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Prediction of Austenit ic Weld MetalMicrostructure and PropertiesWith advent of new stainlesssteels,a wider range ofalloys

mu st be con sidered in predicting ferrite

BY D. L. OLSON

ABSTRACT. Diagrams, such as theSchaeffler and DeLong diagrams, havebeen used to assist in the proper selection and use of austenitic filler materialsand to predict weld metal microstruc-tures and properties. These diagramshave been very successful in predictingthe amount of delta ferrite in stainlesssteel weld metal. This paper is conce rnedwith the predictabil i ty of austenit ic weldmetal microstructure and properties overa larger composit ional range.There are two main different types ofphase transformations associated withaustenitic weld metal. Existing analyticalmethodology has been successful at predicting quantitatively the nature of theliquid to delta ferri te transformation. Butthe austenite to martensite transformation for high al loy weld metal needs to bebetter unders tood i f we ld ing consumables for new high manganese ferrousalloys are to be developed to achieveoptimum properties and service behav

ior. In this paper, new expressions areintroduced to predict the martensite startroom temperature compos i t ion or themartensite start temperature. Some ofthese high manganese ferrous alloys arethe bas is fo r the new no ch rom iumstainless steel. Various available diagram s,which al low for the prediction of weldmetal microstructure, wil l be given.New mathematical forms for expressions to predict we ld m etal phase stabil ityand microstructure, based on solutionthermodynamics and kinetics, wil l beintroduced. These new expressionalforms should al low for better predictabili ty over a larger al loy range. The non-

homogen eous (cored) nature o f the w eldmetal composit ion wil l a lso be considered . These new forms can al low fundamental al loying and solid solution informat ion to be obta ined f rom the micro-structure or property correlations withthe weld metal composit ions.I n t r oduc t i on

As new engineering materials aredeve lo ped i t is important to deve lop themethods and materials for welding them.Austenit ic weld metals are frequently util ized for jo ining various engineeringmaterials and for a variety of reasons.Austenit ic consumables have been extensively used to form the transit ion weldmetal in dissimilar ferrous alloy joints, tojoin stainless steel, in weld repair, inhardfacing, and in corrosion resistantcladdings. If properly al loyed, austenit icweld metal is strong, ducti le, resistant tohot-cracking, and capable of retainingpotential ly troublesome contaminants insolid solution. Austenit ic consumablewire is readily cold formed, faci l i tating itsproduction. But some austenit ic compositions are characterized by a high thermalexpansion coeff ic ient which often leadsto the development of high residualstresses in the w e l d . Weld meta l micro-structures based on traditional austeniticweld metal composit ions can be predicted from empirical diagrams, such asthe Schaeffler diagram (Ref. 1). Difficultiesarise, however, when the weld meta lcompos i t ion ex tends beyon d the app l ication range of the original empirical relationships. This situation is the case, for

instance, when the Schaeffler diagram isapplied to weld metal of a differentthermal history, or of a vastly differentchromium concentra t ion , than that o f theoriginal study. It would, therefore, beadvantageous to uti l ize the fundamentalsof materials science to develop expressions which would be more generallyapplicable to predict weld metal micro-structure and properties.

The Fe-Cr-Ni Weld Metal SystemIn 1906 Guillet (Ref. 2) first introducedthe Fe-Cr-Ni alloy system as a potentialengineering material for corrosion resistance and mechanical applications. Theworks of Gieson (Ref. 3), Monnartz (Ref.4), and Maurer and Strauss (Ref. 5) led tothe commercial ization of the 18Cr-8Nialloys, the basis for most of the 300 seriesstainless steel alloys. By 1934, the understanding that low carbon contents (0.03)

gave austenitic stainless steel a superiorintergranular corrosion resistance wasestablished.Strauss and Maurer (Ref. 6) introduceda nickel-chromium diagram, which waslater modif ied by Scherer, Riedrich andHoch (Ref. 7), that al lowed prediction ofthe various phases in the microstructure

This paperisth e1984 AdamsLecture,presented at theAWS 65thAnnual Meeting,heldApril9-13,1984, inDallas, Tex.D. L.OLSON is Professorand Director at theCenter for Welding Research, ColoradoSchool ofMines,Golden, Colo.

