Oxid Met (2013) 79:239–247 DOI 10.1007/s11085-012-9347-x ORIGINAL PAPER
Grain-Size Effects on the High-Temperature Oxidation of Modified 304 Austenitic Stainless Steel Ju-Heon Kim • Dong-Ik Kim • Satyam Suwas Eric Fleury • Kyung-Woo Yi
•
Received: 18 April 2012 / Published online: 24 December 2012 Ó Springer Science+Business Media New York 2012
Abstract The high-temperature oxidation behavior of modified 304 austenitic stainless steels in a water vapor atmosphere was investigated. Samples were prepared by various thermo mechanical treatments to result in different grain sizes in the range 8–30 lm. Similar R3 grain boundary fraction was achieved to eliminate any grain-boundary characteristics effect. Samples were oxidized in an air furnace at 700 °C with 20 % water vapor atmosphere. On the fine-grained sample, a uniform Cr2O3 layer was formed, which increased the overall oxidation resistance. Whereas on the coarse-grained sample, an additional Fe2O3 layer formed on the Cr-rich oxide layer, which resulted in a relatively high oxidation rate. In the fine-grained sample, grain boundaries act as rapid diffusion paths for Cr and provided enough Cr to form Cr2O3 oxide on the entire sample surface. Keywords Austenitic stainless steel Grain size effect Oxidation resistance Grain boundary J.-H. Kim D.-I. Kim (&) E. Fleury Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seoungbuk-gu, Seoul 136-791, South Korea e-mail:
[email protected];
[email protected] J.-H. Kim e-mail:
[email protected] E. Fleury e-mail:
[email protected] J.-H. Kim K.-W. Yi Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-744, South Korea K.-W. Yi e-mail:
[email protected] S. Suwas Indian Institute of Science, Bangalore 560 012, India e-mail:
[email protected]
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Introduction High temperature oxidation resistant austenitic stainless steels are widely used for the power plant components to improve life time of the power plants [1]. Critical issues for new high temperature resistant steels towards realization of ultra super critical (USC) power plant, which will be operated at a steam pressure of 246 kg/cm2 and at a steam temperature of 593 °C or more, are the enhancement of creep rupture strength and humid oxidation resistance [2]. The presence of water vapor changes the oxidation behavior of austenitic stainless steels compared to the dry oxygen case by enhancing the high temperature oxidation [3]. Modified 304 austenitic stainless steels with Cu addition developed in 1990s, which were mainly suggested to make superheat and reheat tubes of USC boilers with a higher humid condition [4]. One feature of Cu added austenitic stainless steel is the addition of copper and niobium for the formation of copper precipitation in matrix and stable niobium carbides instead of non-stable chromium carbides, which results in obvious enhancement of creep rupture strength [5, 6]. The mechanical characteristics of Cu added austenitic stainless steel has been described by previous researchers [6], but the oxidation behavior of this steel has not systematically investigated. A number of investigations, at high temperature, reported chromium forming alloys such as AISI 304 austenitic stainless steels present an initial oxide growth leading mainly to Cr2O3 and spinel type oxide [3, 7–9]. Cr2O3 protective films grown on stainless steel effectively reduce high temperature oxidation even in aggressive environment [10, 11]. Before the understanding of the mechanisms of Cr2O3 scale growth, it is important to know the minimum bulk concentration of Cr, which is necessary to form a protective scale on the entire surface in order to prevent oxidation of stainless steels. It is known that Cr content of the range between 18 and 20 wt% is required to form a protective, continuous Cr2O3 scale [12–14]. In the initial oxidation stage, protective Cr-rich oxides formation is promoted by the fast outer flux of Cr along the substrate grain boundaries [15, 16]. During the later stage of oxidation, if Cr2O3 oxide layer does not densely cover the entire surface, the oxide scale is converted to the Fe-rich oxides by the outward Fe diffusion leading to the formation of Fe2O3 and inward O transport leading to the formation FeCr2O4 [17–21]. In this work, the oxidation characteristics of Cu added austenitic stainless steels at the 700 °C and 20 % humid condition are investigated in terms of the effect of the grain size on the oxidation behavior. With this objective, three Cu added austenitic stainless steels of fine-, medium- and coarse-grain size are studied.
Experimental Procedures Cu added austenitic stainless steel of basically 18 % Cr–8 % Ni added and 0.1 % C and 3 % Cu containing steel was vacuum induction melted. Chemical composition of investigated steel is given in Table 1.
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Table 1 Chemical composition of Cu added austenitic stainless steel (in wt%) Fe
Cr
Ni
Mn
Cu
Nb
Si
C
N
Bal.
