NR 5/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 223

The microstructure of the Sanicro 25 steel after steam oxidation studied by

advanced electron microscopy and spectroscopy methodsBogdan Rutkowski1*, Aleksander Gil2, Wiktoria Ratuszek3, Barbara Woźnik3,

Aleksandra Czyrska-Filemonowicz1

1International Centre of Electron Microscopy for Materials Science and Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Kraków, Poland, 2Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Kraków, Poland, 3Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Kraków, Poland,

*rutkowsk@agh.edu.pl

Increase of the coal fired power plants efficiency is inseparable with an increase of the steam conditions. Currently used 9÷12% Cr steels are not able to withstand pressure of 30 MPa at 700°C for a long time due to their microstructure instability leading to fast damage. Development of new Fe-based materials able to work under advanced ultra-supercritical (A-USC) conditions for a long time is the key of importance. Present paper deals with a microstructure of the prospective, 22% Cr austenitic steel, Sanicro 25, heat treated or oxidized in water vapour at 700°C. Detailed characterization of the steel was performed using X-ray diffractometry as well as scanning and transmission electron microscopy techniques. Investigation led to establish the effect of temperature and water vapour environment on the microstructure stability of this modern austenitic steel. The results showed that the microstructure of the aged steel consists of M23C6 and Laves phase precipitated on the grain boundaries as well as ε-Cu, NbN, M23C6 and Z-phase precipitated within the grains. After oxidation at 700°C up to 5000 h in water vapour, Sanicro 25 developed a thin protective oxide scale at the surface, consisting mainly of Cr2O3 plates, characteristic for steels oxidation in vapour.

Key words: Sanicro 25, TEM, oxidation, water vapour, A-USC.

Inżynieria Materiałowa 5 (213) (2016) 223÷227DOI 10.15199/28.2016.5.3© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Nowadays, environmental protection takes a particular significance due to high emission of CO2, which is one of the major greenhouse gases causing global warming. Major CO2 sources are coal-fired power plants, therefore particular efforts are made in order to de-crease the CO2 emission by increasing the thermal efficiency of the power plants due to enhancing temperature and pressure of the steam to 700°C and 30 MPa, respectively. Such change, however, has strong impact on the microstructure stability and lifetime of the currently used materials. Commonly used 9÷12% Cr steels are not suitable to operate at these conditions. Sanicro 25 austenitic steel, developed by Sandvik [1, 2], exhibits excellent corrosion resistance in fireside and steam environments [3, 4]. Moreover, in connection with the high creep strength and good weldability it could be a good successor of steels currently used for steam superheaters and re-heaters of coal fired power plants, where materials possessing high corrosion and creep resistance have to be used [5÷9].

The aim of a present study was to investigate the microstructural stability of the Sanicro 25 steel (22Cr25NiWCoCu) after ageing at 700°C up to 1000 h as well as microstructural changes induced by water vapour oxidation at the same temperature up to 5000 h by means of advanced electron microscopy and spectroscopy methods. In particular, the oxide scale developed at the steel surface during steam oxidation at 700°C as well as microstructural changes of the bulk material were characterized in detail.

2. EXPERIMENTAL

The Sanicro 25 (SANDVIK) was delivered as a solution-annealed (1220°C/5 min/water cooled) tube. Table 1 shows the chemical composition of the Sanicro 25 (wt. %, manufacturer data).

Table 1. Chemical composition of Sanicro 25 steel, wt. %Tabela 1. Skład chemiczny stali Sanicro 25, % mas.

Element C Si Mn P S Cr Ni

wt. % 0.064 0.18 0.51 0.016 0.0005 22.35 25.36

Element W Co Cu Al Nb B N

wt. % 3.37 1.44 2.98 0.023 0.49 0.0035 0.23

Investigations were performed using two different types of sam-ples, sectioned from the as-received Sanicro 25 tube: – blocks, embedded in a quartz glass under lowered pO2 pressure

and isothermally aged (700°C, up to 1000 h), – 20×15×2 mm polished plates, exposed to the water vapour con-

ditions (700°C up to 5000 h). Heat treated and oxidized samples were investigated by X-ray

diffractometry (XRD) as well as scanning and transmission electron microscopy (SEM, TEM, respectively) techniques.

