Dynamic Strain Ageing and Eac of Deformed Nitrogen-alloyed Aisi 316 Stainless Steels

Intergranular stress corrosion cracking has occurred in BWR environments in non-sensitised, cold-worked austenitic stainless steel materials. The affecting parameters are so far not fully known, but deformation mechanisms may be decisive. The effect of cold deformation and nitrogen content on the deformation behaviour of austenitic stainless steels was investigated. The materials were austenitic stainless steels of AISI 316L type with different amounts of nitrogen (0.03...0.18%) and they were deformed in tension 0, 5 and 20% before further investigations. The investigations were focused on the dynamic strain ageing (DSA) behaviour as a function of nitrogen level and deformation. A few crack growth rate measurements are performed on nuclear grade AISI 316NG stainless steel with different degrees of deformation (0, 5 and 20%). The effects of DSA on mechanical properties of these materials were evaluated based on peaks in tensile strength and minimum in ductility in the DSA temperature range. Also the strain rate sensitivity in the DSA temperature range was investigated. Internal friction measurements were performed in the temperature range of -100...600 C for determining nitrogen interactions with other alloying elements and dislocations. The results show effects of nitrogen on the stainless steel deformation behaviour, i.e. clear indications of dynamic strain ageing: serrated yielding, negative strain rate sensitivity and changes in the internal friction behaviour as a function of nitrogen content and amount of deformation. The apparent activation energy calculated from the DSA serration occurrence map corresponds to that of nitrogen diffusion. Fully intergranular cracking was obtained in the 20% deformed, nonsensitised AISI 316NG stainless steel in simulated BWRenvironment. The crack growth data and the mechanism are discussed based on the unstable plastic flow of the material.