18.06
9.25
0.755
2.96
0.38
0.198
0.112
0.1621
The ingot was solution treated for 24 h and forged at 1200 °C. Three samples with dimension of 6 9 4.7 9 1.2 mm were cut from the forged specimen and annealed at 1200 °C for 30 min then water quenched (as received sample). Second sample was deformed by 5 % reduction per pass and annealed at 1000 °C for 30 min between passes. Total reduction was 80 % (multi step and annealed sample). Last sample deformed by cold rolling with 80 % reduction by one pass and annealed at 1000 °C for 1 min (one step and annealed sample). The samples were ground using 1200 grit SiC paper and electro polished to get the fine surface for electron back scattered diffraction (EBSD) analysis. Then the samples were ultrasonically cleaned in acetone and finally dried prior to oxidation. The oxidation tests were performed at 700 °C in 20 % humidity condition. After 1, 2, 6, 12, 28, 56, 80, 100, 200, 300, 400 and 500 h of oxidation, the samples were removed from the furnace and weighed using a microbalance with a resolution of 10-5 g after room temperature cooling. The oxidation mechanism of these samples was analyzed by weight gain, scanning electronic microscopy (SEM, Hitachi S-4300SE), energy dispersive X-ray spectroscopy (EDS, Horiba-Oxford), and EBSD (Horiba-HKL Technology). Grain size distribution and number fraction of coincidence site lattice (CSL) boundaries were analyzed using EBSD data reprocessing software (HKL Technology Channel 5). For the cross-sectional analyses, samples were coated using epoxy resin on the sample surface in order to protect the brittle oxide scale on the substrates and milled by cross sectional ion miller (Hitachi E-3500). Results and Discussion In order to measure the average grain size and CSL boundary fractions, more than 100 grains in each sample were analyzed using EBSD with a step size from 0.1 to 0.5 lm. As shown in Fig. 1, all investigated steels have more than 50 % R3 boundary fraction. According to the author’s previous research [24], these R3 boundaries don’t show different oxidation behaviors from the intra grain area. Therefore grain size was analyzed with neglecting R3 boundary condition. The average grain size of the samples neglecting R3 boundaries was 27.1, 17.0 and 8.92 lm for as received, multi step and annealed and one step and annealed sample, respectively. On the contrary, R3 boundary fractions of the observed samples were all similar. Figure 2 shows the weight gain of Cu added austenitic stainless steels with different grain sizes during oxidation at 700 °C with 20 % humidity for 500 h. The oxidation behavior of the samples shows a weight gain increase during the first initial period, after which it follows the typical trend of weight gain decrease in the later stage of oxidation. In as received sample, where severe spallation occurs,
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Fig. 1 EBSD band contrast map overlayed with grain boundaries including special boundaries (red R3, green R5, blue R7 and black random boundaries). a As received (9600), b multi step and annealed (91000), c one step and annealed (92000) (Color figure online)
Fig. 2 Weight gain curve obtained in as received, multi step and annealed and one step and annealed samples during 500 h oxidation, at 700 °C, in 20 % humidity condition
Fe-rich oxide formed and weight gain is gradually increased up to 500 h. After 100 h of oxidation, weight gain of both multi step and annealed and one step and annealed samples is saturated and weight gain does not increase. The weight gain curves clearly show a significant improvement in the oxidation resistance for the one step and annealed sample. Whatever the temperature and the atmosphere, the weight gain curve follows a parabolic law given by (DM/S)2 = a?KPt [22, 23]. In all cases, the oxidation rate present a linear regime 2.518 9 10-2 and 2.227 9 10-4 mg2/cm4 h for as received and one step and annealed, respectively, during the 500 h of oxidation. The surface morphologies of the oxides formed on the Cu added austenitic stainless steels are shown in Fig. 3. In as received sample, the entire surface was covered with Fe-rich oxides. In one step and annealed sample, the entire surface is covered with Cr-rich oxides. But, in multi step and annealed sample, both Fe-rich oxides and Cr-rich oxides are mixed.
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Fig. 3 Secondary electron (SE) images obtained on the surface of the scale formed after 500 h oxidation, at 700 °C, in 20 % humidity. (a) As received, (b) multi step and annealed, (c) one step and annealed
Fig. 4 EDS spectrums acquired from the oxide scale formed on the multi step and annealed sample surface (Fig. 3b area) after 500 h oxidation, at 700 °C, in 20 % humidity
In order to analyze the oxide composition, EDS analyses on the sample surface were performed in multi step and annealed sample. Figure 4 shows EDS spectrums acquired from the region of Fig. 3b. It shows the presence of oxygen, chromium, iron. According to the EDS analyses, two types of oxides are formed on the outer surface: the Cr and Fe contents in scale on nodule is *2 wt% Cr and *50 wt% Fe (spectrum 1 and spectrum 2). It reveals that Fe-rich oxide has formed in the nodule area. While, the Cr and Fe contents of the bottom side oxide is *18 wt% Cr and *20 wt% Fe (spectrum 3 and spectrum 4) show that the bottom oxide is enriched in chromium. Size and distribution of these nodules depended on the grain size of the samples.
