An experimental and thermodynamic study of the Fe-Cr-C-N system at 1273 K
- PDF / 2,752,256 Bytes
- 14 Pages / 594 x 774 pts Page_size
- 51 Downloads / 179 Views
I.
INTRODUCTION
THE phases occurring
in the Fe-Cr-C-N system at 1273 K in the investigated composition range are ferrite ( a ) , the body-centered cubic Fe modification which is ferromagnetic below 1043 K; austenite (y), the face-centered cubic modification of Fe; M23C6, a Cr-rich carbide with a bcc structure occurring at low C activities; M7C3, the hexagonal Cr carbide stable at intermediate C activities; and e-nitride, the hexagonal Cr-rich M2N nitride which precipitates at lower N activities than the face-centered cubic MN nitride (M = Cr or Fe). The Fe-Cr-C system has been the subject of several attempts to find a self-consistent thermodynamic description. Waldenstr~Sm and Uhrenius 1 performed an assessment and reviewed earlier experimental work and theoretical analyses. Their analysis was based on information from the Fe-Cr-WC and Fe-Cr-C systems and covered a wide temperature range. 2 The experimental information on the C content of austenite in equilibrium with ferrite and M23C 6 at 1273 K (1000 ~ used in that work 3 was less accurate. A further experimental study of this C content has been performed here. The Fe-Cr-N system was recently assessed by Hertzman and Jarl. 4 It was based on an analysis of the binary Fe-Cr system 5 in which a new model was used for describing the magnetic contribution to the Gibbs energy. In a study of the interaction between N and other alloying elements in ferrite and austenite, Jarl 6 suggested a set of parameters for the C-N interaction in the Fe-C-N system. The Fe-C-N phase diagram has been experimentally determined in the temperature range 773 to 973 K (500 to 700 ~ by Naumann and Langenscheid. 7 Experimental data on the Cr-C-N system have been reported by Kieffer et al. 8 and others. Imai et al. 9-12 performed experiments on the Fe-Cr-C-N system. A number of alloys were heat treated and examined STAFFAN HERTZMAN, formerly with Royal Institute of Technology, Stockholm, is with Swedish Institute for Metals Research, Drottning Kristinas V~ig 48, S-114 28 Stockholm, Sweden. Manuscript submitted January 18, 1985 METALLURGICALTRANSACTIONS A
metallographically to determine the extent of individual phase fields. Unfortunately, some of their alloys had significant Si and Mn impurities. The aim of the present work was to obtain a thermodynamic description of the Fe-Cr-C-N system at 1273 K. For this purpose an experimental study was performed involving measurements of C and N activities, phase identification, and phase analysis by microprobe measurements. In addition, it was necessary to redetermine the o ~ / T / M 2 3 C 6 equilibrium in the Fe-Cr-C system. The thermodynamic evaluation was performed using a computer-based optimization technique. II. E X P E R I M E N T A L STUDY OF T H E Fe-Cr-C SYSTEM The Fe and Cr contents of austenite (y) in the three-phase o t / ' y / M 2 3 C 6 equilibrium were determined by microprobe
measurements. Determination of the C content by this method is difficult owing to the low emission and high absorption of X-rays with long wave lengths, so the
Data Loading...
