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CHROMIUM CONCENTRATION IN AND AROUND FERRITE-AUSTENITE PHASE BOUNDARIES IN
DUPLEX STAINLESS STEELS
V. Gadgil, J. Swens, B. Kolster
To cite this version:
V. Gadgil, J. Swens, B. Kolster. CHROMIUM CONCENTRATION IN AND AROUND FERRITE-
AUSTENITE PHASE BOUNDARIES IN DUPLEX STAINLESS STEELS. Journal de Physique Col-
loques, 1989, 50 (C8), pp.C8-361-C8-364. �10.1051/jphyscol:1989861�. �jpa-00229959�
COLLOQUE DE PHYSIQUE
Colloque C8, Supplement au nO1l, Tome 50, novembre 1989
CHROMIUM CONCENTRATION IN AND AROUND FERRITE-AUSTENITE PHASE BOUNDARIES IN DUPLEX STAINLESS STEELS
V.J. GADGIL*, J.J. SWENS*" and B.H. KOLSTER"
" "
ateri rials
Science Section, Department o f Mechanical Engineering, y ? i v e r s i t y of Twente, P O . Box 217, NL-7500 AE Enschede, The NetherlandsSGM (Foundation f o r Advanced Metals S c i e n c e ) , P O . Box 8039, NL-7550
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KA Hengelo, The Nether1 andsAbstract
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Austenitic-ferritic duplex stainless steels, tested for pitting corrosion, show pit initiation on ferrite-austenite phase boundaries. Hot worked, solutionised and water quenched material was investigated. The AP FIM was used to determine the local composition, and the chromium concentration in and around the ferrite-austenite grain boundaries. The analysis indicates a phase boundary carbide. It was found that there is a region of Chromium depletion on either side of the phase boundary.1
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INTRODUCTIONThe duplex stainless steels are alloys of Fe-Cr-Ni system with a microstructure mainly consisting of austenite and ferrite. The unique features that distinguish duplex steels from other families of stainless steels, are their ability to accommodate a high Cr and Mo content, (elements that affect pitting and crevice corrosion resistance), their good resistance to chloride stress corrosion cracking, and their high room temperature strength. The nominal composition of commercially available duplex stainless steels is given below.
Table - 1 Nominal composition of duplex stainless steels
0.04 22-27 4-6 2-3 1.5 c0.25 0.02 balance
Pitting corrosion is,a form of localised corrosion, occuring at one part of a metal surface at a much higher rate than over the rest of the surface. Pits are nucleated whenever the oxide film which protects the metal is likely to be discontinuos and where local environmental conditions are most suitable. The pits may be nucleated at points determined not only by faults in the surface film, like emergent screw dislocations, shear etc. but also at sites determined by the underlying metal. Such sites may arise from alloy heterogeneities in the surface, or be associated with grain or phase boundaries. The size of such metallurgical faults may vary considerably from as small as a few atomic diameters to millimeters, perhaps even visible to the eye [I]. Although the initial process of localized breakdown of the passive film on stainless teels has been studied for years, the mechanism has not been completely understood. Several mechanisms have been proposed, explaining why pits form in one phase steel alloys at the grain boundaries or at the non metallic inclusions. That sulfide inclusions are particularly susceptible, as observed not only in austenitic but also in ferritic stainless steels. Sulfide dissolution kinetics are presumed to control the pit initiation process. The dissolution kinetics are influenced by the morphology and composition of sulfides and by the composition of the matrix. Stainless steels with duplex structure, in addition, show another mode of localised corrosion. The alloying elements which act as stabilizers of austenite, e.g. Ni, C, N are partitioned to the austenite phpse, whereas in the ferrite phase, a partitioning of the elements stabilizing ferrite such as Cr, and Mo occurs [2]. The phase boundaries are often preferred sites for impurity segregation and forprecipitation of new phases. In the pitting corrosion range, this phenomenon leads to an increased susceptibility to pit formation at the phase boundaries [3,4].
The effect of the chemical composition on resistance to pitting corrosion has been investigated by Sakai et. al. [ S ] . Corrosion resistance is known to depend on both Cr and Ni contents.
Minimum contents of 21% Cr and 4% Ni are required to obtain significant corrosion resistance.
However the degree of improvement given by increasing Cr and Ni is interdependent. Sakai et.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989861
al. observed: as Ni content increases, more Cr is required to obtain good resistance. Molybdenum and nitrogen additions are also known to improve the pitting corrosion resistance.
The pitting resistance increases drastically with increase of Mo upto 3% [ 4 , 5 , 6 , 7 ] . Nitrogen contents of 0.1-0,15.are optimum for resistance to both pitting and general corrosion [4].