The D500 X-ray diffractometer (Siemens), operating under monochromatic Cu Kα1 (1.54 Å) radiation was used in order to identify the phase composition of the oxide scale grown during cor-rosion tests. The applied grazing incident method (α = 1°) uses the lower penetration depths of X-ray beam to conduct phase analy-sis in outer surface layers. Additionally, the texture analyses were performed with Empyrean diffractometer (PANalytical) with Ni filter, operating under Cu radiation. In order to collect texture data, sample was tilted and rotated with Eulerian cradle, maintaining the constant 2θ angle of 39.7720, 36.0600 and 33.4810° for 006, 110 and 014 planes, respectively. Collected data were processed by the X’Pert Texture (PANalytical) software.

Scanning electron microscope (Merlin Gemini II of ZEISS) was used in order to investigate the morphology of the oxide scale as

224 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

well as the microstructure of as-received, heat treated and oxidized samples. Energy dispersive X-ray spectrometry at SEM (SEM–EDS) investigation was performed under low accelerating voltage (5 kV). That value was high enough for excitation of characteristic Lα (0.705 keV for Fe, 0.573 keV for Cr, 2.166 keV for Nb) and Mα (1.774 keV for W) lines of analyzed elements required for crea-tion the elemental maps. Four times lower incident electron energy (relative to 20 keV) resulted in around 10 times lower electron pen-etration depth for Sanicro 25 steel, allowing for imaging of fine (approx. 50 nm in size) precipitates.

SEM investigation of the oxide scale surface was performed di-rectly on oxidized samples. Other SEM analyses were done using metallographic cross-sections, prepared in a conventional method: before embedding in the acrylic resin, oxide scale was protected against damage by two-stage coating process (Au deposited by magnetron sputtering, followed by electrochemical deposition of Ni). Afterwards, specimens were ground using abrasive papers and polished using diamond suspensions.

Analytical (scanning) transmission electron microscopy meth-ods (S)TEM comprise: bright and dark field imaging (TEM–BF and TEM–DF, respectively), high angle annular dark field imag-ing (STEM–HAADF), STEM–EDS and selected area electron dif-fraction (SAED). (S)TEM analyses were conducted using a probe Cs-corrected Titan3 G2 60-300 (FEI) equipped with ChemiSTEM™ system (including a modern Super-X detector, consisting of 4 SDD detectors), allowing for very fast counting rates (~100 kcps/detec-tor), resulting in high quality EDS elemental mapping and signifi-cantly improved quantification results due to higher signal-to-noise ratio in every pixel of the map.

Thin foils for TEM analyses were prepared by conventional jet electropolishing method utilizing Tenupol 5 equipment of Struers. Lamellae of oxidized samples were prepared by Focused Ion Beam (FIB) facility (NEON CrossBeam 40EsB of ZEISS). Phase identi-fication was performed by XRD and electron diffraction (SAED) methods supported by STEM–EDS. Diffraction patterns were in-dexed with the help of JEMS software [10].

3. RESULTS AND DISCUSSION

3.1. As-received steel

The microstructure of as-received Sanicro 25 consists of the super-saturated γ matrix with some primary Nb-rich M(C, N) carbonitrides.

3.2. Aged steel

Figure 1 shows the microstructure of Sanicro 25 steel annealed at 700°C for 1000 h. Fine precipitates of ε-Cu are visible. Their pres-ence was also reported for other Cu-containing alloys [11÷13].

The characteristic “coffee bean” contrast of these particles proofs that precipitates are coherent with the matrix.

Due to their size (some tens of nanometer) and homogeneous distribution within the matrix they have the share in improving the strength of the steel, however, the major strengthening effect is achieved by the precipitation of fine NbX (where X = C and/or N) precipitates within the matrix and the M23C6 (where M = Cr, W, Fe) carbides at the grain boundaries (Fig. 1a). Laves phase was also found in the heat treated steel (characteristic streaks due to Laves phase’ internal structural defects are visible in diffraction patterns, as shown in Figure 1d). SAED results are shown in Figures 1b÷d.

