Microstructural characterization of a carbonitrided heat resisting alloy using focused ion beam-based techniques

Abstract

An austenitic Fe–25Cr–20Ni (wt.%) alloy was first nitrided in N₂–H₂ then carburised at 1000°C. The nitridation produced lamellar internal precipitates of Cr₂N which grew by discontinuous precipitation at a recrystallised austenite boundary formed at the precipitation front. Subsequent carburisation converted the Cr₂N to a mixture of chromium-rich M₇C₃+CrN, and released nitrogen into the austenite matrix. The submicron CrN precipitates were stabilized by a matrix supersaturated with nitrogen and exhibited a strong orientation relationship with the surrounding austenite. The rejected nitrogen diffused deeper into the alloy to form new, more finely spaced Cr₂N lamellae. Carbon diffusion overtook the nitride structure and precipitated finely spaced, lamellar M₂₃C₆ by a discontinuous precipitation process. This process is available to the carbides only when a prior boundary is present. This phenomenon, and the ability of nitridation to form a boundary and the inability of carburisation to do so, are interpreted using the energetics of nucleation.

Keywords:

• microstructure
• carbonitrided heat resisting alloy
• focused ion beam