Influence of sulphidation and carburisation on creep

Abstract

The mechanical properties of structural materials can be strongly affected by environmental conditions. In particular, during long term exposure at elevated temperatures, i.e. under typical creep conditions, technical atmospheres must be expected, in general, to cause corrosive attack. Thus, to obtain reliable design data it is the interaction of corrosion and mechanical behaviour which must be considered as the lifetime determining factor. The present work is an overview of the influence of sulphidation and carburisation on the creep behaviour of high temperature alloys with emphasis on heat resistant steels. Sulphur is known to be a particularly aggressive species and severe attack on material due to sulphidation is encountered in gas turbines (nickel base alloys) as well as in coal gasification or fluidised bed combustion plants (heat resistant steels). Sulphidation is found to reduce the creep strength of high temperature alloys and the creep ductility in particular can be drastically reduced as a result of accelerated creep crack propagation. Carburisation, which is observed, for example, in ethylene cracker tubes, seems to be less dangerous than sulphidation. Creep experiments on heat resistant steels have shown that carburisation can degrade, but can also improve the high temperature creep properties, depending on the alloy. However, at low temperatures, complete embrittlement of the material always occurs when severe carburisation has taken place. For multicomponent gas mixtures containing, for example, sulphidising, carburising, and oxidising potentials, the corrosion–creep interaction can be rather complex. A high carbon level may eventually overcompensate any deleterious effect caused by sulphidation.MST/1147