Quantitative mapping of time-temperature-strain effects on sensitization of type 316 stainless steels

Abstract

Austenitic stainless steels (SS) are susceptible to the development of sensitization, or grain boundary chromium depletion (GBCD), during thermomechanical (TM) processing and fabrication treatments. Precipitation of chromium-rich M{sub 23}C{sub 6} carbides occurs on grain boundaries during TM treatment, leading to chromium-depletion in the vicinity of the interface. The induction of sensitization has been a recurring cause for intergranular (IG) corrosion and IG stress corrosion cracking (SCC) of primary and secondary power plant components for many years, and is also now being recognized as a potential cause for failure in SS containers used for storage and transport of nuclear and toxic waste materials. Research on sensitization development is, thus, important for understanding phenomena of IG corrosion and IGSCC in SS. Evaluation of thermal and strain cycle effects on carbide precipitation-chromium depletion development in SS also provides a key towards fundamentally understanding mechanisms of precipitate nucleation-growth in materials. While thermal effects on GBCD in SS have been studied for a range of isothermal and continuous cooling heat treatments, research on strain-induced sensitization has only been performed for limited strain and heat treatment schedules. Bain et al observed that straining of Type 304 SS about 50% caused development of accelerated, but less severe,more » sensitization due to precipitation of carbides within transgranular (TG) versus grain boundary regions. Later Tedmon, Vermilyea and Broecker found that TG depletion could also be rapidly desensitized (healed) during heat treatment. Bariant and Ritter observed TG corrosion in heavily deformed Types 304 and 316 SS during (450-800{degrees}C) heat treatment, but considered the onset of sensitization to be enhanced over annealed SS due to absence of previously reported rapid healing.« less