Abstract
An interesting change of scale sequence occurred during oxidation of nanocrystalline surface layer by means of a surface mechanical attrition treatment. The three-layer oxide structure from the surface towards the matrix is Fe3O4, spinel FeCr2O4 and corundum (Fe,Cr)2O3, which is different from the typical two-layer scale consisted of an Fe3O4 outer layer and an FeCr2O4 inner layer in conventional P91 steel. The diffusivity of Cr, Fe and O is enhanced concurrently in the nanocrystalline surface layer, which causes the fast oxidation in the initial oxidation stage. The formation of (Fe,Cr)2O3 inner layer would inhabit fast diffusion of alloy elements in the nanocrystalline surface layer of P91 steel in the later oxidation stage, and it causes a decrease in the parabolic oxidation rate compared with conventional specimens. This study provides a novel approach to improve the oxidation resistance of heat resistant steel without changing its Cr content.
Highlights
Provide theoretical supports for the improvement of oxidation behavior in heat resistant steels exposed to high-temperature and high-pressure water vapor
Cross-sectional morphologies of the specimens were observed by using SEM with back-scattered electron (BSE) and Electron probe micro-analyzer (EPMA)
It is reported that the grains in the surface layer region within 100 μ m thick are refined into nanometer scale, and the thermal stability of nanocrystalline microstructure is excellent until the temperature above 1033 K7
Summary
Provide theoretical supports for the improvement of oxidation behavior in heat resistant steels exposed to high-temperature and high-pressure water vapor. The data analysis of weight gain was considered as a starting from the 24 h oxidation specimen, the oxidation kinetics of the nanocrystalline exhibited parabolic law after the formation of the (Fe, Cr)2O3 inner layer on the nanocrystalline surface layer of P91 steel as shown in Fig. 2b–d, which causes a decreasing in the oxidation rate constant k from 0.044 in the conversional specimen to 0.026 in the nanocrystalline specimen (Fig. 1b), and this denotes a notable improvement of oxidation resistance on the nanocrystalline surface layer.
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