Ageing and high temperature oxidation phenomena in three high alloyed stainless steels

Abstract

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.