Effect of Morphology of Carbide and Nitride Precipitates on the Ductility of Fe-30 mass%Cr Alloys

Effects of morphology of carbide and nitride precipitates on the ductility of Fe-30 mass%Cr alloys containing C from 0.002 to 0.044 mass% and N from 0.004 to 0.13 mass% were investigated. Three kinds of processes including heat treatments and cold rolling were adopted to obtain different types of morphology of carbide and nitride precipitates. Tensile tests were carried out at temperatures ranging from 4.2 K to room temperature. Ductility was evaluated by transition temperatures in reduction of area.The results obtained are summarized as follows:(1) Aging after solution-treatment gives rise to grain boundary precipitation of raft-like carbide or continuous plate-like nitride, while annealing without solution-treatment produces coarse globular carbide particle (5 μm) or fine spheroidal nitride particle (≤0.5 μm) within the grains. Aging after cold rolling of solution-treated specimen produced fine spheroidal carbide particle (≤0.5 μm) within the grains.(2) Even a small amount of plate-like grain boundary nitride brings about a drastic increase in transition temperature. Initiation of microcracks for brittle fracture may be caused by decohesion at the interfaces between the matrix and grain boundary precipitates, due to their continuous flat plate-like morphology. On the other hand, the effect of raft-like grain boundary carbide is less remarkable than grain boundary nitride.(3) Fine spheroidal nitride and carbide particles within the grains do not exert any detrimental effect on transition temperature, while coarse globular carbide leads to moderate increase in transition temperature.(4) In order to obtain improved ductility, it is recommended to choose a process condition in which precipitates are dispersed as fine particles within the grains.