Ageing and high temperature oxidation phenomena in three high alloyed stainless steels
Keywords:Stainless steels, Refractory austenitic steels, Air oxidation, High temperature, Thermal ageing, σ-phase, Precipitates
The oxidation kinetics in air up to 50 h at 1,373 and 1,473 K of three refractory stainless steels (25Cr20Ni, 21Cr11Ni(1), 21Cr11Ni(2)) are studied. At 1,373 K, the best results are those of 25Cr20Ni followed by those of 21Cr11Ni(1), with 0,036 % lanthanides, and the worst behaviour is that of 21Cr11Ni(2), with 0,010 % lanthanides. A 2 h oxidation treatment at 1,323 K in air produces oxidation layers from 1 to 3 μm in thickness for the three materials. Scanning electron microscopy and energy dispersive microanalysis (SEM-EDX) and X-ray diffraction (XRD) characterisation of the oxidation layers give the same characteristics for the three steels: Mn Cr2O4 spinel type in the outer part, Cr2O3 in the inner one, with a SiO2 barrier in the oxide-metal interface and a large amount of internal and intergranular precipitates (also Si02) in 21Cr11Ni materials. After ageing tests, at 1,073 K for 400 h, about 10 vol. % of the σ-phase is present in the 25Cr20Ni (AISI 310 S) material, while no a-phase is present in the 21Cr11Ni materials perhaps due to the high nitrogen content. The application of field emission SEM with EDX allows the analysis of σ-phase and carbides, nitrides and carbonitrides present. The steel 25Cr20Ni only forms, in addition to the σ-phase, M23C6 type carbides, while the steel 21Cr11Ni(1) forms these carbides and M15N nitrides (M8N in the surface) and the steel 21Cr11Ni(2) forms mainly carbonitrides M6(CN). The relatively higher nitrogen level in the 21Cr11Ni(1) alloy could explain the different behaviour of both 21Cr11Ni steels.
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