Dispersion of complex carbides at the oxidation front during the oxidation of 9Cr-1Mo steel

The effect of microstructure on the oxidation and carburisation of 9Cr-1Mo steel exposed to CO2

“…The HRA material (Figure 2b) shows multiple carbide types with larger blockier carbide precipitates along grain boundaries and smaller carbide precipitates inside the grains. The carbide development of HRA material has been previously studied [9], [11], [12] showing the elemental development of carbide precipitates during both pre and post breakaway oxidation of 9Cr-1Mo steel exposed to CO 2 , and this work found that area fraction of carbides increases. The complexity of the carbide precipitates also increases, with the carbide elemental composition developing with exposure time, and complex multiphase, multi-component carbides forming [12].…”

“…The carbides forming are Cr and Mo rich which leads to depletion of these elements within the matrix. The carbide precipitates forming are generally M23C6 and M7C3, these carbides have previously been discussed in greater detail [9], [11], [12]. These changes in the substrate chemistry will affect the oxidation characteristics of the 9Cr-1Mo steel.…”

“…Carburisation of the substrate has been studied extensively and has been linked to changes in the oxidation characteristics of the 9Cr-1Mo steel [9]- [12]. The driving force for substrate carbide formation is attributed to the Boudouard reaction which is shown in equation ( 4)…”

“…Continued exposure at high temperatures leads to microstructural changes within the substrate, with work investigating the microstructural evolution finding that there is significant coarsening of precipitates within the substrate over exposure times, leading to a decrease in hardness [20]- [22] and coarsening of the precipitates also leads to a reduced creep strength [22]. These carbides also alter the substrate chemistry, causing the elemental segregation of elements within the matrix to the carbides, which is typically focused on the grain boundaries [9], [12]. The carbides forming are Cr and Mo rich which leads to depletion of these elements within the matrix.…”

“…This paper will qualitatively compare the two microstructures and the effect it has on the oxidation and carburisation characteristics of 9Cr-1Mo steel, building on previous work where modelling of the carburisation of 9Cr-1Mo steel was performed [11] Experimental Experimental 9Cr-1Mo steel samples were aged on behalf of EDF Energy in autoclaves for varying lengths of time with the intent to better understand the processes taking place in the lead up to breakaway oxidation. The samples investigated are finned samples which have been previously studied [7], [9]- [12]. The work undertaken, unless specified, was completed towards the fin tips.…”