Microstructural and microchemical aspects of the solid-state decomposition of delta ferrite in austenitic stainless stee
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I.
INTRODUCTION
1600
MOSTaustenitic stainless
steels contain 15 to 25 pct Cr and 8 to 10 pct Ni. For these alloys the pertinent phase equilibria can be represented approximately by a pseudobinary section of the Fe-Cr-Ni temary diagram, as shown in Figure 1. Just below the solvus, an alloy will transform completely to austenite for temperatures below approximately 1100 °C. The system, therefore, has the potential for the composition-invariant transformation of ferrite to austenite. It is found, however, for most continuous cooling conditions achieved in practice, that duplex ferrite-austenite microstructures are formed. It is, therefore, of interest to study systematically the transformation segregation on cooling initially homogeneous delta ferrite, and the consequent stabilization to room temperature of the delta ferrite. To the authors' knowledge, this area of research has not been eraphasized by previous workers, who have chosen instead to study the transformations in simulated or actual weldments, and who have, therefore, been forced to deal with the overlaid effects of rapid solidification and solid-state phase transformations. ~'2 The effects of solid-state diffusion on solidification segregation in the high-conductivity delta phase are not insignificant, and are difficult to predict with precision. 2'3 In the present work, therefore, an initial condition was chosen which is less dependent on prior processing history, in the expectation that the results obtained would allow the fuller understanding of previous studies.
II.
EXPERIMENTAL
PROCEDURES
An Fe-20 pct Cr-10 pct Ni alloy was prepared by argon arc melting constituent elements of 99.99 pct purity. The cast alloy was homogenized in vacuum for four days at 1100 °C, water quenched, and reduced to sheet of 0.1 mm thickness by cold rolling. Coupons, 8 mm x 5 mm, were prepared from the rolled sheet, and to each was welded a Pt-13 pct Rh thermocouple fabricated from 0.08 mm diameter wire. These coupons were suspended from the thermocouple wires, in vacuum, and surrounded by a tungsten J. SINGH is Research Associate, Department of Metallurgy, University of Illinois, Urbana, IL 61801. G. R. PURDY is Professor, Department of •Metallurgy and Materials Science, McMaster University, Hamilton, ON, Canada. G. C. WEATHERLY is Professor, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada. Manuscript submitted June 1, 1984. METALLURGICAL TRANSACTIONS A
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Fig. 1 - - A 70 pct Fe pseudobinary section of the Fe-Cr-Ni system.
heating coil. They were heated to 1400 °C, then cooled at various rates by mixtures of helium and argon gas, directed at both sides of the specimen. The thermocouple output was recorded using a microcomputer, which also served to switch the current to the heating coil, and to initiate and reg
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