WELDING RESEARCH SUPPLEMENT I281-s

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28 T 1 1 1 1 1 rMAURER 1939 modified

- FERRimTROOSTOSORBITE+PEARL.ITE,

Fig8 1 12 14 16 1820 22 24 26CHROMIUM PERCENT

/The nickel-chromium diagramused by Maurer to predict m icrostructure. Notice that thephase boundarylines are curved

within the composit ion range of 0 to 26weight percent chromium and 0 to 25weight percent nickel. If carbon, s i l iconand manganese contents were held w i t h in specific limits, the lines of this nickel-chromium d iagram were usefu l in pred ic t ing micros truc ture for a g iven com posit ion. The diagram had curved l ines, asseen in Fig. 1, defin ing reg ions of austen-i te, ferri te, martensite, troostosorbite(very f ine pearl i te), and regions of combinations of these phases. The diagram wasdeveloped based on wrought materialsand not solidified materials.

Newell and Fleischmann (Ref. 8) werefirst in developing an expression fordefining austenite stability as a function ofal loy content for this system. They werea lso concerned wi th wrought product .Their constitu it ive expression for predicting the austenite-austenite plus ferriteboundary is given as:

Ni = (C r + 2 M o - 1 6 ) 212+ 30 0 .10 -C) + 8M n

2 1)

where the chemical symbols representweight percent of that element. Notice inth e Newell-Fleischmann equation thatmanganese is reported to be one-half aseffective in stabilizing austenite as nickel.Carbon was reported to be 30 t imesmore effective than nickel. Also, chromium and mo lybdenum were bo th foundto have a nonlinear relationship w ithnickel, which is consistent with the curve

line for the boundary for the austeniteand austenite plus ferrite regions on theMaure r d iag r am -F ig . 1 . The Newe l l -Fleischmann equation was reported todescribe the austenite stability curve inthe 14 to 19 percent chromium and the10 to 16 percent nickel range.The science of welding with austenitefiller materials became a high interesttop ic jus t pr io r to and dur ing World WarII. Besides the need to produce qualitystainless steel consumables, the activity in3 0

281-ro+ 26I 24?22

austenit ic welding during this period hadto do with welding high strength (armor)materials for the national defense efforts(Refs. 9-24). The use of austenit ic weldmetal in welding diff icult ferrous assemb l ies was based on the knowledgeobtained during the previous decade thataustenitic stainless steel can m aintain highducti l i ty and moderate strength over alarge temperature range with fair ly widecomposit ional variations. What wasrequired for proper application of Fe-Cr-Ni austenit ic consumables was somequant i ta t ive method to pred ic t the max imum amount of base metal di lution thatcan be realized and sti ll achieve the we ldmeta l compos i t ion which wi l l p roduce aductile austenitic matrix and not a brittleweld metal martensit ic structure.

Feild, Bloom and Linnert (Ref. 10)applied the Newell-Fleischmann expression to predict weld metal microstructureand found that the expression did notaccurately predict sol id if ied microstructure. Their specif ic concern was in predicting austenit ic weld metal microstructure that was being used to weld armorsteel.They repo rted that the we ld shou ldcontain some ferri te to assist in preventing root bead cracking. Feild, Bloom andLinnert (Ref. 10) repor ted that a modif icat ion to the Newell-Fleischmann expression,by changing the constant, 8, to 14 inequation 1, gave a better predic tion o faustenite stabil i ty for the composit ionalrange where ch romium var ied f rom 18 to21 percent and nickel varied 9 to 11percent . Mov ing the austen i te promoterto the left s ide, the expression becomes

