Cellular precipitation involving both substitutional and interstitial solutes: cellular precipitation of Cr 2N in CrNi austenitic steels

Abstract

This paper describes a part of our series of studies on the cellular precipitation involving the diffusion of both substitutional and interstitial solutes. Following a brief review of a few previous studies on the cellular precipitation of alloy carbides and nitrides in steels and superalloys, the reason is given why the cellular precipitation of Cr 2N in CrNi austenitic stainless steels with high nitrogen concentrations is best suited for the study on this type of cellular precipitation. The microstructural features of the present cellular precipitation of Cr 2N, which have largely been ignored in the previous studies, are illustrated. Experimentally determined growth features are summarized in well-controlled specimens with four concentration levels of nitrogen. Several growth features were measured on the same specimens. They are the volume fraction of the cellular precipitate, the free surface area of the moving cell boundary, the average migration rate of the moving cell boundary, the interlamellar spacings and the lattice parameter of both the untransformed austenite matrix and the austenite within the cell. The concentration profiles of metallic elements as well as of nitrogen were also measured. These growth features are compared with those established in the “typical” cellular precipitation in binary substitutional systems, such as PbTi, FeZn and AlMg alloys. A qualitative explanation is offered for the observed non-steady state growth features of the present cellular precipitation. They are the transfer of the faster-diffusing element, nitrogen, from the untransformed matrix to the cell via long-range volume diffusion, the deceleration of the migration rate of the moving cell boundary with reaction time and the stoppage of the migration of the cell boundary at a stage when nitrogen supersaturation still remained in the untransformed matrix. The importance of the volume diffusion of the slower-diffusing element, chromium, ahead of the moving cell boundary is emphasized.