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C-Fe-N-Nb-Ti (Carbon-Iron-Nitrogen-Niobium-Titanium) V. Raghavan

The information on this quinary system is limited to the recent thermodynamic assessment by [2001Lee] and some experimental data on the composition of the Nb-Ti carbonitride in equilibrium with austenite at various temperatures in microalloyed steels.

Binary Systems There are 10 binary systems of relevance to this quinary system. The reader may refer to the brief discussion of eight of these binary systems in the updates of the lower order systems that precede this review, e.g., C-Fe-N-Nb and C-Fe-N-Ti. In the Nb-Ti system [1998Sau], Nb and ␤Ti (both bcc) form a continuous solid solution. The (␤Ti) → (␣Ti) transformation temperature is lowered by the addition of Nb. The thermodynamic descriptions of the binary systems chosen by [2001Lee] are: C-Fe [1985Gus], C-Nb [2001Lee], C-Ti [1999Dum], Fe-N [1991Fri], Fe-Nb [2001Lee], Fe-Ti [1998Dum], N-Nb [1996Hua], Nb-Ti [1998Sau], and N-Ti [1996Zen].

Ternary Systems There are 10 ternary systems that are relevant to the assessment of this quinary system: C-Fe-N, C-Fe-Nb, C-Fe-Ti, C-N-Nb, C-N-Ti, C-Nb-Ti, Fe-N-Nb, Fe-N-Ti, Fe-Nb-Ti, and N-Nb-Ti. For brief descriptions of these, see the lower order systems that precede this review, e.g., C-Fe-N-Nb and C-Fe-N-Ti. [2001Lee] assessed or reassessed nine of these ternary systems except C-Fe-N. The aim of the large number of reassessments of the lower order systems was to obtain the best possible fit of the computed quinary data with the experimental results. Due to the scatter in the experimental data, there can be a number of thermodynamic descriptions of a binary system. For example, six different sets of thermodynamic parameters are known for the Fe-Ti system! Many of the descriptions fail in predicting the ternary properties when combined with the assessments of other binary systems. Similarly, a ternary description can fail in describing higher order systems when combined with the description of other ternary systems. Lee assumed that quaternary and higher order interactions have a negligible effect on the Gibbs energy and, therefore, can be ignored. This assumption appears valid for the C-Fe-N-Nb and C-Fe-N-Ti quaternary systems, where NbC-NbN and TiC-TiN form continuous solid solutions and can be assumed to behave ideally. In this quinary system, however, NbC-TiN do not form a continuous solid solution and a miscibility gap exists between them [2001Ino].

The Quinary Phase Equilibria [1998Zaj] prepared six Nb-microalloyed steels having various levels of Nb, C, and N. Also, two more steels containing additional Ti and Ti+V, respectively, were prepared. The full chemical analysis of the eight steels is listed by [1998Zaj]. All Nb-microalloyed steels contained residual Ti of about 0.003% and residual V of about 0.010-0.015%. The eight steels were solution treated at 1300-1350 °C and water quenched. They were then annealed for 24-48 h in the temperature range of 1300-850 °C. The amount of Nb present in Nb(C,N) was measured by the inductively-coupled plasma emissi