Ni + 0 .5M n + 30C(Cr + 2M o - 16)212 + 14 (2)

-

A(A +

+M)

I

r i - i r

AUSTENITE

M 7^-^ + M + F ) 'I ^ I i i

i i i

ssA + U

I I T

1 1 1 1 i i 1 1 1

SCHAEFFLER 1947

Maure i^ / / ^ h a e f f l e r

/// / AUSTENITES +FERRITE

SchaefflerMaurer

181614

d 129 10z 8 14 15 16 17 18 19 2 0 21 22 23 24 25 26 27 28 29 30 31 32 33CHROMIUM EQUIVALENT (Cr +2-5Si +1-8Mo + 2 Cb)Fig. 2 The Schaeffler diagram of 1947, whichindicates some of the primaryphaseboundaricomparesthe curve fromMaurer's nickel-chromium diagramto theSchaeffler diagram whichusesnickel and chromium equivalent equations. Notice the c oefficient used for the chromiumequivalent equation and that thelinesare notlinear

282-sIOCTOBER 1985

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Post and Eberly (Ref. 22), who wereconcerne d wi th austen ite to pseudo-mar-tensite transformation during cold working, reported the fo l lowing equat ion foraustenite stability:N i + 0 .5Mn + 35C =( C r + 1 . 5 M o - 2 0 ) 2

12 + 15 (3)

Th e Post-Eberly equation was used toexplain austenite stabil ity in the c hrom iumrange of 14 to 25 percent and a nickelrange of 7 to 21 percent. Thus, i t wasshown that the Newell-Fleischmannexpressional form was satisfactory in theprediction of austenite stability relative toboth delta ferri te and martensite.The concept of equivalence started totake a mo re established for m wh enCampbell and Thomas (Ref. 15) reportedthat 25 chromium 20 nickel weld metalmicrostructure and mechanical propert ies could be c orrelated to small addit ionsof molybdenum and co lumbium byusing a chromium equivalent expression, wh ich was wr i t ten as chromiumequivalent = Cr + 1.5Mo + 2Cb. Binder,Brown and Franks (Ref. 24) reportedaustenite stability relative to delta ferriteis given by:Ni + 30C + 26 N = 1.3Cr - 11.1 (4)

Thomas (Ref. 19) suggested the followingmo re inclusive l inear equation for predicting the austenite stabil i ty boundary relat ive to de l ta ferr i te fo rmat ion:Ni + 0.5M n + 30C = (5)

1.1 (Cr + M o + 1.5Si + 0.5 Cb ) - 8.2These were the f irst steps towards thelinearization of the final Schaeffler andDeLong diagrams.Schaeffler (Ref. 17), using the aboveconcepts for microstructural correlationand an extensive experimental effort,mad e a diagram which had co mpos it ionalvariables on the axes and ranges for thespecif ic weld metal microstructuralphases plotted in the diagram. The coordinates of the diagram were given asnickel-equivalent and chromium-equivalent, on the vertical and horizontal axes,respectively. This choice of axes allowscorrelation of the effects of the auste nite for me rs (Ni, M n, C, etc.) and theferrite fo rme rs (Cr, M o, e tc .) on thefinal microstructure. One of the originalSchaeffler diagrams is seen in Fig. 2.

The original Schaeffler nickel equivalent equation, which has composit ionsgiven in weight percent, is described (Ref.17) as follows:Ni(eq) = Ni + 0.5Mn + 30C 6)

This equ ation is consistent w ith t he earlierf ind ing o f Newel l and Fleischmann (Ref.8) . This empirical expression indicatesthat, in comparison to manganese, nickel

4 4o 4 0IO

36in 3 26+ 28zr -24zy 20

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Ferrite2 4 6 8 10 12 14 16 18 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4 3 6 3 8CHROMIUM EQ UIVALE NT (Cr + Mo + 1-5 S i + 0 5 C b )

