Abstract:The formation of α' martensite at the surface of an AISI 304 stainless steel subjected to cyclic heating in humidified air is reported. The α' martensite formed during the cooling part of the cyclic tests due to local depletion of Cr and Mn and transformed back to austenite when the temperature again rose to 650• C. The size of the α' martensite region increased with increasing number of cycles. Thermodynamical simulations were used as basis for discussing the formation of α' martensite. The effect of the α' m… Show more
“…Austenitic stainless steels with relatively high Cr and Ni, on the other hand, can be used in higher temperatures due to their combination of relatively good creep strength and high temperature oxidation resistance [9]. Extensive studies [10][11][12][13][14][15][16][17][18][19] in recent years on the oxidation of austenitic stainless steels (e.g. 304, 310) in wet gases have reported the breakaway oxidation.…”
“…Austenitic stainless steels with relatively high Cr and Ni, on the other hand, can be used in higher temperatures due to their combination of relatively good creep strength and high temperature oxidation resistance [9]. Extensive studies [10][11][12][13][14][15][16][17][18][19] in recent years on the oxidation of austenitic stainless steels (e.g. 304, 310) in wet gases have reported the breakaway oxidation.…”
“…In a sensitised steel, the austenite near these boundaries and interfaces tends to transform to martensite phase. In addition, the martensite phase exists at surface compared to austenite phase [29]. This may result in the situation that martensite phase become more pH sensitive than austenite phase.Fig.…”
Effect of heat-treatment on the pH sensitivity of uncoated stainless-steel electrodes was investigated to comprehend the pH sensitivity of metal-oxide coated stainless-steel electrodes as novel pH sensors. The pH sensitivity of stainless-steel electrodes as-received and heat-treated at 500 °C, 600 °C and 700 °C for 24 h were 91 %, 94 %, 102 % and 91 %, respectively. The pH sensitivity tended to increase with increasing heat-treatment time at a given temperature. Thus, the most suitable heat-treatment condition for the stainless-steel electrodes was 600 °C for 24 h. The austenite phase (fcc) was the main phase on the surface of the heat-treated stainless-steel electrodes. Unexpectedly, the change in the martensite phase (bcc) as the second phase with heat-treatment temperature was similar to the pH sensitivity, with the martensite phase affecting the pH sensitivity. Therefore, it appeared that the pH sensitivity of the metal-oxide coated stainless-steel electrodes was affected by the underlying stainless-steel as well as the outer metal-oxide film coating. A prototype stainless-steel tube electrode was used as a working electrode for demonstrating the depth profiling of pH. The stainless-steel tube electrode showed good performance for measuring pH depth profiles compared to commercially available glass electrodes.
“…In fact, many studies have shown that high-temperature oxidation can lead to the formation of the BCC phase in the surface layer of austenitic steel. [28][29][30][31][32] It is known that during the oxidation, a lot of alloying elements are consumed to form the oxide layer, resulting in the reduction of alloying elements in the substrate surface layer. Besides, when the oxidation temperature is high, the formed oxide layer is relatively thick.…”
Section: Microstructure Of the Oxide Layersmentioning
The high temperature oxidation behavior of medium manganese austenitic steel (Fe‐Mn‐Cr, Fe‐Mn‐Al, Fe‐Mn‐Si, Fe‐Mn‐Cr‐Al, and Fe‐Mn‐Cr‐Si) was investigated. The double‐layered oxide scale is formed in the investigated samples. The outer oxide layer consists of (Fe,Mn)‐rich oxides, while the inner oxide layer is mainly composed of AB2O4 spinel oxides. When the oxidation temperature is 800 °C, a region composed of oxide and ferrite is formed in the substrate surface layer. When the oxidation temperature is 900 °C and 1000 °C, there is no such region observed in the substrate surface layer. Although the content of Al in the Fe‐Mn‐Al sample and the content of Si in the Fe‐Mn‐Si sample are relatively lower than the content of Cr in the Fe‐Mn‐Cr sample, the oxidation resistance of the Fe‐Mn‐Al and Fe‐Mn‐Si samples is still better than that of the Fe‐Mn‐Cr sample. The oxidation resistance of the Fe‐Mn‐Cr‐Al and Fe‐Mn‐Cr‐Si samples is also better than that of the Fe‐Mn‐Cr sample. The addition of Al and Si can improve the oxidation resistance of the Fe‐Mn‐Cr sample. Besides, the oxidation resistance improvement effect of adding Si to the Fe‐Mn‐Cr sample is better than that of adding Al to the Fe‐Mn‐Cr sample.This article is protected by copyright. All rights reserved.
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