Influence of carbon on the transformation kinetics of delta-ferrite in type 316 stainless steel weld metals

Scripta METALLURGICA et MATERIALIA

Vol. 27, pp. 313-318, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

INFLUENCE OF CARBONON THE TRANSFORMATION KINETICS OF DELTA-FERRITE IN TYPE 316 STAINLESS STEEL WELDMETALS T.P.S. G i l l , V. Shankar, M. Vijayalakshmi and P. Rodriguez Metallurgy and Materials Programme Indira Gandhi Centre for Atomic Research Kalpakkam 603 102, Tamilnadu, India

(Received March 17, 1992) (Revised May 29, 1992) Introduction Type 316 austenitic stainless steel is the preferred structural material for fast breeder nuclear reactors because of i t s superior corrosion resistance and attractive high temperature properties. However, the w e l d metal of Type 316 material is found to be more Drone to embrittlement during exposure to elevated temperatures than the base m e t a l (1). This is attributed to the presence of a small amount of metastable d e l t a - f e r r i t e phase (usually between 3 to 10 percent) in the austenite matrix, i n t e n t i o n a l l y retained to avoid the occurrence of s o l i d i f i c a t i o n cracking in the weld metal. During exposure to high temperatures, the deltaf e r r i t e constituent of the duplex weld metal is known to transform to various secondary phases (2,3). Important secondary phases influencing the mechanical and corrosion properties are sigma, chi and M^~C_ carbide. Much work has been devoted to the understanding of the formation of the sigma ~a~e from d e l t a - f e r r i t e and the effect of composition on the transformation reactions. I t is now widely recognised that carbon plays a primary role in inhibiting the f e r r i t e to sigma transformation reaction. Therefore, several investigators have attempted to optimise the weld metal composition for improved high temperature s t a b i l i t y (3-5). However, thus far no universally acceptable approach has been proposed. This paper presents somenew findings on the role played by carbon in influencing the f e r r i t e transformation behaviour in the temperature range 873 to I023K. These results may not only lead to a better understanding of the phenomenonbut also help in optimisation of Type 316 stainless steel weld metal chemistry for enhanced s t a b i l i t y at high temperatures. Experimental Four bead-on-plate w e l d pads were deposited using Type 316 stainless steel welding consumables. The carbon content in the weld metals varied from 0.015 to 0.062 wt.%. The chemical composition and average d e l t a - f e r r i t e content (as measured by Magne Gage) are given in Table I. The four different weld metal samples, referred to as Weld-1 (C-0.015 wt.%), Weld-2 (C-0.029 wt.%), Weld-3 (C-0.059 wt.%) and Weld-4 (C-0.062 wt.%), were aged at 873, 923, 973 and I023K for various durations t i l l the d e l t a - f e r r i t e present was nearly f u l l y transformed. Since the f e r r i t e content in a weld deposit varies from point to point, a large number of readings (greater than 30) were taken on each sample before and after ageing to minimise scatter in the results. A few samples from Weld-1 and Weld -3 and aged at 973K were prepared for optical and transmission electron microscopy studies. TABLE I: Chemical Composition of 316 Stainless Steel Weld Metals (wt.%) .

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C Weld-1 Weld-2 Weld-3 Weld-4 .

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S

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P

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Ni

0 . 0 1 5 0.002 0.010 0 . 4 1 0 . 0 2 9 0.006 * 0.27 0 . 0 5 9 0.013 0.031 0 . 5 4 0.062 * * 0.55 .

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Si

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Cr

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6.3 5.4 5.1 5.6

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0.023 0,060 0.059 0.060

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2.12 2.23 2.20 2.00

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FNa

1.72 0.28 1.36 1.28

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N

18.89 18.00 18.80 18.50

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Mo

12.96 12.60 11.53 12.10 .

