475°C Embrittlement and Sigma Phase Formation in Ultra-ferritic Stainless Steels

M.B. Cortie1, V.J. Bhatia1, K. Premachandra2, and A. Dowd1

1Institute for Nanoscale Technology, University of Technology Sydney,

Sydney, 2007, Australia

2 CertainTeed Gypsum Canada Inc.,Ontario, Canada

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Iron-chromium-molybdenum alloys with ~36 wt% Cr can potentially offer significant resistance to corrosion. They can be prepared to be ferritic at room temperature by heat treatment and, if pure, will have good impact toughness in this form [1]. However, alloys of this type are susceptible to embrittlement by spinodal decomposition in the temperature range 450 to 500°C and, at higher temperatures, to the formation of brittle sigma (s) phase [2, 3]. Generally, the starting point for construction of the phase diagram in this system is taken to be body-centred cubic (Fe,Cr) solid solution. Here we use in situ powder diffraction measurements in a synchrotron to investigate whether prior de-mixing of the (Fe,Cr) solid solution in a Fe- 35.5% Cr - 4% Mo-2.5% Ni alloy into Fe-rich a and Cr-rich a¢ phases will have an effect on the subsequent formation of sigma phase. In addition, the accepted binary phase diagram and associated thermodynamic parameters in databases such as that of ThermoCalc imply that, given sufficient time, s should revert to a mixture of a and a¢ below 480°C. We test this assumption. The results imply that the current Fe-Cr phase diagram is not strictly correct.

The authors thank the Australian Synchrotron for the beam-time on the powder diffractometer, and Dr J.Kimpton for experimental assistance.

Figure 1 Effect of temperature on constitution of samples. (a) Relative proportions of a and a¢ phases in embrittled sample, and (b) ferrite and sigma phases in brittle sample compared to quenched sample.

References

[1] M. Cortie, I. Wolff, K. Premachandra, M. Tullmin and E. DeMarsh, in Stainless Steels '91, 555-564, The Iron and Steel Institute of Japan (1991).

[2] Premachandra, K., and Cortie, M.B. (1992), Materials Science & Technol. 8, 437 (1992).

[3] Cortie, M.B., and Pollak, H., Mater. Sci. Eng A. 199, 153 (1995).