Role of alloying elements in steels on metal dusting

摘要

Metal dusting of Fe-Ni-Cr alloys has been observed in industrial processes in strongly carburizingatmospheres at temperatures from 450 ℃ to 800 ℃. At temperatures below 650 ℃ the alloys aregenerally not able to form dense, well adherent oxide layers in spite of relatively high Cr-contents,therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidatethe mechanism and to compare the resistance against metal dusting for high alloy steels [1]. Theintention of this study was to obtain additional information concerning the role of alloying elementsand the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. Thesusceptibility to metal dusting was determined by measuring the metal loss under metal dustingconditions of Fe-20%Cr-32%Ni alloys modified with additions of different carbide formers (W, Mo,Nb) or oxide formers (Si, Al). The samples were exposed at 600 ℃ in a CO-H{sub}2-H{sub}2O-gas mixture for repeated periods up to 500-1500 h. The attack by the oxidizing and carburizingatmosphere leads to the precipitation of internal carbides and metal dusting and more or less toformation of an oxide layer. In comparison to the undoped material, the addition of carbide formersretards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dustingunder the given laboratory conditions. When carbides are present at the metal surface, they affect theinitial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effectof Ce, this rare earth element is generally known to favour Cr-oxide formation and to improve theadherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting andmetal wastage.