Phase equilibria in duplex stainless steels

Phase equilibria in duplex stainless steels

Journal of the Less-Common PHASE EQUILIBRIA Metals, 114 (1985) 89 - 96 IN DUPLEX STAINLESS 89 STEELS F. H. HAYES Joint University of Mancheste...

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Journal of the Less-Common

PHASE EQUILIBRIA

Metals, 114 (1985)

89 - 96

IN DUPLEX STAINLESS

89

STEELS

F. H. HAYES

Joint University of Manchester-UMZST Department Grosvenor Street, Manchester Ml 7HS (Gt. Britain)

of Metallurgy and Materials Science,

Summary Computed and measured austenite and ferrite phase equilibria data in the temperature range 850 “C to 1300 “C for four commercial duplex stainless steels are presented and compared. The level of agreement between calculation and experiment shows that computation of phase equilibria based on assessed thermodynamic data is a powerful tool, capable of providing useful information on higher-order systems that can be used as a basis for alloy design in iron-based and other alloy systems.

1. Introduction Computer calculation of phase relations and phase equilibria in multicomponent systems are increasingly being used as a basis for alloy design both for new alloys and for making improvements to existing practical alloy systems [ 1,2]. In such calculations, thermodynamic data for the pure components, the edge binary systems and, where available, ternary systems are used to yield quantitative information about the compositions and stabilities of both equilibrium and metastable phases under various conditions of interest. Compositions can thus be identified which may be of practical interest thereby reducing the amount of experimental work required to develop new and improved alloys. The technique is not limited to metallic systems and has been applied in recent years to a range of materials, including engineering ceramics [ 3, 41, ternary compound semiconductors [ 51 and non-oxide glasses for fibre optics [ 61. This approach has recently been applied by Kaufman and coworkers at ManLabs to a family of alloys known as the duplex stainless steels [7]. These two-phase austenitic-ferritic materials exhibit an attractive combination of properties which are of current interest in a number of applications [8]. Some of the advantages that duplex stainless steels have over conventional grades of austenitic stainless steels such as the well-known 300 Series are (i) proof strengths higher by more than a factor of two, (ii) excellent localized corrosion resistance including a good resistance to pitting and crevice corrosion in aggressive environments e.g. those high in Cl-, CO* or Hz S, (iii) high resistance to stress corrosion cracking in chloride environments oozz-5088/85/$3.30

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and (iv) good weldability including excellent resistance to sensitization without the need for stabilization. For many applications, duplex stainless steels offer a cheaper ~ternative to expensive high-nickel-cont~ning alloys [9]. Thus in recent years, considerable interest has arisen in understanding phase relationships and phase equilibria in existing commercial duplex stainless steels and in developing new alloy compositions with enhanced properties. A computer program for this purpose has been developed at ManLabs [lo] as part of an extensive research and development programme on duplex stainless steels. To demonstrate its usefulness in both teaching and research we have applied the ManLabs program to four currently available commercial duplex stainless steels. In addition we have studied the same steels experimentally, In this paper the computed features of the high temperature austenite-ferrite equilibria in these steels are compared with the experimental results.

2. Duplex stainless steels In addition to iron, chromium and nickel, duplex stainless steels commonly contain further alloying elements such as molybdenum and nitrogen. Copper, tungsten and niobium are less commonly added (ref. 8, p. 695). Manganese contents up to two weight per cent are common but in some cases higher levels are used. The iron, chromium and nickel contents are such that the composition lies in the two phase austenite-ferrite field of the Fe-Cr-Ni system [11,X2] at the annealing temperature which typically lies in the range 1050 - 1100 “C. The various alloying elements partition between the austenite and ferrite and thereby modify the relative amounts and compositions of these phases depending in turn on the amounts and nature of the alloying additions made to the melt. The overall composition is balanced in the sense that a duplex structure consisting of approximately 50% austenite and 50% ferrite is the stable microstruct~e at the annealing temperature. The steels are rapidly quenched to room temperature to retain this phase balance. The two phases can be seen in Fig. 1 which shows the microstructure of a duplex stainless steel of the Zeron series produced by Mather and Platt Ltd. This particular steel was solution treated at 1150 “C and water quenched following casting. The light phase is austenite and the dark phase is ferrite. With increasing temperature above the annealing temperature the percentage of austenite decreases until at temperatures between about 1340 “C! and the solidus temperature the steels become completely ferritic. With decreasing temperature below the normal annealing temperature the percentage of austenite increases to a maximum and then decreases. At temperatures in the region of 900 “C ~termet~lic phases such as c~bonit~des, u phase, x phase, Laves phase and, at lower temperatures, the cy’ phase can become stable (ref. 8, p. 695). For any given duplex steel, the actual phases which can exist, their stability temperatures and compositions, and the sequence in

