Figure 1 Fe-C phase diagram for Low-Ti steel showing the composition dependence of the formation temperature of titanium nitrides and carbonitrides.
Figure 3 compares the formation temperature of TiN precipitates in Low-Ti (red curves) and High-Ti (blue curves) steels. Higher Ti concentration increases the temperatures of TiN precipitation. It should be noted that the Ti/N ratio varies considerably with the Ti content. At a given Ti/N ratio, Ti nitrides precipitate in the High-Ti steels at temperatures approximately 70K higher than the Low-Ti steels. Considering the effect of diffusion on the precipitates size, TiN precipitates which form at lower temperatures are anticipated to be smaller in size. A study [19] on the effect of weld thermal cycles on precipitates of Ti-Vanadium (V)-microalloyed steels with Ti/N ratios of 0.8-1.8, indicated that only TiN precipitates survived high energy weld cycles and determined the Austenite grain size. Smaller, coherent precipitates with a higher concentration of V than Ti formed in these steels, but dissolved during the welding process. This is in agreement with the findings of this study, confirming that carbonitride precipitates which form at lower temperatures, are smaller in size and mostly dissolve during heat treatments, although the time is as limited similar to welding methods.
The optimum Ti/N ratio is found to be around stoichiometry [1, 9, 11, 20]; however, high contents of Ti and N promote the formation of coarse TiN precipitates which deteriorate toughness [1, 20]. Therefore, a reduced content of these elements is suggested while simultaneously retaining a minimum Ti/N ratio to improve the mechanical properties of these steels [1]. The presence of V, Nb, Molybdenum (Mo), Zirconium (Zr) and Boron (B) nitride forming elements in the microalloyed steels might vary the optimum Ti/N ratio due to interaction of N with a combination of these elements [20]. Therefore, calculated Fe-N phase diagrams for these alloys provide advice on the optimum Ti/N ratio.