Microstructural analysis of carbonitrided austenitic steels

An austenitic heat resisting steel containing 25% Cr and 19% Ni was interna lly carbonitrided in an atmosphere of 0.5% C3H6, 9.5% H-2 and 90% N-2. Imme diately below the alloy surface, globular precipitates were observed. At gr eater depth, two zones of lamellar morphology precipitates were observed to form. M7C3 was found directly beneath the surface and M23C6 was found clos er to the reaction front. The lamellar carbide morphology is more typically characteristic of the precipitates formed during nitriding rather than car burising, suggesting that nitrogen plays some role in the precipitation pro cess. A focused ion beam (FIB) miller was used to create thin, electron tra nsparent locations at specific regions in the precipitation zone in order t o characterise, via transmission electron microscopy, the crystallography a nd composition of the phases present. In this study, thin areas were produc ed in the lamellar region at the transition between the M23C6 and M7C3 regi ons. The composition, crystal structure, and orientation relationships are reported and compared to those found previously at both the internal precip itation front and the sample surface. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.