Abstract
The structure, chemical composition and behaviors of tantalum-carbides and nitrides in steels were-studied by means of metallographic observation and chemical analysis as well as X-ray diffraction analysis of the residues extracted from various specimens of vacuum melted high purity Fe-Ta-C, Fe-Ta-N and Fe-Ta-C-N alloys containing tantalum up to 3.16%. The specimens were prepared from forged materials by solution treatment and long time holding at various temperatures from 1000 to 1300°C. The quantitative analysis of soluble-tantalum, -carbon and -nitrogen was also conducted in-order to calculate the solubility products of tantalum-carbide and-nitride in austenite.Results obtained are as follows;
1) The precipitates of tantalum-carbides or-carbonitrides were observed predominantly on austenite grain boundaries as finely dispersed particles, while the tantalum-nitrides were precipitated as comparatively large particles.
2) It was confirmed that these precipitates can be quantitatively isolated from the iron matrix by dissolving the chips in 6N-HC1 solution at room temperature.
3) The precipitated tantalum-carbide or-carbonitride in these samples were identified to be f. c. c.δ-phase and their lattice constant decreased as the content of nitrogen in sample increased.On the other hand, the precipitated tantalum-nitrides in various specimens were identified to be h. c. p.γ-phase with almost same lattice constant.Intermetallic compound Fe7Ta3 was also found in the sample containing tantalum of more than 3%.
4) The precipitated δ-phase was not pure carbide but carbonitride even in the Fe-Ta-C alloys containing little nitrogen and its chemical composition was determined to be TaC0.85N0.06.The precipitated r-phase was estimated to be pure tantalum-nitride TaN0.85.
5) The solubility products of the tantalum-carbonitride and -nitride in austenite were given respectively by the following equations;
log[%Ta] [%C]0.85 [%N]0.06=-5, 800/T+2.02
log[%Ta] [%N]0.85=-7, 400/T+2.09
6) It has been proved that oxygen has the stronger affinity to tantalum than carbon and nitrogen. in austenite.The preferential precipitation of a little amount of tapiolite [Fe, Ta (Nb)] O2 was recognized in all specimens.
