Assessment of the Fe-Ti-C system, calculation of the Fe-TiN system, and prediction of the solubility limit of Ti(C,N) in
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I.
INTRODUCTION
TITANIUM has a very strong affinity for carbon and nitrogen and combines with these elements to form titanium carbonitride, Ti(C,N), in a complete range from TiC to TiN. Due to the high thermal and chemical stability of the carbonitride, its solubility limit in Fe is very low and, as a result, particles of titanium carbonitride are precipitated in steels at very low concentrations of titanium even at high temperatures. These precipitates retard recrystallization and grain growth during thermomechanical treatments, which improves the transformation characteristics of the steel and leads to the superior mechanical properties of microalloyed steels. In stainless steel, Ti is added in order to avoid the detrimental precipitation of Cr carbides in the grain boundaries after welding, which would otherwise reduce the corrosion resistance and possibly lead to intercrystalline corrosion. In order to benefit from the advantages of Ti additions, it is, therefore, of considerable technical importance to predict the precipitation behavior of Ti(C,N) in steel. In order to perform calculations in the Fe-Ti-C-N system, the underlaying subsystems must be assessed and combined, by thermodynamic modeling, to a consistent thermodynamic description. In order to obtain good predictions in the quaternary system, it is, thus, important to have good descriptions in all underlying binary and ternary systems. In fact, two of the four ternary systems have been assessed. The Fe-C-N system was assessed by Du[1] and the Ti-C-N system by Jonsson.[2] The two remaining systems, the FeTi-C and Fe-Ti-N, will be treated in the present article. Today, no complete thermodynamic treatment of the FeTi-C system seems to exist. In the work by Ohtani et al.,[3] the Fe-Ti system was considered on the Fe-rich side only up to 33 at. pct Ti. In addition, a considerable solubility of Fe in TiC was predicted, which is very unlikely. In other treatments, only parts of the system were modeled. The liquid properties, for instance, were modeled by Bouchard and Bale,[4] and the fcc(Fe)-TiC equilibrium was calculated by Balasubramanian and Kirkaldy.[5] STEFAN JONSSON, Group Leader-Heat Treatment, is with the Department of Technological Properties, Swedish Institute for Metals Research, Drottning Kristinas va¨g 48 S-114 28 Stockholm, Sweden. Manuscript submitted June 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
The Fe-Ti-N system has been modeled by Ohtani and Hillert,[6] but with rather strange results. The Fe solubility in TiNx (where x denotes the nonstoichiometric nitride) is approximately 10 at. pct Fe, and for the stoichiometric nitride TiN, the solubility is as high as 15 at. pct Fe. These results seem very unlikely. In the present work, the Fe-Ti-C and Fe-Ti-N systems will be assessed using recent descriptions by Jonsson for the Fe-Ti,[7] Ti-C,[8] and Ti-N[9] systems. The reassessed descriptions will then be combined with the descriptions of the Fe-C-N and Ti-C-N systems mentioned earlier and, finally, the solubility limit of Ti(C,N) in
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