EFFECT OF MORPHOLOGY OF CARBIDE AND NITRI DE PRECIPITATES ON THE DUCTILITY OF FE-30 MASS PERCENT CR ALLOYS

Effects of morphology of carbide and nitride precipitates on the ducti lity of Fe-30 mass%Cr alloys containing C from 0.002 to 0.044 mass% an d N from 0.004 to 0.13 mass% were investigated. Three kinds of process es including heat treatments and cold rolling were adopted to obtain d ifferent types of morphology of carbide and nitride precipitates. Tens ile tests were carried out at temperatures ranging from 4.2 K to room temperature. Ductility was evaluated by transition temperatures in red uction of area. The results obtained are summarized as follows: (1) Ag ing after solution-treatment gives rise to grain boundary precipitatio n of raft-like carbide or continuous plate-like nitride, while anneali ng without solution-treatment produces coarse globular carbide particl e (5 mu m) or fine spheroidal nitride particle (less than or equal to 0.5 mu m) within the grains. Aging after cold rolling of solution-trea ted specimen produced fine spheroidal carbide particle (less than or e qual to 0.5 mu m) within the grains. (2) Even a small amount of plate- like grain boundary nitride brings about a drastic increase in transit ion temperature. Initiation of microcracks for brittle fracture may be caused by decohesion at the interfaces between the matrix and grain b oundary precipitates, due to their continuous flat plate-like morpholo gy. On the other hand, the effect of raft-like grain boundary carbide is less remarkable than grain boundary nitride. (3) Fine spheroidal ni tride and carbide particles within the grains do not exert any detrime ntal effect on transition temperature, while coarse globular carbide l eads to moderate increase in transition temperature. (4) In order to o btain improved ductility, it is recommended to choose a process condit ion in which precipitates are dispersed as fine particles within the g rains.