Auger electron spectroscopy of stoichiometric chromium carbides and carbide precipitates at grain boundaries of type 304 stainless steel

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METALLURGICA

V o l . 13, pp. 8 5 7 - 8 6 2 , 1 9 7 9 P r i n t e d in the U . S . A .

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AUGER ELECTRON SPECTROSCOPY OF STOICHIOMETRIC CHROMIUM CARBIDES AND CARBIDE PRECIPITATES AT GRAIN BOUNDARIES OF TYPE 304 STAINLESS STEEL*

Steven Danyluk, Jang Yul Park and Donald E. Busch Materials Science Division Argonne National Laboratory Argonne, Illinois 60439

( R e c e i v e d J u n e 13, 1 9 7 9 ) ( R e v i s e d J u n e 22, 1 9 7 9 )

Introduction Chromium depletion due to chromium carbide precipitation at grain boundaries in the austenltic stainless steels is believed to be a prime cause of intergranular stress-corrosion cracking (ISCC). The carbide precipitates have previously been identified as (Fe,Cr)23C 6 by extraction-repllca transmission electron microscopy (i). The iron content in the carbide was measured to be up to 35% (2)° The amount of chromium incorporated in the chromium carbide precipitates will influence the kinetics and extent of the chromium depletion and, therefore, the susceptibility to ISCC. Impurity segregation may also significantly influence the kinetics of chromium depletion and the susceptibility to ISCC. Recently, surface analysis techniques such as Auger electron spectroscopy (AES) have been applied to the analysis of chromium depletion and impurity segregation at the grain boundaries (3). In the AES technique, an electron beam of several kilovolts is incident ~n the surface and secondary and Auger electrons are emitted from a surface layer I0-i00 A thick. The energy of the Auger electrons identifies the element and the Auger-electron number density is proportional to the atomic concentration of the element at the surface. The derivative of the Auger-electron number density with respect to energy is usually recorded versus energy, and the peak-to-peak height (pph) of this derivative is proportional to the atomic fraction° In order to determine the absolute elemental composition of the surface or grain boundary, a normalization factor is needed, which can be obtained from experimental Auger-electron emission data on pure elements or on compounds of known composition. In addition to data on the elemental composition of the surface, AES can sometimes provide ~nformatlon about the nature of the chemical state. In this paper we report AES data on carbide phases at the grain boundaries of a Type 304 stainless steel; these data confirm the earlier replica results (i). The carbide phase was identified from the unique shape of the carbon peaks in the AES spectrum, and composition of the carbide was obtained from a calibration of Auger spectra from stoichiemetric chromium carbides, iron and nickel carbide and the pure elements graphite, iron, chromium and nickel. This represents the first reported use of AES to determine the stolchiometry of carbides found at grain boundaries of Type 304 stainless steel. Experimental Procedures Several uniaxial creep-rupture specimens with a gauge length of 2.22 cm and a width of 0.559 cm were prepared from Type 304 stainless steel of the composition shown in Table I. The specimens were subjected to creep-deformation conditions (5.5 MPa at 700eC for 4 Ms in an argon environment) to enhance the probability of subsequent intergranular fracture. During creep at 700°C, carbides were precipitated and creep cavities formed at the grain boundaries. After creep deformation, a 0.l-cm-thick slice of a specimen that had gone to secondary-stage *Work supported by the Electric Power Institute

857 0036-9748/79/090857-06502.00/0

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creep was cooled in a holder (part of a scanning AES system from Physical Electronics Industries) at liquid-nitrogen temperature. The specimen was then fractured in the ultrahighvacuum (1.33 x I0~8 Pa) AES chamber by an impact energy estimated to be ~ 2°7 J. The fracture mode was predominantly intergranular (3). The AES data were taken from grain-boundary faces that did not contain creep cavities or microvoids. The primary electron-beam voltage and current were 5 keV and i ~A, respectively; the peak-to-peak modulation voltage was 3 eVo AES spectra were also obtained from chemically prepared carbides: High-purity (99%) chromium carbides [Cr23C6, Cr7C 3 and Cr3C 2 powders (325 mesh)], Fe3C , NIC and graphite (99.999% spectro grade) (4) were pressed into ultrapure nickel foil; the specimens were then subjected to argon sputtering (180 s, i0 m A a t i kV) to remove atmospheric contamination from the surfaces. For the purpose of calibration, AES spectra were also taken from hlgh-purity nickel, iron and chromium. All the samples were mounted on a carousel holder and the data were taken sequentially without reexposing the surfaces. This procedure minimized variability due to atmospheric contamination and instrumental factors. Results i.

