Carburisation and its effect on the oxidation characteristics of 9Cr–1Mo steel

Abstract

During operation within an Advance Gas Cooled Reactor (AGR) 9Cr-1Mo steel is subjected to high temperature CO2 rich gas. This leads to complex interactions taking place between the CO2 and the steel. The steel is subjected to simultaneous carburisation and oxidation changing both the chemistry and the physical properties of the steel. Experimental samples were exposed on behalf of EDF Energy to better understand the processes taking place and to better understand the factors which affect time to breakaway oxidation taking place.A range of microscopy techniques have been used to study experimental samples exposed to samples aged using accelerated exposure conditions including Scanning Electron Microscopy SEM), Focused Ion Beam Microscopy (FIB) and Transmission Electron Microscopy (TEM) to better understand the corrosion processes taking place under these conditions.Carburisation of the substrate takes place with carbides forming preferentially along grain boundaries or martensitic laths depending on substrate microstate. These carbides increase in complexity with exposure time with the carbide distribution favouring higher C containing carbides at later exposures (M7C3 vs M23C6). The increase in carbide volume throughout the spinel is associated with the internal oxidation characteristics of the steel, the porosity of the oxide scale and is linked with breakaway oxidation. Some of these substrate carbides have been shown to resist oxidation at the main oxidation front through the growth of a Cr rich shell, this allows the carbide to remain within the spinel for potentially thousands of hours. The porosity across the oxide scale changes with exposure time, and a mechanism of porosity development has been linked with the substrate carbide distribution and frequency.The oxidation behaviour of the steel has been shown to be dependent on both microstructure and carbide distribution, with both the internal oxidation zone and spinel oxide showing evidence of the prior microstructure and carbide distribution. Breakaway samples have been shown to have a thinner internally oxidised regions relative to the pre-breakaway samples.Predictions for time to breakaway have been made based off the growth rate of the IOZ across the fin tips and extrapolated to lower temperatures using the Arrhenius Equation and activation energy calculated from the fin tips. These predictions take the geometry of the fins into account, important as breakaway is always observed to initiate at the fin tips and not the main body.