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Mn

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• not determined, a Ferrite Number

313 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

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314

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Results and Discussion Variations of the fraction of d e l t a - f e r r i t e transformed, X, as a function of ageing time, t , at 8 7 3 , 9 2 3 , 973 and I023K for Weld-] and Weld-4 are shown in Figs. ] (a) and I (b) respectively. In low carbon weld metal (Weld-l), the transformation follows a sigmoidal behaviour, Fig. 1(a), whereas in the high carbon weld (Weld-4), i t is represented by two slopes, Fig. 1(b). Weld-2 and Weld-3 exhibited behaviour similar to Weld-1 and Weld-4 respectively. In Weld-3 and Weld-4, the d e l t a - f e r r i t e transforms rapidly during i n i t i a l stages of ageing followed by a region of sluggish transformation, thus leading to a two-slope transformation curve. The value of X where the change of slope occurs increases with ageing temperature. The two-slope transformation behaviour has not been reported thus far, though f e r r i t e transformation in high carbon w e l d metals has been studied extensively (5-8). However, the two-slope behaviour has been reported in other types of transformation processes (9,10). The influence of temperature and carbon content on the dissolution kinetics of deltaf e r r i t e can be studied by variations in the rate constant of a suitable rate equation. Of the many rate equations proposed for describing different reactions, most heterogeneous reactions of f i r s t order that are encountered in metallic alloys are best described by the Johnson-Mehl equation of the formX=1-exp(-bt n) [1] where n is the time exponent and b is the rate constant. The Johnson-Mehl equation has been also employed by other investigators to describe the f e r r i t e transformation in stainless steel weld metals (11,12). The quantity l n [ - l n ( 1 - X ) ] is plotted against In t for Weld-1 and Weld-4 as shown in Figs. 2(a) and 2(b) respectively. The two-slope behaviour for Weld-4, observed in Fig. 1(b), is again evident in Fig. 2(b). In order to i d e n t i f y the exact cause for the observed two-slope behaviour in high carbon welds, detailed studies on the transformation mechanisms of d e l t a - f e r r i t e were carried out using optical and electron microscopy and X-ray d i f f r a c t i o n of extracted residue of precipitates. The results for low carbon welds have been published elsewhere (3). The salient features of these studies are as follows. In low carbon welds (Weld-1 and Weld-2), the carbide precipitation kinetics is sluggish. In Weld-l, isolated carbide particles were found to decorate the austenite/ferrite interface in the i n i t i a l stages of ageing as shown in Fig. 3(a) for a sample aged at 973K for O.2h (X=O.2). These particles appeared to exert a pinning force on the austenite/ferrite interface thus retarding the transformation kinetics of f e r r i t e . Since the carbon content is low, a r e l a t i v e l y small amount of carbide can only form in this material, and therefore, significant depletion of Cr and Mo do not occur in d e l t a - f e r r i t e . Thus f e r r i t e preferentially transforms to the sigma phase, as shown in Fig. 3(b) for a sample aged at 973K for 2h (X=0.66). In Weld-3 and Weld-4, wherein carbon content is high, precipitation of carbides is rapid. The TEM observations on Weld-3 aged at 973K for O.2h (X=0.34) indicate the growth of alternate l amellae of M^~C. and austenite inside the d e l t a - f e r r i t e , Fig. 4(a), resulting in rapid transformation ~ ~errite. On further ageing, the coarse carbide particles were found in deltaf e r r i t e regions, as shown in Fig. 4(b) for a sample aged at 973K for 2h (X=0.72). The carbide precipitation in d e l t a - f e r r i t e leaves the f e r r i t e particles depleted of Cr and Mo. Smith and Farrar (5) reported similar observations on high carbon w e l d metals and determined the composition of f e r r i t e after the carbide precipitation ceased. The composition was found to be very close to that of the austenite matrix. Depending on the weld metal composition the f e r r i t e may transform either to the austenite or to the sigma phase. The above features suggest that the dissolution of f e r r i t e in low carbon welds is predominantly by i t s replacement by sigma in contrast to the presence of two competing reactions in high carbon welds namely, replacement by carbides and austenite at i n i t i a l ageing times followed by transformation to sigma and/or austenite at longer ageing times. Therefore, i t can be inferred that the two slopes, referred to as nI and n., represent the rapid and sluggish ferrite transformation regions, corresponding to th~ precipitation of carbides and sigma/austenite respectively. Thus i t is reasonable to consider the single slope obtained in Weld-1 and Weld-2, where sigmoidal transformation was observed, as n2. Table 2 l i s t s the constants nI and n2 for a l l the four samples. In general, there i s a systematic decrease in n^ as the carbon content is increased, thus indicating the i n h i b i t i n g influence of carbon content on the transformation of f e r r i t e to secondary phases. The value of nI also shows an appreciable decrease when carbon content is

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315

TABLE 2: Values of nI and n2 for Different Weld Metals .