Alloys in water quenched condition show pit initiations preferentially at the ferrite-austenite interphase boundaries. Scanning electron microscope examinations showed the phase boundaries to be rich in chromium. The region adjacent to the phase boundaries was hence thought to be denuded of chromium. In order to aquire a full understanding of the mechanism of the pit initiation at the phase boundaries in duplex stainless steels, it is necessary to determine the local.
chemical composition in and around the phase boundaries.
2
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EXPERIMENTAL METHODS.The material studied was in the form of hot worked plate which was solutionised between 1050 C and 1150 C and water quenched. The compositibn of the material is given in table 2.
Table - 2 Chemical composition
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C Si Mn Cr Mo Ni N
The samples were cut using EDM machine in to bars of 0.2 x 0.2 mm. FIM AP samples were prepared using floating layer technique for initial necking (25% perchloric acid in acetic acid on tetra chloromethane), polishing in 6% perchloric acid in 2 butoxy ethanol until parting and finally backpolishing in a circular cathod with 2% perchloric acid in 2 butoxy ethanol. TEM was used to examine the tips and to determine the phase by electron diffraction. Selected specimens were then introduced in to the AP FIM and DC field evaporated until1 the phase boundary appeared in the FIM image. AP analysis was carried out at the phase boundary. Areas of analysis are shown in fig. 1.
fig. 1. Areas of analysis at and on both fig. 2. TEM micrograph, clearly showing a phase sides of the phase boundary. boundary
Numbers correspond to the numbers of analysis (table 3)
3 - RESULTS AND DISCUSSION
Duplex stainless steel FIM specimens, appeared to be very difficult to prepare. Specially specimens containing a grain boundary. Furthermore these.specimens were found to be sensitive to fracture unde: pulsing conditions. Because of the difficulties only one specimen could be investigated. A TEM micrograph of the investigated specimen is shown in fig. 2. The phases of the two grains were determined to be ferrite (tip area), and austenite (shaft area), using electron diffraction (fig. 3,4). A FIM micrograph is given in fig. 5 , showing the bright imaging area which is the phase boundary.
AP analyses are given in table 3. These show a good agreement with an expected analysis of ferrite and austenite. The Cr content however showed a peak at the phase boundary and a lower than expected content (20.4 in austenite and 24.8 in ferrite) close to the phase boundary (fig.
5). The high Cr content at the phase boundary coincided with an increase in carbon from 0.02 average to 1.1%. This could indicate the existance of a carbide. The existence of a Cr depletion zone near a phase boundary is found to be in agreement with the proposed mechanisms of pit initiation. However the Cr depletion was found to be on both austenite and ferrite, sides of the phase boundary. A more detailed study of several phase boundaries is required to confirm these observations.
Table
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3 Local composition in atomic percentage in the austenite (analysis 1 , 2 and 3) at the phase boundary (analysis 4) and in the ferrite (analysis 5 , 6 and 7)Analysis Distance from Cr Ni Mo Mn C Ni/Si Fe
no. phase boundary (nm)
*
Errors are calculated versus the overall chemical C-content of 0.02%Distance from Phase Boundary (nm)
fig. 6. Local chromium and carbon concentrations as a function of the distance from the phase boundary
4
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CONCLUSIONSDuplex stainless steels can be successfully investigated by AP FIM. The preparation of these specimens was found to be difficult. Features like phase boundaries and differences in phases can be clearly seen in the FIM image, whereas AP analysis is an appropriate method for analysis of highly localised composition differences.
AP FIM analysis indicates that carbides can form at phase boundaries in non aged duplex stainless steel, resulting in a chromium depletion zone on both, austenite and ferrite, sides of the phase boundary.
REFERENCES
/l/J.C. Scully, 'The fundamentals of corrosion' Int. series of monographs on Mat. Sci. and Tech., ed. D.W. Hopkins, vol. 17, pp 174 (Pergamon, Oxford)
/2/H. Tsuge, Y. Tarutani and T. Kudo, Corrosion NACE 44, 5, May 1988
/ 3 / P . Hronsky, D.J. Duquette, Corrosion NACE, 38, 2, February 1982
/4/P.E. Manning, D.J. Duquette, W.F. Savage, Corrosion NACE
35,
4, April 1979/5/J. Sakai, I. Matsushima, Y. Kamemura, M. Tanimura, T. Osuka, 'Duplex stainless steels (proc.
conf.), ed. R.A. Lula, ASM October 25-28, 1982
/6/ J.G. Parr, A. Hanson, 'An introduction to stainless steels', ASM, 1966
/7/'Handbook of stainless steels', ed. Donald Peckner, I.M. Bernstein, McGraw-Hill 1977