Chemical composition maps of selected elements of aged steel are shown in Figure 2. STEM–HAADF image (Fig. 2a) shows the same area as given in Figure 1, however, in a lower magnifica-tion. Elemental map of selected elements (Fig. 2b) clearly shows chromium enrichment in the M23C6 precipitates grown at the grain boundary. They are surrounded by the Cr-depletion zone (Fig. 2g), therefore in the vicinity of the M23C6 precipitates, precipitation-free zone is present. Tungsten-rich Laves phase particles are also precip-itating at the grain boundaries. Increased amount of copper (Fig. 2e) induces the presence of ε-Cu precipitates. Beside them, secondary NbX or Z-phase (NbCrN) precipitates are forming within the aus-tenite grains (Fig. 2i). The steel is also strengthened by W and Co in the matrix, which contribute to its solid solution strengthening.

3.3. OXIDIZED STEEL

The results of microstructure investigation performed on the Sani-cro 25 oxidized at 700°C up to 5000 h show, that the material exhib-its significant changes with respect to the as-received one.

Fig. 1. Sanicro 25 steel aged at 700°C for 1000 h: a) steel microstruc-ture; TEM–BF image, b)÷d) selected area electron diffraction pattern taken from the matrix, M23C6 and Laves phase, respectivelyRys. 1. Stal Sanicro 25 po starzeniu w temperaturze 700°C przez 1000 h: a) mikrostruktura; obraz TEM–BF, b)÷d) dyfraktogramy elektronowe kolejno od: osnowy, M23C6 oraz faz Lavesa

Fig. 2. Sanicro 25 steel aged at 700°C for 1000 h: a) STEM–HAADF image, b) combined map of selected chemical elements, c)÷j) maps of selected elementsRys. 2. Stal Sanicro 25 po starzeniu w temperaturze 700°C przez 1000 h: a) obraz STEM–HAADF, b) mapa rozmieszczenia kilku wybranych pierwiast-ków, c)÷j) mapy rozmieszczenia wybranych pierwiastków

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Figure 3 shows the results of XRD measurements of the oxide scale grown for 500 h, performed with Bragg-Brentano (BB) as well as grazing incidence angle (α = 1°) geometries.

They indicate a presence of three phases: Fe2O3, Fe3O4 and Cr2O3 within the scale. Figure 4 shows the morphology of this oxide scale. Two types of structures are visible. Thin, chromium and oxy-gen containing plates were identified (by SAED) as Cr2O3. Larger crystals, containing iron and oxygen, were identified as Fe2O3 and Fe3O4 phases. Results of the texture analyses of Cr2O3 phase indi-cated for a fibre orientation and are in agreement with the measure-ments performed using TEM–ASTAR method [14].

Figure 5 shows the results of low voltage SEM–EDS investiga-tion of the cross-section of the sample oxidized at 700°C for 5000 h. The oxide scale, consisting mainly of Cr2O3, has the thickness of about 1 µm, what is very good result, consistent with data obtained by Zurek et al. [15]. The scale is thicker in the grain boundary ar-eas (Fig. 5a), where Cr diffusion was performing faster. Applied low voltage (5 kV) allows for observation of fine (approx. 50 nm in size) precipitates in the EDS maps (Fig. 5b).

Between the oxide scale and the bulk material, the Cr-depleted zone is visible. Local Cr-depletion (Cr content of approx. 9%) is caused by the formation of the protective Cr2O3 scale. Due to the fact that grain boundaries are fast paths of Cr diffusion, some of them are also Cr-depleted, resulting in lack of M23C6 carbides. The latter are replaced by larger Laves phase particles, precipitating at Cr-depleted (M23C6-free) grain boundaries. It is assumed that tung-sten remained after M23C6 dissolution is supporting that process. In the Cr-depleted zone, lack of ε-Cu precipitates is visible.

