Changes of microstructures and high temperature properties during high temperature service of Niobium added ferritic stainless steels

To improve the fuel economy and clean the exhaust gas of automobiles, the temperature of exhaust gas is getting higher and higher. Niobium added ferritic stainless steels are often being used in automotive exhaust systems, because of their excellent heat resistant properties, especially thermal fatigue resistance, which is very important for materials of exhaust manifold. However, coarse precipitates containing niobium, which cause degradation in high temperature strength and thermal fatigue resistance, are unavoidable during high temperature service. In this study, changes of microstructures and high temperature properties in high temperature aging were investigated using several Nb added ferritic stainless steels. It has been found that the microstructure stability of Nb–Ti–Mo alloyed steels in high temperature aging is superior to that of Nb added steels. The microstructure stability leads to less degradation in high temperature strength during high temperature aging and to longer thermal fatigue lives of Nb–Ti–Mo alloyed steels than in Nb added steels.