A Study of Submicron Grain Boundary Precipitates in Ultralow Carbon 316LN Steels

This article reports our efforts in characterization of an ultralow carbon 316LN-type stainless steel. The carbon content in the material is one-third that in a conventional 316LN, which further inhibits the formation of grain boundary carbides and therefore sensitizations. Our primary effort is focused on characterization of submicron size precipitates in the materials with the electron backscatter diffraction (EBSD) technique complemented by Auger electron spectroscopy (AES). Thermodynamic calculations suggested that several precipitates, such as M(23)C(6), Chi, Sigma, and Cr(2)N, can form in a low carbon 316LN. In the steels heat treated at 973 K (700 A degrees C) for 100 hours, a combination of EBSD and AES conclusively identified the grain boundary precipitates (a parts per thousand yen100 nm) as Cr(2)N, which has a hexagonal closed-packed crystallographic structure. Increases of the nitrogen content promote formation of large size Cr(2)N precipitates. Therefore, prolonged heat treatment at relatively high temperatures of ultralow carbon 316LN steels may result in a sensitization.