Cyclic carburizing behaviour of Al modified high Si-containing HP40 alloy

The chromia layer, which is formed during the pre-oxidation process, can suffer from the reduction during the carburizing. However, the oxide coating retards the growth rate of the external carbide, and thus permits the development of the inner silica layer. Nevertheless, wedging crack of the silica layer during cyclic carburizing facilitates the internal carburization. For the comparison, the high grain boundary density within the Ni, Al rich layer of aluminium adopted alloy promotes the severe internal carburization. However, as the ion diffusion has been blocked by the integrity outer alumina layer, the pre-oxidized Al-adopted alloy almost unaffected during cyclic carburizing.

► Al rich grain precipitates adjacent to the chromium carbide during solidification.
► High density of the grain boundary facilitates the severe internal carburization.
► Chromia retards the carburizing rate and thus helps the formation of the silica.
► Inner silica layer can suffer from crack during cyclic carburizing.
► Alumina layer can protect the matrix effectively during carburizing.