A Study of equilibria in the Fe-Cr-Ni-Mo-C-N system at 1273 K
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I.
INTRODUCTION
T H E use of N as an alloying element in stainless steels has attained considerable interest because it improves the mechanical properties and has a favorable influence on the resistance to pitting corrosion. The design of alloys alloyed with nitrogen requires a knowledge of the effect of N on the phase equilibria. The present study was undertaken in order to obtain such information. Experimental, thermodynamic, and phase diagram data were obtained in the Fe rich corner of the Fe-Cr-Ni-Mo-C-N system. Some of the new data was combined with previous information in order to assess the thermodynamic properties of the Fe-Cr-Ni-N system and produce evaluated isothermal sections. The method of calculating phase diagrams using thermodynamic models and computer methods is well established and provides an efficient way to represent phase equilibria. However, it requires information on the thermodynamic properties of the individual phases in all lower order systems involved. Such information has been established for the fcc and bcc phases in the Fe-Cr, ~ Fe-N, 2 Fe-Ni, Cr-Ni, Fe-CrNi, 3 Fe-Ni-N, 4 and Fe-Cr-N 5 systems, and for the bcc and hcp (e nitride) phases in the Cr-N 6 and Fe-Cr-N ~ systems. In order to establish the thermodynamic properties of the fcc, bcc, and e phases in the remaining systems, calculate the quaternary phase diagram from intbrmation on the lower systems, and test the result of this calculation, new experimental information obtained from Fe-Cr-Ni-N specimens in this work and in Reference 7 was used. Abrahamsson and Grant s have reported experiments in the Cr-Ni-N system and Tisinai and Samans 9 in the Fe-Cr-Ni-N system. That information was not used in the present evaluation but was used to check the results.
II.
EXPERIMENTAL TECHNIQUE
The purpose of the experiments was twofold: (a) to establish experimentally the effect of N on phase equilibria in the system Fe-Cr-Ni-Mo at 1273 K (1000 ~ STAFFAN HERTZMAN, formerly with Royal Institute of Technology, Stockholm, is with Swedish Institute for Metals Research, Drottning Kristinas V'fig 48. S-114 28 Stockholm, Sweden. Manuscript submitted January 25, 1985. METALLURGICAL TRANSACTIONS A
(b) to provide data for a thermodynamic description of the individual phases. The isopiestic method is well suited for this purpose and, by combination with a sealed capsule technique, allows invariant equilibria, which require a unique potential of the volatile element, to be studied. This would not be possible in an open system which also has the disadvantage that long heat treatment times (up to 1000 hours) would require extremely stable and clean gas mixtures. By imposing the potential of N and C potentials over a series of alloy compositions in a technically interesting region of the phase diagram, both one-phase alloys and multi-phase alloys can be obtained in a single experiment. This makes possible a very accurate determination of the potentials from the compositions of the one-phase specimens as well as the determination of tie lines, at the ch
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