Precipitation of Second Phases in High-Interstitial-Alloyed Austenitic Steel

Abstract

The precipitation reaction of an austenitic stainless steel containing N + C was investigated using transmission electron microscopy. The main precipitate formed during isothermal aging at 1123 K (850 degrees C) was M(23)C(6) carbide, and its morphology gradually changed in a sequence of intergranular (along grain boundary) -> cellular (or discontinuous) -> intragranular (within grain interior) form with aging time. Irrespective of different morphologies, the M(23)C(6) was consistently related to austenite matrix in accordance with the cube-on-cube orientation relationship. Based on the analysis of electron diffraction, two variants of intragranular M(23)C(6) were identified, and they were related to each other by twin relation. Prolonged aging produced other types of precipitates-the rod-shaped Cr(2)N and the coarse irregular intermetallic sigma phase. The similarities and differences in precipitation behavior between N only and N + C alloyed austenitic stainless steels are briefly discussed.