The microstructure of the bulk material is pronouncedly differ-ent. At the grain boundaries “decorated” by M23C6 (where M = Cr, W, Fe), W-rich Laves phase is significantly smaller than that oc-curring within Cr-depleted zone, so the presence of M23C6 hinders their growth at grain boundaries. Nanometre sized, Cu-rich precipi-tates (ε-Cu) coherent with the matrix, were revealed using the low voltage (5 kV) SEM–EDS map (Fig. 5b). Similar to the aged sam-ple, fine Nb-containing particles and M23C6 carbides are precipi-tated within the austenite grains. Larger precipitates of the Z-phase are also observed. The microstructure of the bulk material after

Fig. 3. XRD patterns of the oxide scale grown at the Sanicro 25 during oxidation at 700°C for 500 h in water vapourRys. 3. Dyfraktogram XRD zgorzeliny na stali Sanicro 25 powstałej pod-czas utleniania w 700°C przez 500 h w parze wodnej

Fig. 4. Morphology of the oxide scale grown at the surface of the Sani-cro 25 during oxidation at 700°C for 500 h in water vapourRys. 5. Morfologia zgorzeliny na powierzchni stali Sanicro 25 powstałej w trakcie utleniania w 700°C przez 500 h w parze wodnej

Fig. 5. Sanicro 25 after oxidation at 700°C for 5000 h in vapour: a) cross-section of the oxidized alloy (SEM), b)÷g) low voltage SEM–EDS el-emental maps of selected chemical elementsRys. 5. Sanicro 25 po utlenianiu w 700°C przez 5000 h w parze wodnej: a) mikrostruktura stali (przekrój poprzeczny; SEM), b)÷g) niskonapięciowe mapy rozmieszczenia wybranych pierwiastków chemicznych (SEM–EDS)

226 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

oxidation at 700°C is qualitatively similar to the microstructure of the aged sample previously described.

4. CONCLUSIONS

– The microstructure of the as-received Sanicro 25 consists of su-persaturated, twinned γ phase with primary Nb-rich MX precipi-tates.

– Isothermal ageing at 700°C up to 1000 h caused significant changes of the steel microstructure. Aged steel exhibits a pres-ence of M23C6 and Laves phase precipitated at the grain bounda-ries as well as ε-Cu, M23C6, NbX and Z-phase precipitates within the grains.

– During oxidation at 700°C up to 5000 h in water vapour, the Sanicro 25 developed a thin protective scale at the surface, con-sisting mainly of Cr2O3 plates, characteristic for steel oxidation in vapor.

– Beneath the scale, a Cr-depleted zone is present. No M23C6 were observed at the Cr-depleted grain boundaries. They were re-placed by large Laves phase precipitates.

– The microstructure of the bulk material after oxidation at 700°C up to 5000 h is qualitatively similar to the microstructure of the aged sample.

ACKNOWLEDGMENTS

The study was supported by the AGH-UST statutory project (No. 11.11.110.299). The authors would like to thank Dr. Alina Aguero (INTA Madrid) for supplying the oxidized samples as well as Adam Gruszczyński, MSc. and Krzysztof Chruściel, MSc. (both AGH-UST) for FIB lamella preparation and XRD analyses, respectively.

REFERENCES

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blätter. (2008).

[3] Rutkowski B., Gil A., Czyrska-Filemonowicz A.: Microstructure and chemical composition of the oxide scale formed on the Sanicro 25 steel tubes after fireside corrosion. Corros. Sci. 102 (2016) 373÷383 doi:10.1016/j.corsci.2015.10.030.

[4] Intiso L., Johansson L. G., Canovic S., Bellini S., Svensson J. E., Halvars-son M.: Oxidation behaviour of Sanicro 25 (42Fe22Cr25NiWCuNbN) in O2/H2O mixture at 600°C. Oxid. Met. 77 (2012) 209÷235 doi:10.1007/s11085-011-9281-3.

[5] Rautio R., Bruce S.: Sandvik Sanicro 25, a new material for ultra supercrit-ical coal fired boilers. Proc. 4th Int. Conference on Advances in Materials Technology for Fossil Power Plants, 25÷28.10.2004, Hilton Head Island, South Carolina, USA, R. Viswanathan et al. (eds) (2005) 274÷281.

[6] Ha V. T., Jung W. S.: Creep behaviour and microstructure evolution at 750°C in a new precipitation-strengthened heat-resistant austenitic stainless steel. Mater. Sci. Eng. A. 558 (2012) 103÷111 doi:10.1016/j.msea.2012.07.090.

[7] Viswanathan R., Henry J. F., Tanzosh J., Stanko G., Shingledecker J., Vitalis B., et al.: U.S. Program on materials technology for ultra-super-critical coal power plants. J. Mater. Eng. Perform. 22 (2013) 2904÷2915 doi:10.1007/s11665-013-0717-6.

