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.
Dynamic Strain Aging of Austenitic High Nitrogen Cr-Ni and Cr-Mn Steels
Dynamic strain aging (DSA) of solution annealed Cr-Ni and Cr-Mn type austenitic high nitrogen steels (HNS) was investigated by means of tensile tests. Nitrogen content of the steels varied from 0.26 to 1.00 wt. %. Tensile tests were performed at strain rates from 0.00005 to 0.03 1/s between room temperature and 800 °C. Austenitic HNSs exhibit DSA at elevated temperatures, which is caused by interactions between diffusing solute atoms and moving dislocations. Low activation energies for plastic deformation in the studied steels indicate that the diffusion mechanism is pipe diffusion below 600 °C, and also bulk diffusion at 800 °C. An activation energy for nitrogen diffusion of about 1.95 eV was determined by internal friction measurements.
Activation energy for serrated flow in a 15Cr5Ni Ti-modified austenitic stainless steel
Serrated flow in a 15Cr15Ni titanium-modified austenitic stainless steel has been investigated at a wide range of temperatures (300–1023 K) and strain rates (10−5−10−2s−1). Three regimes of serrated flow have been identified as low temperature type A (LT-A), high temperature type A (HT-A) and high temperature type C (HT-C). Different methods suggested in the literature for the determination of activation energy, Q were employed and Q was obtained aas 115 ± 9, 140 ± 7 and 178 ± 7 kJ/mol for LT-A, Ht-A and HT-C regimes respectively. Based on the Q values, the mechanisms responsible for the three regimes of serrated flow were identified as vacancy migration for LT-A, C (or N) as controlling diffusing species to form a pair with vacancy causing Schoeck-Seeger locking for HT-A regime and interaction of Ti with C (or N) for HT-C regime. Titanium was found to influence the serrated flow in all regimes.