A Microstructural Investigation of Austenitic Heat Resistant Alloy after 500 h of Steam Oxidation

Abstract

Alloy 709 was oxidized at 700 °C for 500 h in a steam environment. A microstructural analysis of the oxide scale is reported. Modern techniques of advanced electron microscopy were used to characterize the morphology of the oxide scale and recognize its single components. The material developed a complex, multilayered oxide scale. The outermost layer consisting of Fe2O3. Fe2NiO4 tI28 spinel was detected underneath. An internal oxidation zone is present in the innermost layer. High quality SEM-EDS maps give insight into a larger area of the oxide scale at a relatively low magnification.

The Morphology and Microstructure of Oxide Scale Grown on Austenitic Steel during Steam Oxidation at 700 °C for 500 h

“…What is worth mentioning is that the first spallation effects at the lower temperature were noticed after a longer exposure time in comparison to the higher temperature experiments. Our previous investigations on austenitic heat resistant steel [ 21 ] demonstrated very different behavior—spallation occurred while cooling down from the oxidation temperature. According to Galerie et al [ 31 ], the presence of Nb contributes to spallation.…”

“…Therefore, inward oxygen diffusion through Fe-rich oxides is faster than the outward diffusion of Cr. The slow bulk diffusion of chromium in austenite might be confirmed by the shallow Cr-depleted zone in the coarse-grained material, which is attributed to the large grain size of steel [ 21 , 33 , 34 ]. At the Cr 2 O 3 /alloy interface, the oxygen partial pressure is too low for further oxidation of Cr.…”

“…The oxidation test was carried out at 700 °C up to 500 h. The specimen was removed from the chilled furnace after the allotted time. Afterwards, the mass gain was measured, and the specimen was inserted into the furnace and heated up for further oxidation [ 21 ]. The mass gain after a certain time was measured with high resolution (1 µg) MYA 5.4.Y Plus microbalance of Radwag (Radom, Poland).…”

“…Next, the samples were sequentially embedded in the resin and then grounded on the abrasive papers and polished on the SiO 2 suspension. The sample preparation process was described in detail previously [ 21 ]. Such cross-sections underwent further investigations by the methods of scanning electron microscopy.…”

“…In order to ensure the highest quality results, the sample for the STEM analysis has to be thin, and its thickness should be uniform in the whole area. Therefore, it was prepared by the focus ion beam (FIB) method, as described previously [ 21 ].…”