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.
Activation energy calculation and dynamic strain aging in austenitic stainless steel
Dynamic strain aging (DSA) in austenitic stainless steel has been reported on numerous occasions since the paper of Tamhankar et al in 1958. The effect has been attributed to carbide precipitation and to the interaction of atmospheres of substitutional or interstitial atoms with moving dislocations. The theories developed to explain the phenomenon are based on different physical concepts. Some are based on Cottrell's theory and consider that a slow moving dislocation can attract solute atoms. Vacancy assisted diffusion of substitutional atoms and Snoek ordering locking mechanisms have also been used to explain DSA. The consideration that a dislocation moves in a discontinuous manner, being temporarily arrested periodically at obstacles, as in a dislocation forest, plays an important role in explaining the phenomenon. Despite all these studies, there is no single model describing the dynamic strain aging of austenitic stainless steel, because of the large temperature range over which the phenomenon is observed, and in which different mechanisms and atomic species may be involved. Illustrative of the complexity that may be involved with DSA mechanisms in stainless steels are the two temperature ranges for the occurrence of the Portevin-Le Chatelier effect and the athermal behavior for the end of the first,more » shown previously for 304 steel. In this paper the results of tensile tests on 303 stainless steel are reported over the temperature range 200--800 C at strain rates from 3.5 [times] 10[sup [minus]5]s[sup [minus]1] to 3.5 [times] 10[sup [minus]2]s[sup [minus]1]. Analysis is made of the onset and end of serrated flow as a function of temperature and strain rate, and the results compared with results for 304 steels of different interstitial contents, the data being consistent for a wide range of interstitial levels. The activation energy values derived are compared with those for 304 steels and with data from the literature.« less
Compressive and flexural properties of hemp fiber reinforced concrete
The compressive and flexural properties of hemp fiber reinforced concretes (FRC) were examined in this paper. Natural hemp fiber was mixed using dry and wet mixing methods to fabricate the FRC. Mechanical properties of the FRC were investigated. The main factors affecting compressive and flexural properties of the FRC materials were evaluated with an orthogonal test design. Fiber content by weight has the largest effect. The method for casting hemp FRC has been optimised. Under the optimum conditions, compressive strength increased by 4 %, flexural strength increased by 9 %, flexural toughness increased by 144 %, and flexural toughness index increased by 214 %.