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 stainless steels and Ni-base alloys
OF DOCTORAL DISSERTATION AALTO UNIVERSITY SCHOOL OF SCIENCE AND TECHNOLOGY P.O. BOX 11000, FI-00076 AALTO http://www.aalto.fi Author Mykola Ivanchenko Name of the dissertation Dynamic strain aging of austenitic stainless steels and Ni-base alloys Manuscript submitted 04.06.2010 Manuscript revised 22.09.2010 Date of the defence 19.11.2010 Monograph Article dissertation (summary + original articles) Faculty Faculty of Engineering and Architecture Department Department of Engineering Design and Production Field of research Engineering Materials Opponent(s) Prof. David Delafosse and Prof. Tuomo Tiainen Supervisor Prof. Hannu Hänninen Instructor Dr. Yuriy Yagodzinskyy Abstract Dynamic strain aging (DSA) affects mechanical properties of materials and promotes strain localization. DSA results in serrated plastic flow, which was observed by means of constant extension rate tensile (CERT) tests in different grades of AISI 316 austenitic stainless steel and Ni-base alloys Alloy 600 and Alloy 690 in the temperature range of 200 – 600 C and values of strain rate from 10 to 10 s. Negative strain rate sensitivity was reported for the studied materials at temperatures and strain rates where serrated plastic flow appeared. The map for the occurrence of serrated flow as a function of strain rate and temperature was built for the 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 Alloy 600 was reported. The activation enthalpies of nitrogen diffusion in AISI 316NG steel (140 kJ/mol) and carbon diffusion in Alloy 600 (162 kJ/mol), obtained by means of internal friction, correspond well to the activation 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 Alloy 600. 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 exponential decay function with characteristic times of 0.95 and 0.92 ks for AISI 316NG steel and Alloy 600, 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 the diffusive re-distribution of interstitial atoms in the DSA regime affected the deformation behavior of the material by restricting dislocation cross-slip, which in turn promotes strain localization, affecting the mechanical performance of the material.
Diffusion and electrotransport of carbon in γ-phase iron-nickel alloys
The diffusivity and electrotransport behavior of carbon in an iron-32.5 wt pct nickel alloy was investigated for the temperature range of 1220 to 1415 K for two carbon concentrations. The electrical resistivity was measured as a function of temperature between room temperature and 1625 K for carbon contents of 0.002, 0.10, and 0.21 wt pct. Values for the activation energy for diffusion of 32.0 ± 5.8 and 36.0 ± 5.2 kcal per mole were determined for alloys containing 0.10 and 0.21 wt pct carbon respectively, with slightly smaller values being found for the activation energy for electrotransport. The sign of the effective valence indicates that carbon migrates toward the cathode.