Oxidation behaviour of Austenitic stainless steels at high temperature in supercritical plant

Fossil power plant efficiency is improved by increasing steam temperature and pressure. Current martensitic and ferritic steels do not have the required oxidation and spallation properties for boiler tube applications as service conditions increase. Spallation inside the tube can lead to blockage, overheating, creep rupture and turbine erosion. The current steels are limited to 620oC, therefore austenitic stainless steels are proposed for boiler tube applications to enable higher operating conditions. This investigation compared the oxidation and spallation behaviour of the current martensitic steels (T91, T92) with the proposed austenitic stainless steels (super 304H, shot peened super 304H, 347HFG), in air at 600-700oC. Oxide morphology was characterised using SEM and EDX analysis and oxidation kinetics were recorded using specific mass gain and oxide thickness measurements over time. The martensitic steels formed non-protective Fe-rich oxides, which consistently spalled on cooling. The austenitic stainless steels showed little spallation and less mass gain compared to the T92 steel. Double-layered oxides formed on the super 304H and 347H FG steels consisting of an inner protective Cr-rich spinel oxide and an outer Fe-rich oxide. Shot peening increased the oxidation resistance of the super 304H steel forming significantly thinner, single-layer Cr-rich oxides with less mass gain