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.
Manifestations of DSA in austenitic stainless steels and inconel alloys (1-2013)
Dynamic strain aging (DSA) affects mechanical properties of the materials and promotes strain localisation. DSA results in serrated plastic flow, which was observed by means of constant extension rate tensile (CERT) tests in commercial AISI 316NG austenitic stainless steel and Inconel 600 and Inconel 690 Ni-base alloys at temperature range of 200 – 600 C and values of strain rate from 10 to 10 s. Negative strain rate sensitivity, which is another manifestation of DSA, was also reported for the studied materials at temperatures and strain rates where serrated plastic flow appeared. The map for occurrence of serrated flow as a function of strain rate and temperature was built for these materials. The activation enthalpies of dynamic strain aging appearance were found to be 120 kJ/mol for the austenitic stainless steel and 159 kJ/mol for both Ni-base alloys. The internal friction (IF) peak associated with the interstitial atoms of nitrogen and carbon in the solid solution of AISI 316NG austenitic stainless steel and Inconel 600 alloy was reported. The enthalpies of nitrogen diffusion in AISI 316NG steel (140 kJ/mol) and carbon diffusion in Inconel 600 alloy (162 kJ/mol), obtained by means of internal friction, correspond well to the enthalpies of DSA appearance. The height of the internal friction peak increases depending on the pre-straining conditions in the similar way for AISI 316NG steel and Inconel 600 alloy. Annealing in situ at the IF peak temperature results in the decay of the IF peak enhanced by pre-straining for both materials. At the initial stage, the peak annealing processes can be described as an elemental exponential decay function with characteristic times of 0.95 and 0.92 ks for AISI 316NG steel and Inconel 600 alloy, respectively. Transmission electron microscopy was performed on the specimens of AISI 316NG steel after CERT tests at temperatures of 400 and 200 C, where serrated and smooth plastic flow was observed, respectively. Long-range planarity was observed in the dislocation structures of the specimen tested at 400 °C. The microstructure of the specimen strained at 200°C exhibited cellular dislocation structure. It was concluded, that 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.
A Study of the Changes Over Time in State Anxiety in a Computer-Assisted Instructional Program
The purposes of this study were to analyze (1) the changes over time in the state anxiety level of children in a computer-assisted program of reading instruction and in classroom reading instruction, (2) the changes in state anxiety patterns as related to gender, achievement level, and ethnicity, and (3) the difference in anxiety levels of the students while in computer-assisted reading instruction and classroom reading instruction.