To comply with more stringent environmental and fuel consumption regulations in recent years, automotive gasoline engines equipped with turbochargers are increasingly used to improve fuel efficiency. As a result, exhaust gas temperatures are now reaching 1050 degrees C, about 200 degrees C higher than the conventional temperature. Hence, there is an urgent demand in automobile industries to develop novel and economic austenitic heat-resistant steels that are durable against these increased temperatures. In this study, the effects of N addition on creep behavior at 1000 degrees C and 50 MPa are investigated in a series of Nb-bearing austenitic heat-resistant cast steels, which are developed for exhaust component applications. Microstructures before and after creep rupture tests are carefully characterized to illustrate the microstructural evolution during creep deformation. The results of creep tests show approximately an order of magnitude increase in the minimum creep rate among the experimental alloys with variations of N addition. Microstructural analyses indicate that the morphology of NbC and Nb(C, N) is changed from "Chinese-script" to mixed flake-blocks, and then to faceted blocks as N additions increase. The best creep property occurs in an alloy with "Chinese-script" NbC, which could effectively strengthen the grain boundaries and inter-dendritic regions. The Cr-rich phases are adverse to creep properties, in particular those coarsened and coalesced phases along grain boundaries. They act as crack sources and accelerate the propagation of creep cracks. Moreover, the secondary precipitation of Cr-rich phase results in a significant decrease of C concentration in the matrix and thus reduces the solution strengthening ability during creep deformation. This study suggests that the strengthening of these austenitic cast steels can be achieved through the exploit of primary NbC and Nb(C, N) and the elimination of Cr-rich phases, and therefore, N additions should be strictly controlled.