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Ageing Temp. .

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nI Weld-l Weld-2 Weld-3 Weld-4 .

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923K .

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n2 0.78 0.65 0.30 0.21

0.73 0.45 .

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873K

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nI

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973K .

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n2 0.80 0.72 0.29 0.25

0.77 0.49

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nI -

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n2 0.69 0.96 0.36 0.26

0.69 0.47 .

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I023K

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nI

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n2 0.86 0.94 0.50 0.38

0.89 .

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increased. The exact reason for this decrease with a marginal increase in carbon content is not understood. I t is also noticed that there is no significant temperature dependence of nI and n. in the temperature range 873 and 973K; the small changes can be considered to be within th~ experimental errors. However, at I023K, the values of n~ and n deviate from the trend observed at lower ageing temperatures, which has been attributed to a2 change in the transformation mechanism as reported elsewhere (13). The f e r r i t e transformation data were also analysed using an Arrhenius equation of the form b=bo exp(-EA/RT)

[2]

where b is the rate constant derived from Equation [ I ] , EA is the apparent activation energy, and b , R and T have their usual meanings. The variation of In b with reciprocal temperature was found to be linear at a l l the ageing temperatures for Weld-I and Weld-2, whereas for Weld-3 and Weld-4, l i n e a r i t y was observed only in the temperature range 873 and 973K. The apparent activation energy, EA (kJ mole~ ), and parameters n~ and In b° decreased l i n e a r l y with an increase in the car~on content. The following equations were found to best describe these relationshipsE~=-3627C+334; n~=-11.65C+0.99; In b~=-424C+39.9;

(r=-0.968, s=+26.5) (r=-0.994, s:¥0.035) (r=-0.954, s:~3.76)

[3] [4] [5]

where C is expressed in wt.% and r and s are the c o r r e l a t i o n c o e f f i c i e n t and standard error of determination, r e s p e c t i v e l y . Dependence of EA and n~ on composition is also evident from data obtained from l i t e r a t u r e ( / , I I , 1 4 ) ; however, no systematic study has been carried out thus far to q u a n t i f y the r e l a t i o n s h i p s . In order to achieve high temperature s t a b i l i t y of weld metals, the value of n should approach zero. Therefore, from Equation [4] carbon content should be 0.087 wt.%. S~ith and Farrar (5), basing on t h e i r i n v e s t i g a t i o n s on high carbon weld metals, have suggested that the carbon content should exceed 0.0165 wt.% per u n i t volume of d e l t a - f e r r i t e to achieve elevated temperature s t a b i l i t y . Accordingly, minimum carbon content needed for high temperature s t a b i l i t y in the welds used in t h i s study should be 0.104, 0.089, 0.084 and 0.092wt.% for WeldI , Weld-2, Weld-3 and Weld-4 r e s p e c t i v e l y ; these values are very close to the optimum carbon content calculated from Equation [ 4 ] . Equations [ 3 ] to [ 5 ] can also be employed to estimate the f r a c t i o n of d e l t a - f e r r i t e transformed w i t h i n the v a l i d i t y l i m i t s of these expressions. Figure 5 shows a comparison of the observed and calculated values of f r a c t i o n f e r r i t e transformed on ageing between 873 to 973K. I t is observed that transformation of f e r r i t e can be predicted with a good degree of accuracy. In the present study, the c o n t r i b u t i o n s of a l l o y i n g elements other than carbon to E., n^ and In b are not r e a d i l y apparent, as the v a r i a t i o n in the major a l l o y i n g elements in th~ fou~ welds s~udied is not appreciable. In order to include the c o n t r i b u t i o n from major a l l o y i n g elements to Ea, ne and In b and to develop an expression applicable over a wider composition range, i t is e~sential to carry out rigorous analysis on a broader database. Conclusions Increase in the carbon content has been found to change the high temperature transformation of d e l t a - f e r r i t e in Type 316 s t a i n l e s s steel weld metals from a single-slope to two-slope behaviour. Data analysis employing Johnson-Mehl and Arrhenius equations yielded expressions describing the dependence of the apparent a c t i v a t i o n energy and parameters np and In b on carbon content. Using these r e l a t i o n s h i p s , i t was possible to estimate the f r ~ c t i o n of f B r r i t e transformed with good accuracy.

