Magnetic force microscopy and cross-sectional transmission electron microscopy of carburised surfaces

Magnetic force microscopy has been used to determine the origin and spatial distribution of changes in magnetic properties of an HPM alloy carburised by exposure in an ethylene production furnace. In higher activity carburising atmospheres at 400–800 °C, metal dusting can occur in which the exposed surface becomes carburised to such an extent that the tube alloy pits and literally turns into a dust of carbon, graphite and metallic particles. Metal dusting is a significant “remaining life” and design issue in several large petrochemical plants in Australasia. Transmission electron microscopy is being used to observe the element segregation into carbides, coke and graphite to clarify the mechanisms by which metal dusting occurs in 601 and 316 stainless steel alloys.