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Fig. 1. Zeron duplex stainless steel (Mather and Platt Ltd., Manchester) etched electrolytically to reveal austenite (light) and ferrite (dark) regions. (Magnification, 100X.)

which they form will depend on the total alloy composition. Since the presence of such phases usually causes a severe degradation in mechanical properties, it is of considerable interest to alloy designers to be able to predict with some certainty for any given alloy composition (a) the variation in the percentages of austenite and ferrite with temperature, (b) the compositions of austenite and ferrite coexisting at various temperatures and (c) the nature of, compositions and temperatures at which various intermetallic phases become stable. In addition to these equ~ibrium questions, a knowledge of the kinetics of the various possible phase transformations in the form of TTT and CCC curves are also of considerable value in connection with critical cooling rates during processing.

3. Computer program Full details of the computer program used in the present work will be given elsewhere [lo] ; a brief outline only will be given here. The program contains data files which store the following thermodynamic data: (i) coefficients for expressions which describe the variation with temperature of the Gibbs free energy differences between the b.c.c. and f.c,c, forms of the pure components, (ii) coefficients for the excess Gibbs free energy expressions for the b.c.c. and f.c.c. solid solutions of each of the edge binary systems and (iii) ternary interaction coefficients, where known, for the ternary b.c.c. and the ternary f.c.c. solid solutions. During program execution the total composition is read in at the terminal together with the temperature and estimated starting values. The program then finds, using the Ne~on-Raphson iteration method, the amounts and compositions of austenite and ferrite such that (a) the chemical potentials of each component are equal in both phases and (b) the Lever rule is obeyed by each component. When the com-

Manufacturer

Sandvik AB (Sweden) Sandvik AB (Sweden) Bonar-Langley Alloys Firth Brown

Designation

SAF 2205 3RE60 Ferralium 255 FMN

Duplex stainless steels studied in this work

TABLE 1

Cr 22 18.3 25.4 26

Balance Balance Balance Balance

(wt.%)

Fe

Composition MO 3.0 2.90 3.83 1.5

Ni 5.5 4.81 5.09 5.5

Mn 1.7 1.59 0.83 0.80

N 0.14 0.15 0.20

0.4 1.6 0.44 0.7

Si

0.13 1.85 -

cu

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positions satisfying these requirements have been found, the program prints out the percentages and compositions of austenite and ferrite in equilibrium for the particular temperature concerned. The temperature is then incremented and a new ~ompu~tion started using the previous output as the starting values for the new temperature. Thus, the output lists for the particular steel composition and temperature, the percentages of each phase and their compositions together with the chemical potential of each component. The various commercial duplex stainless steel compositions to which the computer program has been applied in the present work and on which experiments have been carried out are listed in Table 1. The computer results are shown in Figs. 2 - 5 where the open circles are the computed percentages

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Fig. 3, Percentage austenite us. temperature for duplex stainless steel 3RE60 : 0, computed values; experimental values from SEM-EDAX (o), and from point counting (m).

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Fig. 5. Percentage austenite us. temperature for duplex stainless steel FMN: o, computed values; experimental values from SEM-EDAX (a), and from point counting (m).

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Fig. 6. Computed (a, 0) and observed (A, l) austenite and ferrite compositions for duplex stainless steel SAF 2205 - iron, chromium and nickel: A and A, austenite;~and l, ferrite. Fig. 7. Computed (A, 0) and observed (A, l) austenite and ferrite compositions for duplex stainless steel SAF 2205 - molybdenum and manganese: A and A, austenite; o and l, ferrite.

of austenite at the various temperatures indicated., Figures 6 and 7 show the computed austenite and ferrite compositions at various temperatures for the case of SAF 2205 as an example of the detail obtained in the computed results.