AES Spectrum at Stainless Steel Grain Boundary

Figure i shows a grain-boundary AES spectrum [dN/dE] vs. E, where N is the number of Auger electrons at energy E. The major constituents (Fe, Cr and Ni), in addition to P, S and C, may be identified in the spectrum from the energy positions of the peak minima. Peaks representing the oxygen and nitrogen transitions (410 and 372 eV, respectively) are absent; this indicates the fracture surface has not been exposed to contaminants from the atmosphere. Figure 2 shows the AES spectrum in the energy range 230-300 eV for a stainless steel grain boundary and graphite standard. Both spectra show a peak minimum at 271 eV, the major transition for carbon. Two additional peaks with minima at 253 and 2 6 1 e V are observed at the stainless steel grain boundary. These additional peaks represent a chemical shift in the energy levels of the carbon due to charge transfer when the compound is formed. The presence of peaks at the low-energy side of the 271-eV carbon peak can be used to distinguish the carbon signals of carbides from those of adsorbed hydrocarbons (5). 2.

Carbide Identification

Figure 3 shows the Auger spectra of graphite and Cr23C 6. The differences in peak shape between these two spectra are evident. The chromium carbides Cr7C 3 and Cr3C 2 had peak shapes similar to those of Cr23C 6. The peak minima for the carbides occurred at 252, 260, 271 and 274 eV, corresponding to KLILI, KLIL2, KLIL3, KL2L2, and KL3L 3 carbon transitions (6). The pph of the 271-eV carbon peak increased as the amount of carbon in the carbide increase; the strengths of the carbon peaks in Cr23C6, Cr7C 3 and Cr3C 2 were in the ratio 1:1o3:1.9. This is expected, since the pph for a given element is proportional to the concentration. Figure 4 shows the Auger spectra of Cr23C6, Fe3C, NiC, and the stainless steel grain boundary in the energy range 240-300 eV. Again, the carbide peaks to the left of the 271-eV carbon peak correspond to KLIL 1 - KL3L 3 transitions. The energy positions of the peak minima for the major transitions, along with the peak shapes, can he used to distinguish between carbides of different chemical composition. As shown in Fig. 4, the major transition occurs at 271 eV for Cr23C 6 and 275 eV for Fe3C and NiC. The positions of the peak minima for the lower-energy transitions were not very different among the four carbides: The minima for Cr23C6, Fe3C and NiC occurred at 252 and 260-261 eV, while for the grain-boundary carbide the minima occurred at 253 and 262 eV. By comparison of the shapes and positions of the carbide peaks it can be concluded that the grain boundaries of the Type 304 stainless steel contain carbides which are composed predominantly of chromium, possibly with some iron or nickel. 3.

Determination of Stolchiometry

The stoichiometry of the carbide at the stainless steel grain boundary may be deduced by a comparison of the carbon-to-chromium pph ratio with these of standards whose composition has been determined independently. The carbon-to-chromlum pph ratios [C(271)/ Cr(489)] obtained for Cr23C6, Cr7C 3 and Cr3C 2 are shown in Table If. The ratio increases with increasing carbon content, as expected. However, for two of the samples, the experimental ratios are lower than the calculated stoichiometric ratios. In Fig° 5, the pph ratios for these three carbides and for pure chromium are plotted versus the calculated ratios. The carbon-to-chromium pph ratio at the stainless steel graln-boundary is also shown in the

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figure. The stoichiometric ratio of 0.16 is obtained graphically from the pph. This value (0.16) differs by 38% from the stoichiometric C/Cr ratio of 0.26 calculated for Cr23C 6. Discussion Satellite Auger peaks at the low-energy side of the main carbon peak (271 eV) are an indication that carbon in the surface region of the sample has chemically combined with other elements; the peak shape can be used as a fingerprint of the chemical state of the carbon (5-9). The energy positions of these carbon-peak minima (core-level KLL transitions) have been derived theoretically be Seigbahn (i0). Table II shows a comparison of the theoretical KLL transitions for carbon and the measured peak positions for a namber of carbides. Although the peak-minlmum positions shown in Table III can be used to distlnguishamong some of the carbides, the peak shape and height are also sensitive to the carbide type and stoichiometry. The carbide peak that we have found at the stainless steel grain boundary is similar in shape to those of the chromium carbides and can be distinguished from those of the iron or nickel carbides. The major nickel carbide transition occurs at 275 eV as opposed to 271 eV for the chromium carbides. Our data are supported by previous work (1,2) which showed that iron was incorporated in the carbide lattice of a stainless steel. In that work, th E lattice parameter of pure bulk chromium carbides was found to decrease with substitution of Pc. The x-ray lattice parameter was 106.23 mmfor the carbides at the grain boundaries and 106.4 mm for pure bulk chromium carbides. Nickel was not found to be incorporated in the carbides. These data are consistent with the present AES results, which suggest that the carbide at the stainless steel grain boundary is mainly chromium carbide with some iron incorporated in the lattice. The creep-deformation heat treatment resulted in heavy sensitization of the steel° At sensitizing temperatures below 900°C, the grain- and twin-boundary carbides in austenitic stainless steels consist of interconnected dendritic sheets (Ii). If we assume that the carbide at the grain boundary of Type 304 stainless steel is Cr23C6, we would expect a carbide coverage of % 62% (0.16/0.26) per unit area. This coverage is consistent with the heavy sensitization conditions that the steel was given. Conclusions The conclusions of this s t u d y m a y be summarized as follows: (i) The carbide at the Type 304 stainless steel grain boundary is (Fe,Cr)23C 6. (2) The ratios between the Auger peak-to-peak heights of carbon (271 eV) and chromium (489 eV) for known chromium carbides, plotted against the calculated stoichiometrlc C/Cr ratios of these carbides, may be used as a calibration curve to estimate the composition of unknown chromium carbides at the grain boundaries of stainless steels from Auger peak-to-peak height data. Acknowledgments RP449-I.

