Microstructure of HK40 alloy after high temperature service in oxidizing/carburizing environment
Crack propagation and oxidation phenomena during high temperature service of tubes made from HK40 alloy have been investigated. The materials were characterized using electron microscopy, X-ray diffraction and microanalysis techniques after being used for an extended period (up to 25,000 h) as furnace tubes in ethylene pyrolysis. The service conditions subjected the materials to oxidizing and carburizing conditions on the surfaces leading to the formation of complex oxide structures in both external and internal oxide scales, and of carbide-denuded zones in subsurface regions. A macrocrack in one of the samples provided an opportunity to study the mechanism of crack propagation and the sequence of oxidation of the constituent elements in the material. The observations imply that silicon segregation during carbide coarsening was an important precursor to crack propagation.