Internal sulfide precipitation in low Cr-Fe alloys
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I.
INTRODUCTION
II.
BACKGROUND
A solid-state diffusion study was performed in the in the Fe-Cr-S system at 600 ~ The objectives of the solidstate diffusion study were as follows: (1) to analyze the phase equilibria in the ternary system; and (2) to describe the phase layer sequence and morphology that developed in the diffusion couple microstructures. During this diffusion study, an internal sulfide precipitate layer was found in Fe-Cr alloy diffusion couple endmembers containing less than 90 wt pct Cr. The sulfide phase morphologies and compositions were documented across the entire Fe-Cr composition range. Alloy compositions containing up to 50 wt pct Cr possessed an internal precipitate layer (IPL) morphology that contained three sulfide phases (Fe~_xS, FeCrS4, and Crl_xS) and showed significant particle size and facet growth.tU Alloy compositions exceeding 50 wt pct showed a triplex interfacial scale with an internal precipitate zone just beneath the alloy surface.m The precipitates in alloy endmembers containing >59 wt pct Cr were very small and closely spaced, so microanalysis was very difficult. The intemal sulfide precipitates in the low Cr-Fe were chosen for more in-depth study because the particle sizes made microanalysis and precipitate size and shape measurements possible. Internal precipitation theory has been used successfully to describe internal sulfidation and oxidation in different systems. However, no analysis of internal sulfide precipitation was found for the Fe-Cr-S system at a temperature as low as 600 ~ The purpose of this article is to analyze the internal sulfide precipitate composition and morphology in low Cr-Fe alloys at 600 ~
Internal sulfide precipitation in Fe-Cr alloys has been reported previously in the temperature range 700 ~ to 1000 ~ and in the Fe-Mn-S systemt3] at temperatures exceeding 1000 ~ Furthermore, in a review of internal oxidation theory by Rapp, t41 high-temperature (>1000 ~ internal oxidation of Fe-Cr alloys was used as an example to illustrate the accuracy of internal oxidation theory. In each case, there were similarities between the current research performed at 600 ~ and the literature in terms of sulfide composition and morphology. A more important characteristic shared by both the internal sutfidation and oxidation work is that both are diffusion controlled and, therefore, their composition profiles and kinetics may be explained using solutions to established diffusion equations.iS.41 The mathematical theory describing internal precipitate formation has already been developed.t95 wt pct Fe data in the middle region). The 600 ~ isothermal sec3196~VOLUME 27A, OCTOBER 1996
tion of the Fe-Cr-S equilibrium phase diagram 161(Figure 1) shows that this layer (marked I~F~Cr)is a-FeCrss. The righthand side of the profiles shows the Cr concentration profile in the original sulfide endmember (marked 1~1). The depth of the IPLs were measured in each low Cr-Fe diffusion couple and are shown in Figure 5. The thickness has been normalized for the square root of time beca
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