High-temperature oxidation of duplex stainless steels in air and mixed gas of air and CH4

TGA, isothermal and thermal-cycle tests were carried out to investigate the oxidation behaviour of duplex stainless steels at 1220 °C in various atmospheres. The weight gain in air is initially in parabolic relation to time. This is followed by a linear relationship for super alloy and a dramatic growth one for standard alloy that is subject to breakaway oxidation by forming nodules. The thickness of γ-layer increases with increasing annealing time and oxygen content in the atmosphere, and it is slightly higher in standard alloy than in super alloys. A single layer of (Fe,Cr)3O4 spinel is noticed in the external oxide scale after annealing for 2 h, and two layers are found after 4 h with an inner layer of the spinel and an outer layer of iron oxides. Nodules grow on the δ/γ packets where the oxidation of austenite is accelerated and that of ferrite is suppressed. Both chains and Si-enriched scales grow inside the γ-layer along grain and phase boundaries.