1 Central Iron and Steel Research Institute, Beijing 100081, China 2 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
Yin BAI, Zhengdong LIU, Jianxin XIE, Hansheng BAO, Zhengzong CHEN. Effect of Pre-Oxidation Treatment on the Behavior of High Temperature Oxidation in Steam of G115 Steel. Acta Metall Sin, 2018, 54(6): 895-904.
In order to improve the steam oxidation resistance of G115 steel (9Cr3W3CoVNbCuBN) at 650 ℃, pre-oxidation treatment was carried out in argon environment with low oxygen partial pressure. The oxidation behaviors of the pre-oxidized and untreated samples were simultaneously investigated by a cyclic oxidation experiment. Weight gains of samples were measured by analytical balance, phases of oxide products were identified by XRD and EDS, morphology and structure of scales were characterized by SEM and EDS. The result showed that pre-oxidation treatment significantly decrease oxidation weight gains in 1800 h. After pre-oxidation treatment, the oxidation kinetics transformed from cubic into linear form, and the scale structures transformed from duplex layers into triple layers. In the scale of pre-oxidized samples, the outermost layer was enriched in Fe, the middle layer was enriched in Cr, and the innermost layer was transformed from the matrix metal. The middle layer had chromium content as high as 46% (mass fraction) and was considered to be conformed of chromite (FeCr2O4). This layer was the most protective layer due to its highest Cr content, and the diffusion of O and Fe though it was the main controlling process of the whole oxidation. It suggested that the stable structure of the middle layer improved the oxidation resistance of pre-oxidation samples. The thickness of the middle layer nearly kept constant during the whole oxidation process, which was the main reason why the pre-oxidized sample had linear oxidation kinetics. The long term effect of the pre-oxidation treatment was evaluated based on the scale structure and oxidation mechanism.
Fig.1 Schematic of high-temperature steam oxidation testing rig
Fig.2 Weight gains of original and pre-oxidized G115 samples as a function of exposure time in steam at 650 ℃
Fig.3 SEM images of the surface oxide of original samples exposed in steam at 650 ℃ for 100 h (a) and 800 h (b)
Fig.4 XRD spectrum of the surface oxide of the original sample exposed at 650 ℃ for 200 h in steam
Fig.5 SEM image (a) and XRD spectrum (b) of the surface oxide of the sample pre-oxidized in argon for 50 h, EDS results of ‘+’ point in Fig.5a (c) and EDS results of area scanning (d)
Fig.6 SEM images of the surface oxide of the pre-oxidized samples exposed in steam at 650 ℃ for 200 h (a), 400 h (b), 1000 h (c) and 1800 h(d)
Fig.7 EDS results of the matrix oxide (a) and whisker caps (b) in Fig.6a, and whisker in Fig.6b (c)
Fig.8 Cross-sectional SEM image of the original sample exposed in steam at 650 ℃ for 400 h
Fig.9 Cross-sectional SEM images of the pre-oxidized samples exposed in steam at 650 ℃ for 0 h (a), 200 h (b), 400 h (c) and 1000 h (d)
Fig.10 Element distribution mappings of the scale of the pre-oxidized sample exposed in steam at 650 ℃ for 400 h
Fig.11 Thickness of the middle layer of the pre-oxidized samples as a function of exposure time
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