[8] Kościelniak B., Hernas A., Staszewski M.: Analiza odporności na utleni-anie w parze wodnej i korozję wysokotemperaturową nowych stali aus-tenitycznych. [in:] A. Hernas (ed.), XII Konf. Nauk. Procesy Niszczenia oraz Powłoki Ochr. Stosow. w Energ., Racibórz (2015) 79÷90.

[9] Hernas A., Wala T., Staszewski M.: Charakterystyka i dobór stali na prze-grzewacze o nadkrytycznych parametrach pary. Inżynieria Materiałowa 3 (2009) 143÷151.

[10] Stadelmann P.: JEMS Java Electron Microscopy Software. (2015) http://cime.epfl.ch/.

[11] Rana R., Bleck W., Singh S. B., Mohanty O. N.: Development of high strength interstitial free steel by copper precipitation hardening. Mater. Lett. 61 (2007) 2919÷2922 doi:10.1016/j.matlet.2006.10.037.

[12] Tan S., Wang Z.-H., Cheng S., Liu Z., Han J., Fu W.: Effect of Cu content on aging precipitation behaviours of Cu-rich phase in Fe–Cr–Ni alloy. J. Iron Steel Res. Int. 17 (2010) 63÷68 doi:10.1016/S1006-706X(10)60101-X.

[13] Ren L., Nan L., Yang K.: Study of copper precipitation behaviour in a Cu-bearing austenitic antibacterial stainless steel. Mater. Des. 32 (2011) 2374÷2379 doi:10.1016/j.matdes.2010.11.030.

[14] Rutkowski B., Galanis A. S., Gil A., Czyrska-Filemonowicz A.: A novel approach to the characterization of thin oxide layers. Mater. Lett. 173 (2016) 235÷238 doi:10.1016/j.matlet.2016.02.104.

[15] Zurek J., Yang S.-M., Lin D.-Y., Hüttel T., Singheiser L., Quadakkers W. J.: Microstructural stability and oxidation behaviour of Sanicro 25 dur-ing long-term steam exposure in the temperature range 600÷750°C. Mater. Corros. 66 (2015) 315÷327 doi:10.1002/maco.201407901.

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Mikrostruktura stali Sanicro 25 po utlenianiu w parze wodnej badana za pomocą zaawansowanych

metod mikroskopowych i spektroskopowychBogdan Rutkowski1*, Aleksander Gil2, Wiktoria Ratuszek3, Barbara Woźnik3,

Aleksandra Czyrska-Filemonowicz1

1Międzynarodowe Centrum Mikroskopii Elektronowej dla Inżynierii Materiałowej oraz Wydział Inżynierii Metali i Informatyki Przemysłowej, Akademia Górniczo-Hutnicza im. S. Staszica, Kraków, 2Wydział Inżynierii Materiałowej i Ceramiki, Akademia Górniczo-Hutnicza im. S. Staszica,

Kraków, 3Wydział Inżynierii Metali i Informatyki Przemysłowej, Akademia Górniczo-Hutnicza im. S. Staszica, Kraków, *rutkowsk@agh.edu.pl

Inżynieria Materiałowa 5 (213) (2016) 223÷227DOI 10.15199/28.2016.5.3© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: Sanicro 25, utlenianie, para wodna, A-USC.

1. CEL PRACY

W pracy przedstawiono wyniki badań stabilności mikrostruktury stali Sanicro 25 (22Cr25NiWCoCu) po starzeniu w temperaturze 700°C w czasie krótszym niż 1000 h oraz po utlenianiu w atmos-ferze pary wodnej w tej samej temperaturze do 5000 h. Szczególną uwagę poświęcono mikrostrukturze zgorzeliny wykształconej na powierzchni stali podczas utleniania w temperaturze 700°C oraz zmianom mikrostruktury materiału rodzimego.

2. MATERIAŁ I METODYKA BADAŃ

Stal Sanicro 25, o składzie chemicznym zawartym w Tabeli 1, była do-starczona w stanie przesyconym (1220°C/5 min/chłodzenie w wodzie).

Badania przeprowadzono na dwóch rodzajach próbek: – materiał zatopiony pod obniżonym ciśnieniem powietrza w rur-

kach ze szkła kwarcowego (starzone w 700°C), – wypolerowane płytki (utlenione w 700°C).

