Precipitation behavior of the super austenitic stainless steel SANICRO® 35 and the effect on impact toughness and pitting corrosion resistance

Abstract: This research extended the knowledge of the solid phase transformation and the resulting influence on impact toughness and pitting corrosion resistance in super austenitic stainless steel (SASS) SANICRO® 35. A time-temperature-transformation diagram (TTT diagram) was assembled by performing isothermal heat treatments in the temperature range of 650-1050 °C for different periods of time, ranging from 5 min to 500 min. Microstructural analysis via LOM-DIC, SEM-EDS shows that the nose temperature of dominating σ phase is located in between 900-950 °C. Minor nitrides including π phase and Cr2N with the nose temperature of 900 °C and 850 °C, respectively, were detected after prolonged heat treatment times. Area fraction of precipitates was calculated by analyzing micrographic images in the software ImageJ. Charpy impact tests indicate that the impact toughness degrades with increasing area fraction of precipitates but at a higher rate at the early stage of precipitation. Despite a much-lessened area fraction, fine precipitates decorating the grain boundaries in a continuous pattern impose significant negative effect on impact toughness. Pitting corrosion resistance was indicated by critical pitting temperature (CPT) as per ASTM G150mod (3M MgCl2). Pitting corrosion resistance deteriorated with increasing amount of σ phase due to the Cr- and Mo-depleted surrounding area, but it is more dependent on the distribution pattern of precipitates, as well as the secondary phase type. The lowest CPTs were measured after heat treatment for 500 min at 800 °C and 850 °C where nitrides including Cr2N and π phase were formed and the small precipitates were distributed on grain boundaries continuously. Auxiliary simulation of TTT diagram via TC PRISMA shows drastic variation from experimental results in regard of time scale. The enhancement pre-factor for the interfacial mobility and interfacial energy can be modified to approach the experimental results.