Modelling carburisation in 9Cr-1Mo ferritic steel tube substrates in experimental CO2 atmospheres

Abstract

By combining modelling and experimental work, new insights have been gained into the combined oxidation/carburisation of 9Cr-1Mo steels in CO2 rich environments. The breakawayoxidation process is preceded by the rejection of carbon into the scale due to the poor solubility of carbon in the metal. Experimental TEM work reveals the formation of a carbide rich layer near the substrate surface which forms a further barrier against carbon ingress. The eventual oxidation of this layer could contribute to breakaway oxidation. A 2D finite-difference based diffusion model developed in combination with Thermo-Calc software underlines the role of specimen geometry on breakaway initiation.

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

“…This grain boundary segregation of carbides has been previously observed [35] with precipitates forming preferentially along the prior substrate grain boundaries, as observed here. The increasing fraction of carbides in the matrix has been reported several times previously on identical 9Cr-1Mo material [9][10][11]. It was found that measurements taken closer towards the fin base (Figure 1) showed a dip towards the middle of the fin increasing at the edge highlighting the increase in carbides from the carburisation reaction taking place between the steel and CO2 rich atmosphere [9].…”

“…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.…”

“…This study will show a direct comparison between two materials with similar compositions but different starting microstructures, one with a martensitic lath microstructure and one which consisted of fully equiaxed ferritic grains which was achieved via two different heat-treatments, to highlight the effect microstructure has on the oxidation characteristics of 9Cr-1Mo steel exposed to a CO2 rich atmosphere. 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].…”

A Novel Design to Enhance the Mechanical Properties in Cu-Bearing Antibacterial Stainless Steel

“…Thermo-Calc is always used to calculate the equilibrium thermodynamics in steel [24][25][26]. A description of the amounts of copper precipitation as a function of temperature can be obtained.…”