Próbki badano metodami dyfrakcji rentgenowskiej (XRD) oraz skaningowej, transmisyjnej i transmisyjno–skaningowej mikro-skopii elektronowej (odpowiednio: SEM, TEM, STEM). W celu zobrazowania nanometrycznych wydzieleń występujących w stali (wielkość około 50 nm) za pomocą SEM zastosowano mikroana-lizę składu chemicznego (EDS) przy napięciu przyspieszającym obniżonym do 5 kV.

3. WYNIKI I ICH DYSKUSJA

3.1. Stal w stanie dostawy

W mikrostrukturze stali w stanie dostawy ujawniono pierwotne wy-dzielenia azotków wzbogaconych w niob występujące w austeni-tycznej osnowie.

3.1. Stal po starzeniu

Rysunek 1 przedstawia mikrostrukturę stali wyżarzonej w 700°C przez 1000 h, w której występują liczne wydzielenia fazy ε-Cu ko-herentne z osnową, biorące udział w umocnieniu materiału, które jednak głównie zależy od niewielkich wydzieleń NbX (X = C lub/i N) oraz wydzieleń M23C6 obecnych na granicach ziaren. W mikro-strukturze stwierdzono także obecność faz Lavesa.

Mapy rozmieszczenia wybranych pierwiastków dla sta-li starzonej przedstawiono na rysunku 2. Obraz mikrostruktury

przedstawiony na rysunku 2a zarejestrowany techniką STEM–HA-ADF ukazuje obszar z rysunku 1 przy mniejszym powiększeniu. Stwierdzono wzbogacenie węglików M23C6 w chrom, otoczonych strefami zubożenia w Cr (rys. 2b). Wydzielenia fazy ε-Cu ujawnio-no na rysunku 2e. Widoczne są również (rys. 2i) wtórne wydziele-nia NbX lub fazy Z (NbCrN).

3.1. Stal po utlenianiu

Wyniki badań XRD zaprezentowane na rysunku 3 dla stali utlenionej w 700°C przez 500 h ujawniają obecność trzech faz składających się na zgorzelinę: Fe2O3, Fe3O4 oraz Cr2O3. Rysunek 4 ukazuje morfo-logię zgorzeliny. Można wyróżnić płatki, które za pomocą SAED zidentyfikowano jako Cr2O3 oraz większe kryształy Fe2O3 i Fe3O4.

Rysunek 5 przedstawia wyniki badania SEM–EDS (przy na-pięciu przyspieszającym zmniejszonym do 5 kV) próbki utlenio-nej w temperaturze 700°C przez 5000 h. Zgorzelina, zawierająca między innymi Cr2O3 odznacza się grubością około 1 μm. Grubość tlenków jest większa w miejscu występowania granicy ziaren (rys. 5), która stanowi ścieżkę szybkiej dyfuzji chromu. Pomiędzy zgo-rzeliną a materiałem rodzimym wykształciła się strefa zubożenia w chrom, w której również granice ziaren są zubożone w ten pier-wiastek. Skutkuje to brakiem wydzieleń M23C6 zastąpionych na granicach ziaren większymi fazami Lavesa. Mikrostruktura mate-riału rodzimego jest podobna do mikrostruktury materiału starzo-nego opisanego wcześniej.

4. PODSUMOWANIE

Przeprowadzone badania pozwoliły na sformułowanie następują-cych wniosków: – mikrostruktura stali w stanie dostawy składa się z austenitu oraz

wydzieleń MX wzbogaconych w Nb, – stal starzona w 700°C krócej niż 1000 h wykazuje obecność

wydzieleń M23C6 i faz Lavesa na granicach ziaren oraz ε-Cu, M23C6, NbX i fazy Z wewnątrz ziaren,

– podczas utleniania w parze w 700°C krócej niż 5000 h na stali Sanicro 25 wytworzyła się cienka warstwa zgorzeliny ochronnej o morfologii typowej dla utleniania w parze wodnej,

– pod zgorzeliną uformowała się strefa zubożenia w chrom, w któ-rej nie występują węgliki M23C6 zastąpione większymi wydzie-leniami faz Lavesa na granicach ziaren.