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Acknowledgements The authors are thankful to Dr. S.L. Mannan, Head, Materials Development Division, Mr. J.B. Gnanamoorthy, Head, Metallurgy Division and Dr. V.S. Raghunathan, Head, Physical Metallurgy Section for their continuous support during the course of this investigation. References (I) C.A.P. Horton, P. Marshall and R.G. Thomas, Mechanical Behaviour and Nuclear Applications, p66, The Metals Society, London (1982) (2) R.A. Farrar, J. Mater. Sci., 20, 4215(1985) (3) T.P.S. Gill, M. Vijayalakshmi, J.B. Gnanamoorthy and K.A. Padmanabhan, Weld. J. Res. Suppl., 65, 122s(1986) (4) J.M. Leitnaker, ibid., 61, 9s(1982) (5) J.J. Smith and R.A. Farrar, J. Mater. Sci., 26, 5025(1991) (6) G.F. Slattery, S.R. Keown and M.E. Lambert, Metals Tech., 10, 373(1983) (7) R.A. Farrar, C. Huelin and R.G. Thomas, J. Mater. Sci., 20, 2828(1985) (8) R.A. Farrar, ibid., 22, 363(1987) (9) R.A. Vandermeer and P. Gordon, Recovery and Recrystallization of Metals, p211, Interscience Publ., New York (1963) (10) H.W. Bergmann, H.U. Fritsh and B. Sprusil, Phase Transformations in Crystalline and Amorphous Alloys, p199, Deutsche Gesellschaft Fur Metallkunde E.V. (1983) (11) R.G. Thomas and D. Yapp, Weld. J. Res. Suppl., 57, 361s(1978) (12) M.D. Mathew, G. Sasikala, S.L. Mannan and P. Rodriguez, Mat. Sci. and Tech., 7, 533(1991) (13) V. Shankar, A. Selvam, T.P.S. Gill and S.L. Mannan, Joining of Materials for 2000AD, p411, The Indian Institute of Welding, Tiruchirapalli (1991) (14) R.G. Thomas and S.R. Keown, Mechanical Behaviour and Nuclear Applications of Stainless Steel at Elevated Temperatures, p30, The Metals Soc., London (1982)

X

1.0

Xl.0 I ~0.8 n~

~0.4 z

o_ .<

0.~ _,.......... 1

.............................

TI

~1

, t (hSO'

(a) FIG.

I=,+l

I

1

b_

1.

,o'

olooo 1023K

O n`

..................................

",-'Yo , o - ' 1

lO TIME, t

(~

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lO

(b)

Transformation Kinetics of delta-ferrite at different ageing temperatures, (a) Weld-1 and (b) Weld 4.

. ........

lO +

Vol.

27, No.

3

TRANSFORMATION KINETICS

1.O 1.O

_~-o.o L-1.0

rI

~-1.0

-2"-03.0' -1'.0 ,

t

|

=

I

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I

1.0

0 In(t~" ,

(a) FIG. 2.

t

I

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[i!:!:;o=I 7.0

5.0

-2"--04.() -2.0 0.0 ,

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t

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2.()t) 40' In([ . t

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l:::: :~I ,.....,.~ i

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6.0

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8.0

(b)

Variation of In[-In(l-X)] with Int for (a) Weld-I and (b) Weld-4.

Ca) FIG. 3.

317

(b)

Microstructures of Weld-1 samples aged at 973K: (a) Electron photomicrograph of the precipitation of M~CA at the austenite/ferrite interface in a sample aged for 0.2h and (b) Optical phb~o~icrograph indicating the conversion of delta-ferrite to sigma phase after ageing for 2h (etchant: modified Murakami reagent).

318

TRANSFORMATION

KINETICS

Vol.

(a) FIG. 4.

27, No. 3

(b)

Transmission electron photomicrographs of Weld-3 samples aged at 973K: (a) Cellular precipitation of Mo~C= and austenite within d e l t a - f e r r i t e in a sample aged for O.2h and (b) complete conve~i~n of d e l t a - f e r r i t e to M23C6 and austenite after ageing for 2h.

1.0

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IX(CAL) - 1.0,~X(OBS) - 0.0~2 ;r =-±

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FRACTION TRANSFORMED,

FIG. 5.

'1.o

X(OBS)

Comparisonbetween predicted and observed fractions of d e l t a - f e r r i t e transformed.