Figure 7 The phase diagram of the Low-Nb steel as a function of (a) carbon and (b) nitrogen concentration
The Fe-C phase diagrams of the low-Nb steel (pink) and High-Nb steel (blue) are compared in Figure 8 . The concentration of nitrogen (0.0021 wt%) and niobium (0.001 wt%) in the Low-Nb steel, is lower than High-Nb steel which contain 0.0048 wt% and 0.11 wt% nitrogen and niobium respectively. This results in precipitation of titanium nitrides at lower temperatures in the Low-Nb steel compared to the High-Nb steel with larger nitrogen and niobium concentrations. It suggests that all precipitates including carbides and nitrides, will dissolve in the matrix where the Low-Nb steel is heat treated above ~ 1673K. This will cause a rapid austenite grain growth.