Sulfidation behavior of austeno-ferritic steels

Abstract

Two austeno-ferritic stainless steels were sulfidized at temperatures of 783, 873, and 963 K under sulfur pressures in the range 4×10−12 to 3×10−5 atm. In all cases a triplex scale developed, consisting of an outermost layer of (Fe, Cr)1−xS, an intermediate layer of FeCr2S4, and an innermost layer of porous (Cr, Fe)1−xS containing particles of Mo2S3. Parabolic kinetics were observed except at the lowest temperature where one of the steels reacted according to irregular kinetics. The Mo2S3 particles in the innermost layer acted as inert markers, imaging the former positions of the steels' ferrite phase in which Mo is enriched. The lamellar microstructure of the steel was thus reproduced in the innermost sulfide layer. The positions of the Mo2S3 particles together with the porosity of the inner layer are taken to imply inward sulfur transport through this layer and outward metal transport through all three layers.