Microstructural studies of the scale on Sanicro 25 after 25,000 h of oxidation in steam using advanced electron microscopy techniques

Abstract

In the present study the effect of long-term oxidation at 700 °C in steam on the microstructure of Sanicro 25® (42Fe22Cr25NiWCuNbN) was investigated in detail by advanced electron microscopy, spectroscopy and FIB-SEM tomography techniques.

Owing to its satisfactory creep and oxidation resistance at high temperature the steel is considered suitable for use in reheaters and superheaters in ultra-supercritical coal-fired steam boilers. Microscopic analyses were performed using electron microscopy, while the chemical composition and elemental distribution were analyzed using energy dispersive X-ray spectrometry. The SEM was used for investigation of the scale surface morphology and in-depth steel microstructure. Further analyses were conducted by (S)TEM using a S/TEM Titan Cubed G2 60-300 equipped with the ChemiSTEM (FEI) system. The FIB-SEM tomography allowed for 3D visualisation of the oxide scale surface and the boundaries between the oxide scale and the underlying steel.

During steam oxidation of Sanicro 25 (up to 25,000 h) approx. 3 μm thick dual-layered scale consisting mainly of Cr2O3 plates had formed. The thicker outer layer of the scale was enriched with Si and Mn. Beneath the scale, Si underwent internal oxidation which resulted in the formation of numerous SiO2 particles at the scale/steel boundary. The growth of the Cr2O3 scale caused the formation of chromium depleted zone underneath the oxide layer. In the Cr-depleted zone, M23C6 was unstable and dissolved in the austenite. In the underlying bulk steel, M23C6, ε-Cu, Fe2W Laves phase as well as NbX and/or (Nb,Cr)N Z-phase particles had precipitated in the austenitic matrix. The study showed that Sanicro 25 steel is very resistant to steam oxidation at 700 °C, but after 25,000 h it exhibits significant microstructural and chemical composition changes (up to a depth of 20 μm) in comparison to the as-received one.