Effect of titanium carbide precipitates on the ductility of 30 mass% chromium ferritic steels

The effect of morphology of Ti carbides on the ductility of 30 mass%Cr ferritic steels containing C and Ti was investigated by tensile test at low temperatures. C and Ti contents were varied from 0.011 to 0.071 mass%, and from 0.094 to 0.60 mass%, respectively. The ratio Ti/C was about nine. Two kinds of heat treatments were adopted to obtain different types of morphology of Ti carbide precipitates, namely numerous fine grain boundary Ti carbide precipitates about 0.05 mum in diameter and coarse globular Ti carbide particles in the 1.5 pro size range. The tensile ductility was evaluated by transition temperatures in reduction of area. The numerous fine grain boundary Ti carbide precipitates lead to an increase in the transition temperature more than uniformly dispersed coarse globular Ti carbide particles. The former brings about the increase in the amount of intergranularly fractured surface in dimple and brittle fracture more than the latter. Cluster-like coarse globular Ti carbides at grain boundaries cause a scatter band of the transition temperature. Microfractographic examinations reveal that initiation of microcracks for brittle fracture is considered to be caused by intergranular fracture cracks which are promoted by decohesion at the interface between the matrix and many grain boundary Ti carbide precipitates and by decohesion at the interface between the matrix and cluster-like coarse globular Ti carbides at grain boundaries.