This work was supported by the Electric Power Research Institute under Contract No. Special thanks are extended to Gabriel M. Dragel for technical assistance. References

i. 2. 3. 4. 5.

C. Dacasa, V.B. Nileshwar, and D.A. Melford, Jo Iron Steel Inst. London, 207, 1325 (1969). H.J. Goldschmldt, J. Iron Steel Inst. London, 160, 345 (1948). J.Y. Park and S. Danyluk, Corrosion 33, 304 (1977). Cerac/Pure, 407 N. 13th Street, Milwaukee, WI 53233. C.C. Hang in Characterization of Solid Surfaces, Eds. P.F. Kane and G.B. Larrabee, Plenum Press, New York (1974). 6. N.J. Taylor, Revo Sci. Inst. 40, 742 (1969). 7. L.C. Isett, "The Binding Energy of Carbon on Ni (100) and a Stepped Nickel Surface from Equilibrium Segregation Studies", Ph.D. Dissertation, Cornell University, Ithaca, NY (January 1975), Report #2340. 8. T.W. Haas, J.T. Grant and G.J. Dooley III, J. Appl. Phys. 43, 1853 (1972). 9. Handbook of Auger Electron Spectroscop~, 2nd Edition, Eds. L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Physical Electronics Inds. Eden Prairie, MN (1976). I0. K. Siegbahn, C. Nordling, A. Fahlman, et al. in Atomic, Molecular and Solid State Structure Studied by means of Electron Spectroscopy, ESCA (Electron Spectroscopy for Chemical Analysis), Almqvist and Wiksell Boktryckeri AB., Uppsala (1967). Ii. F.G. Wilson, J. Iron Steel Inst. London, 209 , 126 (1971).

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TABLE I Chemical Composition of the Type 304 Stainless Steel (Wet Chemical Analysis) Element Fe Ni Cr C S

Wt % Balance 9.3 17.7 0.046 0.012

Element

0.026 0.33 1.17 0.47 0.20

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TABLE II C/Cr Ratios for Chromium Carbides of Known Composition

Wt %

P Mo Mn Si Cu

STEEL

Carbide

Calculated (Stoichiometric Ratio)

Experimental (Auger Peak-toPeak Ratio) a

Cr23C 6

0.26

0.27

Cr7C 3

0.43

0.35

Cr3C 2

0.67

0.52

a

pph at 271 eV pph at 489 eV

TABLE III Measured Energy Positions a of Carbide-peak Minima for Various Carbides Relative to KLL Transition Energies for Pure Carbon KLL Transitions (2s2s) KLIL I

•

(2s2p) KLIL I

(2sZp) KLIL 2

KLIL 2

KLIL 3

(2p2p) KL2L 2 KL2L 3 KL3L 3

Pure Carbon (theoretical)

243

MoC

Reference

252

258

265 266 267

i0

254

262

272 275

8

261

SiC

240

250 255

Wc

240

247 255

TiC

250 253

NiC

246 255

NiC

250

TaC

263 265

259 261

270

5

270

5

270

5

270

5

259

270 272

Present

252

260

271

PHI Handbool

Cr3C 2

252

260

271 274

Present

Type 304 SS Grain Boundary

252

261

272 274

Present

251

260

271 274

Present

254

262

272 275

265

Cr23C6 Cr7C3

FeC NiC aln eV.

244

9

dN

r

8O0 'ZOO ELECTRON ENERGy (eV)

~e Fe

I

ELECTRON

250 ENERGY, ev

KLILz KLzL 2 KLIL 3 KLzL 3

STAINLESS

300

FIG. 2 Auger Electron Spectrum of the Carbon Transitions of a Carbide at a Stainless Steel Grain Boundary, Compared with a Graphite Standard. The transitions are KLL type.

Tg

400

C,

,eco eooo

I 240

I

I 260

I 270

I 280 ELECTRON ENERGY (eV)

250

I 290

500

--

FIG. 3 Auger Electron Spectra of the Carbon Transitions of Graphite and Cr23C6

250

~C 6

cA

FIG. 1 Auger Electron Spectrum of an Intergranular Fracture Surface of a Type 304 Stainless Steel Specimen

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