Since the 0.2% proof strength improved significantly at 600 C for 409 stainless steels with the addition of Nb, aging treatment was performed at 600 C for 2 h. Before aging, except for some large inclusions, nearly no nano-scale precipitates could be found under electron microscope. After aging, precipitates in the size range of 20–50nm were observed. Figure 10 shows the morphologies and the compositions of precipitates observed in Ti–0.15%Nb and Ti–0.3%Nb 409 stainless steels after aging treatment. As can be seen in Figure 10a and b, a spherical precipitate witha diameter of50nmwasobserved in409Ti–0.15% Nb stainless steel, and the Nb, Ti, and C peaks in EDX analysis reveal that the precipitate is a (Nb,Ti)C carbide.Meanwhile, in 409 Ti–0.3%Nb stainless steel, the higher Nb content contributes to more precipitates. A precipitation band containing several spherical and irregular-shaped precipitates in the size range of20– 50nm was observed and the EDX result showed an extremely strong diffraction peak for NbC as shown in Figure 10c and d. In the tensile test, the samples were heated to 600 C within 1 h and held for 10 min before applying strain, and strainwas thenapplied. Therefore, it is reasonable to believe that the heating process and/ or applied strain was conducive to precipitation of abovementioned nano-scale particles. Thus, the improvement of 0.2% proof strength and fatigue resistance with the Nb content increasing were mainly caused by the precipitation strengthening effect offine NbC particles.[15–18]