric specimen. A section of themetal lographic specimen was removedafter each thermal cycle to evaluate themicrostructure at each ferri te level.Table 3 summarizes the testing proce

dures, resulting ferrite contents and CTEof th is specimen. The in it ia l d i latometrictests were run at lower maximum temperatures than the latter tests to limit andcontrol the amount of ferri te that transformed. Figure 7 illustrates the CTE as afunction of delta ferri te content. Asexpected, the CTE decreases withincreasing ferri te content. The as-weldedmaterial conta ining 34.2 delta ferrite hasa CTE of 17.2 ftm/m/C and after f ivethermal cycles the weld contains 3.1ferrite and has a CTE of 18.2 / i m / m /C.Since the residual delta ferrite is distrib

uted as a second phase throughout theaustenite matrix, the CTE of the duplexstructure may fo l low a composi te theorybehavior with delta ferri te content. Areview o f thermal expansion composi tetheory relationships by Nielsen (Ref. 25)compares th ree thermal expansion composite relationships for particulate filledsystems. These relationships are the Kern-er, Thomas and Turner theories for thermal expansion.The Thomas equation was selected torepresent the expected behavior between the CTE and delta ferri te content

because, as Nielsen (Ref. 25) summarizes,the Thomas and the Kerner equations aremore accurate in general than the Turnerequation and because the Kerner equation requires knowledge of the elastic

propert ies wh ich were not measured inthis study. The Thomas theory states:Inctc= FIn

8/12/2019 Dylatometria Ferryt Delta

7/9

Fig. 8 Optical micrograph showing themicrostructureofType 312 stainlesssteel weldin: Aas-welded condition; B thermallycycled condition. KOH electrolytic etch.X625

accomplished using the magnet ic etchingtechnique of Gray, Sikka, and King (Ref.26) and using Murakamai's etch (Ref. 26);how eve r , a 10 KO H elect ro ly t ic etch(1 A / c m 2for 5 to 7 s) pro du ced the sameresults and was less time-consuming.Figure 9 shows the same microstructure at a higher magnification. Figure 9Ashows a region of the microstructurewhere the delta ferr ite t ransformed tosigma phase and austenite in approximately equal amounts; i t is to be notedthat there appears to b e a crystallograph-ic or ientat ion relat ionship between thenewly formed sigma phase and austeniteand the or iginal delta ferr ite. The transformat ion of delta ferr ite was not uniformthroughout the microstructure.

Figure 9B shows a region of the samespecimen where much less of the deltaferr ite t ransformed and the sigma phasethat is present is not as abundant as thatof Fig. 9A. The original ferrite-austeniteboundar ies are decorated wi th whatseem to be precipitates and they indicatethe size of the delta ferrite particle priorto thermal cycling.After six thermal cycles, a point countof sigma phase sho ws 14.9 to bepresent in the microstructure and ferr itemeasurements sho w 2.0 ferr ite lef tuntransformed. This means that of theinitial 34.2 ferrite , only 17.3 of theferr ite in the microstructure transformed

direct ly to austenite.The presence of sigma phase in themicrostructure and its inf luence on theCTE has not been considered up to thispoint . However, i f the regression l ine of

Fig. 7 is extrapola ted to 0 ferr ite, wi th14.9 sigma phase in the austenitic structure , the material has a CTE of 18.4/ u m / m / C . Comparing this value to thatof about 18.5 ^m/m/C for the fullyaustenitic structures shown in Fig. 3 indicates that there is litt le difference in theCTE, although one structure contains14.9 sigma phase and the other is fullyaustenitic. This leads to the conclusionthat sigma phase has a CTE that is nearlythe same as that of austenite.To account for sigma phase, a separateterm could be included in equat ion (3);ho we ve r, the extra term is not necessaryto determine first order effects since theeffective CTE of sigma phase in austeniteis close to that of the austenitic matrix.

Decrease in Specific Volume as Delta FerriteTransforms to AusteniteIrreversible dilatometr ic behavior wasobserved between heat ing and cool ingcycles of the welded specimens when:1. The maximum temperature of thetest went above about 600C (1112F).2. The weld contained delta ferr ite.In all cases where irreversible behaviorwas observed, the cool ing curve wouldlie below the heat ing curve. This type ofdilatometric behavior of stainless steelwelds has also been observed by Blochand Huszar (Ref. 24) and is associatedwith the transformat ion of delta ferr ite.The dif ference between the heat ing andcooling curves represents a contract ionof the specimen, independent of thethermal expansion (contract ion) proper

ties of the material. This behavior is illustrated in Fig. 10, which compares twodilatometr ic curves of the high ferr iteweld ser ies descr ibed in the previoussection.Figure 10A shows the typical dilationbehavior of the weld when 11.0 of theferr ite t ransforms. The heat ing and coo l ing curves do not superimpose; in fact ,the cooling curve l ies signif icant ly belowthe heat ing curve. There is a negat ivedisplacement in the specimen length of0.0008 in. (0.02 mm), measured at roomtemperature af ter the test ; this meansthat the specimen has contracted. Incontrast, Fig. 10B shows that the heatingand cooling curves almost superimposewh en only 1.1 of the ferr ite t ransforms.With subsequent thermal cycling, themater ial would stabil ize and the dilatometr ic curve would be reversible.The change in specimen length is dueto a decrease in specif ic volume of thematerial as the delta ferrite transforms tothe more dense austenite and less densesigma phase. To est imate how muchcontract ion should occur, the densit ies ofType 410 ferrit ic stainless steel (7.7 g m /c m 3Ref. 13), Type 316 stainless steel

(8.0gm/cm3-Ref. 13), and Fe-Cr sigmaphase (7.6 gm /c m 3 Ref. 27) we re usedto approximate the densit ies of thephases present in this we ld .With these density approximat ions, a

we lded mater ial containing 34.2 ferr iteand 65.8 austenite wo ul d have a density of 7 .90 gm/cm 3 , while the same mater ial af ter thermal cycling containing 2fe rr i te , 83 .1 austenite and 14.9 sigmaphase wou ld ha ve a density of 7.93g m / c m 3 . The difference in specific volume between the mater ial in the as-welded and thermally cycled condit ionswo uld be 0.378 .The relat ionship, A V = 3em, (Ref. 28)where AV is the change in specif ic vo l ume andt is the linear strain, is goo d forsmall strains and can be used to calculatethe strain in any direction to be1.26 X 10 ~3 and for the 32.2 change infe rr i te , there should be 3.9 X 10~5 strainfor each percent ferr ite that t ransforms.Note that this strain would be greater ifless sigma phase had been formed aswould be the case if a stainless steel basemetal less susceptible to forming sigmaphase than Type 316 stainless steel wouldhave been used. This value of strain can

now be compared to the measured value.Specimen length as a funct ion of ferr itecontent is summarized in Table 3 andplotted in Fig. 11 .The specimen length ateach ferr ite level was determined bysubtract ing the displacement, measuredfrom the dilatometr ic curve at room temperature, f rom the previously calculatedlength. The specimen length was alsomeasured with a micrometer af ter eachtest and both measurements are includedon this plot.

8/12/2019 Dylatometria Ferryt Delta

8/9

= 2 0EHOzLU 1 0 -UJoz