The development of aligned precipitates during internal carbonitridation of Fe-Ni-Cr alloys

An austenitic Fe-25Cr-19Ni stainless steel alloy was carbonitrided at 1,000 degrees C in an atmosphere with a(c) = 1 and P(N-2)=0.9atm to form two dis crete precipitation zones. Local equilibrium between the precipitates and t he austenite matrix carbon activity was achieved throughout the reaction zo ne. Small, globular Cr7C3 particles were formed immediately beneath the sur face. High aspect ratio Cr23C6 lamellar plates were formed deeper in the pr ecipitation zone and were found to have a cube-cube orientation relationshi p with the austenite matrix. The inward growth of these carbides was facili tated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropr iate orientation and accelerated the segregation of chromium to the carbide tips.