Carbide precipitation associated with carburisation of 9Cr–1Mo steel in hot CO2 gas

Abstract

Experimental ferritic 9Cr-1Mo steels exposed at high temperature and pressure to CO2-based gas undergo oxidation and carburisation. Oxidation proceeds initially as a protective phase with parabolic kinetics (stage I). At high temperatures and longer times, more rapid "breakaway" oxidation occurs (stage II). Post-breakaway oxidation follows linear kinetics (stage III). A virgin sample and three oxidised samples were selected for analysis. Precipitates embedded in the matrix with four different morphologies were observed using focused ion beam imaging, X-ray diffraction and electron microscopy techniques. Coarse carbides (M23C6) were observed as the predominant precipitates in all four samples. Fine needles (M2C), large needles (MC) and cored coarse carbide precipitates (M23C6) were also present in stages (I), (II) and (III), respectively. TEM analysis was used to give the lattice types and orientations of the carbides found. Energy-dispersive X-ray analysis was used to obtain compositions. The precipitation is considered particularly with respect to the formation of cored M23C6 carbide precipitates. These data provide a threshold for the matrix composition that defines the onset of breakaway oxidation.