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.
Microstructural manifestation of dynamic strain aging in AISI 316 stainless steel
Abstract Dynamic strain aging (DSA) affects a material’s mechanical behaviour. In the current study, the deformation microstructures of AISI 316 stainless steel specimens were examined after tensile testing at several temperatures both in and out of the DSA-regime (200, 288 and 400 °C), and the effect of straining on free nitrogen evolution at those temperatures was studied by Anelastic Mechanical Loss Spectrometry (Internal Friction). Analysis of the nitrogen-induced Snoek-like peak after straining indicated that DSA behaviour is associated with the formation of sub-micron scale, nitrogen-enriched zones in the austenitic matrix. Long-range planarity was observed in the dislocation structures at 400 °C and short-range planarity at 288 °C, but at 200 °C the microstructure exhibited cellular dislocation structure. Diffusion re-distribution of nitrogen in the DSA-regime affected the deformation behaviour of the material by restricting cross-slip, which in turn promotes strain localization, degrading the mechanical performance of the material.
Structural and analytical electron microscopy of dislocation loops and precipitates formed during ageing of an α‐bombarded MnCr austenitic steel
Solution annealed samples of the MnCr austenitic steel Nitronic 32 are uniformly implanted at 310 K with 500 atppm He corresponding to a dose of ≈ 0.1 dpa. Specimens are subsequently aged, one set at 773 K and another set at 1023 K; the evolution of the dislocation structure and the related segregation and precipitation phenomena are examined by TEM, EELS, and electron diffraction. Short annealing (2 h) at 1023 K lead to the formation of Frank interstitial dislocation loops which are found enriched in N and Cr. After longer annealing (25 h) at 1023 K, Cr2N precipitated on the loops, small He bubbles becomes visible in the matrix, whereas no lamellar precipitation is observed in contrast to unirradiated specimens. 
 
 
 
Losungsgegluhte Proben von austenitischem MnCr-Stahl Nitronic 32 werden bei 310 K mit 500 atppm He implantiert. Die entsprechenden Strahlungsschaden betragen ≈ 0,1 dpa. Die Proben werden anschliesend bei 773 und 1023 K warmebehandelt. Die Entwicklung der Versetzungsstruktur und die damit zusammenhangenden Segregations- und Ausscheidungsphanomene werden mittels TEM, EELS und Elektronenbeugung untersucht. Kurze Anlaszeiten (2 h) bei 1023 K fuhren zur Bildung von Frankschen Zwischengitterversetzungsringen, in denen N und Cr angereichert sind. Bei langerem Anlassen (25 h) bei 1023 K scheidet sich Cr2N an den Versetzungsringen aus und kleine He-Blasen werden in der Matrix sichtbar. Im Gegensatz zu unbestrahlten Proben wird keine lamellare Ausscheidung beobachtet.