Solubility product for niobium carbide in austenite
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I.
INTRODUCTION
A S part of a comprehensive thermodynamic examination of the equilibrium of niobium carbonitrides with alloyed austenite I we have experimentally determined the solubility product for NbC0.87 in unalloyed austenite between 950 and 1250 ~ This expands, and to a large extent bridges and confirms, the extensive data set already available in the literature. 2-~1 The principal methodology involved mutual equilibration of encapsulated Fe-C and Fe-C-Nb alloy samples in hydrogen. Solubility product determinations and precipitate identifications were also carried out on two high purity vacuum-melted steels using chemical and electro-chemical methods to isolate the precipitated phases.
II. THE GAS EQUILIBRIUM EXPERIMENTAL METHOD The sealed capsule technique, employed by K/illstrom and Omsen, ~2 Heckler and Winchell, ~3 Zupp and Stevenson, ~4 Nishizawa, ~5and Uhrenius and Harvig ~6was chosen for the present study. This consists of a series of diffusion-isolated Fe-Nb alloys and Fe-C alloys enclosed in an evacuated, H2 back-filled, thick-walled quartz tube, annealed at the desired temperature. Carbon is selectively transferred between the Fe-C and the Fe-Nb alloys which are not in physical contact. The carbon activity obtaining in the capsule is determined from the final carbon content of the binary Fe-C alloy samples via the results of Ban-ya et al.17 The main experimental considerations in this method were: (1) High purity Ferrovac electrolytic iron and ferro-niobium with an oxygen content less than 0.0012W pct has been used in this study (Table I). Samples were prepared by melting ferro-niobium and electrolytic iron under an argon atmosphere. High purity H2 gas with a very low H20 content has been used in the equilibration. We therefore conclude that the oxygen contamination of the NbC087 precipitates is negligible. (2) Accurate temperature control and establishment of a uniform temperature zone (+4 ~ for T < 1100 ~ +-6 ~ for T > 1100 ~ (3) Prevention of leakage of H2 from the thick-walled quartz tube via a 1 atm external H2 bath. v .K. LAKSHMANANis a ResearchEngineerwith TataIron and Steel Company,Jamshedpur,India. J. S. KIRKALDYis Professor,Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada, L8S 4K1. Manuscript submittedSeptember1, 1982. METALLURGICALTRANSACTIONSA
(4) Establishment and assessment of equilibrium with respect to carbon content (0.5 mm sheet annealed for times up to 120 hours). (5) Investigation of the possibility of transfer of niobium between the samples or of silicon contamination from the quartz capsule. (6) Carbon analysis (Leco conductometric apparatus for low concentrations). Full details are to be found in References 1 and 12 to 16. III.
STUDIES ON PRECIPITATES EXTRACTED FROM NIOBIUM-BEARING STEELS
Effective electrochemical means for isolating precipitates have been discussed by Blickwede and Cohen, ~8 Andrews and Hughes, 19 Walz and Bloom, 2~ and Gurry et al. 2t The principle in all these techniques is the same, viz., anodie dissoluti
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