4. Experimental details Samples of the various steels listed in Table 1 were heated at a series of temperatures between 850 “C and 1300 “C under flowing argon in a platinumwound high temperature quenching furnace. In each case sufficient time at the desired temperature was allowed for equilibrium to be established prior to quenching into either water or iced-brine as appropriate. Samples were then sectioned and prepared for metallographic examination by both optical and electron microscopy. The percentages of austenite and ferrite were determined in two ways (a) by point counting on etched specimens using an optical microscope and (b) by calculation from composition data for austenite and ferrite. The compositions were obtained using a Philips 505 Scanning Electron Microscope fitted with an EDAX 9100/60 energy dispersive spectrometer. During these measurements appropriate precautions were taken to ensure that individual grains were being sampled by the electron beam in order to obtain meaningful composition data. Nitrogen contents could not be measured with the equipment available.

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5. Results and discussion Experimental results are given in Figs. 2 - 7 in which computed austenite percentages at various temperatures (open points) for the four duplex stainless steels listed in Table 1 are compared with observed results (filled points). Figures 6 and 7 compare for SAF 2205 the computed and observed austenite and ferrite compositions excluding nitrogen. It is seen that for both the austenite percentages and the SAF 2205 phase compositions the computed and observed results are in close agreement. In most cases agreement is within experimental error. In each case the computer calculations predict in a satisfactory manner the magnitude of the austenite percentage and its variation with temperature. The partitioning of each component of SAF 2205 between austenite and ferrite at different temperatures is also well described by the computed results. As one would expect, chromium and molybdenum partition to the ferrite whereas nickel, manganese (and nitrogen) partition to the austenite. The extent of the partitioning varies significantly with temperature a feature which is also mirrored by the computed results. 6. Conclusions The level of agreement obtained between measured and computed results for the multicomponent duplex stainless steels studied in the present work shows that useful and meaningful data can be obtained over wide ranges of stainless steel compositions using computer programs employing existing thermodynamic data. Such calculations can thus be used with confidence as a tool in the design of new alloys and to provide detailed phase equilibrium data for existing alloys. Acknowledgments The author would like to thank the Directors of ManLabs Inc., Cambridge, MA, for granting permission to publish that part of the work carried out in their laboratory, the following who carried out experimental work as Final Year students: Messrs. S. Maropoulos, P. Arundale, J. McGough, M. Clarke, I. Mellas, M. Williams and Miss L. Tsantzalou, Professors K. M. Entwistle and E. Smith for provision of laboratory facilities and Sandvik AB, Bonar-Langley Alloys and Firth Brown for supplying samples of their stainless steels. References 1 T. G. Chart and F. Putland, Calphad, 3 (1979) 9. 2 M.-L. Saboungi and C. C. Hsu, in G. C. Carter (ed.), Applications of Phase Diagrams in Metallurgy and Ceramics, Natl. Bur. Stand. (U.S.), Spec. Publ. 496 (1978) 1109.

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L. Kaufman, F. H. Hayes and D. Birnie, C&_&ad, 5 (1981) 163. L. Kaufman, F. H. Hayes and D. Birnie, High Temp.-High Pressures, 14 (1982) 619. L. Kaufman, J. Nell, K. Taylor and F. H. Hayes, Caiphad, 5 (1981) 185. L. Kaufman, J. Agren, J. Nell and F, H. Hayes, Calphad, 7 (1963) 71. L. Kaufman, F. H. Hayes and J. Agren, unpublished work. R. A. Lula (ed.), Duplex Stainless Steels Co& Proc., St. Louis, Missouri, October 1982, American Society for Metals, Metals Park, OH, 1983. S. 0. Bernhardsson, J. Oredsson and M. Tynell, HzS Corrosion in Oil and Gas Production -A Compilation of Classic Papers, NACE (1981) p. 358. L. Kaufman, J. Agren and F. H. Hayes, to be published. G. V. Raynor and V. G. Rivlin, Bulletin of Alloy Phase Diagrams, 2 (1981) 89. V. G. Rivlin and G. V. Raynor, Znt. Met. Rev., 25 (1980) 79.