Inclusions are believed to be the p

Inclusions are believed to be the precursor sites for pitting corrosion, pits are often initiated at the matrix/inclusion interface. Once it occurs, the cavity will form and propagate through the inclusion dissolution.[12] Additionally, the fatigue crack generally initiates from a large inclusion and the stress localization at the inclusion/matrix interface is the origin of fatigue cracking. Once a larger inclusion debonded at the interface, a larger crack forms and propagates quickly. It is also found that fatigue strength increases with decreasing inclusion size.[13,14] Thus, it is necessary to study the size, composition and distribution of inclusion, to elucidate the correlation between inclusion and properties of the steel.Two kinds of inclusions from the samples with and without Nb were observed through scanning electron microscope. The inclusions found in 409Ti are mainly TiN with a tetragonal shape. While, inclusions obtained in 409Ti–0.30%Nb were identified as irregular-shaped (Nb,Ti)(C,N) carbonitride. The chemistries ofboth the core and boundary parts are investigated through EDS analysis. The core part revealed to be Ti-rich (Nb,Ti)(C,N), and the composition of boundary part proved to be (Nb,Ti)C (as shown in Figure 7).Then, inclusion statistics were performed under a scanning electron microscope with a magnification of 50 times. Fifty inclusions were identified in the 409Ti stainless steel sample, and the average size is 4.09mm2. However, for 409Ti–0.15%Nb stainless steel and 409Ti–0.30%Nb stainless steel, the statistical analysis of 53 inclusions indicated that the average area sizes decrease to 0.61 and 0.62mm2, respectively. Moreover, the maximum size ofthe inclusions also decreases significantly with the increase of Nb content (as shown in Figures 8 and 9). It is obvious that Nb addition contributes to the size refinement of inclusions of 409 ferritic stainless steels. However, the average size of inclusions is not in inverse proportion to Nb content. When the concentration ofNb reaches a threshold, its effect will have a downtrend, which could partially explain the results of cyclic corrosion experiment.The large inclusions of 409Ti can not only easily induce pitting but also make the Cr2O3 passive film suffer from discontinuity at the location of inclusion. Inclusions are incoherence with ferritic stainless steel in crystal structure and crevice corrosion may happen between inclusion and matrix which make the self-reparation of the passive film becomes difficult.[9] Thus, 409Ti stainless steel is more sensitive to condensate corrosion than 409Ti–Nb stainless steels in cyclic corrosion test. And the higher fatigue strength ofNb-added 409 stainless steels is partially due to the smaller inclusions.