| Abstract: | The structure of the oxide layer formed on Alloy 800 at 600 °C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloy/oxide interface. Increasing the number of grain boundaries by 10-90% cold work leads to increasing Cr- and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leads – since the Cr-content in the scale is decreasing with the grain size – to a linear relation of growth rate and grain size. The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 900 °C, these were exposed to a CO-CO2H2O-H2 mixture at 700 °C for long time. The gas mixture was tagged with 14C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure. The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy diffusion paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer. |
