Microstructural and Mechanical Analysis of Intermediate Temperature Equilibrium Phase Formation in Nitronic 60 Stainless Steel

Nitronic 60 is a highly alloyed, nitrogen-strengthened austenitic stainless steel containing additions of manganese and silicon (nominally 8 and 4 wt.%, respectively) which provides excellent galling resistance relative to conventional austenitic stainless steels. While Nitronic 60 is not conventionally heat treatable, applications exist where Nitronic 60 parts within an assembly may be exposed to temperatures between 500-900°C when heat treated along with age-hardenable alloys. These intermediate temperature heat treatments present a general risk for equilibrium phase formation (ex. sigma phase, carbides, etc.) in austenitic stainless steels. Limited information, however, exists in the technical literature for nitrogen-strengthened austenitic stainless steels including Nitronic 60 examining the effects of these nonconventional heat treatments on microstructure and mechanical properties. This works aims to characterize equilibrium phase formation in Nitronic 60 exposed to various time-temperature histories. Optical and electron microscopy in conjunction with analytical electron microanalysis were used to characterize the morphology and composition of sigma phase networks for alloy compositions of Nitronic 60 containing varying amounts of retained delta-ferrite (up to 2 vol.%). Following 1 hour exposures between 800-900°C, chromium- and silicon-rich sigma phase formed along austenite grain boundaries as well as within aligned delta-ferrite stringers. Sigma-phase formation in Nitronic 60 resulted in a precipitous drop in tensile ductility with some specimens exhibiting reduction in area values as low as 10%. Microstructural and fractographic evaluation of tested tensile specimens revealed preferential crack propagation along sigma phase-decorated grain boundaries. The results of this work establish guidelines for thermal processing of assemblies containing Nitronic 60 parts to ensure mechanical properties, especially ductility, are not diminished. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.