Fig. 4 - The Schaeffler diagram of 1949, the diagram which is commo nly used to predictierritic-austenitic dissimilar weld metal microstructure, had coefficient changes in the chromiumequivalent express ion when com pared to the Schaeffler diagram of 1948

weld metal, which also quantif ies thealloying influence of vanadium and ti tanium.The Schaeffler diagram (Fig. 5) isd iv ided into regions based on the transformation behavior of austenite. Liquid to austenite and austenite to martensite transformations are on the left of the diagram,while l iquid to ferri te transformations areon the right. The mechanism of thismartensite (diffusionless) transformationis understandably diffe rent fr om those ofth enucleation-and-growth of ferri te froml iquid.DeLong and Reid (Ref. 28) investigatedthe portion of the original Schaeffler diagram whic h is impo rtant to the com positional range of austenitic stainless steeland cons tructed a diagram. This diagram,

seen in Fig. 7, has been very instrumentalin advancing the utilization of stainlesssteel, in that i t a l lowed for quantitativeand reproducible prediction of theamount of delta ferri te. It has beenacce pted as practice t o expec t that austenitic stainless steels should have a singlephase austenitic structure after rolling andannealing, but 3-8% delta ferrite isexpected in the austenit ic weld metal toreduce the susceptibility of hot crackingin austenitic stainless steel welds. DeLongand Reid introduced a modif ication to thenickel equivalent expression by addingthe influence of nitrogen. The nickelequivalent equation, then, is given asfo l lows:

N ie q = Ni + 0.5Mn + 30C + 30N (12)

Notice that the nitrogen was found tohave the same influence of austeniticstabil i ty as carbon. Long and DeLong(Ref. 29) made further changes to thisdiagram by altering the lines after anextensive experimental and analyticalanalysis to improve its ability to predictdelta ferrite. This diagram is given in Fig.8. They also made som e evaluationof-theexperimental and statistical error in itsuse. They found that with f i l ler metals oftypes 308, 308L and 347 stainless steelsthe Schaeffler and DeLong diagrams areessentially equal, except at high nitrogenlevels, in their ability to predict stainlesssteel weld metal microstructure. TheSchaeffler diagram, as reported in Fig. 9,was found to underes t imate the ferr i tecontent for the filler materials of types316, 316L and 309 stainless steels. Therevised DeLong diagram was determinedto be an improvement for these higheralloyed stainless steels. Long and DeLong(Ref. 29) reported that their diagram isfairly insensitive to typical heat input variations found in arc welding.

Tables 1 and 2 list the re por ted coef f icients for elements in the nickel andchromium equivalent equations for predicting delta ferri te. Much of the apparent variation is due to the broad range ofal loys from which these coeff ic ients weregenerated.Szumachowski and Kotecki (Ref. 30)have recently found better agreementbetween calculated and measured ferri tenumbers fo r an extended manganeserange, up to 12.5 weight percent, byusing a modif ied nickel equivalent equa

t ion. The original DeLong nickel equivalent expression, which has been found tobe very useful for manganese contentscommon to the 300 series stainless steel,was found to seriously underestimate the

30

25

+ 20 -

15

SCHNEIDER

X .\ l + M

-MARTENSITE

I 9 6 0

AUSTENITE

\ / A + M+V/ x . SF ^

/ AUSTENITEDELTA FERRITE -

^^

FERRITE

15 20 25 30Cr = Cr + 2Si + l-5Mo +5V35 4 0

Fig. 5 The Schneider diagram, which was developed for cast materials,reports the influence o f cobalt and vanadium on the nickel andchromium equivalent expressions

5 3 2o

2ino24oo- 16fezUJ

KAKH0VS KII e t . a l .

\

\ M + F >

M

\ ,

A

/ M + F

1980

\ A + M + F \ ^\ 1 \

i y0 / ,

/ 1 0 / 3

8 0 ^ ^i o r j ^ ^