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 discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr7C3 particles were formed immediately beneath the surface. High aspect ratio Cr23C6 lamellar plates were formed deeper in the precipitation zone and were found to have a cube-cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.