Precipitate morphologies and growth kinetics in the internal carburisation and nitridation of Fe–Ni–Cr alloys

Abstract

Model Fe–Ni–Cr alloys were carburised in gas of unit carbon activity, nitrided at p N 2 =0.9 atm, sequentially carburised and nitrided, and simultaneously carburised and nitrided at 1000°C. Internal precipitation kinetics were always parabolic. Carburisation rapidly produced numerous small carbides, and nitridation led to the slow growth of large, sparsely distributed and strongly directional precipitates. In both cases, alloy penetration rates were controlled by volume diffusion of oxidant. The low stability of Cr 2N led to variation in precipitate volume fraction with depth, the distribution being controlled by the diffusion path, not nucleation. Carburisation of nitrided alloys converted prior nitrides to carbides whilst retaining the parent precipitate morphology. The process was diffusion controlled and slow, reflecting the high carbon activity at the nitride–carbide interface. Carbonitriding produced large volume fractions of carbide with, in some cases, nitride-like morphologies, but no obvious nitride phase. Rapid rates reflected fast diffusion along the aligned precipitate-matrix interfaces.