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The size of nodules increased with increasing oxidation time, on the coarse grained sample. During further oxidation, the nodules became progressively enriched in iron. The main oxide formed on the outer surface of the coarse grained is iron oxide while a Cr-rich oxide is formed on the fine grained sample. Medium sized (multi step and annealed) sample shows mixture of both oxides. Fe2O3 outer oxide has formed, but it did not cover the entire sample surface. The cross sectional microstructure of oxide scale of different grain sized samples is shown in Fig. 5. The contrast difference in back-scattered-electron (BSE) image reveals that the as received sample has outer/inner dual layer structure but that one step and annealed sample has single outer oxide. During oxidation, the inner and outer oxide becomes larger in size. After 500 h in as received sample, Fe2O3 outer oxide covers the entire sample surface but in one step and annealed sample, a thin Cr2O3 oxide layer covers entire surface of the sample. Figure 5 includes the through thickness EDS line profiles of oxygen, iron, chromium, nickel, manganese and copper to identify the oxide phases. In as received sample, the outer layer composes of Fe2O3 and the inner layer composes of a FeCr2O4 spinel oxide. Also, there is thin Cr2O3 oxide in the adherent subscale. But in the one step and annealed sample, only a thin protective Cr2O3 scale is formed. In multi step and annealed sample, at some site, this oxide layer is not totally protective. Then it is converted to an outer scale of iron oxide and an inner scale of mixed oxide phases of Fe, Cr, Mn and Ni similar to the oxide nodules formed in as received sample. Figure 6 is the SE image acquired from the 1 h oxidized multi step and annealed sample and the oxide phases identified by EBSD in the intra grain and grain boundary region. The oxides formed at the initial stage of oxidation of multi step and annealed sample are quite different from site to site. At the intra grain region, where a certain distance away from the grain boundary, nodule type oxide forms. The EBSD point analyses reveal the abundance of thin Cr2O3 oxide with a small amount of CrMn2O4 spinel oxide at the grain boundary region. On the contrary at the intra grain area, nodule type thick oxides form, which are identified as Fe3O4
Fig. 5 The back-scattering-electron (BSE) images and EDS line profile obtained from cross-sections of the scales formed after oxidation at 700 °C, in 20 % humidity for 500 h a as received, b one step and annealed
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Fig. 6 Surface of the scale formed on multi step and annealed sample after 1 h oxidation at 700 °C, in 20 % humidity : a SE image, b oxide phases identified by EBSD from grain interior region (site b of a), c oxide phases identified by EBSD from grain boundary region (site c of a)
and Fe2O3 by EBSD. In coarse grains, iron oxide covers the grain interior region but is located about 4 lm from grain boundaries. The grain boundary region had a greater near-surface concentration of chromium compared to the grain interior. Cr2O3 is formed at the grain boundary region during the initial stages of oxidation. It increases oxidation resistance at high temperature when the grain size is below a critical value. At the beginning of oxidation, the total oxidation behavior of both coarse grained and fine grained samples induce the formation of Cr2O3 followed by sideways diffusion of Cr and spreading of Cr2O3 from grain boundaries. In the case of coarse grained sample, the Fe2O3 oxides appear from the beginning of the oxidation. The oxidation of coarse grains in the as received sample is controlled by both external diffusion of metal cation resulting in an outer scale growth, and internal diffusion of O resulting in an inner scale growth. In these cases the outer layer contains Fe2O3 and Fe3O4 while the inner is enriched in Cr due to the formation of the FeCr2O4. Rapid external diffusion of Fe is related to the insufficient Cr supply in the grain interior region. It causes loose Cr2O3 oxide formation and allows Fe-rich oxide formation. Due to the relatively high growth rate of Fe oxides, the whole surface of the material is quickly covered by Fe rich oxide. The enhanced diffusion of Cr in defect regions results in the formation of Cr-rich oxide. With decreasing grain size, the total flux of Cr is increased due to the increase in the grain boundary density. Thus, abundant grain boundaries increases Cr diffusion in the surface region and the sideways spreading of external Cr2O3 from
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grain boundaries is enough to allow formation of a protective layer in the fine grained sample. The grain size in the austenitic stainless steel can play an important role to supply Cr to the alloy/oxide interface, since the diffusivity of Cr along grain boundaries is much higher than that of the grain interior. Grain refinement is another approach to improve oxidation resistance without additional elements addition or increasing Cr content.
Conclusions High temperature oxidation behavior of modified 304 austenitic stainless steel was investigated with focusing on the grain size effect and following conclusions are drawn from the present study. 1.
2.
3.
4.
5.
All modified 304 austenitic stainless steel samples show typical parabolic oxidation behavior regardless the grain size of the samples. Oxidation rate of coarse grained sample is 100 times larger than that of fine grained sample. In fine grained sample, entire surface is covered by protective Cr rich oxide but in coarse grained sample, it covers with non-protective Fe oxide, which results in the large amount of spallation during oxidation. In coarse grained sample, the outer layer of oxide composes of Fe2O3 and the inner layer composes of FeCr2O4 spinel oxide. Thin Cr2O3 oxide in the adherent subscale is also observed. In fine grained sample, only a thin protective Cr2O3 scale is formed. In medium grained sample, these two kinds of oxide structures are mixed site by site. The difference in the oxidation rate between coarse and fine grained samples results from the different growth rate of composing oxide. The medium grained sample shows intermediate oxidation rate due to the mixture of both oxide structures. The grain size refinement is an advantageous approach, without increasing the Cr content or adding additional elements, to increase the resistance to high temperature oxidation in humid air.
Acknowledgments This research was supported by by Seoul R&BD Program (Grant No. CS070157).
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