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.