Metal Dusting-Mechanisms and Preventions

摘要

Metal dusting attacks iron,low and high alloy steels and nickel-or cobalt-base alloys by disintegrating bulk metals and alloys into metal particles in a coke deposit.It occurs in strongly carburising gas atmospheres(carbon activity a_C>1) at elevated temperatures(400℃～1000℃).This phenomenon has been studied for decades,but the detailed mechanism is still not well understood.Current methods of protection against metal dusting are either directed to the process conditions-temperature and gas composition-or to the development of a dense adherent oxide layer on the surface of the alloy by selective oxidation.However,metal dusting still occurs by carbon dissolving in the base metal via defects in the oxide scale.The research work at UNSW is aimed at determining the detailed mechanism of metal dusting of both ferritic and austenitic alloys,in particular the microprocesses of graphite deposition,nanoparticle formation and underlying metal destruction.This work was carried out using surface observation,crosssection analysis by focused ion beam and electron microscopic examination of coke deposits at different stages of the reaction.It was found that surface orientation affected carbon deposition and metal dusting at the initial stage of the reaction.Metal dusting occurred only when graphite grew into the metal interior where the volume expansion is responsible for metal disintegration and dusting.It was also found that the metal dusting process could be significantly changed by alterations in alloy chemistry.Germanium was found to affect the iron dusting process by destabilising Fe_3C but increasing the rate of carbon deposition and dusting,which questions the role of cementite in ferritic alloy dusting.Whilst adding copper to iron did not change the carburisation kinetics,cementite formation and coke morphology,copper alloying reduced nickel and nickel-base alloy dusting rates significantly.Application of these fundamental results to the dusting behaviour of engineering alloys is discussed.